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Sheep INTRODUCTION Like beef cattle, sheep are raised primarily as a means of marketing forages, especially those forages that have limited alternative markets. It is important to predict intake to determine the proportion of their nutrient re- quirements that can be met from forages, so that the amount of supplemental nutrients needed can be esti- mated. In sheep feed intake is controlled by physiologi- cal demand due to maintenance needs and production demands, up to the limits of gastrointestinal (GI) capac- ity (Mertens,1973~. Usually GI capacity is adequate for dry ewes to nearly meet their requirements from low- quality forages, especially until the last 30 days before lambing. GI capacity also allows lactating ewes to meet their own requirements for maintenance and lactation and to meet the requirements of their nursing lambs for near maximum growth when the diet is high-quality for- age and the quantity available is not limiting (K. E. Mc- Clure, C. F. Parker, and S. C. Loerch, Ohio Agricultural Research and Development Center, personal communi- cation, 19851. Intake of weaned lambs that are finished in drylot, however, is limited primarily by the demand for maximum growth potential as well as dietary factors in addition to GI fill. Nutrient requirements are related to rate of production, which depends primarily on the intake of the diet being fed (National Research Council [NRCi, 1975~. Therefore, under most conditions intake must be predicted before diets can be formulated to meet nutrient requirements, unless they are limit-fed a high-nutrient-density diet that would exceed their re- quirements if fed to appetite. This is not the usual case; management problems such as wool eating often occur under these conditions (McClure et al., personal com- munication, 1985~. When sheep are fed to appetite and intake is accurately predicted, nutrient requirements can be expressed as a percentage of the diet, which also 75 most appropriately expresses requirements for rumen fermentation (Fox et al., 1984~. The purpose of this chapter is to identify and discuss the primary factors influencing feed intake in sheep and to present equations and adjustment factors that can be used to predict feed intake of sheep under widely vary ing feeding and environmental conditions. PHYSIOLOGICAL AND DIETARY FACTORS Many of the same fundamental factors that influence feed intake of cattle, as described in Chapter 6 for beef cattle, apply also to sheep. An extensive review by the Agricultural Research Council (ARC; 1980) revealed that for finely processed diets intake/W075 (relative in- take, where Wis live weight) was 90.5 g for sheep ver- sus 89.5 g for cattle. These values were derived from a summary of data reported in six journals. Average diet dry matter (DM) energy values (Meal of NEm/kg, where NEm is net energy for maintenance) were 1.45 for sheep and 1.64 for cattle; both values are within the range of dietary energy values at which intake is maximum, as shown in Chapter 6 for beef cattle. Within this range of energy values, the NRC (1975) suggests intake of grow- ing sheep is 103 g/W075, based on the data of Rattray et al. (1973) in which pelleted diets were fed to sheep. Figure 7-1 shows the relationship of intake to stage of growth, comparing the values reported by the ARC (1980) and that given in Chapter 6 for cattle at approxi- mately the same degree of body fatness. At the same stage of growth, intakes appear to be similar for cattle and sheep, when milled and/or pelleted diets are fed. Thus, as with cattle, cell wall content (neutral detergent fiber, NDF) of the diet is the primary factor limiting intake, unless high-concentrate diets are being fed. In

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76 Predicting Feed Intake FIGURE 7-1 Relationship of stage of growth to intake in sheep. Loo loo - ~90 _ - 111 a it tar 70 t c, ~50 creasing dietary concentration of slowly degraded or indigestible material reduces the rate of passage and physical fill becomes limiting (Mertens and Ely, 1982; VanSoest, 1982; Mertens, 1983~. As digestible energy content is increased, for example, when grain in the diet is increased, metabolic controls become the dominant factors limiting intake. The use of diet energy concen- tration to predict intake describes the combined effects of physical fill and metabolic controls on appetite. There appears to be some unique differences between cattle and sheep when fed long or chopped roughages or silages. ARC (1980) found that dry matter intake was similar (87 versus 90 g/ We 75) for cattle when fed coarse (long or chopped) versus fine (milled and/or pelleted) forage at high energy densities. However, long or coarsely chopped forage gave relative intakes in sheep of 57 g/ We 75 as compared to 90.5 g for fine diets; silages gave 46 g/ W075. Average diet energy densities (Meal of NEm/kg of DM) were 1.39,1.45, and 1.50, respectively; all are within the range over which diet energy density would not be expected to influence intake. Similar results for roughages (441 determinations of ad libitum forage intake across 2,747 individual animal intakes) were reported by Heaney et al. (19681; chopped mixed hay gave 66 g/ W0 75, while the same hay in pelleted form 80 _ 60 _ 40 _ 30 1 1 1 1 1 1 1 1 1 1 00 1 50 200 250 300 350 400 450 500 550 // ~ / / / . 'ant ~ . /- \- N;\~e ~ ~ V. \~ ...- Fox and Black., 1984 (calves) Owens and Gill, 1982 (cattle) - Plegge et al., 1984 (300 kg cattle "initially") - . Plegge et al., 1984 (calves 200 kg "initially") ARC, 1980 (sheep-fine) _ ARC, 1980 (sheep-coarse) BODY WEIGHT, AVERAGE-FRAME-SIZE STEER EQUIVALENT 1 0 1 5 20 25 30 35 40 45 50 55 BODY WEIGHT, AVERAGE-SIZE WETHER gave 135 g/ W0 75. Silage intake averaged 54 g. The over- all coefficient of variation was 16.4 percent; within and between animal variation was high. Demarquilly (1973) found silage intake to be 47 g across 87 silages, a value 33 percent lower than the intake from the same herb- ages prior to ensiling. Intake of silage by sheep is not closely related to its digestibility but is more closely related to its fermenta- tion characteristics (Demarquilly, 1973; Wilkins, 19741. Wilkins (1974) concluded that the depression in intake after fermentation appeared to be due primarily to the increased total volatile fatty acids (VFAs). Part of the differences in the literature are due to the effect of oven drying on loss of VFAs (Fox and Fenderson, 19781. Most of the intake differences between fermented and unfermented forage when fed to cattle are due to this effect. However, the large intake depression in sheep observed from feeding fermented feeds is apparently due to physiological effects. In Chapter 1, it was stated that receptors in the sheep's duodenum are particularly sensitive to the duodenal concentration of lactic acid. Thus, silages that tend to be high in lactic acid (imma- ture or treated with products that extend fermentation) would be expected to depress intake more than those that are low in lactic acid (more mature, not treated with

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Sheep 77 fermentation extenders). In the studies of Johnson and McClure (1968), where corn silages were compared when harvested from the blister (21 percent DM) to the mature (72 percent DM) stages, intakes averaged 54 g/ We 75, a value comparable to those cited previously. In- take was as much as 20 percent lower at the blister and milk stages, however, where lactic acid content aver- aged 12.8 percent compared to 8.4 percent overall. An- other key factor in feeding silages to sheep may be the distribution of nitrogen in silage and the nitrogen sup- plementation used. In these studies, nonprotein nitro- gen (NPN) was used for nitrogen supplementation; protein solubility is usually 60 to 70 percent in NPN- supplemented corn silage (Lomas et al., 1982) and would likely exceed 70 percent in immature silages. However, intakes of 93 g/W075 on a corn silage diet supplemented with soybean meal were achieved in growing lambs (McClure et al., personal communica- tion, 1985~. It may be that the sheep's reticulum possesses stretch receptors that are sensitive to distension of the gut after a meal. This factor, when coupled with the longer lag time before digestion begins, slower rate of digestion, and longer retention time of long versus chopped for- ages, may explain the unique effect of length of forage on intake in sheep. Mertens and Ely (1982) concluded that cattle digest feeds with low digestibilities to a greater extent than do sheep. Lactation also influences feed intake of sheep. An ex- cellent summary on that effect is presented by ARC (1980~. As with cattle, ewes increase intake as lactation progresses to the fourth week; intake then begins to decline by the seventh week of lactation. They are fur- ther influenced by the number of lambs they are nurs- ing. Intake averages are approximately one-third higher when ewes are nursing one lamb and one-half higher when nursing two lambs. PREDICTING DRY MATTER INTAKE OF SHEEP Based on the information presented in Chapter 1, the biological factors that appear to control intake are those related directly to direct dietary effects (distension of body wall, ruminal pH and acetate concentration, he- patic uptake of propionate) and metabolic factors medi- ated by the central nervous system, including size of adipose mass and demand for nutrients. The equation that best represents the biological effect is one that primarily reflects the influence of fill or dis- tension with feeds with slow rates of digestibility and passage, and the influence of amounts of end products of digestion that result when rates of digestibility and passage are expected to be high. Adjustments are then made for size of adipose mass and demand for energy (level of production and climatic effects). Such a predic- tion function should be continuous, because most bio- logical responses are continuous rather than point effects, and it can be used over a wide range of condi- tions. Equations and adjustments suggested in this section are illustrated in Figure 7-2. They were selected from a variety of sources, since it would be next to impossible to identify all of the independent variables from any one data base. The concentration of NEm was selected as the independent variable, since it represents the widest combination of dietary and animal production condi- tions. Even with the best equations and adjustments, however, intake may vary from predicted values by 10 percent or more. The data of Mertens (1973) were used to derive equa- tions to predict intake of sheep fed forages as follows. g of DMI/W075 = 74.4 + 0.877X - 0.017X2, where X is percent forage cell wall, and DMI is dry matter intake (r2 = 0.761. The range of date was 30 to 80 percent cell wall. This equation was developed from 187 intake and digestibility trials from 12 experiment sta- tions in the United States, the Philippines, and Puerto Rico. All but one trial was with sheep. The net energy value for lactation (Mertens, 1983; based on Mertens, 1973), Mcal/kg of DM for legumes is NEE = 2.40 - 0.0232 (NDF percent). (2a) For grasses the NEL is (Mertens, 1983; based on Mer- tens, 1973) NEL = 2.86 - 0.0262 (NDF percent). (2b) The NEm value can then be calculated (Van Soest et al., 1984~: NEm = 0.7SNEE + 0.31. (3) The NEm concentration (Meal/kg) was then related to dry matter intake to give the following equations for grasses and legumes. These equations allow for adjust- ment for fill factors, inherent differences between leg- umes and grasses in rate of passage, and the influence of end products of digestion as forage digestibility in- creases. At the higher energy densities, immature for- ages (in most cases pasture) would be represented. Intake of legumes (g/day) = W075( - 70.4 + 182NEm-53.2NEm2). (4) Intake of grasses (g/day) = W075(-~.3 + 166NE''' - 41.2NEm2). (5) The previous discussion indicates that a separate base equation is needed to predict intake of ensiled forages. The value for relative intake of 46 g, as suggested by the

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78 Predicting Feed Intake FIGURE 7-2 Dry matter intake of sheep as related to diet energy density. 120: 110L 100 90~ 80: 70: 60: 50: 40: 0.6 0.8 1.0 30 1 1 ARC (1980) for ensiled immature forages or those treated with products known to increase lactic acid con- tent, may be the best available estimate. However, si- lage intake by sheep may follow the pattern described by Equation 5 when ensiled at a more mature stage or at a low water content where lactic acid content is likely to be lower or oil meal supplements are used. More re- search is needed to separate out more clearly the inde- pendent effects of these variables. As shown in Figure 7-2 and in the previous discussion, it appears that a separate equation is needed to predict intake of pelleted or milled feeds for sheep. Both NRC (1975) and ARC (1980) indicate that the fill factor is overcome by pelleting. At high energy densities, end products of digestion reduce intake. However, at low energy densities the rate of passage becomes the domi- nant factor; Mertens (1973) found that the rate of pas- sage is more important than the rate of digestion. Both the NRC (1975) and ARC (1980) equations appear to be valid; the NRC equation is based entirely on pelleted diets, while the ARC data base includes both pelleted and finely ground diets. Thus the NRC equation could be used for pelleted diets and the ARC for other finely processed, nonpelleted milled diets. For intake of milled, nonpelleted diets (ARC, 1980~: / / - 1976 NRC (pelleted); equation 7. 1984 NRC (beef cattle) Legumes (Martens); equation 4. Grasses (Martens); equation 5. ARC (fine); equation 6. ARC (coarse) ARC (silages) 1 1 1 1 1 ,,~ 1.2 1.4 1.6 1.8 2.0 2.2 2.4 Dl ET DRY MATTER NEm (Mcal/kg) Intake (g/day) = W 75(129 - 22.3,). The range of data was 1.35 to 2.05 NEm/kg of DM. For intake of pelleted diets (NRC, 1975~: Intake(g/day)= W075(131 - 18.7NE,~?). (7) The range of data was 0.87 to 1.83 NEm/kg of DM. Adjustments are needed for stage of growth; these adjustments represent energy demand for growth, adi- pose mass, and apparent relative rumen capacity changes with body weight in sheep. The data of Hogue (1979) and ARC (1980) were used to develop the relationships for stage of growth shown in Table 7-1. These were then used to develop adjustment factors TABLE 7-1 Determination of Stage of Growth Sheep Frame Size 'do Body Fat Breed Types 21.3 26.5 31.5 Equivalent Weights (kg) 29 36 44 Southdown 36 45 55 Corridale, Dorset 44 55 66 Suffolk Small Average Large

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Sheep 79 TABLE 7-2 Intake Adjustment for Stage of Growth Adjustment (Jo) Lamb Equivalent Weight (kg) 20 30 40 45 50 55 Fine Diet None None -3 -10 -18 -27 Coarse Diet -28 -20 -12 _9 -6 None for stage of growth effects based on the ARC (1980) data base, as shown in Table 7-2. The data of ARC (1980) were used to develop the adjustment factors for coarse forages. As stated in Chapter 1, energy demand is one of the controlling factors of intake. In lactating ewes, as with cattle, intake would be expected to gradually increase with energy demand as lactation progresses and then decline in late lactation. As with cattle, energy demand increases more rapidly than intake early in lactation, but reserves are allowed to be replenished later in lactation when intake exceeds energy demand. Table 7-3 in- cludes intake adjustment factors for lactation based on the ARC (1980) data base. NRC (1981) reviewed environmental effects on in- take. A summary of their findings for sheep are as fol- lows: 1. Exposure to heat reduces voluntary intake in sheep, as it does in other species. Effective ambient temperature, which combines the effects of both tem- perature and humidity, is the value most closely related to this effect. 2. Voluntary intake increases during exposure to cold relative to the animal's thermal neutral zone (TNZ). However, it reaches a maximum before animals are se- verely cold stressed. 3. Daily temperature fluctuations around the TNZ did not influence intake. However, more work is needed in this area. Based on the summary by NRC (1981), temperature adjustment factors were developed (Table 7-4~. These conclusions indicate that adjustments for in TABLE 7-3 Adjustment for Lactation Adjustment Factor for the Following No of Lactation Weeks Lambs 1 2 3 4 5 6 7 8 9 10 One 1.24 1.27 1.30 1.32 1.33 1.33 1.32 1.31 1.31 1.30 Two 1.40 1.45 1.50 1.53 1.54 1.54 1.53 1.52 1.52 1.51 TABLE 7-4 Adjustment Factors for Temperature Temperature (C) Adjustment Factora 1.10 1.05 -5 5 15 25 35 1 0.90 0.75 aBased on TNZ of 15C and a data base of -5C to 35C, with shorn lambs (Brink, 1975). At other TNZs, the adjustment factor relationship may be different. take should use as their base the animal's TNZ. There- fore, the relationships in Table 7-4 may need to be shifted as the animal's TNZ changes. EVALUATION OF EQUATIONS TO PREDICT INTAKE OF SHEEP Independent data were obtained to evaluate the equa- tions presented above. Data from feeding trials from the Ohio Agricultural Research and Development Center (K. E. McClure, C. F. Parker, and S. C. Loerch, personal communication, 1985) were summarized and tested against the various equations presented above. In- cluded were diets ranging from corn silage to all concen- trate fed to growing lambs and diets ranging from corn stover to high-energy lactation rations fed to ewes dur- ing the course of complete reproductive cycles. All were average-frame-size crossbreds, with the lambs in the finishing trials originating from these same ewes. The lamb finishing data were by 14-day intervals, for a total of 129 pen observations with 428 lambs. The ewe data were collected over three complete reproductive cycles at approximately 30-day intervals, for a total of 52 ob- servations with 572 ewes. Table 7-5 summarizes the comparisons that were made. Lambs fed the all-pelleted diets had intakes nearly identical to that predicted by Equation 7. The silage intake was best predicted by Equation 5, as ex- pected. However, the response was not linear with in- creases in weight as these equations imply; equations were developed from these data to take into account this effect. Figures 7-3 and 7-4 compare Equations 7 and 5 with those developed from these data, as follows. For intake of pelleted diets fed to lambs: Intake(kg/day) = -0.414 + 0.303 W0.75 - 0.00755( W075)2 - 0.504NE,77 (r2 = 0.78). (8) For intake of corn silage supplemented with soybean meal: Intake (kg/day) = - 4.19 + 0.297 W 75 - 0.00843( W 75)2 (9) + I.77NE,,, (rim = 0.98).

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30 Predicting Feed Intake TABLE 7-5 Evaluation of Equations to Predict Intake of Sheep Dieta Average Weight (kg) Average NE'' (Meal/kg) Prediction Equation Number Actual 4; Legumes 5; Grass 6; Milled 7; Pelleted Intake of growing lambs (kg/day) Pelleted 33 1.85 1.36 1.14 1.15 1.21 1.33 Silage 31 1.78 1.05 1.06 1.07 1.17 1.28 Intake of ewes (kg/day)/' Overall 62 1.27 1.70 1.78 1.53 2.45 2.62 Early gestation 60 1.03 1.42 1.31 1.00 2.29 2.41 Mid-gestation 61 1.11 1.31 1.42 1.12 2.28 2.41 Late gestation 65 1.42 1.71 1.82 1.61 2.22 2.38 Lactations 61 1.60 2.44 2.73 2.54 3.02 3.27 Maintenance 60 0.90 1.19 1.07 0.74 2.35 2.47 aThe pelleted diets were whole or pelleted corn based, with alfalfa meal as a roughage. The silage was whole plant corn silage. All were average-frame-size crossbred ewe and wether lambs from the ewes reported in this table. The data were evaluated by 14-day intervals. Diving a total of 129 Den observ~tinn.~ with 42R lumen rarer fn''r ~vT~f~rimPntc The return Cicero ,l~t~ were available in only one of these trials. bData were collected over three complete reproductive cycles at approximately 30-day intervals, for a total of 129 pen observations with 572 ewe observations. Adjustment factors were applied from Table 7-3 for the number of lambs nursed, which ranged from 1.3 to 2.5 per ewe across the groups fed . , . over the three reproductive cycles represented. SOURCE: K. E. McClure, C. F. Parker, and S. C. Loerch, Ohio Agricultural Research and Development Center, personal communication, 1985. Equation 4 was adequate in predicting the intake of the ewes, and the overall average was within 2 percent of the actual intake. The ewes had been fed to maintain a constant condition score, so their intake met their nutri- ent requirements when feeds of the energy density shown in Table 7-4 were fed to appetite. Equation 5 consistently underpredicted intake with the low-energy diets. All of these diets were supplemented as needed with soybean meal and minerals to meet nutrient re- quirements, however. The data of Vona et al. (1984) were used to evaluate FIGURE 7-3 Pelleted diet intake of growing lambs. AS _ 1.6 `1 ~1 4 Z 1.2 111 ~ 1.0 uJ 0.8 0.6 15 .: ~C: the intake equations presented for predicting intake of hay. These data included 16 hays (cultivars of switch- grass, big bluestem, and tall fescue) harvested in five states across the central and eastern United States at three stages of growth (vegetative, early head, and bloom) over a 2-year period. They were fed to wethers in the chopped form in one location to determine the effect of forage stage of growth on intake and digestibility. Actual intake was compared to that predicted by the intake equations presented previously for legumes and grasses. The results are summarized in Figure 7-5. At ,.~ O Actual Predicted 20 25 30 35 40 45 50 BODY WEIGHT (kg)

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Sheep 8 1 car 1.6 ~ 10L 0.8 0.6 0.4 ~- 15 1 20 25 30 BODY WE IGHT (kg) - O Actual Predicted 1 1 1 35 40 45 most energy densities the legumes equation was within 5 to 10 percent of actual intake. Thus across both the ewe and wether evaluations, the legumes equation 4 appeared to be more consistently related to actual in- take. MANAGEMENT TO MAXIMIZE INTAKE IN SHEEP The physiological control mechanisms reviewed ear- lier that influence intake in sheep, and the prediction so r 80 ' 40 _ 30 20 10 _ 0.68 O Actual Legumes Equation ~ Grass Equation 1 1 1 1 ~ 0.96 1 .00 1.10 1 .17 1 .36 GRASS NEm (Mcal/kg) FIGURE 7-5 Dry matter intake of chopped grass by wethers. Actual data are those of Vona et al. (1984~; data include 16 hays (cultivars of switchgrass, big bluestem, and tall fescue) collected in five states (Iowa, Kentucky, New [er- sey, New York, and Pennsylvania) at three stages of growth (vegetative, early head, and early bloom). 50 FIGURE 7-4 Silage intake of growing lambs. equations and adjustment factors that have been pre- sented, suggest a number of management practices that influence intake as follows. 1. For diets with high grain content, manage feeding to avoid a rapid drop in rumen pH and the rapid lactic acid buildup that follows because of the rapid prolifera- tion of Streptococcus bovis at low pH. This is accom- plished by: a. Maintaining a uniform, constant intake. b. Gradually moving to a full feed of grain by start- ing at 50 to 60 percent grain and then not increasing the grain level by more than 5 percent per day. c. Using a minimum coarse roughage level of 8 to 10 percent. 2. To maximize the use of high roughage diets, do the following: a. Avoid rapid filling. The stretch receptors will give a negative feedback, causing the animal to go off feed. Sheep appear to be more sensitive in this regard than are cattle. b. Consider processing to increase the rate of pas- sage, especially in sheep. Digestibility will be reduced 5 to 10 percent, but the benefit of greater energy intake to more nearly meet maintenance needs or intake over maintenance that is available for productive functions may offset this effect. In many circumstances, the re- duced wastage plus the greater rate of passage benefits may more than offset digestibility reduction and eco- nomic costs. Pelleting and fine milling of roughages will clearly be more beneficial in sheep than in cattle.

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82 Predicting Feed Intake 3. Balance dietary protein so that both rumen and animal needs are met. Diet formulation systems under development will take this into account in a dynamic way. An inadequate supply of ammonia and some amino acids will limit ruminal microbial growth, reducing the rate and extent of digestion and thus the rate of passage. Metabolizable protein must also be balanced relative to metabolizable energy intake and the animal's potential for tissue or milk production. 4. When feeding high-forage diets to growing sheep, intake can be improved by feeding some grain continu- ously (ARC, 1980~. The primary effect apparently is to increase the rate of passage of the total diet, avoiding the effect of the sensitive stretch receptors that sheep have. Although there will be some loss in digestibility of the forage due to associative effects, the benefits in in- take in sheep, especially growing sheep, may be much greater than the loss due to associative effects. 5. Blending off-flavored feedstuffs into a total mixed ration will overcome the olfactory appetite control mechanism. Many by-products or waste feeds can be utilized when restricted to a limited percentage of the diet, which varies with the particular feed. A good guideline if that level is not known is to start at 10 per- cent of the diet DM, increase the level slowly until in- take begins to be reduced, then stop just short of that point. 6. Protecting sheep from direct exposure to weather effects (storms and mud, for example) will allow intake to increase during cold weather and thus reduce their critical temperature. Protecting sheep from direct expo- sure to heat stress will prevent intake reduction, allow- ing energy intake over maintenance and, thus, performance to be maintained. 7. Identifying sheep selected for relative growth rate potential will likely improve intake due to a greater nu- trient demand. Both sheep and cattle industries need to improve methods of identifying animals with this ability and methods of maintaining identity so that production of this type can be recognized and encouraged. REFERENCES Agricultural Research Council. 1980. The Nutrient Requirements of Ruminant Livestock. Farnham Royal, Slough, England: Common wealth Agricultural Bureaux. Brink, D. R. 1975. Effect of Ambient Temperature on Lamb Perfor mance. M.S. thesis. Kansas State University, Manhattan. Demarquilly, C. 1973. Chemical composition, fermentation, charac- teristics, digestibility and voluntary intake of forage silages: Changes compared to the initial green forages. Ann. Zootech.22:1. Fox, D. G., and J. R. Black.1984. A system for predicting body compo- sition and performance of growing cattle. J. Anim. Sci. 58:725. Fox, D. G., and C. L. Fenderson. 1978. Influence of NPN treatment, oven temperature and drying time on error in determining true corn silage dry matter. J. Anim. Sci. 47:1152. Fox, D. G., C. J. Sniffen, and P. J. VanSoest. 1984. A net protein system for cattle. P.20 in Proceedings of the 1984 Georgia Nutrition Conference, Atlanta. Heaney, D. P., G. I. Pritchard, and W. J. Pigden.1968. Variability in ad libitum forage intakes by sheep. J. Anim. Sci. 27:159. Hogue, D. E. 1979. Sheep Nutrition. Animal Science Mimeograph Series 45, Cornell University, Ithaca, N.Y. Johnson, R. R., and K. E. McClure. 1968. Corn plant maturity. IV. Effects on digestibility of corn silage in sheep. J. Anim. Sci.27:535. Lomas, L. W., D. G. Fox, and J. R. Black.1982. Ammonia treatment of corn silage. I. Feedlot performance of growing and finishing steers. J. Anim. Sci. 55:909. Mertens, D. R. 1973. Application of Theoretical Mathematical Models to Cell Wall Digestion and Forage Intake in Ruminants. Ph.D. dissertation. Cornell University, Ithaca, N.Y. Mertens, D. R.1983. Using neutral detergent fiber to formulate dairy rations and estimate the net energy content of forages. Pp.60-68 in Proceedings of the Cornell Nutrition Conference, Ithaca, N.Y. Mertens, D. R., and L. O. Ely. 1982. Relationship of rate and extent of digestion to forage utilization-A dynamic model. J. Anim. Sci. 54:895. National Research Council. 1975. Nutrient Requirements of Sheep. Washington, D.C.: National Academy of Sciences. National Research Council. 1981. Effect of Environment on Nutrient Requirements of Domestic Animals. Washington, D.C.: National Academy Press. National Research Council.1984. Nutrient Requirements of Beef Cat- tle. Washington, D.C.: National Academy of Sciences. Owens, F. N., and D. R. Gill.1982. Influence of feed intake on site and extent of digestion. Proceedings of the National Beef Symposium and Oklahoma Cattle Feeders Seminar. Division of Agriculture, Oklahoma State University, Stillwater. Plegge, S. D., R. D. Goodrich, S. A. Hanson, and M. A. Kirick. 1984. Predicting dry matter intake of feedlot cattle. P.56 in Proceedings of the Minnesota Nutrition Conference, University of Minnesota, St. Paul. Rattray, P. V., W. N. Garrett, N. Hinman, I. Garcia, and J. Castillo. 1973. A system for expressing net energy requirements and net energy content of feeds for young sheep. J. Anim. Sci. 36:115. VanSoest, P. J. 1982. Nutritional Ecology of the Ruminant. Corvallis, Oreg.: O and B Books. VanSoest, P. J., D. G. Fox, D. R. Mertens, and C. J. Sniffen. 1984. Discounts for net energy and protein. 4th ed. P. 121 in Proceedings of the 1984 Cornell University Nutrition Conference. Vona, L. C., G. A. Jung, R. L. Reid, and W. C. Sharp. 1984.11utritive value of warm-season grass hays for beef cattle and sheep; digest- ibility, intake and mineral utilization. J. Anim. Sci. 59:1584. Wilkins, R. J. 1974. The nutritive value of forages. Pp. 167-189 in Proceedings of the Nutrition Conference of Feed Manufacturers, University of Nottingham, England.