Optimum utilization of pastures by sheep is very difficult to attain. As pasture forage matures, the protein content declines, fiber increases, and both forage intake and digestibility decline. The combination of internal parasites and the inability of young lambs to consume adequate dry matter invariably results in weight gains on pasture being 40 to 60 percent less than when lambs are grain-fed in drylot (Jordan and Marten, 1968a). Lambs 4 to 6 weeks old are particularly sensitive to heavy internal parasite infestation with which their dams normally infect the pasture. Pasture forage is best suited for maintenance of ewes who are significantly more tolerant of internal parasite infestation.
In some areas of the country, pastures are often underutilized and much forage is trampled and wasted. Limiting grazing time to a few hours a day or restricting grazing time to around 60 percent of normal (Jordan and Marten, 1968b) reduces selective grazing, reduces forage intake, increases pasture carrying capacity by 50 to 100 percent, and prolongs the period of available feed.
Legume forages rotationally grazed provide more nutrients over a longer growing period than nonlegume forages, although in some areas bloat precludes their use by sheep of any age. Where nonlegumes are the major forage they must be augmented with supplementary annual pastures.
Rape, a cool-season species, is an excellent summer and fall pasture for both ewes and lambs, resulting in an average daily gain of 0.20 to 0.25 kg. Forage peas have a low carrying capacity. Sudan grass or sudan-sorghum crosses produce high yield but result in very selective, spotty grazing and low lamb performance and are far better suited for mature sheep, in which maintenance rather than increases in weight is paramount (Wedin and Jordan, 1961).
Timothy, fescues, wheat grasses, and blue grass become unpalatable on reaching maturity in early summer. Orchard grass is less palatable than brome grass in midsummer but produces far more forage (Table 10). New varieties of low-alkaloid-containing canarygrass produce more digestible nutrients over a long grazing season than either orchard grass or brome grass and appear very promising (Marten et al., 1981).
Range sheep do not differ physiologically from pen-raised sheep; however, the nutrient needs of the two types differ widely.
Type and composition of plants at any one location on pasture or range is dependent on type and composition of the parent soil, as well as moisture, radiant energy available for growth, and previous and present management of the area. Soils inherently low in a given element often will produce plants low in that element, and thus, deficiencies of the element may occur. Where high levels of specific elements exist in soil (plants may accumulate these elements), it is likely that if they are toxic to sheep, toxicities will be seen. Although maps of states or of the United States are available that describe areas of mineral deficiencies and toxicities, it is important to understand that local environmental and topographical factors can influence the occurrence of toxicities and deficiencies.
Typically, range land is evaluated based on its stage of ecological succession toward climax vegetation for a specific type of vegetative community. Although range land classification is used in allocating forage for game and livestock, classification or score may not accurately predict animal performance. Important considerations for range use and expected animal performance on range are distribution of water, topography, season of use, presence
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Page 29 5 Other Aspects of Sheep Nutrition Pastures Optimum utilization of pastures by sheep is very difficult to attain. As pasture forage matures, the protein content declines, fiber increases, and both forage intake and digestibility decline. The combination of internal parasites and the inability of young lambs to consume adequate dry matter invariably results in weight gains on pasture being 40 to 60 percent less than when lambs are grain-fed in drylot (Jordan and Marten, 1968a). Lambs 4 to 6 weeks old are particularly sensitive to heavy internal parasite infestation with which their dams normally infect the pasture. Pasture forage is best suited for maintenance of ewes who are significantly more tolerant of internal parasite infestation. In some areas of the country, pastures are often underutilized and much forage is trampled and wasted. Limiting grazing time to a few hours a day or restricting grazing time to around 60 percent of normal (Jordan and Marten, 1968b) reduces selective grazing, reduces forage intake, increases pasture carrying capacity by 50 to 100 percent, and prolongs the period of available feed. Legume forages rotationally grazed provide more nutrients over a longer growing period than nonlegume forages, although in some areas bloat precludes their use by sheep of any age. Where nonlegumes are the major forage they must be augmented with supplementary annual pastures. Rape, a cool-season species, is an excellent summer and fall pasture for both ewes and lambs, resulting in an average daily gain of 0.20 to 0.25 kg. Forage peas have a low carrying capacity. Sudan grass or sudan-sorghum crosses produce high yield but result in very selective, spotty grazing and low lamb performance and are far better suited for mature sheep, in which maintenance rather than increases in weight is paramount (Wedin and Jordan, 1961). Timothy, fescues, wheat grasses, and blue grass become unpalatable on reaching maturity in early summer. Orchard grass is less palatable than brome grass in midsummer but produces far more forage (Table 10). New varieties of low-alkaloid-containing canarygrass produce more digestible nutrients over a long grazing season than either orchard grass or brome grass and appear very promising (Marten et al., 1981). Range Sheep Range sheep do not differ physiologically from pen-raised sheep; however, the nutrient needs of the two types differ widely. Type and composition of plants at any one location on pasture or range is dependent on type and composition of the parent soil, as well as moisture, radiant energy available for growth, and previous and present management of the area. Soils inherently low in a given element often will produce plants low in that element, and thus, deficiencies of the element may occur. Where high levels of specific elements exist in soil (plants may accumulate these elements), it is likely that if they are toxic to sheep, toxicities will be seen. Although maps of states or of the United States are available that describe areas of mineral deficiencies and toxicities, it is important to understand that local environmental and topographical factors can influence the occurrence of toxicities and deficiencies. Typically, range land is evaluated based on its stage of ecological succession toward climax vegetation for a specific type of vegetative community. Although range land classification is used in allocating forage for game and livestock, classification or score may not accurately predict animal performance. Important considerations for range use and expected animal performance on range are distribution of water, topography, season of use, presence
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Page 30 of poisonous plants, occurrence of predation, and system of grazing management. Range sheep must frequently be supplemented with phosphorus, protein, and energy for optimum performance. For example, forage available to range sheep during gestation (late fall, winter, and early spring) is often at its lowest concentration of nutrients (Cook et al., 1954; Huston, 1983; Huston et al., 1981). Ranges classified as fair to poor are unlikely to provide adequate energy, protein, or phosphorus. Excellent to good ranges generally supply adequate energy (except when snow covered), but sheep may need supplemental protein and usually are lacking in phosphorus (Bryant et al., 1979). It has been estimated that maintenance requirements for energy of grazing sheep are 60 to 70 percent greater than for comparable pen-fed sheep (Young and Corbett, 1972). The greater need for energy by grazing sheep results largely from the impact of environmental factors and an increased activity increment. The environmental factors are discussed elsewhere in this publication and by NRC (1981). The higher energy requirement due to an increased activity increment for grazing sheep results from the energy costs of grazing, horizontal movement, vertical movement, and other activities such as rumination time. As range changes from essentially flat to rolling, the change in energy needs for travel will change because the energy cost of vertical travel is approximately 10 times the energy cost for horizontal travel (6.86 cal/m/kg body weight versus 0.59 cal/m/kg body weight; Clapperton, 1964). Also, as density of grazable forage decreases or distance to water increases, energy needs to satisfy daily requirements increase. Because it is difficult to measure feed intake and selectivity by range sheep, management must rely on knowledge of nutrient composition of range forages at various stages of growth and during various seasons of the year, as well as on the ability of the sheep to achieve adequate quantity and quality of forages. Ewe condition in relation to previous condition and projected desired condition and the sheep's general vigor and activity are the usual criteria used to assess adequate feed intake. Although proper nutritional management may indicate supplementation, it is at times physically impossible to get sufficient supplemental feed to sheep; under these conditions sheep must rely on body stores to sustain them through periods of shortages. To be economically successful the range sheep operator must manage sheep so they meet their nutrient needs largely from grazing rather than from supplements, grain, and hay. Formulating Supplements for Range Ewes In formulating supplements for range ewes it is necessary to assess the composition of the available diet and the condition, status, and stage of production of the ewe. A diet consisting largely of dead grass will require a different supplement composition than one consisting of sage and browse. Alfalfa hay, which may contain a good source of energy, protein, and b-carotene, discourages grazing. Its use is more suitable when the range is snow covered and there is a need for increased DM intake as well as for energy and protein. The usual supplement is fed as cubes that provide in a concentrated form whatever nutrients are deficient in the range forage (Weir and Torell, 1967). This type of supplement will generally encourage grazing and enhance the utilization of the nutrients provided by the range feed. The usual amount fed per ewe per day is 0.1 to 0.2 kg to provide 30 to 50 percent of the protein requirements, 75 percent of the vitamin A and phosphorus requirements, and 20 to 30 percent of the energy requirements. To avoid consumption of poisonous plants when trailing sheep or when feed is snow covered, the amount of supplement fed daily may be increased 2 to 4 times. The usual supplements include 30 to 40 percent protein equivalent, 1.5 to 2.0 percent phosphorus, 3.5 to 4.0 Mcal DE/kg, and 15 to 20 mg carotene/kg. Although range ewes must be managed quite differently than intensively raised sheep, their physiological needs and responses to nutrients are no different from those of confined sheep. Range ewes respond to flushing, are equally susceptible to pregnancy disease, and lactate at levels dictated by nutrient intake. Thus, they should be managed so that either grazing or a combination of grazing and supplementation will meet their needs during those critical periods. Table 11 presents formulas for range supplements for different ranges and nutrient needs. Flushing The practice of increasing nutrient intake or the dynamic effect that influences body weight (BW) change and condition prior to and during breeding is called flushing. Its purpose is to increase the rate of ovulation and, hence, the lambing rate. Although flushing is a husbandry practice used in major sheep-producing countries, the response to flushing is variable and an explanation for the response is not evident. A high level of hepatic steroid metabolizing enzymes (SME) is thought to be associated with an increased clearance rate of steroids, and a decrease in steroids is associated with an increase in gonadotropins and thus an increase in ovulation (Thomas et al., 1984). Increased intake of nutrients, particularly protein, effectively increases levels of hepatic SME. Phenobarbital is also an effective inducer of hepatic SME. Thomas et al. (1984) reported that 1 g phenobarbital daily
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Page 31 per ewe increased the ovulation rate 0.24 ova per ewe, whereas 0.45 kg grain per ewe per day had no effect. Lambing rate, as affected by nutritional alteration prior to and during breeding (flushing), is influenced not only by the number of ova fertilized but by embryo survival, which affects the number of ewes lambing. The first month after fertilization is very critical to embryo survival. Robinson (1983) divides that period into two, a preimplantation period of 15 days and a 14-day embryo implantation phase. During the first 15 days after conception, a balance in the distribution of embryos between the two horns of the uterus is accomplished and the implantation process is initiated linking the rapidly developing trophoblast and the epithelial cells of the maternal caruncles (Boshier, 1969). Loss of fertilized ova during this preimplantation period results in a high incidence of repeat estrous cycles occurring at normal intervals or a lowered lambing rate. Unless the ewes are subjected to severe undernutrition at this time, nutrition is likely to be only a minor factor affecting embryo survival (except very high levels of energy intake have detrimental effects on embryo survival) (Robinson, 1977; Doney, 1979). Nutrition does exert some effect on the concentration of progesterone in maternal plasma (Parr et al., 1982), and progesterone does influence embryo growth during this preimplantation period (Lawson, 1977). During the implantation period (14 to 28 days), nutritionally related deaths have a wider range of effects on pregnancy. These include a higher-than-normal number of ewes returning to estrus at more than 19 days after a fertile mating, a reduced lambing rate, and reduced lamb birth weights. The last effect is due to embryo death in the third and fourth weeks of pregnancy disturbing the balance in the distribution of the fetuses between the two uterine horns. This increases within-litter variability in fetal growth as a result of the surviving embryos' inability to utilize the vacated maternal cotyledons and also reduces the birth weight of the fetuses that do survive (Robinson, 1983). Extremes in nutrition are detrimental to embryo survival, suggesting that ewes should be kept at maintenance levels of nutrition during the first month of pregnancy. Another aspect of conditioning ewes for breeding is referred to as static effects or ewe size embracing metabolic mass and condition. Exceptionally poor body condition or severe undernutrition during the immediate premating period, irrespective of condition, may delay onset of seasonal estrus, lengthen the estrous cycle, cause ovulation failure, or result in ovulation unaccompanied by estrus (Doney and Gunn, 1981). Foote and Mathews (1983) reported a very high correlation between body weight and body size (0.999), prolificacy (0.992), and weight of lambs born per ewe lambing (0.998). Correlations between ewe body weight and weaning rate and weight of lambs weaned were 0.336 and 0.672, respectively. The response to flushing is affected by the age of the ewe (mature ewes show a greater response than yearlings), its breed, and the stage of the breeding season. Flushing during the seasonal peak in ovulation rate is less effective than during early or late in the breeding season. Ewes in fleshy condition during breeding have a significantly higher ovulation rate and greater follicle size but a lower embryonic survival rate (El-Sheikh et al., 1955). The lower embryonic survival rate is likely affected by and related to ovulation rate; that is, a higher ovulation rate would result in more ova subject to loss (Edey, 1969). Foote et al. (1959), however, found that maternal cotyledon weight increased when ewes were changed from full to limited feeding, probably to obtain a greater nutrient supply for the fetus from the mother, thereby protecting the fetus from a nutrient shortage imposed by the limited feeding. Ova loss is complicated by nutrition effects after mating and by interaction between pre- and postmating (Edey, 1976). Both severe undernourishment or overnourishment postmating may be associated with ova loss and may have more severe effects than a static intermediate level (Doney and Gunn, 1981). The placenta generally attains 95 percent of its final weight during the first 90 days of gestation, whereas the fetus attains about 15 percent of its weight in 90 days (Russell, 1979). Thus, nutrition level during early gestation may have its greatest effect on maintenance of the integrity of fetal membranes, which in turn affect the retention of the fetus. Grain feeding and increased ewe weight also resulted in higher plasma glucose levels and greater adrenal and pituitary weight (Bellows et al., 1963; Howland et al., 1966; Memon et al., 1969) and consequently greater total follicle stimulating hormone and luteinizing hormone potency. Larger ewes, irrespective offatness, had larger pituitaries and greater follicular fluid weight. Virtually all prenatal deaths occur within the first 25 days after breeding (Foote et al., 1959), and the integrity of the cotyledons and placental membranes are logically a contributing factor. Doney (1979) suggests that the efficiency of reproduction depends on the average nutrient intake level over the year as well as on the actual level at different stages in the annual cycle. Ovulation rate is affected by factors operating up to the time of mating or during the recovery period between lactation and breeding, whereas ova loss or prenatal mortality is affected by nutrition during recovery and also during pregnancy. Thus, both the static (during the recovery period) and dynamic (flushing) aspects of nutrition influence lambing rate (Coop, 1966). Changing nutrient intake from a high prebreeding level to a low postbreeding level appears to contribute more
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Page 32 to prenatal mortality than if the ewes are maintained at a low level throughout the whole period. This suggests that extremes are to be avoided and that body condition throughout the year is as critical as during a short flushing period. Flushing is usually accomplished by providing ewes with fresh pasture, supplemental harvested forage, or up to 0.50 kg of grain per ewe daily, depending on environmental stress, availability of forage, and body condition. This level of nutrient intake should approach the energy and protein levels normally provided during late gestation. Special feeding usually begins around 2 weeks prior to mating and continues 2 to 4 weeks into the breeding season. The practice is especially beneficial for thin ewes that have not recovered from previous lactation stress. It should not continue too long, because an extended period of high feeding is unnecessarily costly, and overconditioning during pregnancy should be avoided. Drastic or severe decreases in the plane of nutrition should be avoided. Creep Feeding The practice of providing supplemental feed to nursing lambs in an area that cannot be entered by their dams is called creep feeding. Lambs usually commence creep feeding around 10 to 14 days of age, and the amount consumed is inversely proportional to the amount of milk consumed. Inadequate energy intake by suckling lambs is the major cause of slow weight gains. Greater efficiency and lamb weight gains occur if lambs are creep fed than if only the ewes are grain fed. Jordan and Gates (1961) fed hay to ewes but did not creep feed lambs, for a lamb ADG of 0.15 kg; fed hay plus grain to ewes but did not creep feed lambs, for a lamb ADG of 0.20 kg; gave hay to ewes and did creep feed lambs, for a lamb ADG of 0.30 kg; and gave hay plus grain to ewes and did creep feed lambs, for a lamb ADG of 0.33 kg. Creep feed consumption by the lambs approximated the amount of corn fed the ewes. The amount of creep feed consumed by lambs 2 to 6 weeks of age is affected by the palatability of the ration (ration composition and ration form) and the location and environment of the creep area. A well-bedded, well-lighted area located close to where ewes congregate is preferred. Low milk yield tends to encourage creep consumption, but lamb size as affected by birth weight and milk consumption has a significant effect on daily creep feed consumption. Initially, lambs prefer ground creep rations to pelleted rations. After 4 or 5 weeks of age, lambs show a preference for pelleted rations, and after 5 to 6 weeks, lambs should be fed unground grains. Ørskov (1983) reported that ground, pelleted barley, corn, wheat, or oats versus whole grains did not affect weight gains or feed-conversion efficiency but did lower rumen pH approximately 1 point and increased the proportion of propionic acid to acetic acid to a level that exceeds the metabolic capacity of the liver, giving rise to odd- and branched-chain fatty acids resulting in soft fat and reduced carcass quality. Unprocessed grain alleviates these problems. The deterrent to feeding whole grains is the separating out of various supplements that are usually finely ground. Pelleting only the supplement alleviates this problem. Soybean meal is an important ingredient in creep diets because of its high protein content and palatability. Bran is well liked by lambs, as are most sweet feeds. Oats, while consumed readily, are less well liked than corn as the lambs get older. Acceptability of ground feed may be increased slightly by adding 2 to 5 percent molasses. Typical creep diets are suggested in Table 12, but other formulations may perform equally well. For rapid weight gains, creep diets must be palatable and high in energy and must contain adequate protein (12 to 14 percent), minerals (especially calcium, since grains are low in calcium), and vitamins. The most important physiological factor determining successful early weaning and ability to utilize solid food is the state of rumen development (Ørskov, 1983). Rumen development is stimulated by the intake of solid feed, which, on fermentation, yields volatile fatty acids. Lambs suckling heavy-milking dams are less inclined to eat solid feed. Restricting protein intake of the ewe reduces milk flow and thus encourages creep feed intake (Robinson et al., 1974). To achieve satisfactory performance and encourage rumen growth, lambs should receive a diet that ferments rapidly and does not lead to an accumulation of indigestible fibrous material within the rumen. Corn satisfies both requirements, whereas oats are high in indigestible hulls and result in pot-bellied lambs (Ørskov, 1975). High-quality legumes degrade rapidly in the rumen and also stimulate rumen growth. Unless the transition from a stage of high milk-low creep feed intake to low milk-high creep feed intake is completed prior to weaning at 4 to 6 weeks, a check in growth will occur and lambs will not gain for 7 to 10 days (Ørskov, 1982; Frederiksen et al., 1980). Thus, the level of solid food intake is a better guide to weaning than lamb weight, since lambs suckling heavy-milking dams may meet the weight criteria but, because they have consumed little solid feed, may have less-developed rumens (Ørskov, 1983). Early Weaning Lactating ewes normally reach their peak in milk production around 3 to 4 weeks postpartum and produce 75
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Page 33 percent of their total milk yield during the first 8 weeks of lactation. While milk production during early lactation can be stimulated through proper selection of feeds, after 6 to 8 weeks milk production declines markedly and high nutrient intake fails to stimulate production (Jordan and Hanke, 1977). Early weaning as used in this report refers to the practice of weaning lambs at 6 to 8 weeks of age. There is considerable interest in early weaning because of possible early marketing of lambs, out-of-season lambing, multiple lamb crops per year, and use of prolific breeds. Early weaning can be cost-effective because it enables higher and more-efficient gains while the lambs are young and also reduces ewe cost because the ewes can be maintained on a limited feed allowance for longer periods of time between parturitions. Lambs to be early weaned should receive creep feed from the time they are old enough to eat solid feed (7 to 14 days of age). At weaning, stress on the lambs should be minimized by removing the ewes and leaving the lambs in familiar surroundings. The postweaning ration should be a high-concentrate ration with a minimum of 16 percent crude protein, 0.6 percent calcium, and 0.30 percent phosphorus. Since their source of protein from milk has been removed, the level of protein in the dry diet of a 6- to 8-week-old weaned lamb should actually be higher than that for a 3- to 5-week-old suckling lamb (Jordan and Hanke, 1970) and certainly higher than for older lambs. Artificial Rearing The practice of removing lambs from their dams when they are 8 to 24 hours old and rearing them on milk replacer for 3 to 4 weeks is referred to as artificial rearing. Although milk replacers are expensive, artificial rearing is feasible in such cases as orphan lambs and ewes with insufficient milk supply because of mastitis and in cases of more prolific breeds that give birth to litters larger than can be adequately suckled (Frederiksen et al., 1980; Gorrill et al., 1982). Lambs intended for artificial rearing should be allowed to obtain their mother's colostrum for a minimum of 8 hours after birth before being weaned. Frozen colostrum, warmed to body temperature and bottle fed, is an adequate alternative. At least 50 ml of colostrum per kilogram of lamb weight is necessary to provide an effective level of disease resistance (Frederiksen et al., 1980; Gorrill et al., 1982). Although frozen ewe colostrum is preferred, research has shown that frozen cow colostrum also provides adequate antibodies for rearing lambs (Larsen et al., 1974; Logan et al., 1978; Franken and Elving, 1982). Maximum performance during artificial rearing is obtained by feeding specially formulated lamb milk replacers containing at least 24 percent fat and 24 percent protein, with all of the protein provided by spray-dried milk products (Heaney et al., 1982a; Gorrill et al., 1982). Similar milk replacers in which part of the skim milk powder is replaced by casein and whey or cerelose are also being used successfully, but with this type of milk replacer it is recommended that lactose content be limited to 30 to 35 percent (Glimp, 1972; Frederiksen et al., 1980). Lambs can also be successfully reared with a high-quality milk replacer designed for calves that contains at least 20 percent fat and 20 percent protein. It is very important that only a high-quality calf milk replacer with all the protein provided by skim milk powder be used. It is unlikely that lambs could adequately utilize lower-quality ingredients at the reduced protein level. Gains on calf milk replacer are around 90 percent of gains reported with lamb milk replacer. Nevertheless, such a system could be economical because the lower cost of calf milk replacer could offset the marginal reduction in performance (Heaney et al., 1982b). During a 3- to 4-week artificial rearing program, a lamb will consume an average of 400 to 500 g of dry milk replacer per day when 1 part milk replacer is mixed with 4 to 5 parts water. Lambs should be fed the milk replacer ad libitum at 2° to 4°C to minimize digestive disturbances, particularly abomasal bloat (Large and Penning, 1967; Peters and Heaney, 1974; Frederiksen et al., 1980; Gorrill et al., 1982). Lambs should be provided constant access to fresh water and high-quality, palatable solid feed to accustom them to eating dry feed and to minimize weight losses during the transition from a liquid to a solid feed diet at around 3 to 4 weeks. Weight gains should approximate 0.25 to 0.30 kg (0.55 to 0.66 lb) per lamb daily during the period that milk replacer is fed. Because milk replacer is expensive, the liquid-feeding period should be as short as possible. Lambs can be successfully weaned from milk replacer at 3 weeks of age, although a growth check will occur in which the lambs lose some weight the first week before resuming normal gains (Heaney et al., 1982a, b, 1984). The growth check is primarily a reflection of reduced nutrient intake (Frederiksen et al., 1980). Delaying weaning to 4 weeks of age reduces the growth check and results in lamb weights approximately 1 kg (2.2 lb) heavier at 70 days of age. The extra weight is not sufficient, however, to offset the extra costs of the 3.0 to 3.5 kg of milk replacer required for the extra week of feeding (Heaney et al., 1984). The postweaning diet should be high energy and should contain 18 to 20 percent protein (as-fed basis) for the first 3 weeks and then 14 to 17 percent protein thereafter. It is doubtful whether higher levels of protein will result
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Page 34 in sufficiently better lamb performance to justify the cost (Meat and Livestock Commission, 1976; Frederiksen et al., 1980; Gorrill et al., 1982; Heaney et al., 1983). Feed Additives Antibiotics may improve performance when added to creep diets and growing-finishing diets for lambs (Ott, 1968). Responses to antibiotics seem to be markedly affected by differences in management and in the amount of stress to which animals are subjected (Hays, 1969). At present only chlortetracycline and oxytetracycline are approved by the Food and Drug Administration for nutritional uses in sheep diets (Anonymous, 1984a). Chlortetracycline can be used at levels varying from 22 to 55 mg/kg of feed to promote growth and improve feed efficiency. Oxytetracycline is approved for use at levels of 11 to 22 mg/kg of feed for the same purpose. The polyether antibiotic, lasalocid, which was recently approved for prevention of coccidiosis in sheep held in confinement (Anonymous, 1984b), has also been shown to improve rate of gain and feed efficiency in lambs (Foreyt et al., 1979; Horton and Stockdale, 1981). Lasalocid is approved for use at levels of 22 to 33 mg/kg of the total diet. There is some evidence that antibiotics help reduce the incidence of enterotoxemia (Ott, 1968). Chlortetracycline can be used at a level of 22 mg/kg of feed for this purpose, and oxytetracycline can be used at a level of 25 mg per lamb per day. In addition to the above, a number of feed additives are approved for treatment of specific sheep diseases. Information for the approved usage of these antibiotics can be obtained by consulting the Feed Additive Compendium (Anonymous, 1984a). Poisonous Plants Many poisonous plants grow on pastures and range areas in the United States (Kingsbury, 1964; Sperry et al., 1964; James et al., 1980). In some areas, such as the western states, poisonous plants are a major cause of economic loss to the sheep industry (Dwyer, 1978; Schuster, 1978). Most losses occur when desirable forage is scarce and poisonous plants are abundant (Binns, 1974). Losses result from death, abortions, photosensitization, decreased production, emaciation, and birth defects (James et al., 1980). Since there are no known specific treatments for animals poisoned by most poisonous plants, proper management of pastures and animals is the best approach to preventing losses (Merrill and Schuster, 1978). The best protection against poisonous plant problems is to become familiar with the poisonous plants that grow in pastures and learn under what conditions these plants are dangerous to sheep. Sheep that have been under stress or that are overly hungry or thirsty should not be permitted to graze in areas infested with poisonous plants. Sheep introduced into a new area that contains poisonous plants with which they may not be familiar should be watched closely. Salt and supplemental feed should be provided to grazing animals as needed. Control of poisonous plants (spraying, grazing management, hand pulling) or of animal access to areas containing poisonous plants (fencing, pasture rotation) should be practiced where feasible. Effective treatment of poisoned sheep requires identification of the specific plant causing the problem, removal of sheep to a feed source free of the poisonous plant, and administration of an antidote if one is available. In cases where a specific treatment is unknown, the only course of action is to treat the signs. Ration Alternatives Although the daily nutrient requirements for ewes presented in Table 1 are specific, the sources of nutrients available to meet these requirements are many. Confinement feeding of diets low in energy to ewes on slotted floors at high density levels and often with inadequate feed bunk space makes it difficult to provide adequate nutrient intake to all sheep and to deal with the accumulation of refused feed under the slotted floor. Alternatives to the typical high-forage diets and various management approaches are available to circumvent these problems and to minimize labor and facility costs. For example, feeding several groups of ewes at different times with one common feed bunk eliminates the problems of inadequate bunk space. Feeding gestating ewes on alternate days or 3 times weekly accommodates feeding groups of ewes at different times. Ewes fed three times weekly the same amount of feed per week were equal in weight gains and in lamb and wool production to ewes fed daily (Jordan and Hanke, 1963; Jordan, 1966). Another ration alternative is to feed ewes higher-than-normal grain rations. The digestible energy (DE) values used for forages are overvalued in relation to the DE in grain, and since grains are often a lower-cost source of energy than hay, high-grain rations may offer advantages for intensively managed sheep. Gestating ewes fed rations consisting of equal parts of hay and corn (69 percent total digestible nutrients) in amounts equal to one-half the weight of an all-hay ration showed weight gains and lamb and wool production equal to ewes fed the all-hay diet (Jordan, 1966). However, dry matter or bulk is lacking, which results in wool picking. A more reasonable approach is to feed 3 parts of hay and 1 part of corn at 75 percent of an all-hay ration. This is more apt to provide
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Page 35 adequate bulk, protein, and minerals and still reduce feeding costs. It normally takes ewes 3 to 4 hours to eat enough long hay to meet their nutrient requirements, but when fed a 40 percent corn-60 percent hay ration, they can consume enough feed to meet their requirements in 20 to 30 minutes. The reduced time for feeding is particularly advantageous when feeding three or four different groups of ewes per day with one common feed bunk. To prevent esophageal choke, the corn should be mixed with the hay (or spread on top of long hay) to prevent too-rapid consumption. Also, hay must be of good quality with 15 to 18 percent protein so that protein and calcium deficiencies do not develop. The composition of two important sheep feedstuffs, corn silage and haylage, are on a DM basis, but these feedstuffs contain 40 to 70 percent moisture on an as-fed basis. To compare the ''as-fed" nutrient content of silage or haylage with the values presented in Table 13, multiply those values by the DM content in the silage or haylage being fed. Corn silage with 70 percent TDN, on a DM basis, × 35 percent DM contains 24.5 percent TDN on an as-fed basis. Haylage with 56 percent TDN, on a DM basis, × 50 percent DM contains 28 percent TDN on an as-fed basis. There are numerous feedstuffs that can be used as ration alternatives to the conventional legume hay-grain feedstuffs generally used by producers. These include many grain, vegetable, fruit, and food industry by-products as well as damaged grains and roughages. The major consideration in using alternative feedstuffs is their cost relative to more conventional feeds. Frequent use of alternative feedstuffs requires careful attention to correcting whatever nutrient deficiencies may exist.