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5 Minerals INTRODUCTION horses, balance studies need to be conducted that compare the amount of mineral consumed by the animal to the While constituting only a minor part of the equine diet amount of mineral lost in the feces and urine and deter- by weight, minerals play a critical role in the health of mined via total collections of both feces and urine. Collect- horses. Minerals are involved in a number of functions in ing the total amount of urine voided in a 24-hour period can the body, including physiological roles such as in acid-base be labor intensive. The use of a urinary fractional electrolyte balance, formation of structural components, enzymatic co- excretion test (FE test) has been used, which requires col- factors, and energy transfer. Some minerals are integral lecting only a single urine sample. Unfortunately, fractional parts of vitamins, hormones, and amino acids. The horse excretion values of minerals do not always agree with re- obtains most of the necessary minerals from forages and sults obtained through total volumetric urinary collection, concentrates. The mineral content of feeds and the avail- suggesting less accuracy (McKeever et al., 2002; McKenzie ability of minerals vary with soil mineral concentrations, et al., 2003). Some minerals are present in such minute con- plant species, stage of maturity, and conditions of harvest- centrations that determining requirements through balance ing. The resulting variations in feed mineral content should studies is extremely difficult. Furthermore, mineral balance be considered in assessing an animal’s mineral status and studies have not been done in all ages and classes of horses. formulating appropriate diets, as minerals are elements that In such cases, requirements are estimated by examining cannot be created or destroyed under normal circumstances what amounts have been fed without negative effects being and must be provided in the ration. Minerals are typically observed and, as a result, might be more appropriately classified as macrominerals—those typically found and termed “recommendations.” Sulfur and many of the mi- needed in concentrations in the diet measured in g/kg or crominerals would fit this description. For minerals such as percentage—and microminerals—those measured in ppm these, the research has tended to examine concentrations in or mg/kg. While the amounts of individual minerals in the the diet. Besides still being provided as a concentration, the ration are important, the ratios of all minerals should be committee has made a transformation, based on expected taken into consideration, as minerals often influence the ab- daily intakes, in an attempt to determine what would be a sorption, metabolism, and/or excretion of other nutrients. reasonable recommendation for a daily allotment on a body Therefore, excesses or deficiencies of certain minerals can weight basis. The committee encourages researchers to re- alter the requirements of others. When appropriate, the port data from future studies on a body weight basis instead maximum tolerable concentration of a mineral is provided of simply as a dietary concentration to aid in determining from the publication Mineral Tolerance of Animals (NRC, actual requirements. The committee also encourages more 2005). This concentration is defined as the dietary amount studies aimed at determining mineral requirements for that, when fed for an extended period of time, will not im- growth, exercise, and pregnancy, as these are areas in which pair animal health and performance. This differs from a data seemed to be insufficient to adequately determine min- toxic amount. Also, this tolerable concentration is based eral requirements. For instance, in pregnancy, it is recog- upon all other nutrients in the diet being at or near the ani- nized that additional minerals are needed for embryonic and mal’s requirement. When other minerals are provided at fetal growth, including those needed in the uterine fluid and higher or lower concentrations, absorption of the mineral in for growth of the uterus. Though the additional amount question can be altered, thus influencing the maximum tol- needed might be small, it is likely important, but not enough erable concentration. To determine mineral requirements of data are available to make strong conclusions. 69

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70 NUTRIENT REQUIREMENTS OF HORSES When sufficient data are available, the apparent di- Serum ferritin status is believed to be a better indicator of gestibility of a given mineral can be calculated by subtract- iron status than is blood or plasma iron concentration. With ing the amount of a mineral in the feces from the amount fed most minerals, ration evaluation and case history are still to the horse, then dividing that difference by the total often needed to confirm problems. Additionally, proper amount ingested. While easier to determine than the true di- sampling techniques and analysis need to be performed to gestibility, the apparent digestibility often is lower as some ensure the greatest likelihood of meaningful results. of the nutrient found in the feces can come from endogenous In some studies on mineral requirements of horses, the losses instead of a mineral that was not absorbed. To deter- daily intake of the mineral of interest is reported on a body mine the true digestibility of a mineral, endogenous losses weight basis. However, in other studies, intake is reported as need to be determined and, to do so, various concentrations a concentration of the diet. Whenever possible, daily intake of a mineral are fed. The regression line obtained by plotting was converted to a body weight basis using feed intake and the amount of a mineral apparently absorbed (Y-axis) body weight information presented in individual studies. against the amount of a mineral fed (X-axis) can be used to When feed intake was not reported, assumptions were made estimate endogenous mineral losses, or the point where the as to the average intake of each class of horse so that it was mineral intake is at zero. In this procedure, various assump- possible to provide an estimate of the amount of a given tions have to be made. One assumption is that absorption of mineral needed per kg of body weight (BW). Obviously the mineral follows a linear function. Another assumption is these estimates of feed intakes are not constant among that endogenous secretions are constant. True digestibility is horses but are needed to provide estimates of daily nutrient then represented as the amount absorbed (which is the intakes. Horses at maintenance were assumed to eat 2 per- amount of mineral fed minus the amount of mineral in feces cent of their BW/d in dry matter feed, while growing horses and in the endogenous secretions) divided by the amount were assumed to eat 2.5 percent, pregnant mares were as- fed. Alternatively, true digestibility of the nutrient can be sumed to eat 2 percent of their nonpregnant body weight, represented by the slope of the regression of the amount of and lactating mares were assumed to eat 2.5 percent feed on mineral absorbed on dietary mineral intake (Pagan, 1994). a dry matter basis. With increasing intensities and durations By plotting retention (Y-axis) against intake (X-axis), a pre- of exercise, dry matter intake typically increases, though this diction can be made as to what dietary intake would result does not necessarily occur—particularly if fat is substituted in no mineral being retained and the mineral balance would for carbohydrates in the diet. For light exercise, a dry matter be zero, thus representing minimal requirements (Hintz and intake of 2 percent was assumed, while 2.25 percent and 2.5 Schryver, 1972). Limitations to these methods include im- percent of BW/d were assumed to be the intake of moderate proper mineral recovery from the urine or feces due to im- and heavily working horses, respectively. As noted previ- proper laboratory procedures (O’Connor and Nielsen, 2006) ously, these assumptions were only used when there was in- or not accounting for minerals lost through avenues other sufficient information reported in an individual study to cal- than the urine or feces. Concentrations of minerals such as culate mineral intake on a body weight basis. These sodium and chloride need to be examined in other body se- estimates of intake are somewhat liberal and thus may over- cretions such as sweat, particularly in the exercising animal estimate the actual feed intake of study animals. However, (Coenen, 2005). Animals retaining more mineral than they by using a liberal estimate of intake, it seemed less likely are losing are considered to be in a positive mineral balance. that a requirement expressed on a body weight basis would This is expected when an animal is growing and accreting be underestimated. As there is an apparent lack of research minerals in various tissues. Once an animal is mature and in in determining mineral requirements of breeding stallions, a homeostatic state, it is more likely that an animal should they can be considered to be similar to maintenance values, be in a near zero balance, though a slight positive balance though it is possible some differences do exist. may exist due to unaccounted-for mineral losses in tissues, such as hair or hoof growth. While analyzing substances, MACROMINERALS such as the hair of horses, has been performed by some in- vestigators in an attempt to determine mineral status of Calcium horses (Asano et al., 2002), such techniques are of question- able use as they can be influenced by factors, such as coat Function color, that are independent of nutrition (Cape and Hintz, 1982; Asano et al., 2005). In contrast, mineral concentra- About 99 percent of the calcium (Ca) in the body is found tions in the blood can sometimes be useful in determining in the bones and teeth, with calcium constituting about 35 whether a dietary deficiency or excess is present. Blood percent of equine bone (El Shorafa et al., 1979). Calcium samples are more likely to provide information about phos- also plays an important role in various functions within the phorus, magnesium, potassium, sodium, chloride, copper, body such as muscle contraction, the function of cell mem- manganese, selenium, and zinc status of a horse than they branes, blood coagulation, and the regulation of many en- are the calcium, iodine, and cobalt status (Lewis, 1995). zymes. Calcium homeostasis within the blood is critical. The

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MINERALS 71 skeleton, besides serving as structural support for the body, cium digestibility of 72 percent for alfalfa compared to only can serve as a readily available storage location for calcium. 40 percent for Bermudagrass. When mature ponies were fed 316 and 535 mg Ca/kg BW/d, van Doorn et al. (2004b) re- ported the apparent digestibility of calcium was around 28 Sources and Factors Influencing Absorption percent as compared to 42 percent when the ponies were fed Calcium carbonate, sulfate, and oxide are common inor- 148 mg Ca/kg of BW/d (dietary concentration equal to 0.78 ganic forms of calcium (Highfill et al., 2005). A calcium- percent calcium, which is still excessive for a maintenance amino acid proteinate did not differ from calcium carbonate diet). All ponies were in positive calcium balance, though in absorption rate though more calcium was absorbed in the calcium retention was greatest at the highest concentration two supplemented groups than in a nonsupplemented diet of calcium fed. As mature animals would be expected to (Highfill et al., 2005). Free-choice feeding of calcium supple- maintain a balance close to zero by adapting the digestibil- ments is not an effective means of ensuring adequate intake ity or urinary excretion, van Doorn et al. (2004b) suggested (Hintz, 1987a). Calcium supplements should be mixed with that when the amount of absorbed calcium exceeds the grain or other palatable materials to help ensure consumption. amount that can be excreted with urine, the additional cal- The portion of dietary calcium that is absorbed varies in cium is retained, resulting in a positive balance. In what tis- order to maintain normal calcium homeostasis (Jones and sue(s) the extra calcium is retained is unclear, though it is Rasmusson, 1980). In response to lowered ionized serum expected that the animals would need to eventually return to calcium concentrations, parathyroid hormone (PTH) secre- a homeostatic state if they were examined long enough, un- tion is stimulated in order to reestablish normal concentra- less calcium is lost from hair, hoof, or sweat and not ac- tions through stimulation of bone resorption and renal tubu- counted for. While Hintz and Schryver (1973) reported that lar calcium reabsorption (Bushinsky and Monk, 1998). increasing the dietary concentration of magnesium in- Vitamin D mediates intestinal absorption of calcium and creased calcium absorption, the absorption efficiency of cal- adaptation to dietary intake in most species, though Brei- cium decreases as phosphorus increases in the diet due to denbach et al. (1998) suggest that it does not play a key role the competitive nature of calcium and phosphorus absorp- in regulating calcium and inorganic phosphate homeostasis tion in the small intestine. This likely varies somewhat de- in the horse. Horses have a moderate or lacking increase of pending upon mineral source. Schryver et al. (1987b) re- plasma calcium and at the same time a pronounced increase ported true calcium digestibility increasing from 51 to 69 in plasma inorganic phosphate during vitamin D intoxifica- percent as the percentage of sodium chloride in the diet in- tion, potentially suggesting a different regulatory role of vi- creased. Dietary oxalate decreases absorption of calcium tamin D in horses than in other species (Harmeyer and quite dramatically and can play a role in a deficiency of ab- Schlumbohm, 2004). More research would need to be done sorbed calcium when not taken into consideration when bal- to confirm this theory. ancing diets. Swartzmann et al. (1978) reported a reduction An absorption efficiency of 50 percent is used for all ages of calcium absorption by about 66 percent by the inclusion of horses. The true absorption efficiency can be as high as of 1 percent oxalic acid in equine diets. McKenzie et al. 70 percent with young horses but appears to decline as a (1981) reported total dietary oxalate concentrations of 2.6 to horse matures. Throughout a horse’s life, calcium absorp- 4.3 percent decreased calcium absorption as evidenced by a tion can vary depending upon many factors, so determining negative calcium balance resulting from a doubling of fecal a specific absorption rate in an individual animal is difficult. calcium and decreased urinary calcium in comparison to Additionally, many mineral balance studies last 10 days or control horses. Similar negative balances for calcium were less, and horses likely are able to modulate the absorption observed by Blaney et al. (1981) in horses fed various trop- efficiency of calcium to account for lowered dietary intakes ical grass hays containing more than 0.5 percent total ox- (Hintz, 2000). Thus, a 50 percent absorption efficiency may alate. When the calcium:oxalate ratio on a weight-to-weight be low, especially when calcium concentrations in the diet basis was less than 0.5, it was concluded that horses could are low. However, Hintz (2000) contended making a liberal be at risk for nutritional secondary hyperparathyroidism. No estimate is better than making a conservative estimate, as it difference in calcium absorption was reported by Hintz et al. will decrease the likelihood of developing a calcium defi- (1984) from alfalfas containing 0.5 and 0.87 percent oxalic ciency. Pagan (1994) reported true calcium digestibility es- acid in which the calcium:oxalate ratios were 3 and 1.7, re- tablished over the course of many trials to be around 75 per- spectively. By contrast, there have been a few reports of cal- cent in mature horses. Other factors affecting calcium cinosis in horses due to the consumption of calcinogenic absorption include concentrations of calcium, phosphorus, plants (Mello, 2003). The plants that have been documented phytate, and oxalate in the diet. As calcium concentrations to affect horses are Cestrum diurnum (in Florida) and Trice- increase, the absorption efficiency typically decreases; how- tum flavescens (in Austria). These plants increase release of ever, apparent calcium absorption was higher in horses fed 1,25(OH)2D3, and can greatly increase calcium absorption, alfalfa than grass hays (Cuddeford et al., 1990; Crozier et thereby resulting in hypercalcemia. Fortunately, such situa- al., 1997). Sturgeon et al. (2000) reported an apparent cal- tions are rare.

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72 NUTRIENT REQUIREMENTS OF HORSES Both intake and type of diet influence renal calcium ex- maintain blood calcium concentration within such a tight cretion (Meyer, 1990). Calcium is not absorbed from the range renders total serum calcium a poor indicator of cal- large intestine, emphasizing the need to have sufficient cium status (Krook and Lowe, 1964) so even when an un- amounts to ensure adequate absorption from the small intes- balanced diet is fed, calcium concentrations in the blood can tine (Stadermann et al., 1992). Dietary constituents can in- remain relatively constant (de Behr et al., 2003). When frac- fluence digestibility as prececal apparent digestibility of cal- tionated out using a micropartition system, Lopez et al. cium is higher with alfalfa hay than with concentrate (2006) reported total calcium in the serum of healthy horses (Stadermann et al., 1992). Hoffman et al. (2000) reported to be composed of ionized calcium (48.5 ± 0.7 percent), lowered bone mineral content of the third metacarpus when protein-bound calcium (47.4 ± 0.9 percent), and calcium feeding a fat- and fiber-based diet compared to a sugar- and complexed with weak acids (4.1 ± 0.9 percent). The ionized starch-based diet, and thus expressed concerns over the po- fraction is thought to be the only physiologically active form tential binding of calcium by fat and fiber. A follow-up study and thus is the form that should be evaluated for clinical showed that supplementing more than 0.9 percent calcium work. A normal reference range for total serum calcium has in a sweet feed had no effect on mineral content of the third been given as 10.8 to 13.5 mg/dl serum and for ionized metacarpus in fat- and fiber-fed foals, thus leading the au- serum calcium as 6.44 to 6.74 mg/dl serum (Garcia-Lopez et thors to conclude that the transient lower bone mineral con- al., 2001). An ionized serum calcium concentration of less tent observed in the prior study may be better attributed to than 6 mg/dl serum has also been used to define horses as nutrient-endocrine interactions than to calcium insufficiency hypocalcemic by Toribio et al. (2001). Calcium deficiencies (Hoffman et al., 2001). Grace et al. (2003) reported no often have a dramatic impact on skeletal integrity. A defi- change in apparent absorption of calcium when weanlings ciency of calcium in the developing foal can lead to os- were fed diets ranging in concentration from 3.5 to 12 g teopenia. This condition is characterized by poor mineral- Ca/kg DM. Additionally, no changes in bone strength were ization of the osteoid tissue and the probability of enlarged related to dietary calcium concentration. Though hypothe- joints and crooked long bones. A survey of the severity of sized that diets high in protein increase urinary calcium ex- metabolic bone disease in yearlings and diet analysis on 19 cretion with potential detriment to bone, no influence on Ohio and Kentucky horse farms revealed a negative linear bone density was reported due to varying protein intakes relationship between dietary calcium intake and perceived (Schryver et al., 1987a; Spooner et al., 2005) or protein severity of metabolic bone disease in young horses (Knight quality (Smith et al., 2005). Besides diet, other factors can et al., 1985). Farms with yearlings having the lowest inci- influence absorption. Cymbaluk et al. (1989) reported true dence of metabolic bone disease were fed diets containing calcium digestibilities decreased substantially from 6 to 24 1.2 percent calcium, whereas yearlings with the most severe months of age (though this was confounded with a change metabolic bone disease were on farms that fed diets with 0.2 in calcium source), and the efficiency of calcium absorption percent calcium. It should be noted that this report did not was influenced by varying requirements during different have data on what the horses were consuming prior to the stages of training (Stephens et al., 2001). At times when survey and it did not report differences in exercise afforded mineral deposition is occurring in bone, requirements would to the young horses—both of which likely could have influ- be expected to increase. Bone serves as a reservoir for enced results. In the mature horse, inadequate dietary cal- calcium, and more calcium is needed from the diet when cium can result in weakening of the bones and an insidious calcium is being placed into bone as compared to when cal- shifting lameness (Krook and Lowe, 1964). cium is being removed from bone during periods of disuse Whitlock et al. (1970) fed diets with calcium:phosphorus (Bronner, 1993). During initial periods of disuse, the cal- ratios of 1.16:1 (0.43 percent calcium) and 4.12:1 (1.96 per- cium goes into the plasma, from which excess is secreted cent calcium) and observed a greater proportion of lamellar from the body. This appears to decrease calcium require- bone than osteonic bone in high-calcium horses; however, ments (Nielsen et al., 1998a). Likewise, when mineral is no clinically deleterious effects or gross morphological dif- being deposited into bone at times when exercise intensity is ferences were detected. Krook and Maylin (1988) proposed increasing, requirements are likely higher than when inten- that osteochondrosis may be associated with excess dietary sity of training is relatively constant. Exercise in mature calcium (e.g., from alfalfa hay) by producing hypercalci- horses did not alter apparent calcium digestibility (about 54 toninism. However, calcium has been fed at more than five percent: Pagan et al., 1998). times the required concentration without detrimental ef- fects, provided the phosphorus concentration is adequate (Jordan et al., 1975). When provided adequate, but not ex- Signs of Deficiency or Excess cessive, amounts of phosphorus, the maximum tolerable Removing calcium from the skeleton to meet metabolic concentration of calcium in horse feed has been given as 2 demands when dietary calcium intake is inadequate can re- percent of the diet (NRC, 2005). Recent work in other sult in a weakened skeleton if done in excess. The need to species has shown that calcium influences gastrin secretion

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MINERALS 73 (Cheng et al., 1999; Dufner et al., 2005), indicating unnec- quirements likely go up, even if horses are just returned to essarily high dietary calcium may be implicated in gastric a pasture setting to compensate for bone loss associated ulcers and should be avoided. More details on the cal- with disuse. cium:phosphorus ratio are given in the phosphorus section Using the estimate of Schryver et al. (1974) that growing and in Chapter 12. foals deposit approximately 16 g Ca/kg of gain, a 215-kg Some work has been done in utilizing dietary cation- foal gaining 0.85 kg/d and having a calcium absorption effi- anion difference (DCAD) to influence calcium utilization. A ciency of 50 percent would require 27.2 g/d (16 g × 0.85 kg low DCAD (defined as mEq (sodium + potassium) – chloride/ of gain/0.5) of dietary calcium for skeletal growth plus 8.6 kg dry matter) has been shown to increase urinary calcium g/d (215 kg × 20 mg/0.5) to meet endogenous losses. How- loss in exercising horses (Wall et al., 1992) and perhaps ever, a study by Cymbaluk et al. (1989) estimated endoge- leads to a calcium deficiency in growing horses (Wall et al., nous fecal calcium to be 36 mg/kg BW in growing Quarter 1997). However, Cooper et al. (2000) reported that when fed horses. While Hintz (1996) suggested there is little evidence a lower DCAD (defined as mEq (sodium + potassium) – for the calcium requirements of growing horses to be greatly (chloride + sulfur)/kg of diet DM), horses increased intes- changed from the 1989 NRC’s recommendations, the appar- tinal calcium absorption to compensate for increased urinary ent greater endogenous losses in growing horses compared excretion of calcium. Using that same definition, Baker et al. to mature horses would increase the requirements of a 215- (1998) indicated a low DCAD diet formulated with ammo- kg foal by an additional 6.9 g/d over what the 1989 NRC nium chloride resulted in a decrease in apparent calcium suggested—up to 42.7 g. While these horses were on a rela- balance. Cooper et al. (2000) suggested the horse is able to tively high-forage diet during the study (about 50 to 90 per- maintain metabolic equilibrium despite the ratio of cations cent), it is reasonable to believe growing horses have greater to anions in the diet and that horses can tolerate a wide vari- endogenous losses than a mature horse. In addition, Moffett ation in DCAD without experiencing adverse effects on et al. (2001) reported increased daily gains, as well as in- skeletal growth. Likewise, McKenzie et al. (2002) found creased calcium retention, when yearlings were fed 0.48 that varying DCAD did not alter daily balance of calcium percent calcium compared to 0.32 percent. Thus, the cal- and phosphorus. Data from Cooper et al. (1998) obtained cium requirement for growing horses not in training was in- after a 21-day diet adaptation period suggested horses are creased to meet endogenous losses of 36 mg Ca/kg BW with able to maintain normal acid-base status regardless of the di- a 50 percent absorption rate and to meet growth require- etary cation-anion balance of the diet. Whether this would ments of 16 g Ca/kg BW gain with a 50 percent absorption be true in a longer duration study is not known. rate. The resultant requirement for calcium is (0.072 g × kg BW) + (32 g × ADG in kg). In late gestation, calcium requirements for the mare are Recommendations increased to meet the needs of fetal growth and tissue de- Endogenous losses of calcium have been estimated to be velopment. Approximately 11.1, 25.3, and 11.4 mg of Ca/kg 20 mg Ca/kg BW/d (Schryver et al., 1970, 1971a). Using of mare body weight are deposited daily in the fetus and that estimate and the absorption efficiency of 50 percent, membranes of mares in months 9, 10, and 11, respectively the 1989 NRC proposed a 500-kg horse would require 20 g (Meyer and Ahlswede, 1978; Drepper et al., 1982). Though (500 kg × 20 mg/0.5) of dietary calcium (0.04 g of Ca/kg the substantial increase at 10 months of gestation is suspect, BW) or 1.22 g Ca/Mcal DE (digestible energy)/d for main- their findings may be supported by a study by House and tenance. Using data compiled from many studies, Pagan Bell (1993) that examined fetal calves from slaughtered (1994) estimated endogenous calcium loss to be 17.4 g/d cows. At the end of gestation, calcium accretion in the fetus (R2 = 0.94) for a 550-kg horse (about 31.6 mg/kg BW), was 10.3 g/d at 280 days of gestation. Using the average cow which is higher than the 1989 NRC values. The true di- weight of 714 kg, 14.4 mg of calcium were being deposited gestibility of calcium was estimated at 74.7 percent, which per kg of BW, which is comparable to the 11.4 mg Ca/kg of is also higher than 1989 NRC values. However, the result- BW reported in horses at the end of gestation. Given that ing requirement for a 500-kg horse (21.2 g Ca/d) is similar fetal calf growth increased from 329 g/d at 200 days post- to the 1989 NRC requirement (20 g/d). Regression analyses mating to 456 g/d at 242 days post-mating and then declined on the calcium data of Buchholz-Bryant et al. (2001) using to 296 g/d at 280 days of gestation, it seems plausible that 12 sedentary geldings of three different age groups at four the increased deposition of calcium in the 10th month of time periods gives a calculated requirement of 0.043 g gestation in horses also represents the greatest period of Ca/kg of BW. This would represent 21 g Ca/d for a 500-kg fetal foal growth and the greatest need for calcium. Using an horse, which is similar to the recommendation of the 1989 absorption efficiency of 50 percent, the calcium requirement NRC. Thus, the subcommittee saw no reason to change the during months 9, 10, and 11 of gestation for a 500-kg mare maintenance calcium requirements. As noted in the section would be 11, 25, and 11 g/d, respectively, for fetal develop- on exercise, after extended periods of stall rest, calcium re- ment. As data on the deposition rate of minerals in the fetus

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74 NUTRIENT REQUIREMENTS OF HORSES are very limited, a mean deposition rate (15.9 mg Ca/kg partum week to 0.8 g/kg of fluid milk during weeks 15 to 17 BW) was used for the last 3 months (NRC, 1989). Using the postpartum. This is consistent with the data of Baucus et al. 50-percent absorption rate, 0.032 g Ca/kg BW is needed in (1987). For a 500-kg mare producing 16 kg milk/d in early addition to maintenance requirements during the last 3 lactation and with a calcium absorption efficiency of 50 per- months of gestation. Martin et al. (1996) evaluated prepar- cent, the daily dietary calcium requirement would be 38.4 g tum mares fed either 0.55 percent calcium (0.076 g Ca/kg (16 kg × 1.2 g/0.5) for milk production, in addition to 20 g BW; above the 1989 NRC suggested minimum of 0.45 per- for maintenance. This estimate of calcium requirement for cent) or 0.35 percent calcium (0.045 g Ca/kg BW; below the milk production is consistent with that of Jarrige and suggested minimum) and concluded that the optimal dietary Martin-Rosset (1981). When fed at the 1989 NRC recom- calcium concentration for prepartum mares was closer to mended concentration, mares still lost bone density from the 0.55 percent than 0.35 percent. The dietary concentration of third metacarpal bone during the first 12 weeks of lactation 0.45 percent calcium recommended by the 1989 NRC for but density was restored by 24 weeks postpartum (Glade, pregnant mares was not evaluated in this study, which ap- 1993). Mares fed 20 percent less calcium had not fully re- peared to confirm that 0.35 percent dietary calcium was not covered third metacarpal bone density by 20 weeks after adequate for prepartum mares. However, in the study by milk production had ceased (40 weeks after parturition). Martin et al. (1996), the two treatment groups were main- This suggests that rebred mares not fed sufficient calcium tained on separate farms and the treatment groups were not during lactation would have a greater demand for calcium balanced for breed. In addition, the dietary calcium concen- throughout gestation as they attempt to restore the mineral tration was determined from estimated pasture intake lost from the skeleton during lactation. This work reaffirms (which was very different for the two groups), so the results that the dietary calcium recommendation of the 1989 NRC may have been influenced by factors other than dietary cal- for lactation should not be reduced, though it is uncertain if cium concentration. However, Glade (1993) confirmed that the recommendation should be increased. Thus, the recom- the dietary amount of calcium recommended for brood- mendations remain unchanged, although new estimates of mares is a minimum and should not be reduced. Foals of milk production are used. Milk production is estimated at mares fed calcium lower than the 1989 NRC requirements 0.032 kg milk per kg BW from foaling to 3 months, 0.026 had mechanically weaker bones at birth compared to foals kg milk per kg BW from 4 to 5 months, and 0.020 kg milk of mares fed the 1989 NRC recommended amounts. While per kg BW after 5 months. The resultant equations for cal- the 1989 NRC based its recommendation on DE, the current cium requirements during lactation are: method of determination results in a similar requirement and, at this time, there is no strong evidence to suggest a re- Foaling to 3 months = (0.04 g × kg BW) + vision of that requirement. However, considering there is (0.032 × kg BW × 2.4 g) substantial fetal growth during months 7 and 8 of gestation, the requirement for those months is suggested as being an 4–5 months = (0.04 g × kg BW) + (0.026 × kg BW × 1.6 g) average of maintenance and late gestation. In summary, to meet endogenous losses of 20 mg Ca/kg BW with an ab- > 5 months = (0.04 g × kg BW) + (0.020 × kg BW × 1.6 g) sorption rate of 50 percent and to meet fetal deposition rate of 15.9 mg Ca/kg BW with a 50 percent absorption rate, the Previously, it had been assumed that any additional cal- calcium requirement during months 9, 10, and 11 of preg- cium required for exercise would be met through the addi- nancy is calculated as (0.04 g Ca × kg BW) + (0.032 g Ca × tional feed consumed to meet energy demands (NRC, 1989). kg BW) or simply (0.072 g Ca × kg BW). To meet endoge- The need for calcium is greatly influenced by bone develop- nous losses of 20 mg Ca/kg BW with an absorption rate of ment. Bone formation is impacted by exercise, or the lack 50 percent and to meet fetal deposition rate of 8 mg Ca/kg thereof. When net bone deposition is increased, so is the BW with a 50 percent absorption rate, the requirement for need for calcium. When bone loss is occurring, requirements pregnancy during months 7 and 8 of gestation is calculated for calcium are lower as calcium is removed from the skele- as (0.04 g × kg BW) + (0.016 g × kg BW) or simply (0.056 g ton, goes into the plasma, and is lost through the urine and × kg BW). feces with an accompanying decreased need for calcium ab- Though commonly recognized, additional demand for sorption (Nielsen et al., 1998a). Unfortunately, plasma cal- calcium during lactation was reconfirmed by Martin et al. cium concentrations are of little use in assessing net calcium (1996), who reported an increase in serum parathyroid hor- balance as horses in a negative calcium balance commonly mone concentrations with a lactation-induced decrease in have normal plasma or serum values (Rose, 1990) despite serum total and ionized calcium beginning 3 days prepartum measurable changes in bone mineral content. Estimates of and continuing until 2 days postpartum. Schryver et al. mineral content of the lower leg are reduced in young horses (1986b) reported that the daily calcium requirements for lac- that are stalled without sufficient access to exercise as com- tation range from 1.2 g/kg of fluid milk during the first post- pared to horses kept on pasture (Hoekstra et al., 1999; Bell

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MINERALS 75 et al., 2001; Brama et al., 2002). Porr et al. (1998) reported 1989 NRC recommended calcium amount were fed com- stall rest for 12 weeks in mature Arabians similarly de- pared to 97 percent and 136 percent. Likewise, Michael et creased third metacarpal bone mineral content estimates, al. (2001) reported increased markers of bone formation and which was not prevented by dietary calcium intake at twice decreased markers of bone resorption in horses fed the two the 1989 NRC recommended concentration (an excess of higher percentages of calcium compared to the two lower about 20 g of calcium). To compensate for mineral loss from percentages. Unfortunately, neither Nolan et al. (2001) nor the skeleton, Meyer (1987) suggested feeding 20 percent Michael et al. (2001) reported weight of the horses or feed above the recommended amounts of calcium and phospho- intake, so calculating absolute intake on a body weight basis rus if horses have a prolonged period without activity once is not possible. If the feed intake of the horses was relatively normal activity has returned. Short sprints are sufficient to low, the absolute amount taken in may have been closer to stimulate bone formation, while exercising for longer dis- the amount recommended by the 1989 NRC than the con- tances, but at slower speeds, is less efficient. Vervuert et al. centrations would make it appear. Stephens et al. (2004) re- (2005a) reported draught load exercise with low velocities ported calcium retention to be maximal when calcium intake had no effect on calcium metabolism. was 123 mg Ca/kg BW/d, which represents a 36 percent in- In contrast, despite receiving the exact same diet, a single crease over the 1989 NRC recommendations for exercise. 82-m sprint five days per week increased indirect measures However, variation was great (R2 = 0.23) for calcium reten- of third metacarpal bone mineral content and improved third tion vs. intake. Total concentrate and hay intake averaged metacarpus architecture in stalled weanlings compared to 18 g/kg BW. Calcium intake was 135 ± 6 mg/kg BW/d at the stalled weanlings afforded no exercise (Hiney et al., 2004). start of the study and was 123 ± 4 mg/kg BW/d at the com- These studies demonstrate the importance of exercise in pletion of the study at day 128. In these studies, the effects stimulating calcium deposition in the skeleton. Interestingly, of additional calcium are often confounded with varying a 10-year study in humans showed that calcium intake over amounts of phosphorus and magnesium but demonstrate po- that period was not associated with bone gain or bone tential improvements in bone mineral content, and poten- strength (Lloyd et al., 2004). Instead, only exercise during tially strength, by feeding additional calcium over what was adolescence, as determined by a sports exercise question- recommended by the 1989 NRC. When only the calcium naire, was significantly associated with increased bone min- concentration was altered, Schryver et al. (1978) suggested eral density and bone bending strength (P < 0.01). Similarly, that 0.6 percent dietary calcium was not needed, compared Nielsen (2005) reported a much greater role in influencing to 0.4 percent calcium, when fed to yearling Standardbreds markers of bone turnover in studies related to differences in entering training. For horses consuming 2 percent of their exercise as compared to nutrition. Hence, the importance of body weight per day, as was the case in many of these stud- exercise in young horses to maximize bone strength is un- ies, this would equate to 0.08 g Ca/kg BW. This is also sup- derscored. If sufficient exercise is provided, extra dietary ported by the data of Buchholz-Bryant et al. (2001). When calcium can likely be utilized to improve bone strength. Ex- their data for calcium intake for young horses in training are ercise, particularly in the young horse, appears to increase plotted against the amount retained, the linear regression re- calcium requirements above maintenance requirements. sponse shows a requirement of 0.079 g Ca/kg BW. For Gray et al. (1988) found horses in training had lower urinary horses that are rapidly growing (typically under 24 months calcium excretion, despite receiving more calcium, than of age), these requirements would usually be met by the re- horses not working, suggesting exercise increases the need quirements for growth ((0.072 g Ca × kg BW) + (32 g Ca × for calcium. Exercised horses had a higher calcium balance ADG in kg)) that have been increased from the 1989 NRC. than horses that were sedentary for 2 months and then exer- As a result, the requirement for growing horses that are ex- cised for 2 months (Elmore-Smith et al., 1999). In response ercising is the same as the requirement for growth. Typi- to previously measured declines in mineral content of the cally, this is equal or greater than the apparent recommen- third metacarpal bone (Nielsen et al., 1997, 1998a), calcium dation for heavy exercise (0.08 g Ca × kg BW) only when balance and retention were determined in 2-year-old race- average daily gain drops to 0.1 kg. It is likely the calcium re- horses placed into training receiving a total diet containing quirement associated with exercise is less for mature horses. either 0.31 percent calcium (0.063 g × kg BW) or 0.38 per- Mansell et al. (1999) found little difference in the response cent calcium (0.074 g × kg BW) on an as-fed basis and con- of bone to varying concentrations of dietary calcium but suming about 2 percent of their body weight daily (Nielsen found bigger differences depending upon the age of horses. et al., 1998b). Calcium retention was greater, as was mineral When calcium intake data of Buchholz-Bryant et al. (2001) content of the third metacarpal bone, in horses fed 0.38 per- for mature horses are plotted against retention, linear re- cent calcium (0.074 g × kg BW). Other studies also support gression reveals a requirement of 0.04 g Ca/kg BW/d—the the need for additional calcium for horses in training. Nolan same as is required for maintenance. While it is likely the et al. (2001) reported increased mineralization of the third calcium requirement for mature horses is less than that of metacarpal bone when 151 percent and 169 percent of the the younger horse, an insufficient number of studies have

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76 NUTRIENT REQUIREMENTS OF HORSES been performed to warrant lowering the requirement for in dietary sodium chloride from 1 percent to 5 percent in- heavy exercise in mature horses at this time. Because forces creased phosphorus absorption from 28 to 40 percent applied to the skeleton primarily regulate bone mineral dep- (Schryver et al., 1987b). Despite concern with aluminum osition, which is one of the major needs for additional cal- interference in phosphorus absorption, phosphorus absorp- cium, establishing calcium requirements for exercise is dif- tion was not affected by feeding an aluminum (Al) supple- ficult and will vary depending upon the intensity of training ment (931 ppm Al/kg feed or 12 mg of Al/kg BW/d) for 1 and the influence upon the skeleton. The requirement will month (Roose et al., 2001). likely be closer to maintenance for horses not experiencing Diets containing high concentrations of oxalates depress any high-speed work and for horses that are not increasing phosphorus retention (Blaney et al., 1981; McKenzie et al., their intensity of training. Thus, the calcium recommenda- 1981). Phytate phosphorus, the salt of phytic acid and a pre- tions for light and moderate exercise can be expected to dominant form of phosphorus in plants, is poorly absorbed be lower than that of heavy exercise and have been set at by horses, though some phytase exists in the hindgut (Hintz 0.06 g Ca × kg BW for light exercise and 0.07 g Ca × kg BW et al., 1973). To meet phosphorus requirements, inorganic for moderate exercise. While a heavily exercising horse may phosphates are often added to the equine diet and this could have only a small increased need for extra calcium com- be at least partially avoided if the absorbability of phytate pared to a lightly exercising horse if the exercise intensity phosphorus could be increased by an appropriate use of phy- has been constant, these recommendations are established to tates (Eeckhout and De Paepe, 1994). Despite an abundance minimize or eliminate problems as the intensity of exercise of research on the use of phytase to increase phosphorus increases. availability in other species, Morris-Stoker et al. (2001) re- ported no benefit to horses of feeding phytase (200 units/g) at the rate of 1 g/kg of feed. Patterson et al. (2002) also Phosphorus found no improvement in phosphorus apparent digestibility with the addition of phytase up to 900 Phytase unit Function (FTU)/kg of diet. When horses were fed four different diets Like calcium, phosphorus (P) is a major constituent of (Coasta Bermudagrass with whole oats, alfalfa cubes, a tex- bone, making up 14 to 17 percent of the skeleton (El Shorafa tured sweet feed, or a pelleted concentrate), Hainze et al. et al., 1979). In addition, it is required for many energy (2004) found phytase decreased fecal phosphorus only for transfer reactions associated with adenosine diphosphate the sweet feed-based diet, due largely to decreased fecal out- (ADP) and adenosine triphosphate (ATP), and for the syn- put of the insoluble fraction of phosphorus. Because horse thesis of phospholipids, nucleic acids, and phosphoproteins. manure contains a lower proportion of total phosphorus as the water-soluble phosphorus fraction compared with that from other farm animals, phosphorus in horse feces may be Sources and Factors Influencing Absorption less prone to runoff in typical pasture-based management True phosphorus absorption is quite variable and typi- systems. Hainze et al. (2004) concluded that under normal cally ranges from 30–55 percent. It varies depending upon feeding conditions, phytase probably has limited potential other dietary constituents, how much and what type of for decreasing concerns over environmental phosphorus phosphorus is fed, and the age of the horse. High calcium contamination as the proportion of fecal phosphorus repre- concentrations in the diet depress phosphorus absorption. sented by the soluble fraction is increased. However, Warren In diets averaging 0.89 percent calcium, Pagan (1994) re- (2003) cautioned that, though equine research has not shown ported true phosphorus absorption to be around 25 percent. phytase to be useful in reducing phosphorus excretion, eval- Similarly, phosphorus absorption of a high-phosphorus diet uated diets have had sufficient phosphorus. A better test is to (0.125 g P/kg BW) decreased from 25 percent in horses determine if phytase can increase phosphorus utilization by receiving a diet containing 0.148 g Ca/kg BW/d to 11 per- the horse when dietary phosphorus concentrations are below cent and 13 percent when fed diets containing 0.316 required amounts (Warren, 2003). and 0.535 g Ca/kg BW/d, respectively (van Doorn et al., Phosphorus absorption is assumed to be higher by foals 2004b). All horses were in positive phosphorus balance. consuming milk than it is in mature horses, though Grace et High dietary phosphorus concentrations (1.19 percent al. (1999a) suggested creep feed be fed to nursing foals as phosphorus) have been shown to increase phosphorus re- milk may be insufficient in phosphorus (as well as calcium) tention and plasma phosphorus concentrations (Schryver et for optimal growth of foals. A phosphorus absorption effi- al., 1971b). Buchholz-Bryant et al. (2001) reported similar ciency of 35 percent is used for mature horses (except for findings when examining the effect of calcium and phos- lactating mares) as they primarily consume plant sources of phorus supplementation in young, mature, and aged horses phosphorus. For lactating mares and growing horses, a 45- with different training regimens. All groups retained phos- percent efficiency is used because their diets are often sup- phorus regardless of whether they were fed normal (0.24 plemented with inorganic phosphorus. Horses at 8 months percent) or high (0.57 percent) phosphorus diets. Increases were more efficient at utilizing phosphorus than horses at 12

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MINERALS 77 months (Cymbaluk, 1990). Environment appears to influ- often in the United States, but Hintz (1997) cautioned that ence phosphorus absorption as horses housed in a warm horse owners still need to be made aware of this potential barn had greater true phosphorus digestibility than those problem, particularly if horses are fed large amounts of housed in a cold barn (Cymbaluk, 1990). Similar to the find- grain-based feedstuffs such as wheat bran or oats. While this ings for calcium, Stephens et al. (2001) reported the effi- condition is rare, it can still occur in situations when horses ciency of absorption can vary with requirements. Absorp- are fed grains not supplemented with calcium and are re- tion, at least to a degree, apparently increases when demand ceiving forage relatively low in calcium or that contains sub- for phosphorus increases and may change with stage of stantial amounts of oxalates (Mason et al., 1988; Ronen et training though more studies are needed. al., 1992; Luthersson et al., 2005). Though typically not a concern with forages, certain types, such as orchardgrass, can have inverted calcium:phosphorus ratios. With grains Signs of Deficiency or Excess being naturally higher in phosphorus than calcium, it is quite Inadequate dietary phosphorus will, like calcium and vi- easy to feed too much phosphorus in relation to calcium tamin D, produce rachitic-like changes in growing horses when raw grains or grain byproducts are fed in large quanti- and osteomalacic changes in mature horses. Excess phos- ties. Clinical signs vary depending on the degree of the phorus reduces the rate of calcium absorption and leads to imbalance of calcium to phosphorus, oxalate content of chronic calcium deficiency and nutritional secondary hyper- the diet, age of the horse, performance level, and environ- parathyroidism (NSH). Clinically, NSH is characterized by mental conditions (Ramirez and Seahorn, 1997). While a shifting lameness and, in advanced cases, by enlargement of low calcium:phosphorus ratio can be quite detrimental, ra- the upper and lower jaws and facial crest (Krook and Lowe, tios as high as 6:1 in the growing horse may be acceptable if 1964). Savage et al. (1993a) reported that foals fed 388 per- phosphorus intake is adequate (Jordan et al., 1975). The cent of the 1989 NRC recommendation for phosphorus calcium:phosphorus ratio in milk has been reported to range showed numerous, severe lesions of osteochondrosis but no between 1.8 and 2.5:1 between weeks 16 and 24 of lactation clinical signs of NSH, though Savage et al. (1993b), in a his- (Sonntag et al., 1996). tomorphometric assessment of bone biopsies from the foals, reported that the changes were consistent with NSH. Recommendations Both organic and inorganic phosphorus exist in serum, though serum inorganic phosphorus is derived entirely from Endogenous losses of phosphorus by the mature horse inorganic compounds and is the value utilized in clinical have been estimated at 10 mg/kg BW/d (Schryver et al., practice (Lau, 1986). Serum inorganic phosphorus concen- 1971b). Combined with a 35 percent absorption efficiency, trations are high at birth and decline over several months to the 1989 NRC estimated the maintenance phosphorus re- normal adult values. Serum inorganic phosphorus values quirements for a 500-kg horse to be 14.3 g (0.028 g P/kg may be more indicative of dietary phosphorus status than BW). Using data from a number of studies, Pagan (1994) serum calcium is of calcium status because homeostatic estimated endogenous phosphorus loss to be 4.7 g/d (R2 = mechanisms for phosphorus are less sensitive than for cal- 0.33) for a 550-kg horse (about 8.5 mg/kg BW), which is cium (Schryver et al., 1970, 1971b). Caple et al. (1982) slightly lower than the 1989 NRC estimates. The true di- determined that horses excreting more than 15 µmol of gestibility of phosphorus was estimated to be 25.2 percent, P/mOsm of urine solute and having a phosphorus: creatinine which is also lower than the 35 percent used by the 1989 clearance ratio greater than 4 had excessive phosphorus in- NRC. However, the resulting requirement for a 500-kg take and were subject to NSH. It has been suggested that a horse was estimated at 17 g P/d which, although slightly maximum tolerable concentration of dietary phosphorus in higher, is similar to 1989 NRC values. This emphasizes the horses fed adequate dietary calcium is 1 percent assuming importance of knowing the availability of a phosphorus an appropriate calcium:phosphorus ratio (NRC, 2005). source when formulating rations as the absorption effi- ciency can greatly influence the amount of phosphorus needed in the diet. Due to concerns with extra phosphorus Calcium:Phosphorus Ratio being introduced into the environment, the subcommittee The absolute intakes of calcium and phosphorus by has chosen to remain with the lower estimate for phospho- horses must be adequate, but secondarily, it is important to rus requirements. evaluate the calcium:phosphorus ratio of equine rations. If As foals deposit about 8 g P/kg BW gain (Schryver et al., calcium intake is less than phosphorus intake (ratio less than 1974), the 1989 NRC suggested growing horses require 17.8 1:1), calcium absorption may be impaired. Even if the diet g (8 g/0.45 percent efficiency) for each kg of gain in addi- contains adequate calcium, excessive phosphorus intake tion to the maintenance requirements. Thus, a 215-kg foal may cause skeletal abnormalities (Schryver et al., 1971b). gaining 0.85 kg/d would require about 15.1 g of phosphorus Nutritional secondary hyperparathyroidism is covered more (8 g/0.45 × 0.85 kg gain) in addition to its maintenance re- completely in Chapter 12. This condition does not occur quirement of 4.8 g (215 kg × 10 mg/0.45). While Furtado et

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78 NUTRIENT REQUIREMENTS OF HORSES al. (2000) estimated endogenous fecal losses in growing absorption) for lactation would be 26.7 g for a mare produc- horses to be 10.3 mg P/kg of BW/d, Pagan (1989) argued ing 16 kg milk/d in early lactation and 11.1 g for a mare pro- that endogenous phosphorus losses in young horses are ducing 10 kg milk/d during late lactation. At these rates of double those used by the NRC (1989) in estimating dietary milk production, a 500-kg mare would require 37.8 and phosphorus requirement. This agrees with Cymbaluk et al. 22.2 g P/d in early and late lactation, respectively. No data (1989), who estimated endogenous fecal phosphorus to be have been found suggesting the 1989 NRC recommendation 18 mg/kg BW/d in growing Quarter horses. Using the higher for lactation should be changed other than to account for a estimate for endogenous loss (215 kg × 18 mg/0.45), the revision of milk production estimates. To meet endogenous total phosphorus requirements would be increased to 23.7 g losses of 10 mg P/kg BW with an absorption rate of 45 per- P/d. Grace et al. (1999b) suggested 21 g of P/d is the re- cent and to meet milk production needs estimated at 0.032 quired amount compared to the 16 to 20 g recommended by kg milk/kg BW containing 0.75 g P/kg milk that is absorbed the 1989 NRC for growing horses expected to reach 500 kg. at a 45 percent rate, the phosphorus requirement for lacta- Their calculations were based upon a 200-kg horse gaining tion from foaling to 3 months is (0.022 g × kg BW) + (0.032 1 kg/d and using an absorption efficiency of 0.5. With a × kg BW × 1.67 g). To meet endogenous losses of 10 mg lower absorption efficiency, requirements would be even P/kg BW with an absorption rate of 45 percent and to meet greater. The results of these studies suggest that the endoge- milk production needs estimated at 0.026 kg milk/kg BW nous losses, and hence, maintenance phosphorus require- containing 0.5 g P/kg milk that is absorbed at a 45 percent ments, for young horses are greater than previously assumed rate, the phosphorus requirement for lactation from 4 to 5 and an endogenous loss of 18 mg/kg BW/d was used in de- months is (0.022 g × kg BW) + (0.026 × kg BW × 1.11 g). termining the requirements. The requirement for growth To meet endogenous losses of 10 mg P/kg BW with an ab- recommended in this publication thus contains both a main- sorption rate of 45 percent and to meet milk production tenance component (0.018 g/0.45 absorption efficiency × kg needs estimated at 0.020 kg milk/kg BW containing 0.5 g BW) and a growth component (8 g/0.45 percent absorption P/kg milk that is absorbed at a 45 percent rate, phosphorus efficiency × kg gain). Phosphorus requirements increase requirement for lactation after 5 months is (0.022 g × kg during late gestation and lactation. Phosphorus requirements BW) + (0.020 × kg BW × 1.11 g). for the products of conception for mares in months 9, 10, The influence of exercise on phosphorus requirements and 11 of pregnancy have been estimated to be 7, 12, and 6.7 has been studied in combination with calcium. During the mg/kg BW/d, respectively (Drepper et al., 1982). At 35 per- first 4 months of race training in 2-year-old Quarter horses, cent absorption efficiency, the daily phosphorus require- Nielsen et al. (1998b) reported phosphorus retention re- ments for a 500-kg mare for products of conception during mained relatively constant over a range of phosphorus con- gestation months 9, 10, and 11 would be 10, 17.1, and 9.6 g, centrations in the diet (0.21 to 0.30 percent on an as-fed respectively (mean 12.2 g/d). The mean daily phosphorus basis). Likewise, 0.24 percent of the total diet (average deposition for the last 3 months of gestation ((8.6 mg/kg amount fed to control group) appeared to be adequate and BW)/0.35 absorption efficiency) was added to maintenance no apparent benefit was seen by feeding 0.29 percent, needs to determine the requirements. An average of mainte- though both the minimum and maximum amounts are above nance and late gestation was used for the phosphorus re- the 1989 NRC recommended amount based upon a percent- quirements during months 7 and 8 to allow for fetal growth age of the diet. In similarly trained horses, Stephens et al. occurring during that period. Thus, to meet endogenous (2004) reported phosphorus requirements to be at least 66 losses of 10 mg P/kg BW with an absorption rate of 35 mg/kg BW/d, which was 32 percent over the 1989 NRC rec- percent and to meet fetal deposition rate of 4.3 mg P/kg ommendation. While feeding various amounts of calcium, BW with a 35 percent absorption rate, the requirement for phosphorus, and magnesium to horses in race training, pregnancy during months 7 and 8 is (0.028 g × kg BW) + Nolan et al. (2001) reported diminished mineralization of (0.012 g × kg BW), or simply 0.04 g × kg BW. To meet the third metacarpus when feeding phosphorus at the 1989 endogenous losses of 10 mg P/kg BW with an absorp- NRC suggested amount, even when additional calcium and tion rate of 35 percent and to meet a fetal deposition rate of magnesium was provided. Only when phosphorus was at 8.6 mg P/kg BW with a 35 percent absorption rate dur- 130 percent of the NRC recommendation (calcium and mag- ing months 9, 10, and 11 of pregnancy, the requirement nesium both above 150 percent) was mineralization in- is (0.028 g × kg BW) + (0.0245 g × kg BW), or simply creased. Furthermore, exercised horses had a higher phos- 0.0525 g × kg BW. phorus balance than did horses that were sedentary for 2 The phosphorus concentration of mares’ milk ranges months and then exercised for 2 months (Elmore-Smith et from 0.75 g/kg of fluid milk in early lactation to 0.50 g/kg al., 1999). Young et al. (1989) also found an increase in daily of fluid milk in late lactation. If the absorption efficiency of phosphorus retention when miniature horses were exercised. lactating mares is 45 percent, the daily phosphorus require- In contrast, Lawrence et al. (2003) reported no differences in ments above maintenance (adjusted for the higher percent phosphorus retention between exercising horses and seden-

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MINERALS 79 tary horses in a review of studies reporting phosphorus in- sorption (Kapusniak et al., 1988), though the apparent di- take and excretion. Likely, the majority of the horses from gestibility of magnesium was still between 41 and 45 per- the studies that were reviewed were mature horses and un- cent when mature ponies were fed excess phosphorus of ap- dergoing a relatively constant training protocol in contrast to proximately 125 mg P/kg BW/d (van Doorn et al., 2004b). many of the studies reporting requirements higher than the In a small study, Weidenhaupt (1977) reported high potas- 1989 NRC that were conducted in young horses just enter- sium concentrations slightly decreased magnesium apparent ing training. Like with calcium, any potentially higher re- digestibility. In contrast, high concentrations of aluminum quirements associated with exercise in the rapidly growing did not alter magnesium absorption (Schryver et al., 1986a) horse (typically under 24 months) should be met by the and neither did varying the consumption of salt (Schryver et phosphorus requirements for growth that have been in- al., 1987b). Wall et al. (1992) reported that varying DCAD creased from the 1989 NRC. With no strong evidence to did not alter mean daily urinary excretion of magnesium. suggest mature exercising horses have a higher requirement Schryver et al. (1987b) found that the calculated true ab- than was suggested by the 1989 NRC, the requirements re- sorption of magnesium was between 62 and 67 percent. Ols- main unchanged and are 0.058 g P × kg BW for heavy exer- man et al. (2004) reported a diet rich in sugar beet pulp did cise, 0.042 g P × kg BW for moderate exercise, and 0.036 g not alter magnesium absorption even though pH of intestinal P × kg BW for light exercise. contents should have theoretically been lowered, resulting in increased mineral solubility. Pagan et al. (1998) reported that the apparent digestibility of magnesium in mature Magnesium horses did not differ between exercised (28.5 percent) and nonexercised (35 percent) groups. Magnesium is absorbed Function from both the small and large intestine, though the majority Magnesium (Mg) constitutes approximately 0.05 percent appears to be absorbed from the small intestine (Kapusniak of the body mass. Sixty percent of magnesium in the body is et al., 1988). found in the skeleton and about 30 percent can be found in muscle (Grace et al., 1999b). Magnesium is an important ion Signs of Deficiency or Excess in the blood, plays a role as an activator of many enzymes, and participates in muscle contractions. Meyer (1990) suggested that renal creatinine/magnesium quotients greater than 7.5 indicate an insufficient dietary supply. Stewart et al. (2004) reported that determination of Sources and Factors Influencing Absorption urinary magnesium excretion during a 24-hour period was a Many commonly used feedstuffs contain 0.1–0.3 percent better method to indicate decreased magnesium intake as magnesium, and magnesium absorption from these feed- compared to serum total and ionized magnesium, as well as stuffs appears to be 40–60 percent (Hintz and Schryver, muscle magnesium concentrations, but a spot sample of the 1972, 1973; Meyer, 1979). According to Harrington and fractional clearance of magnesium can be conveniently used Walsh (1980), inorganic supplemental sources such as mag- to identify horses consuming a magnesium-deficient diet. nesium oxide, magnesium sulfate, and magnesium carbon- Hypomagnesemia was reported in foals with magnesium in- ate appear to be essentially equivalent as supplemental di- take at 7–8 mg/kg diet/d (Harrington, 1974). Assuming a etary sources of magnesium for growing foals and have a feed intake equivalent to 3 percent BW, magnesium intake higher absorption rate (70 percent) than the magnesium would have been only 0.2 mg/kg BW in comparison to the found in natural sources. Data from human studies indicate control animals receiving 390 mg/kg diet or 11.7 mg/kg the absorption rate of magnesium oxide to be lower than BW. Clinical signs of magnesium deficiency include nerv- magnesium citrate (Lindberg et al., 1990; Walker et al., ousness, muscle tremors, and ataxia, with the potential for 2003) and magnesium chloride, magnesium lactate, and collapse, hyperpnea, and death. Meyer and Ahlswede (1977) magnesium aspartate (Firoz and Graber, 2001). Stadermann indicated that a magnesium intake of 5–6 mg/kg BW/d re- et al. (1992) reported apparent digestibility of magnesium sulted in hypomagnesemia (less than 1.6 mg/dl serum) and was higher with alfalfa hay (51 percent) than with concen- a marked reduction in renal excretion of magnesium, trate (31 percent). McKenzie et al. (1981) reported magne- whereas 20 mg of Mg/kg BW/d resulted in normal serum sium to be 42–45 percent digestible and not affected by ox- magnesium values of 1.6–2.0 mg/dl. Hypomagnesemia in- alate. Supplemental phytase also did not affect magnesium duces mineralization (focal calcium and phosphorus de- absorption (van Doorn et al., 2004a). Further, van Doorn et posits) in the aorta. Histologic changes occur within 30 days al. (2004b) reported no differences in the apparent di- of initiation of a low-magnesium diet (Harrington, 1974). gestibility of magnesium (approximately 41–45 percent) The 1989 NRC indicated pastures that are conducive to when horses were fed rations ranging from 0.148–0.535 g magnesium deficiency, tetany, and death in ruminants do not Ca/kg BW/d. Excess phosphorus decreased magnesium ab- affect horses similarly, though no evidence was found in the

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98 NUTRIENT REQUIREMENTS OF HORSES Recommendations forms have been reported to range from 0.4–3 percent (An- derson, 1987), while chromium from sources such as Harrington et al. (1973) demonstrated that 40 mg Zn/kg brewer’s yeast has been reported to be absorbed at rates as purified-type diet was sufficient to prevent zinc deficiency high as 10–25 percent by rats (Underwood, 1977). in foals. Schryver et al. (1974) reported that foals fed 41 mg While no evidence has been found of a chromium defi- Zn/kg natural diet grew at acceptable rates and maintained ciency in horses, a chromium deficiency leads to symptoms normal body stores of zinc. Drepper et al. (1982) and Jar- associated with adult-onset diabetes and cardiovascular dis- rige and Martin-Rosset (1981) indicated that 50 mg Zn/kg ease in humans (Vincent, 1999). From data in other species, dry matter was adequate for all classes of horses. Accord- the maximum tolerable concentration of chromium in the ra- ing to Knight et al. (1985), based on farm surveys, the op- tion of horses is set at 3,000 mg/kg DM for the oxide form timal dietary zinc concentration in equine rations to mini- and 100 mg/kg DM for the chloride form of the trivalent mize the incidence of metabolic bone disease approaches forms of chromium (NRC, 2005). The hexavalent forms of 90 mg/kg feed. Although these workers suggested that the chromium appear to be much more toxic (NRC, 1997). 1978 NRC recommendations on zinc requirements be Chromosome damage in hamsters has been noted after treat- reevaluated, no controlled studies support a dietary zinc re- ment with chromium picolinate but not with chromium quirement greater than 50 mg/kg ration DM and the 1989 nicotinate, nicotinic acid, and trivalent chromium chloride NRC did not change the recommendations from 40 mg/kg hexahydrate at the same doses (Stearns et al., 1995). dietary DM. Likewise, Wichert et al. (2002b) reported that At this time, insufficient information is available to de- only 42 percent of 106 sampled horses had zinc intake of 1 termine if there is a chromium requirement for horses, mg/kg BW and 25 percent of the horses were fed < 0.5 mg though the recommended safe and adequate daily intake for Zn/kg BW with none of the horses showing signs of zinc humans is 50–200 µg/d (Anderson and Kozlovsky, 1985). deficiency. Mare’s milk contains 1.8–3.2 mg Zn/kg fluid Jackson (1997) suggested chromium requirements may be milk (NRC, 1989; Anderson, 1992). Thus, foals drinking 15 higher for exercising horses than for sedentary horses. While kg of milk/d would consume 27–48 mg of Zn/d. On a dry not strongly supported by research in horses, this is sup- matter basis, this is equivalent to 17–30 mg Zn/kg DM in- ported by Lukaski (2000) for humans in endurance training. take. The zinc in milk is assumed to be highly available. Pagan et al. (1995) reported that exercising horses supple- Kavazis et al. (2002) concluded that supplementing late mented with 5 mg of chromium from a chromium yeast gestating and lactating mares with zinc at higher concentra- product had lower plasma glucose concentrations during tions than recommended by the 1989 NRC did not have any several stages of a standardized exercise test. Likewise, sup- influence on foal growth and development or the zinc con- plemented horses had lower blood insulin 1 hour after grain centrations of mare milk or foal serum. feeding than did control horses. In contrast, Pagan et al. Pagan (1994) estimated endogenous zinc loss to be 54 (1995) indicated no response to supplementation in un- mg/d for a 550-kg horse (about 0.1 mg/kg BW), which is trained, sedentary horses. Ott and Kivipelto (1999) reported similar to Hudson et al. (2001). They estimated endogenous chromium tripicolinate supplementation resulted in peak zinc losses to be between 65 and 70 mg Zn/d for horses av- plasma glucose concentrations decreasing more rapidly fol- eraging 534 kg (about 0.12–0.13 mg Zn/kg BW). Pagan lowing an intravenous insulin sensitivity test. Likewise, (1994) calculated the true digestibility of zinc to be 20.8 per- mean glucose fractional turnover rate values increased in re- cent. The resulting requirement for a 500-kg horse (236 mg sponse to the chromium tripicolinate supplementation. Gen- Zn/d) is much lower than the 1989 NRC recommendation as- try et al. (1997) reported chromium tripicolinate supplemen- suming a 2 percent of BW intake (400 mg/d). However, until tation had only a marginal effect on metabolic, hormonal, studies are performed demonstrating lower amounts can be and immune responses. Vervuert et al. (2005b) reported that fed without problems for longer periods, the decision was horses supplemented with a chromium yeast product did not made to keep the requirements the same at 40 mg/kg of DM. have improved glucose “handling” capabilities. Chromium- supplemented horses also had higher heart rates and blood lactate at the end of a standardized exercise test suggest- OTHER MINERALS OF INTEREST ing that the exercise capacity of chromium-supplemented horses was compromised. Supplementation of chromium-L- Chromium methionine to geriatric mares for a period of 4 weeks did not Chromium plays a role in carbohydrate and lipid metab- alter most immune parameters (Dimock et al., 1999). In ad- olism. It acts as a potentiator of insulin to facilitate glucose dition, 4 mg/d of chromium propionate supplementation did clearance and is considered an essential nutrient in humans not alleviate elevations in plasma leptin and insulin or alter (Mertz, 1992). glucose dynamics and insulin sensitivity in horses with a Trivalent and hexavalent are the two most common forms high body condition (Cartmill et al., 2005). While it has of chromium. Organic chromium appears to be more been suggested that dietary supplementation with chromium bioavailable than inorganic forms. Absorption of inorganic can calm horses and may be beneficial in horses with recur-

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MINERALS 99 rent exertional rhabdomyolysis, because horses with trol animals, and had fewer injuries compared to controls. polysaccharide storage myopathy display abnormal sensitiv- Feeding a natural zeolite, as compared to the synthetic SZA, ity to insulin, chromium supplementation may be counter- did not increase plasma silicon (Mazzella et al., 2005). productive in those animals (Valberg, 2005). Porter et al. Greater rates of bone formation (Lang et al., 2001b) and (1999) suggested that, at least in humans, routine use of lower rates of bone resorption (Lang et al., 2001a) as chromium supplements is not warranted based upon current compared to the control horses were reported in SZA- data. supplemented horses, suggesting an overall increase in bone production. Turner et al. (2005) reported an increase in bone turnover in calves fed SZA and hypothesized that the reduc- Fluorine tion in injuries reported previously was due to rapid repair Fluorine (F) is known to be involved in bone and teeth of subclinical injuries. These studies suggested benefits as- development, but its dietary necessity for horses has not sociated with supplemental available silicon. Silicon ap- been established. pears to reduce aluminum absorption and toxicity (Jug- Forages often contain between 2 and 16 mg F/kg DM and daohsingh et al., 2000), though it is not certain to what cereal grains usually contain between 1 and 3 mg fluo- degree each element inhibits the absorption of each other. ride/kg DM (NRC, 1980). Phosphorus supplements that The relative abundance of silicon makes the deficiency of have not been adequately defluorinated are a common this element difficult to achieve. Likewise, silicon naturally source of excess dietary fluorine. found in the environment is not very absorbable and thus is A fluorine deficiency is not known to have been reported not likely to cause toxic effects through dietary intake. The in horses. Horses appear to be more tolerant than cattle of NRC (2005) did not set a maximum tolerable concentration excess fluorine (Buck et al., 1976). Fluorine intoxication for silicon in horses, but set one at 0.2 percent for cattle and may result from long-term ingestion of feed or water con- sheep. taminated by certain industrial operations or from consump- As a result of research demonstrating silicon can interact tion of water or mineral supplements that contain high con- with other nutrients for apparent beneficial effects (Nielsen, centrations of fluorine (Schryver, 1990). Excess intake 1991), the American Institute of Nutrition reformulated its results in discolored teeth (fluorosis), bone lesions, lame- published formulas of purified diets for experimental ro- ness, and unthriftiness. Shupe and Olson (1971) indicated dents and added silicon to the required nutrient profile at the horses can tolerate 50 mg F/kg ration for extended periods rate of 5 mg/kg ration (Reeves, 1997). Difficulty in produc- without detrimental effects, but the maximum tolerable con- ing a silicon-deficient purified diet for horses will make de- centration of fluorine has been set at 40 mg/kg DM intake termining a minimum requirement difficult, though a need based upon data with other species (NRC, 2005). for silicon in the ration likely exists for the equine. An equine requirement for fluorine has not been estab- lished, but given the concentrations of fluorine in normal Other Elements feedstuffs, any potential requirement is likely met and addi- tional supplementation is not suggested. Further elements are, in principle, accepted as essential constituents in a mammalian diet (e.g., boron, nickel, and vanadium). Based on the current status of knowledge, their Silicon natural occurrence is sufficient to ensure the required very Despite silicon being the second most common element small amounts of these elements. The supplementation of of Earth’s crust (Carlisle, 1972), surprisingly little is known these elements, including the rare earth elements, is not about the nutritional importance of it in the diet of mam- based on scientifically elaborated, valid data and has the po- malian species. 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