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A Beef Cable INTRODUCTION The dominant factors that determine dry matter in- take of beef cattle are physiological demand due to maintenance needs and potential for production, and limitations in gastrointestinal capacity. Beef cattle are kept in the United States primarily as a means of mar- keting feed especially feed with little or no market value. A large proportion of the total intake during the life cycle of beef cattle, therefore, is forage. For the beef cattle herd, voluntary intake must be predicted to deter- mine the proportion of their requirements that can be met from low-quality feeds. Then the absolute amounts of supplemental feeds needed daily can be calculated. During early postweaning growth, calves are typically used to market forages. Although forages fed to calves are of higher quality than those usually fed to the beef herd, most calves fed high-forage diets are not able to consume enough digestible energy to allow physiologi- cal demand to control intake because of limits in gastro- intestinal capacity. During the finishing period, however, limits in physiological demand for growth can be the dominant factor controlling intake. The National Research Council (NRC, 1984) empha- sizes the expression of nutrient requirements as amounts per head daily. At any particular stage of growth, however, nutrient requirements are related to rate of gain (NRC, 1984), which depends upon intake of the particular diet being fed (Fox and Black, 1984~. Therefore, intake must be predicted before the diet can be formulated to meet requirements for growth. With accurate prediction of intake and rate of weight gain, nutrient requirements can be expressed as a percentage of the diet, which may also be more appropriate for describing requirements for rumen fermentation (Fox et al., 1984~. The purpose of this chapter is to identify and discuss 56 the primary factors influencing feed intake of beef cattle and to present equations and adjustment factors that can be used to predict intake under widely varying feed- ing and environmental conditions. Many producers have predictions for intake from historical information that reflects their ration, management, cattle types typi- cally fed, and environmental conditions. They may wish to only estimate the effect of one variable for which they have no independent data. Therefore, the approach used here will be to present separate equations and ad- justment factors for each variable where possible. PHYSIOLOGICAL FACTORS Body Size and Production Demands The relationship of body size to feed intake has been a subject of much debate. Gastrointestinal size is related to the 1.0 power of body weight, while energy intake is related to weight raised to the 0.75 power (VanSoest, 19821. This implies a more rapid turnover of rumen con- tents at lighter weights. Other studies in which the best fit of intake data with body weight was examined re- sulted in powers of 0.5 to 0.8 (Colburn and Evans, 1968~. Owens and Gill (1982) found intake to be related to the 0.47 power of body weight; Thornton et al. (1985) found that this relationship varied with the time on a high- energy ration. Preston (1972) concluded that intake of beef cattle was 95 g/ W0 75, with a 95 percent confidence interval of 88 to 102, where Wis body weight. The NRC (1984) developed equations to describe feed intake in beef cattle that reflect intake as a function of the 0.75 power of body weight, as did Plegge et al. (1984) and Fox and Black (1984~. Figure 6-1 summarizes the relationship of dry matter intake and dietary energy concentration by comparing systems proposed by the

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Beef Cattle 57 120 r 110 100 90 80 cr UJ t 70 CC . Hi.' 60 ~ / / 350 kg / ,/ ./ /300 kg / / .' , ' / 50 _ . 40 1 1 1 1 1 1 1 o.s 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 / 200 kg / / / ~ _ / . . . \ . _ _ \ ~- N _ __ ~ \ A. \ \ NRC, 1984 _ Plegge et al., 1984 ~ ~ ~ - Fox and Black, 1984 -- ARC, 1980 (processed diets) D I ET N Em (Meal/kg DM) Agricultural Research Council (ARC, 1980), NRC (1984), Fox and Black (1984), and Plegge et al. (1984~. With most diets fed to the breeding herd and during early postweaning growth, cell wall content (neutral de- tergent fiber) is the dominant characteristic limiting in- take. An increase in the dietary concentration of slowly degraded or indigestible material causes a reduction in the rate of passage and physical fill becomes limiting (VanSoest, 1982; Mertens, 1983~. As the net energy concentration in the diet is increased, as in finishing diets, at some point metabolic controls become the dom- inant factors limiting intake (Figure 6-1~. The use of dietary energy concentration to describe energy density effects on intake represents the combined effects of rate of passage and metabolic controls on appetite. Proce- dures for predicting feed energy values from feed analy- sis have been outlined (Mertens, 1983; NRC, 1984; VanSoest et al., 19841; these procedures can be used to predict dry matter and energy intake. The data of Owens and Gill (1982), Fox and Black FIGURE 6-1 Relationship between di- etary energy concentration and dry mat- ter intake (DM) in growing cattle NEm, net energy for maintenance. (1984), and Plegge et al. (1984) indicate that intake/unit of metabolic weight begins to decline at about 350 kg average-frame-size steer equivalent weight (Figure 6- 2~. These data indicate that the degree of fatness and/or a reduction in demand for growth influence voluntary intake. Song and Dinkel (1978), using the data of Smith et al. (1976), found a significant decline in intake with degree of maturity. Increased body fat likely reduces appetite as a result of competition for abdominal space (Taylor, 1969) or feedback from adipose tissue (Chapter 1~. It is clear that the effect of body fat on intake must be taken into account. It is especially important when pro- jecting gains at the end of a feeding period to make marketing decisions. The optimum sale weight is pro- jected to be considerably lighter if intake is predicted to decline as body fatness increases. As shown in Figure 6-2, there appears to be additional factors influencing intake at early stages of growth. Fox and Black (1984) found relative intake to be constant until later stages of

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58 Predicting Feed Intake FIGURE 6-2 Relationship of stage of growth and weight of a steer when 50~- placed on a high-energy diet to dry mat ter intake. growth; Owens and Gill (1982) and Plegge et al. (1984) found the weight when growing cattle are started on a high-energy ration to be related to intake patterns over the feedlot finishing period. Further work is needed to describe the basis for this effect. Nursing or milk-fed calves consume less total dry mat- ter than weaned calves. The data of Wilton (1980) and Wyatt et al. (1977) indicate an average total DM intake of 60 g/ W075 for nursing or milk-fed calves consuming 5 to 10 kg of milk/day with access to dry creep feed or calf starter feed. Calves with continuous access to whole milk or milk substitutes at 12 to 15 percent DM consume 70 to 80 g/ W075 but 60 g when fed only twice daily (ARC, 1980~. The data of Le Du et al. (1976) indicate a grazed forage DM consumption of 33 g/W075 at a milk DM consumption of 36 g/ W075, increasing to a forage intake of 101 g/W075 at a milk DM intake of 6 g/W075. The relationships among body size, body composi- tion, and intake discussed previously suggest that ma- ture dry cows may have voluntary intakes similar to those of growing cattle of similar body composition. Beef cows in average flesh condition (condition score 5) contain about 22 percent body fat (George, 1984), sug- gesting that intake would be expected to be similar to those of calves weighing up to 364 kg. A number of studies (Bines et al., 1969; Taylor, 1969; Lusby et al., 1976; ARC, 1980) indicate that intake of cows is re- duced as body fat increases. Lactating cows consume 35-50 percent more than nonlactating cows of the same weight and on the same diet, with an average increase of 120: 110 ~ . . - - ~100 ~ cn flu 90 _ y in UJ t ~70 to c, 80 601 3 200 kg / / 350 kg - . ~ arm:\-\ I. i'\ \. ~\ . Plegge et al., 1984 - - .. Fox and Black, 1984 Owens and Gill, 1982 . O ARC, 1980 (milk fed calves) ~ Wilton, 1980 (nursing calves) 1 1 1 1 1 1 1 1 1 100 150 200 250 300 350 400 450 500 550 BODY WEIGHT, AVERAGE-FRAME-SIZE STEER EQUIVALENT 0.2 kg of DM/kg of fat-corrected milk (ARC, 1980). Vol- untary intake declines by 2 percent per week during the last month prepartum, increases to a peak 4 to 6 months postpartum, and then declines as milk production is re- duced (ARC, 1980 and Chapter 5 in this volume). A comparison of the two intake equations of the NRC (1984) with the data from several studies (Streeter et al., 1974; Lusby et al., 1976; Lemenager et al., 1978; Hollo- way et al., 1979; Horn et al., 1979; ARC, 1980) indicates that voluntary intake of beef cows is similar to that of growing cattle when adjusted for the effect of milk pro- duction. The information presented in this section, considered in light of that presented in Chapter 1, indicates that the primary factors that control intake in beef cattle are those related to direct dietary effects (distension of the rumen wall, rumen pH and acetate concentration, and hepatic uptake of propionate) and metabolic factors me- diated by the central nervous system, including size of adipose mass and demand for satisfying maintenance and production functions. Subsequent sections will ex- amine these effects. Mature Size and Sex Cattle varying in mature size and sex differ in the weights at which they reach a given degree of fatness (Smith et al., 1976; Byers, 1980; Fortin et al. 1980; Fox and Black, 19841. Thus, they would be expected to dif

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Beef Cattle 59 fer in the weights at which intake/W075 begins to de- cline. Harpster (1978) found that heifers corresponding to diverse breed types of steers had 3 percent higher intakes/W075 when fed to the same stage of growth. Heifers fed to the same finish (percentage of final body fat) as steers consumed 5 percent more in studies by Klosterman and Parker (19761. Growing bulls and heif- ers had similar intakes/ W075 in the studies of Fox et al. (1984~. Ayala (1974) found similar relative intakes among bulls, steers, and heifers. A system for predict- ing the intakes of cattle varying in frame size and sex has been developed by Fox and Black (1984~. Intake for alternative frame sizes is based on that used for an average-frame-size steer of equivalent body composi- tion. Age Owens and Gill (1982) found that daily dry matter intake increased 0.20 kg for each 50 kg above 277 kg of initial weight when placed on a high-energy diet, and i decreased by this same amount for initial weights under 277 kg. Similar trends were obtained by Plegge et al. (1984), Ralston et al. (1970), and Thornton et al. (19851. The NRC (1984) concluded that growing cattle started on feed as yearlings consume an average of 10 percent more than calves with similar weights and frame sizes. Abdalla (1986) found that compensating cattle whose rate of growth had been retarded to about half of that at which maximum daily protein gain could be expected consumed an average of 10 percent more DM/W075 when placed on a full feed. The yearling ef- fect on intake may be the same as that obtained during compensatory growth, as the cattle's older age for lesser weight indicates a previous period of retarded growth. Abdalla (1986) found that rumen size rapidly increases following retarded growth; the impetus for compensatory growth appeared to be increased demand for nutrients, with an increase in appetite as well as increased efficiency of utilization of nutrients. Field ob- servations indicate that previous diet is a key factor in determining subsequent eating patterns. For example, calves that previously grazed on wheat pasture con- sume more than calves from drylot feeding back- grounds or drought conditions (D. L. Gill, Oklahoma State University, personal communication, 1985~. Genetic Variance Reid (1962) suggested that the best opportunity for improving feed efficiency through selection was to se- lect for greater appetite; the likelihood that cattle that could be identified that metabolize consumed nutrients more efficiently was small. Across four studies (Swiger et al., 1961; Brown and Gifford, 1962; Koch et al., 1963; Brown and Gacula, 1964) heritability estimates reported for feed intake var- ied from 0.43 to 0.76, with a mean of 0.62. Thus cattle of a given type are likely to vary in their appetite, and intake and gain may be considerably under- or overpro- jections due to genetic variance. Intake data collected over 9 years from individually fed bull and heifer calves were summarized in which the effects of weight, frame size, sex, and body condition could be separated in a herd of cattle in which selection had been practiced for feed efficiency (D. G. Fox, Cornell University, unpub- lished data, 19831. Across five frame sizes, intake was 12 to 14 percent greater than predicted by the system of Fox and Black (1984~. These data suggest that pre- dicted intake can be increased by 12 to 14 percent for cattle purchased from herds where selection pressure for growth rate independent of frame size or environ- ment is practiced. In these same cattle, feed efficiency independent of intake effects was 18 to 34 percent greater than expected, indicating that progress can be made in intake and use of consumed nutrients by select- ing for increased relative growth rate. Increased genetic potential for growth likely stimulates intake as a result of a greater demand for production. Breed Type Intake differences among beef cattle breeds and their crosses may largely be accounted for by differences in mature size. Much of the data on this effect were con- founded because the cattle were not usually fed to the same degree of fatness and, thus, over the same stage of growth. In the experiments of Smith et al. (1976), cross- breds averaged 2 percent greater intake than straightbreds fed to the same stage of growth. Harpster (1978) and Lomas et al. (1982) did not find any differ- ences in intake/ W075 due to breed types. Holsteins may be an exception. Examination of a number of studies in which Holsteins were compared with beef breeds (Garrett, 1971; Crickenberger et al., 1978; Thonney et al., 1981) suggest that at the same stage of growth Holsteins consume an average of 8 per- cent more DM/W075; similar results were reported by Plegge et al. (1984~. However, recent studies (Thonney et al., 1981; Fox and Black, 1984) indicate that this dif- ference disappears at weights beyond 450 kg, and in fact, relative intake of Holstein steers declines at a faster rate with increased fatness than with beef breeds. It can be speculated that the tendency of Holsteins to deposit a higher proportion of their body fat internally may be involved. It is possible that differences in rearing management between Holsteins and beef breed calves may play a role

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60 Predicting Feed Intake i. n the usually higher feed intake of Holsteins during most of the growth period. Beef calves usually nurse cows until at least 7 months of age; Holsteins are weaned by 6 weeks of age, resulting in a higher intake of ruminally fermented feeds at early stages of growth. End products of fermentation stimulate rumen develop- ment (VanSoest, 1982~. In addition, Holstein steers are likely to be fed a high-forage diet for a relatively long time on dairy farms before being placed on feed in feed- lots. A large part of their higher intake, however, may be due to a high demand for production due to their higher body protein while on feed (Crickenberger et al., 1978) and their higher growth potential (Owens et al., 19851. ENVIRONMENTAL EFFECTS Temperature and Weather The effects of environment on dry matter intake have been summarized (NRC, 1981~. The primary environ- mental effects on voluntary intake of cattle occur at temperatures greater than 25C and less than 15C an-d by exposure to wind, storms, and mud. These effects FIGURE 6-3 Environmental effects 120 on dry matter intake. 1 1 0 _ 100 - to - Z 90 J o at 80 70 60 are summarized in Figure 6-3. Adjustment for these effects is more accurate if the average environmental state for a period of a week or month rather than daily fluctuations is used. Intakes are more variable and diffi- cult to predict under these conditions, especially if the changes are abrupt. Breed differences in voluntary intake under various environmental conditions have not been clearly identi- fied. However, it appears that intake changes due to environmental conditions vary with changes in the ani- mal's critical temperature (the point at which it must increase or decrease heat production to maintain a nor- mal body temperature). This temperature is a function of age, body mass, hide and external fat thickness, hair coat density and depth, and dietary energy density (NRC, 19811. Thus, shifts of the curve to the right or left in Figure 6-3 for various breed types can be predicted by estimating their deviation from the average critical tem- perature, which can be calculated as outlined by the NRC (19811. Photoperiod and Timing of Feeding Light to dark ratios influence eating patterns (Tucker et al., 1984~. Many feedlots illuminate their lots at night, . . Mild Mud Rain Deep Mud Storms ~ . / . \ . Cool \ Night No Night \ Cooling \ . \ . -15 -5 5 15 25 35 45 TEMPERATURE (C)

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Beef Cattle 61 with the assumption that it stimulates eating activity. However, there appears to be an optimum ratio of light (L) to dark (D) over a 24-h period. Sheep and heifers have been shown to consume up to 13 percent more feed and grow faster when the ratio is 16L to 8D than when the ratio is 8L to 16D or continuous light (Forbes et al., 1975; Peters et al., 1980; Schanbacher and Crouse, 1980; Petitclerc et al., 1983, Zinn et al., 19831. Heifers and sheep on the 16L to 8D ratio had more total eating events, with more occurring during lighted hours. Heif- ers on the 8L to 16D ratio increased their eating activity 1 to 2 h before lights came on; those on the 16L to 8D ratio did not reinitiate eating activity until lights were actually on. Eating activity was also stimulated when fresh feed was offered. Eating pattern has been directly related to increased rate of weight gain, however, only when feeding occurs once per day, soon after dawn or in the middle of the day compared to 1 h before dark (Zinn et al., 1983~. Feeding twice daily improved intake 2 per- cent and daily gain 5 percent across five experiments (Embry and Burkhardt, 1971~. Photoperiod appears to have an important effect independent of intake. Short days stimulate fat deposition, while long days stimulate protein accretion in cattle (Tucker et al., 1984~. Forage Availabilityfor Grazing Cattle The two major factors influencing intake by grazing cattle are quantity and quality of available forage. The quantity of available forage is the first limiting factor. In pastures or ranges with abundant available forage, ani ''n ~ . . _ 100 90 80 70 60 50 40 30 20 10 o O 0 O ~ O / O / O o Jo /40 / O o v , I I I I I 0 0.45 0.9 1.35 1.8 2.25 . . FORAGE STANDING CROP (thousands) (kg/ha) O L ambs Calves O Cows 2.70 3.1 5 3.60 mats can selectively graze large mouthfuls of the most nutritious plant parts, usually leaves. As the quantity declines, the amount of intake per grazing bite declines. In addition, as the grazing pressure increases and/or the plants mature, the animal is forced to consume plant parts with a slower rate and extent of digestion. A sum- mary of data by Rayburn (1986) indicates that intake of cattle and sheep is maximum at a forage availability of about 2,250 kg/ha, or 40 g of organic matter (OM)/kg of live weight (LW), and then rapidly declines to 60 percent of maximum by 450 kg/ha or 20 g of O M/kg of LW (Figures 6-4 and 6-5~. Pastures containing legumes have usually given higher intakes (Holloway and Butts, 1983; Rayburn, 19861. The rate of digestion for legumes is higher than that for grasses of the same total digestibility (Mertens, 1983; Rayburn, 1986~. The impact of these effects on animal performance and production per hectare on a pasture that begins at ~ 2,250 kg of DM/ha is shown in Figure 6-6. Reduction in animal performance is noticeable as 50 percent utili- zation is reached, but production per hectare is maxi- mized at 80 percent utilization. DIETARY FACTORS Diet Water Content Nonlactating cattle consume an average of 3 parts water to 1 part dry matter up to a 4.4C environmental FIGURE 6-4 Effect of forage standing crop on the relative forage dry matter intake (relative DMI) of lambs, calves, and dairy cows grazing pasture under continuous grazing management (Ray- burn, 19861.

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62 Predicting Feed Intake FIGURE 6-5 Effect of daily forage al- lowance on the relative forage dry mat- ter intake (relative DMI) of lambs, calves, and dairy cows grazing pasture under rotational grazing management (Rayburn, 19861. 1 1 0 100 Z 70 LU t tr: > at: llJ 60 50 40 20 10 temperature and then an increased amount of water proportional to increases in the ambient temperature (Winchester and Morris, 19561. Thus, restricting water reduces dry matter intake (Utley et al., 1970), and any factor that affects water consumption could reduce in take. Voluntary free water intake plus water in the feeds consumed is approximately equal to the water require- ments of cattle (NRC, 1984~. Thus, dietary water con- centration per se would not be expected to influence dry matter intake until total expected water intake per unit of dry matter is exceeded. Rumen contents contain about 85 percent water; wa- ter added to the rumen has little effect on dry matter intake since it is rapidly absorbed and excreted (Van- Soest,1982~. The data of Winchester and Morris (1956) FIGURE 6-6 Effect of grazing pres- sure under rotational grazing on relative production (Rayburn, 19861. 0~ 0 0 0 o'' 0 /o /o / 1 1 o O Lambs Calves O Cows 1 1 1 1 1 1 1 1 0 20 40 60 80 100 120 140 160 18~) 200 DAILY FORAGE ALLOWANCE (9 OM/kg LW) can be used to estimate the percentage of water in the total diet intake per day. For lactating cows, 0.87 kg of water must be added to the expected daily intake for each kg of milk produced. Forced water intake above these levels could reduce intake. In milk-fed calves intake is reduced 32 percent as dry matter content of milk falls from 15 to 5 percent (ARC, 19801. Degree of Fermentation When data are corrected for errors in dry matter de- termination, it has been shown that a desirable fermen- tation during ensiling does not reduce dry matter intake in cattle (Fox and Fenderson, 1978; ARC, 1980~. However, when silage is unusually wet or dry, unde 1 .0< 0.8 O- z O o 111 -0.2 ax: UJ to 0.6 0.4 0.2 0.0 -0.4 -0.6 -0.8 _ -1 .0 o I;. ~1 1 ~ O Production/Head/Day Production/ha 20 40 60 80 100 PASTURE UTILIZATION (%)

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Beef Cattle 63 sirable fermentation may occur. In silages with greater than 65 percent DM, the potential for molding in- creases, which could reduce intake (Mertens, 1979~. In silages with less than 30 percent DM, a pH of higher than 4.4 may be indicative of proteolytic fermentation and the development of amines and excessive butyric acid, which may reduce intake (VanSoest, 1982~. Dietary Protein Diet digestibility, and thus rate of passage, is reduced if the nitrogen requirements of rumen bacteria are not met (VanSoest, 1982~. Nitrogen requirements for maxi- mum microbial growth are primarily a function of di- gestible organic matter intake (VanSoest et al., 1982~. Diet protein solubility and degradability influence di- etary protein availability to meet microbial nitrogen needs. Thus, the level of nitrogen needed in the rumen to support the maximum rate of passage would be ex- pected to vary with carbohydrate digestibility in the rumen. An evaluation of data from several studies (Fig- ure 6-7) indicated that most diets satisfy this require- ment at 6 to 8 percent crude protein, but 9 to 11 percent crude protein may be required for calves, especially when highly digestible forages are the primary diet. Di- etary protein can be overprotected to the point where rumen nitrogen requirements are not met (Fox et al., 1984; VanSoest et al., 1984~. Feed Processing Reducing particle size and collapsing of the cell struc- ture by finely grinding and pelleting fibrous feeds re 130 120 ~0 100 _ 90 _ 80 _ _ 30 _ 80 70 60 50 40 20 0 1 ,. 0/ / ,~ w/~ e o~ ~ ~ 0 O A O _.~ __ ~ ~ ~ O ~3 o o C. 0~08 - Gutherie et al. (1 984b) (Prairie Hay and Soybean Meal O job/ in 250 kg Calves) O / 00 0/ O / O 1 8 ~ 1 1 1 1 2 3 - Gutherie et at. (1984a) (Prairie Hay and Soybean Meal in 480 kg Steers) Fox et al. (1984) (Corn Silage in 150 kg Calves) -_-- Fox et al. (1984) (Corn Silage in 250 kg Calves) ....... Fox et al. (1984) (Corn Grainin 150 kg Calves) Milford and Minson (1965) (Tropical Grasses) 1 1 1 1 1 1 1 1 1 7 8 9 10 11 12 13 14 15 4 5 6 PROTEIN IN THE RATION DRY MATTER (%) duces rumination time and increases the rate of passage and thus feed intake (VanSoest, 1982), by up to 50 per- cent (Greenhaugh and Wainman, 1972~. Digestibility can be depressed by up to 3 to 8 percent per increase in multiples of maintenance, but utilization of digestible energy may be improved because the acetate:propio- nate ratio may be decreased. The NRC (1984) recently summarized the influence of processing feedstuffs on intake and utilization. They concluded that intake is improved most with processing where roughage is the major constituent, and the impact increases with increasing concentrations of plant cell wall and with alkali, ammoniation, or other treatments that increase the potential for cell wall digestion. In- creasing the rate of passage of indigestible material can improve intake of forages high in cell wall content by up to 50 percent. Generally, however, intake is reduced if grains are processed and if digestibility is increased. The ARC (1980) summarized data from six journals to separate out the interaction of diet energy concentration and degree of processing. Based on their summary, the change in intake due to fine processing can be predicted to be 47.2 percent at 0.92 NEm, 20.8 percent at 1.33 NEm, O at 1.73 NEm, and -17.2 percent at 2.10 NEm. These predicted values generally are consistent with the effects suggested by the NRC (1984~. This summary does not attempt to define the opti- mum degree of processing, as changes in digestibility and the utilization of digestible energy are influenced by processing, and vary with treatment used, age, grain type, and grain variety. The reader is referred to the NRC (1984) for recommendations in this regard. FIGURE 6-7 Influence of diet type and protein level on dry matter intake.

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64 Predicting Feed Intake AN EVALUATION OF ALTERNATIVE SYSTEMS FOR PREDICTING INTAKE BY BEEF CATTLE A number of systems for predicting intake of beef cattle based on summaries of different data bases have been published recently. Each may be useful under dif- ferent conditions, when the limitations are taken into account or when they are not applicable. The published equations and factors for each and a summary of likely limitations based on the previous discussion are given. 1. NRC (1984J. The data base is published informa- tion from various studies. DMI (kg/day) = We 75 (0.1493NEm - 0.046NEm2 - 0.0196), where DMI is dry matter intake; add 10 percent for large-frame steer calves and medium-frame yearling steers; add 5 percent for large-frame bulls; subtract 10 percent for medium- frame heifers. Limitations a. Needs more refined adjustment for frame size and sex. b. Needs adjustment for stage of growth. c. Needs adjustments for feed additives, dairy breed type, and environment. d. Needs adjustment for diet processing. 2. Plegge etal. (1984J. Data base is a summary of feed- ing trials conducted in Minnesota between 1966 and 1984, including 617 pens (14,199 cattle); cattle were fed mostly under sheltered conditions. All of these were used to develop an intake equation based on mean weights and intake during the feeding period. Cattle in 158 of the pens (5,244 cattle) were used to develop an intake equation based on intake and weights for each 28- or 42-day period during the trials. a. Intake at a particular weight (kg/day) = -43.18 - 0.004(IW) + 0.00003(IW2) + 36.8326(RW) TABLE 6-1 (1984) 20.8356(RW)2 + 24.5011(ME) - 4.4019(ME2), where IW is initial weight (in kg), RW is the proportion of finished weight (assumed to be low choice grade), and ME is diet metabolizable energy concentration (in Meal/ kg); r2 = 0.80. b. Intake based on overall average weight and intake (kg/day) = -7.65 + 0.0063(AW) + 0.0000189(AW2) + 9.4106(ME) - 1.9011(ME2), where AW is the overall average weight from start to finish, and ME is the same as in a above (~2 = 0.78~. The adjustments in Table 6-1 need to be applied to the value from the equation in a orb. Limitations a. b. May underestimate feed intake for cattle that have low initial weights for their frame size or may over- estimate feed intake of cattle that have high initial weights for their frame size. Does not appear to be applicable to cattle under 250 kg initial weight, especially weaned calves fed high-forage diets. c. May overestimate the effect of growth stimulants. d. Need to apply environmental adjustments appro- priate for the region. Not clear whether heifer effect is the result of dif- ferences in final body fat or a unique sex effect. f. Need adjustment for diet processing. 3. Owens and Gill (1982J. Data base is a summary of 15 feeding trials (about 1,500 cattle) at Oklahoma State University, Stillwater, with weights and intakes taken at 28- or 56-day intervals. DMI (kg/day) = -5.08 + 0.0636W- 0.000072(W2) + 0.0039 (IW - 276.7~. Limitations a. Need to adjust for diet energy concentration. b. Need to adjust for differences in frame size. c. Need to adjust for feed additives and environmen- tal conditions. Adjustments for Sex, Age, Breed, Feed Additive, Growth Stimulants, and Seasons for Plegge et al. Adjustments (kg/day) for: Weight Heifers Steers Calves Yearlings Beef Holstein Particular= -0.255 0.255 -0.055 0.055 - 0.310 0.310 0verall = - 0.185 0.185 - 0.205 0.205 - 0.265 0.265 Monesin Growth Stimulants Summer Winter With Without Without With Particular = -0.165 0.165 - 0.300 0.300 - 0.450 0.450 0verall = -0.185 0.185 -0.295 0.295 -0.145 0.145

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Beef Cattle 65 d. May not apply to calves weighing under 250 kg fed a high-forage diet. e. Needs adjustment for diet processing. 4. Thornton etal. (1985)and Owensetal. (1985J. Data base is 675 pens (total of 119,482 head) of yearling steers of English breeding, implanted with a growth stimulant, fed an ionophore-supplemented high-concentrate diet in a large unsheltered feedlot in western Kansas from December 1981 through November 1982. a. Adaptation (first 14 days): DM intake (kg/day) = 0.0217 Wl.02; ~ = 0.54. b. After 14 days: DM intake (kg/day) = 6.93 + 0.019 (days on feed) - 0.00013 (days on feed2) + 0.0000248 IW2; r2 = 0.38. c. Mean DM intake over 112 days on feed (kg/day) = 0.197 ~ 656; r2 = 0 54 d. Adjustments, based on 48 pens of heifers (5,012 head) and 22 pens of Holsteins (2,056 head): heifers, none; Holsteins, 10 percent. Limitations a. Does not adjust for variations in diet energy con- centration. b. Does not adjust for nonuse of ionophores and growth stimulants. c. May not apply to calves since the data base was yearlings. d. Diet-processing effects not considered. e. Environmental effects not considered. f. r2 values were low. 5. Fox and Black (1984J. Data base is information from feeding trials reported in various sources, primar- ily experiment station bulletins and research reports. DMI (kg/day) = 0.1 W0 75 for average-frame-size steer equivalent weight of 364 kg and then decreased by 2 g/ W0 75 for each additional 22 kg. Base intake decreased 2 g/W075 for each 0.02-Mcal/kg increase in diet NEm above 1.27 Mcal/kg. There was an intake increase of 10 percent for yearlings and 17 percent for Holsteins and a decrease of 10 percent for monensin and 2 percent for lasalocid. There was an intake decrease of 35 percent for temperatures > 35C if no night cooling and 10 per- cent for temperatures > 35C with night cooling, or 25- 35C; there was an intake increase of 3, 5, 7, and 16 percent as temperature fell from 5 to -5C, 15 to 5C, - 5 to - 15 C, and below - 15 C, respectively. Intake was decreased by 15 percent for mild mud and 30 per- cent for severe mud. Limitations a. Not applicable to diets of less than 1.54 Mcal NEm/ kg. b. More refined adjustment is needed for age (initial weight). c. Overestimates intake of young Holstein steers, es- pecially above 22 percent body fat. d. Need to extend adjustment for heavier than low choice grade. e. Needs adjustment for diet processing. 6. ARC (19809. Data base is a summary of data re- ported in six journals. DMI (kg/day) = W0 75 (0.1168 - 0.01059 ME), where ME is in Mcal/kg. Limitations Data are not refined enough. No adjustments are given for any of the effects identified previously, except diet energy density. 7. NRC (1984J, for breedingfemales. Data base is in- formation taken from various drylot feeding trials with beef cows, primarily reported in experiment station bul- letins and research reports. DMI (kg/day) = W~ 75 (0.1462NEm-0.0517NEm2 - 0.0074~. Limitations a. Not clear why intake for breeding females should be higher at low energy densities and lower at high- energy densities than that for growing and finish- ing cattle. b. Needs adjustment for level of milk production. c. Needs adjustment for pregnancy. 8. Fox (NRC, 19879. Data base from Vona et al. (1984) includes 16 hays (cultivars of switchgrass, big bluestem, and tall fescue) collected in five states (Iowa, Kentucky, New Jersey, New York, and Pennsylvania) at three stages of growth (vegetative, early head, and early bloom). DMI (kg/day) = W0 75 (19.4 + 54.5 NEm); r2 = .64. In developing this equation, a linear equation was as closely related to intake as a nonlinear one. This indi- cates that over the range of forage energy densities usu- ally fed to beef cows, a linear equation adequately describes intake. Limitations a. Needs adjustment for level of milk production. b. Needs adjustment for pregnancy. SUMMARY OF FACTORS INFLUENCING INTAKE OF BEEF CATTLE This section summarizes the factors influencing in- take of beef cattle, based on the review presented in this chapter. Users of this information may choose to use

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66 Predicting Feed Intake TABLE 6-2 Adjustment of Actual Weight to Average Frame Equivalent Weight Adjustment Factors for the Following Frame Sizes Sex 1 3 4 5 ~7 ~l 8 9 Steers 1.25 1.19 1.13 1.06 1.00 0.95 0.92 0.87 0.83 Heifers 1.56 1.47 1.39 1.32 1.25 1.19 1.14 1.09 1.04 Bulls 1.04 0.98 0.93 0.88 0.83 0.79 0.76 0.73 0.69 some combination of systems 1-5 presented in the pre- ceding section and the factors presented here. 1. Diet energy concentration: NRC (1984) and Plegge et al. (1984) equations for growing cattle and Fox (NRC, 1987) equations for beef cows given in previous section. 2. Body fat (Fox and Black, 1984~. Relative intake of beef cows may decline above a con- dition score of 5 (1 = extremely shin, 9 = extremely fat), which is a body fat value of 22 percent (George, 1984~. Adjustment factors can be determined for alternative frame sizes of growing cattle by multiplying actual weight by the adjustment factor in Table 6-2 and then by using the adjusted weight (equivalent weight) to get the intake adjustment factor from Table 6-3. Then the ad- justed intake/W075 is applied to the animal's actual weight to predict kg of DM/day. For example, a frame size 9 steer weighing 500 kg has an equivalent weight of 500 x 0.83 = 415 kg. The appropriate body fat adjust- ment factor is -3 percent. 3. Initial weight, when placed on finishing diet (Thornton et al., 1985; Gill, personal communication, 19851: 1.5 kg/100 kg increase in initial weight above 225 kg. Monensin @ 33g/ 1,000 kg of diet Monensin ~ 22g/ 1,000 kg of diet Lasalocid in diet TABLE 6-4 Adjustment for Finely Processed Diets Diet NEm (Meal/kg) 1.00 1.35 1.70 2.05 Adjustment ('ho) 47 20 None -17 Feed additives (Fox and Black, 1984): Additive Adjustment factor (%) -10 - 6 - 2 8. Finely processed diet (ARC, 1980) (Table 6-4). 9. Environmental factors (NRC, 1981) (Table 6-5). 10. Forage availability for grazing cattle (Rayburn, 1986). Rotationalgrazing: Relative DMI (%) = 103-(710/ DFA), where DFA is daily forage allowance in g of or ganic matter/kg of live weight (r2 = 0.86; standard error tSE] = 1.8). Continuous grazing: Relative DMI (%) = 4. Unique breed effects (Fox end Black, 1984; Plegge 94-(13,500/FSC), where FSC is kg of forage standing et al., 1984; Owens et al., 1985). All beef breeds: none Holsteins continuously on high-energy diet after 8 to 12 weeks of age: none. Other Holsteins: 8 percent 5. Genetic variance (D. G. Fox, Cornell University, unpublished data, 1983). High selection pressure for relative growth rate: 12 percent. 6. Nonuse of an anabolic stimulant (Plegge et al.. 1984): - 8 percent. TABLE 6-3 Adjustment for Body Fat Average Frame Steer Equivalent Weight (kg) s 350 400 450 500 550 Empty Body Fat (%) 21.3 23.8 26.5 29.0 31.5 Intake Adjustment None -3% - 10% - 18% -27% crop/ha (r2 = 0.54; SE = 6.5~. 11. Milk production (George, 1984; ARC, 1980) (Table 6-6~. 12. Forage intake of nursing calves. Fox (NRC, 1986) based on LeDu et al. (19761. (Table 6-7~. TABLE 6-5 Adjustment for Environmental Conditions Temperature (C) or Lot Condition Intake Adjustmenta (%) > 35, with no night cooling > 35, with night cooling 25to35 15to25 Stow -5to5 -15to -5 < -15 Mild mud (10 to 20 cm) Severe mud (30 to 60 cm) -35 -10 -10 None 16 -15 -30 Adjustments assume that cattle are not exposed to wind and storms.

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Beef Cattle 67 TABLE 6-6 Adjustment for Milk Production of Beef Cows Breed Range in Milk Production Increased Intake First 60 Days Postpartum (kg/day) (kg/day)a 3-9 2-6 3-8 2-9 3-8 2-6 3-9 6-17 6-17 3-9 Angus Brahman Charolais Chianina Hereford T . LlmOusln Salers Shorthorn Simmental South Devon aO.2 kg/kg of milk produced (ARC, 1980). SOURCE: George, 1984; ARC, 1980. 0.6-1.8 0.4-1.2 0.6-1.6 0.4-1.8 0.6-1.6 0.4-1.2 0.6-1.8 1.2-3.4 1.2-3.4 0.2-1.8 Based on forage dry matter that is 81.9 percent and milk dry matter that is 92.9 percent organic matter, re- spectively. 13. Pregnancy (ARC, 1980~. Minus 2 percent per week during the last month. 14. Water intake (Winchester and Morris, 1956) (Table 6-8~. TABLE 6-7 Forage Intake of Nursing Calves Month of Lactation 1 2 3 4 5 6 Intercept Slope 35.6 26.7 24.1 17.9 22.8 22.4 22.4 -3.11 -2.12 -2.19 - 1.71 -2.21 -2.60 -2.60 NOTE: y = g of forage organic matter grazed/kg of live weight, and x = milk organic matter intake/kg of live weight. TABLE 6-8 Expected Percentage of Water in Total Daily Intake of Cattle Environmental temperatures (C) To 4 10 16 21 27 32 74.7 76.2 78.6 81.2 83.2 87.6 NOTE: Add 0.87 kg of water to total intake for each kg of milk produced. Water intake = (dry matter intake/percentage of water in intake) - dry matter intake. For example, if dry matter intake = 10 kg, total water intake at 10C = [lOt(1 - 0.762)] - 10 = 32 kg. If the ration contains 50 percent dry matter, then the free water intake = 32 - 10 = 22 kg. APPLICATION OF EQUATIONS AND ADJUSTMENTS The usefulness of intake predictions depends on the ability to use them to predict rate and cost of gain and to evaluate management practices. To further assess the application of the equations presented, several evalua- tions were made, as follows. Yearlings Average values from the data of Thornton et al. (1985) were used to evaluate the ability of the equations given above to predict the intake and rate of gain by 14-day intervals. Table 6-9 summarizes the results of these cal- culations. To estimate the average weight for each pe- riod, the rate of gain was estimated from the given intakes. Predicted daily gain from actual intake ex- ceeded actual overall gain (1.35 kg/day) by 27 percent, when the calculated diet net energy values given were used. The intakes appeared to exceed three times main- tenance for most periods. Owens and Gill (1982) re- ported a depression of 4.8 percent in diet digestibility per multiple of maintenance intake on a high- concentrate ration. As intake increases above mainte- nance, this effect is magnified, up to about three times maintenance, at which point the effect plateaus. Van- Soest et al. (1984) developed discount factors to adjust for the intake effect on digestion for various feeds. A value of 4 percent per multiple of maintenance intake was applied to the calculated diet net energy values for use in projecting period gains, using the system of Fox and Black (1984~. These adjustments must be made at high intakes to determine the effective diet net energy value, as most NRC net energy values are estimated from total digestible nutrients, which is commonly de- termined at a feed intake near maintenance (VanSoest et al., 1984~. When these adjusted net energy values were used, average daily gain projected from actual in- takes was within 5 percent of that actually obtained (1.42 versus 1.35 kg/day). The same calculations with the NRC (1984) energy requirement equations resulted in an overprediction of daily gain (1.35 versus 1.71 and 1.54 kg/day for the yearling and calf equations, respec- tively). The Fox and Black (1984) system was used for predicting gains to evaluate the various intake equa- tions because of its smaller deviation from actual gain with this data set. Adjusted energy values and predicted interval weights were then used to predict intake with the vari- ous systems summarized previously. Intakes at a partic- ular weight exceeded those predicted by the NRC (1984), Plegge et al. (1984), and Fox and Black (1984) by 21 to 28 percent for days 15 to 42, with the differences between actual and predicted values being reduced as the cattle increased in weight (Figure 6-81. The NRC equation, which is based on overall average weights and intakes, clearly was inappropriate for describing intake at a particular weight. Intake obviously declines as the cattle approach the degree of fatness of the current grade of low choice, yield grade 3. Under the conditions

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Beef Cattle 69 lo.4r 10.2 10.0 9.8 9 6 w - u~ y ~ 9.0 At Al A: 9.4 9.2 8.8 8.6 8.4 8.2 8.0 l 7.8 7.6 . 7.4 320 340 360 380 c, ctl OP/ <~9/ ~C/ / '_ .. / ~ ~.. . / ~Image ~ / ',,-0~.~.~.. 1 1 1 1 1 1 1 1 1 1 400 420 BODY WEIGHT (kg) of this feedlot, which is probably typical of most high plains commercial feedlots in the United States, year- ling cattle appear to reach near maximum intake by 28 to 42 days on feed, which then plateaus until they are near finished (28 percent fat, low choice grade) weight, at which time intake begins to decline rapidly. Thornton 13.6 Hi, 1 1 .4 - u~ y by 9.1 to t 6.8 / ,...\ oi3AN ~ - _ _ 440 460 4 = Initial Weight of 182 kg 5 = Initial Weight of 227 kg 6 = I n itia I Weight of 272 kg 7 = Initial Weight of 318 kg 8 = Initial Weight of 364 kg 9 = Initial Weight of 409 kg 10 = Initial Weight of 454 kg 5 _, . FIGURE 6-8 Predicted intake of year- ling steers. SD, standard deviation. et al. (1985) concluded that under these conditions in- take patterns were usually established by 28 days on feed and can be used along with initial weight and days on feed to predict intake beyond that point (Figure 6-91. However, variability was large as evidenced by r2 values of 0.30 to 0.47, even though the data were restricted to FIGURE 6-9 Intake versus initial Jim' weight. Source: Thornton et al. (19851. 10 _,0 9- 10 -9 9 8_84 8~9~8 - 1~7i~7'7 7\ 9 / /~5-5~4~5-5 \ 1 6 4.51 1 1 1 1 1 1 1 182 272 364 454 545 WEIGHT (kg/d)

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70 Predicting Feed Intake FIGURE 6-10 Predicted gain without discounting diet NE. t 1 .3 ~ 1.2 _ 1.1 _ yearling steers of British breeding. It is apparent that variations in frame size, previous nutritional treatment, selection practices, and other factors yet to be identified are involved. It is not clear why the other systems grossly underpredicted intake from 28 to 84 days on feed and to a lesser degree at heavier weights. It may likely be related to the fact that their data are based on experimental conditions such as a smaller pen size, cat- tle were usually fed in confinement, there was less ge- netic variation (as evidenced by an '2 of 0.78), and a smaller proportion of cattle came from backgrounds known to increase intake, such as wheat pasture and thin body condition. When the equations for mean intake were used, the predicted intake values were much closer to actual in- take values (Table 6-91. These results indicate that most of the mean intake equations can be used to predict overall performance whenever expected intake for the feedlot is not known, with the period equations being useful primarily to determine if the cattle are consuming more or less than that which is expected, for use in making daily management decisions. The usefulness of each equation in predicting weight gains and feeding efficiency was examined, assuming that actual intakes were not known. The results of this evaluation are summarized in Table 6-9 and Figure 6- 10. The equation of Thornton et al. (1985) resulted in an overprediction of the rate of gain. Except for that of NRC (1984), the other systems predicted the rate of gain . . .e 7 ~ql _ ~`~` 16 it\ _ ~~-\ ~~ _ OCR for page 56
Beef Cattle 71 sensed in Table 6-10. The NRC (1984) equation aver- aged within 4 to 7 percent of actual intake (r2 = 0.83), and the Fox and Black (1984) equation averaged within 2 percent of actual intake ore = 0.86~. It appears that both of these adequately describe average intake, but the Fox and Black (1984) equation was more accurate at a particular weight because of the adjustment for stage of growth. This is of importance when simulating cost of gain to incremental final weights to identify the most profitable sale weight. The equation of Plegge et al. (1984) underestimated intake by an average of 13 to 18 percent Ire = 0.66), which appeared to be an effect of the initial weight function. Further work is needed to deter- mine the reasons why, under some conditions, initial weight is an important factor. Differences in frame size, protein nutrition, and environmental conditions are pos- sibilities. Predicted versus actual daily gains were determined with the Michigan data, using the NRC (1984) equations for the various frame sizes and sexes and the equation of Fox and Black (1984~. Daily gains were based on weight adjusted to a constant dressing percentage; all were fed to a similar level of empty body fat (28.1 percent). Ac- tual daily gain averaged 1.06 kg/day; daily gain pre- dicted from actual intakes by the NRC (1984) and Fox and Black (1984) equations averaged 1.06 and 0.98 kg/ day, respectively, over a range of actual daily gains from 0.75 to 1.47 kg/day. In an evaluation with seven pens of calves (474 head) in a commercial feedlot in Florida, the Fox and Black (1984) equation predicted daily gain within 1 percent with steers and 3 percent with heifers, using actual intakes. Thus under research conditions the NRC (1984) equations were very accurate in predict- ing overall daily gain, but the Fox and Black equation was more accurate in both yearling and calf commercial feedlot evaluations. Rouse and Loy (Iowa State Univer- sity, personal communication, 1985) found that the NRC equations overpredicted daily gain of small- and average-frame-size cattle in commercial feedlots in Iowa but was accurate on large frame-sized cattle. Both the Fox and Black and NRC equations use the same maintenance requirement; the NRC system has a lower gain requirement. Since the NRC gain equations appear TABLE 6-10 Evaluation of Dry Matter Intake Prediction for Calves to be more accurate under research conditions, it is im- plied that maintenance requirements are typically higher under commercial feedlot conditions. This is likely due to the elimination of the start-up period and sick and low-performing cattle under research condi- tions, and in some cases the fact that cattle were fed in a less stressful environment. These evaluations indicate that adjustments to net energy values for level of intake and appropriate energy requirement equations must be used along with the best dry matter intake prediction equations to accurately predict the performance of growing cattle. Beef Cows Two data sets were used to evaluate the usefulness of the NRC (1984) equations for predicting intake of dry and lactating beef cows. Fox (NRC, 1987) developed the first data set from Vona et al. (1984) in which dry beef cows (Angus, Hereford, and Charolais crossbreeds) were used to determine the intake and digestibility of warm-season grasses (fed in the long form) over a period of 2 years at different stages of growth across the east- ern and central United States. (See Item 8 in the section entitled "An Evaluation of Alternative Systems for Pre- dicting Intake by Beef Cattle.") The data base included 16 grass hays (cultivars of switchgrass, big bluestem, and tall fescue) harvested in five states at three stages of growth (vegetative, early head, and early bloom). The results are presented in Figure 6-11. Intake was lower than that predicted by either equation at lower energy densities (late-cut hay) and was higher than that pre- dicted at higher energy densities (vegetative stage, which is typical of well-managed pastures); ret values for the NRC (1984) growing cattle and NRC beef cattle equations were 0.65 and 0.47, respectively. These data covered the range in qualities of forages usually fed to beef cows, a number of forages and locations were in- cluded, and intake and energy values were determined under standardized conditions. The data of Holloway and Butts (1983) was used to evaluate the adequacy of Fox (NRC, 1987) and NRC (1984) equations to predict the intake of grazing beef Predicted Dry Matter Initial Final Actual Intake (kg/day) Data Weight Weight Frame Diet NEm Diet NEg Dry Matter Plegge Fox and Source (kg) (kg) Sizer (Meal/kg) (Meal/kg) Intake (kg/day) NRC (1984) et al. (1984) Black (1984) Pennsylvania studiesb 241-284 454-545 4-7 1.50-1.82 0.99-1.39 8.18 8.51 7.27 8.31 Michigan studiesC 135-275 356-578 3-8 1.50-2.05 0.95-1.52 7.72 8.27 6.35 7.88 aBased on actual final body composition and the system of Fox and Black (1984). bIncludes 207 pen observations taken at 28-day intervals (Walburg, 1981; Loy, 1983). CIncludes 371 pen observations taken at 28-day intervals (Woody, 1978; Harpster, 1978).

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72 Predicting Feed Intake 120r JO L loo 90 80 70 60~ 1 50 0.60 0.78 0.96 1.14 1.30 1.47 1.63 1.78 _ /p' REV $ N R C Cows D I ET N Em (Mcal/kg) FIGURE 6-11 Dry matter intake of dry beef cows. The equa- tion for actual values was developed from Vona et al. (1984~; DMI = W0 75 (19.4 + 54.5 NEm) r2 = 0.64. The data base includes 16 hays (cultivars of switchgrass, big bluestem, and tall fescue) collected in five states (Iowa, Kentucky, New Jersey, New York, and Pennsylvania) at three stages of growth (vegetative, early head, and early bloom). cows nursing calves. This data base was collected from studies on Angus cows grazing fescue-legume pastures over a period of 5 years. The stocking rate was 0.81 ha/ cow-calf pair, with a weekly rotation between two pas- tures per treatment group. Legumes and fertilizer were added each year to keep forage availability high; nurs- ing calf weights averaged 273 kg after 150 days of graz- ing, indicating that forage availability was not limiting. The results of this evaluation are presented in Figure 6-12. The Fox (NRC, 1987) equation was within 4 per 104,c cry _ Y O Actual '_ 92 - O NRC Grovving Z 90 _ O NRC Cows \ Fox, 1986 \ Q 88 _ \- 86 1 1 1 1.54 1.50 1.46 1.42 PASTURE NEm (Mcal/kg) 1 ~ 1.38 1.33 FIGURE 6-12 Dry matter intake of grazing beef cows nurs- ing calves. Actual data are those of Holloway and Butts (1983~; includes 90 cow years collected over 5 years of grazing fescue-legume pastures (10 cows with calves on 8.1-ha pas- tures, rotated weekly), from the second 30 days of lactation (6.2 kg of milk/day, 151-kg calves) to the last 30 days of lacta- tion (3.5 kg of milkIday, 273-kg calves). U.S. Department of Agriculture. cent of actual intake at the highest energy density and was within 6 percent at the lowest energy density, was similar at the middle energy density, and averaged within 3 percent overall. The ARC (1984) equations were more variable (Figure 6-121. This evaluation sug- gests that when forage availability is not limiting, the equation given above can be used to estimate the intake of grazing beef cows with nursing calves. The addition of the adjustments given for milk pro duction would have added 13 percent to intake during early lactation, declining to an additional 8 percent by late lactation. It is not clear under what conditions this adjustment should be made; predicted intake plus this factor could represent the difference between grazing and drylot intakes. In the studies evaluated here, nursing calf forage intakes/W0 75 were 67 g at 175 kg, 93 g at 199 kg, and 108 g at 248 kg body weight; intakes were not taken at lighter weights. Total intake of forage and milk at these same weights were 81, 104, and 115 g/W075, respec- tively, with actual/predicted ratios of 0.98, 1.20, and 1.15. REFERENCES Abdalla, H. 0. 1986. Compensatory Gain in Calves Following Protein Restriction. Ph.D. dissertation. Cornell University, Ithaca, N.Y. Agricultural Research Council. 1980. The Nutrient Requirements of Ruminant Livestock. Commonwealth Agricultural Bureaux, Sur- rey, England: The Gresham Press. Ayala, H.1974. Energy and Protein Utilization by Cattle as Related to Breed, Sex, Level of Intake and Stage of Growth. Ph.D. disserta- tion. Cornell University, Ithaca, N.Y. Bines, J. A., S. Suzuki, and C. C. Balch.1969. The quantitative signifi- cance of long-term regulation of feed intake in the cows. Br. J. Nutr. 23:695. Brown, C. J., and W. Gifford. 1962. Estimates of heritability and gel netic correlation among certain traits of performance tested beef bulls. Ark. Agric. Exp. Stn., Bull. 653. Brown, C. J., and M. Gacula. 1964. Estimates of heritability of beef cattle performance traits by regression of offspring on sire. J. Anim. Sci. 23:321. Byers, F. M. 1980. Systems of beef cattle feeding and management to regulate composition of growth to produce carcasses of desired composition. Ohio Agric. Res. Dev. Cent., Res. Circ. 258. Colburn, M. W., and J. L. Evans.1968. Reference base, W. of growing steers determined by relating forage intake to body weight. J. Dairy Sci. 51:1073. Crickenberger, R. G., D. G. Fox, and W. T. Magee. 1978. Effect of cattle size, selection and crossbreeding on utilization of high corn silage or high grain rations. J. Anim. Sci. 46:1748. Embry, L. B., and J. B. Burkhardt. 1971. Dry and high moisture corn as affected by processing and type of diet. 1971 South Dakota Cattle Feeders Day Report Series 71-21. Forbes, J. M., P. M. Driver, A. A. El Shahat, T. G. Boaz, and C. G. Scanes. 1975. The effect of daylength and level of feeding on serum prolactin in growing lambs. J. Endocrinol. 64:549. Fortin, A. S., S. Simpfendorfer, J. T. Reid, H. J. Ayala, R. Anrique, and A. F. Kertz. 1980. Effect of level of energy intake and influence of breed and sex on the chemical composition of cattle. J. Anim. Sci. 51:604.

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