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Nutrient Requirements of Goats: Angora, Dairy, and Meat Goats in Temperate and Tropical Countries
6
NUTRITION-RELATED METABOLIC DISORDERS
ABORTION
The goat is more susceptible to abortion than other species of domestic livestock. Most of the work relating to abortion in goats has been with the Angora (Van Heerden, 1963; Van Rensburg, 1971; Shelton and Groff, 1974), in which the problem is more severe. Infectious diseases such as brucellosis are also capable of causing abortion in goats (Alton, 1973). The fact that the goat is a corpus-luteum-dependent species predisposes the animal to abort whenever there is an interference with a functional corpus luteum (Wentzel et al., 1975). A low level of abortion is common with the Angora under normal production conditions, but catastrophic losses sometimes occur. Most abortions occur in response to stress between 90 and 110 days of gestation. Undernutrition during the critical stage of rapid fetal development and competition for nutrients between fetal and maternal organisms appear to be one explanation. The incidence of abortion is reduced in flocks in which replacement does are fed for proper size and development prior to the first breeding season and during gestation (Shelton and Stewart, 1973).
A series of studies from South Africa appears to provide a physiological explanation for the type of abortion observed in that country with the Angora. Parturition, either at or prior to term, is normally initiated by elevated corticosteroids of fetal or maternal origin (Wentzel and Roelofse, 1975). Two types of abortion have been identified in the Angora. One is known as stress abortion, which is triggered by low maternal blood glucose (Wentzel et al., 1976). This type is normally induced by poor nutritional condition of the doe (Wentzel et al., 1974), but other stress factors are also involved. Stress abortion is identified by the expulsion of a live or fresh fetus. Low maternal glucose appears to trigger hyperactivity of the fetal adrenal. The cause of abortion in the period 90–110 days of pregnancy is apparently explained by the fetal adrenal gland’s producing elevated levels of estrogen precursors (Wentzel et al., 1976), and estrogens are known to be potent abortifacients (Wentzel et al., 1975). After 110 days the fetal adrenal is more mature and produces corticosteroids, which are slower acting or less potent abortifacients. A second type of abortion is that by the habitual aborter. These goats can be identified by a history of abortion, and by the expulsion of a dead edematous or autolyzed fetus. This type of abortion apparently results from maternal hyperadrenalism. Both types of abortion may be triggered by undernutrition resulting in low blood glucose. Initial or stress abortions can be almost totally prevented by adequate nutrition and the elimination of stress.
ENTEROTOXEMIA
It has been said with reason that it is impossible to manage a herd of good dairy goats without experiencing some incidence of enterotoxemia, also known as toxic indigestion or overeating disease (Guss, 1977). Diarrhea, depression, lack of coordination, digestive upsets, coma, and death may be observed after excessive feeding on the part of both baby kids and mature animals. Excessive feeding may occur after sudden changes in feeds; with access to palatable, readily fermentable feeds relished by hungry goats; and under conditions of calcium insufficiency and acidosis. Enterotoxemia is a toxic reaction to Clostridium perfringens type C or D, against which antitoxins and vaccination programs with toxoid or bacterins are effective. However, the best prevention in stable-fed goats is frequent feeding of milk, grain, and forage in small amounts. Large meals given once a day should be avoided. Changes of concentrates and forages in the ration should be introduced gradually over several days, especially when the protein or energy content of the diet is increased. When urea or other nonprotein nitrogen is to be part of the diet, then the gradual adaptation should take at least three weeks.
Acute indigestion with a rumen pH of less than 4.8 indicates lactic acidosis. It can follow high levels of grain feeding in early lactation and may lead to the secondary complication of enterotoxemia. Recent research with sheep and cattle on the sensitivity of Streptococcus bovis,
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Nutrient Requirements of Goats: Angora, Dairy, and Meat Goats in Temperate and Tropical Countries
the initiator of acute acidosis, to various antibiotics gives hope that powerful aids in the prevention of enterotoxemia may be available for goats (Muir et al., 1981).
KETOSIS
Ketosis is a metabolic disorder defined by increased levels of ketone bodies (acetone, betahydroxybutyric acid, and acetoacetic acid) in blood, milk, and urine, and is associated with elevated blood plasma nonesterified fatty acids (NEFA), which are precursors of ketone bodies. Lactation ketosis is observed primarily in high-producing dairy cows and to a lesser extent in dairy goats (Leach, 1971; Mackenzie, 1973; Schultz, 1974; Guss, 1977). Late-pregnancy ketosis is encountered in sheep and goats carrying multiple fetuses. Goats appear to be more resistant than cows or ewes to ketosis. Treatment is similar to that for cows: intravenous glucose, glucocorticoid steroids, adrenocorticotrophic hormone (ACTH) injections, oral drenching with sodium propionate, propylene glycol or chloral hydrate.
Experimentally, phlorizin injections in goats have simulated ketosis by causing glucosuria, hypoglycemia, ketonemia, and blood plasma NEFA level increases (Menahan, 1966). Forcing goats to go without food also results in increases of blood NEFA levels, especially in late pregnancy or during lactation. However, the fat depot is the ultimate source of ketosis, and dairy goats appear to lack the fat reserves of cows and other animals, which might explain why ketosis is unusual in goats.
Increased ketogenesis has been produced by infusion of butyric acid into the rumen of goats with phlorizin-induced hypoglycemia; the condition was corrected by intravenous injections of glucose or propionate, or intraruminal administration of propionate (Menahan, 1966). Most ketogenesis was produced, however, from butyric acid infusion into the rumen in late pregnancy and when the goats were forced to fast.
Diabetes accompanied by elevated blood NEFA levels has also been simulated in goats by the administration of intravenous alloxan (Menahan, 1966). A glucose drain during late multiple pregnancy and heavy lactation is the triggering stimulus to lipolysis and ketogenesis. A feedback effect from ketonemia in the presence of insulin appears to prevent further increases of fat mobilization and may be important to the survival of the animal.
Increased plasma NEFA levels were a more sensitive indicator of undernutrition in goats than blood ketones or blood glucose levels (Radloff, 1964). Growth hormone, epinephrine, glucocorticoid steroids, and ACTH have direct effects on and relationships to blood ketone levels in goats.
PARTURIENT PARESIS (MILK FEVER)
The incidence of paralysis-type conditions of this metabolic hypocalcemic disorder differs among genetic groups of dairy cattle, but is also reported for other species, including goats (Littledike, 1974; Guss, 1977). However, it is not observed as frequently in goats as in cows. Signs and treatments are similar to those for cows. Prevention has been tried with different contents of calcium in the diet during the dry period, and with hormone treatment and vitamin D therapy, but no generally accepted management practice has evolved. Parturient paresis has been related to greatly increased mammary blood flow immediately after parturition (Reynolds, 1970).
The relationship of the kinetics of calcium pool size and calcium turnover rate to dietary phosphorus levels were studied and discussed by Twardock et al. (1970) and Anderson et al. (1970). It was noted that different dietary regimes of goats, including changed calcium-to-phosphorus ratios, had significant effects on the size and biological half-life of the readily exchangeable calcium pool. It was suggested that the response time of the parathyroid hormone and the removal of calcium from the so-called nonexchangeable bone pool was too slow for the immediate calcium needs of parturition and lactation onset in goats; and when the readily exchangeable calcium pool was inadequate to meet these needs, then parturient paresis resulted. A low or high calcium diet over time may be a predisposing factor for a reduced readily exchangeable calcium pool in goats. Dietary phosphorus levels influence the effects of such diets, and also the level of intestinal calcium absorption and available calcium in goats.
POSTHITIS
Posthitis, also known as sheathrot or pizzlerot, has been reported in male goats (Shelton and Livingston, 1975). This problem has been studied extensively in Merino wethers in Australia (Osborne and Widdows, 1961; McMillan and Southcott, 1973). The causative agents are thought to be a high protein ration in combination with the presence of a urea-hydrolyzing organism such as Corynebacterium renale (McMillan and Southcott, 1973; Barajas and Biberstein, 1974; Shelton and Livingston, 1975). The problem appears to be aggravated by confinement to areas where irritation or infection are more likely to occur. The problem is not likely to be a serious one with goats, except with mature Angora wethers kept for hair production. The problem may also occur with individual breeding bucks kept in confinement.
TOXIC PLANTS
The problem of poisonous plants is of great importance to owners of much of the world’s goat population. It is not known, nor can it be inferred, that goats are either more or less susceptible to toxic plants than other animal species. However, their grazing habits and the environment under which many of them are kept place them in wider contact with toxic plants. Many goats are found in arid areas, and
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Nutrient Requirements of Goats: Angora, Dairy, and Meat Goats in Temperate and Tropical Countries
are noted for eating a variety of plants under these conditions. Many native forage species found under arid conditions have natural protective mechanisms, including toxic principals that retard evaporation and protect against livestock. Goats are thought to be less sensitive than cattle to the toxic effects of tannic acid. Goats can live for extended periods of time on oak species with high tannic acid, whereas cattle are very susceptible to this material (Dollahite, 1961). Goats are also not bothered by bitter-weed (Hymenoxys odorata), which causes severe losses with sheep in some areas (Hardy et al. 1931). And goats have been used at times to reduce the availability of toxic plants to other animal species (Dollahite, 1972). Some references to toxic plants and their effects include Sperry et al. (1964), Kingsbury (1964), Lindahl (1972), and Keeler et al. (1978).
UREA TOXICITY
Urea is an important natural compound in the physiological processes of goats, but can be highly toxic if consumed in excess. Although most of the urea that is formed in the liver is excreted through the kidney, a portion passes into the rumen where it is hydrolyzed to ammonia and used by rumen microorganisms for protein synthesis (Vercoe, 1969; Hume et al., 1970). Therefore, urea is frequently included in ruminant diets to partially replace protein ingredients. Producers and feed formulators must exercise caution when feeding goats urea, since excessive amounts can result in a buildup of ammonia to toxic levels in the bloodstream (Morris and Payne, 1970; Kromann et al., 1971). It is recommended that urea supply no more than one-third of the total crude protein in forage or roughage-type diets and not more than one-half in the concentrate portion of the diet. Also, an adaptation period of at least three weeks is required for the animal to utilize urea efficiently. It is generally believed that 44 g/100 kg body weight at a single feeding will result in acute toxicity. Producers should assure that daily consumption levels at that rate do not occur.
UROLITHIASIS
Goats are known to be susceptible to urolithiasis (urinary calculi), and serious losses can occur when valuable breeding males are placed on calculogenic rations (Sato and Omori, 1977). It is not known whether they are more susceptible or less susceptible than other ruminant species or whether the predisposing factors are different. For the purpose of this discussion it will be assumed that goats do not differ from cattle or sheep with respect to calculus formation. Nutritional imbalances are generally considered the primary cause of stone formation, but infection has been identified as a predisposing factor with some species (Griffith et al., 1975). The problem is largely restricted to the male because his urinary tract is much more susceptible to blockage, and it is seen infrequently in grazing goats. The problem is important only in confined animals, which represent a small portion of the world’s goat population but include some of the more valuable stud bucks. The chemistry of calculus formation is complex and is not completely understood. One of the more important predisposing factors is a high phosphorus content in the diet, or a content high relative to calcium or potassium content (Rabbins et al., 1965; Hoar et al., 1970). In dry lot rations the potassium levels should be maintained at an adequate level and the calcium-to-phosphorus ratio should be maintained at 1.5:1 or greater. Additional protection may be obtained through the use of ration additives such as ammonium chloride (Crookshank, 1970) or potassium chloride (Shelton and Ellis, 1965; Crookshank, 1966), which will acidify the urine. If infection plays a part in calculus formation, it may be through its effect on pH of the urine. Using medication to combat infection with a view to preventing calculus formation is not generally recommended, but such an action may be a secondary benefit of using antibiotics in the ration for other reasons.
Representative terms from entire chapter:
calcium pool
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