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Sheep INTRODUCTION Comparatively speaking, sheep are more tolerant of climatic extremes than other farm animals. For example, Alexander (1974) calculated from data of Bennett (1972) that adult sheep when dry, in a calm wind, and with 10 cm of fleece could withstand -120°C ambient temperature. However, calculated cold tolerance for freshly shorn adult sheep (7 mm of fleece) would be only -15°C, and, when wet and exposed to a 7 m/s wind, similar sheep could withstand only 13°C. For lambs, Alexander (1968) suggested that heat loss would exceed summit metabolism at 23°C when a small lamb with a short fleece was wet and exposed to a 5.5 m/s wind. For a large lamb in the same condition, metabolism would match heat loss at approximately 4°C. Obvi- ously, survival during cold is highly variable, depending on both the ani- mal's ability to increase heat production and aspects of the environment af- fecting effective ambient temperature (EAT). Heat tolerance is even less defined than cold tolerance for sheep. Mount (1979) reports that sheep will survive acute periods of 40-60°C dry-bulb temperature, but successful growth and reproduction at that temperature has not been demonstrated and is unlikely. Since homeotherms must rely more on evaporative heat loss when exposed to effective ambient temperatures (EAT) in the hot zone, factors affecting rate of evaporation (wind and humid- it~y) are of major importance in determining survival during heat. 85
86 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT THERMAL ZONES FOR SHEEP The thermal zones for sheep depend largely on amount of external insulation provided by the fleece. As noted in Table 1, shorn growing lambs on a main- tenance ration have a lower critical temperature (LCT) of about 25°C, but LCT is estimated as low as -20°C in similar fleeced animals (Blaxter, 1967; Webster et al., 1969~. Calculation of lower critical temperature indicates that increased intake and the concurrent increase in heat production reduces LCT as much as 20°C from ad libitum feeding to fasting. Clearly, LCT can be in- fluenced by wetting of fleece because insulatory value is reduced by wetting (Bennett, 19724. Wind increases rate of heat loss (Joyce and Blaxter, 1964) and has an additive effect when sheep are wetted (Blaxter et al., 19661. Since TNZ iS, in fact, a descriptive term relating balance of heat production and loss, it is obvious that any factor affecting either of these two determi- nants will consequently alter TNZ and LCT. ENVIRONMENTAL EFFECTS ON FEED ENERGY INTAKE Many factors affect the amount of metabolizable energy (ME) available to the animal. These include digestibility of the foodstuff, amount of the foodstuff consumed, and/or the ability of the animal to acquire food. In general, fac- tors influencing amount of metabolizable energy available for consequent use may be considered as feed intake. Exposure to heat reduces voluntary intake in sheep as it does in other spe- cies (see page 31~. Bhattacharya and Hussain (1974) reported that high ambi- ent temperatures coupled with high humidity during the day, reaching a max- imum of 32°C and 98 percent, respectively, reduced ad libitum intake in sheep, with depression most severe when the diets contained high levels of roughage. However, a later report from the same laboratory (Bhattacharya and Uwayjan, 1975) showed conflicting results when temperature was cou- pled with low humidity. The diversity of those findings supports the need to discuss results in terms of EAT rather than separating the effects of tempera- ture and humidity during heat stress. In general, voluntary intake increases during cold compared to TNZ. Web- ster et al. (1969) reported increased food intake of sheep housed indoors as still air temperature fell. Soderquist and Knox (1967) reported higher dry matter intake at 0°C compared to 23°C in growing lambs. Ames and Brink (1977) reported increased dry matter consumption in lambs, whose critical temperature was 13°C, to be statistically higher at 10, 5, and 0°C, but no fur- ther increase was noted at - 5°C. Those data clearly show that voluntary in- take is increased above thermoneutral values during mild cold, but that a limit in voluntary intake was reached before animals were severely cold stressed.
Sheep 87 For shorn lambs whose lower critical temperature was 13°C, Brink (1975) related dry matter consumption over a wide range of ambient temperatures - 5 to 35°C) and found the following linear relationship: DMl = 1 1 1.3 - 0.52 T. where DMI = daily intake (g/W075), T = temperature (°C). The impact of fluctuating temperatures and the validity of mean daily tem- perature when the standard deviation of daily temperature is high has been recently studied by Giacomini (1979) where lambs exposed to a constant thermoneutral temperature (15°C) were compared with lambs in fluctuating environments with a mean temperature of 15°C. When fluctuations were from 10 to 20°C, 5 to 25°C, or 0 to 30°C, feed intake was not different from constant 15°C temperature. Certainly the area of fluctuating temperature de- serves more study. This is particularly important in the design of confine- ment systems where constant temperatures may result in patterns of feed in- take different from those established during fluctuating temperatures. Assuming some nutrient (e.g., vitamin and mineral) requirements are con- stant over wide ranges in temperature raises a concern that difference in in- take can have a major effect on vitamin and mineral intake. When these nu- trients represent a constant percentage of the diet, it is obvious that reduced intake may lead to deficiency. Care must be taken (and adjustments made when necessary) to meet the animal's requirement when intake varies. The same consideration must be made for various nonnutrient additives (antibi- otic premixes, growth promotants, etc.) when they are to be ingested at a constant rate daily. Obviously, diet adjustment for components that are not affected by the thermal environment should be altered in proportion to changes in rate of feed consumption. For this reason it is important that accu- rate estimates of voluntary intake during thermal stress be established. NUTRIENT DIGESTIBILITY Many authors have reported a positive relationship between ambient temper- ature and nutrient digestibility for ruminants (see Table 21. A general discus- sion is found on page 31. Although information relating diet digestibility to heat stress is not in total agreement, most data tend to support the hypothesis that digestibility is in- creased during heat stress. Some believe this results from decreased volun- tary intake rather than from a direct effect of increased EAT. Ames and Brink
88 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT (1977) used shorn lambs fed in controlled environmental chambers to deter- mine the effect of ambient temperature on the digestibility of diet compo- nents. This study showed increased digestibility of dry matter, crude protein, and nitrogen-free extract as temperature rose from 15°C to 35°C (LCT was 13°C). Crude fiber digestibility increased when temperature rose from 15 to 30°C, but was not increased at 35°C. No difference was found in ether ex- tract digestibility during heat stress. Bhattacharya and Hussain (1974) re- ported that during heat stress sheep diets had lower digestibilities except for crude fiber and nitrogen-free extract. They found that higher roughage (75 percent) diets were most affected. Perhaps variations in findings relating heat stress to diet digestibility are altered by roughage to concentrate ratio. Obvi- ously, more knowledge is needed for different rations. Several authors (Blaxter and Wainman, 1961; Christopherson, 1976; Kennedy and Milligan, 1978; Young and Christopherson, 1974) indicate lowered dry matter digestibility during cold. Examples of depressed digest- ibilities during cold are shown in Table 2. Consequently, when increased in- take during cold is considered in combination with decreased digestibility, the advantages of the former would be partially offset by the latter. For ex- ample, increased consumption per unit of metabolic size from 10 to 0°C is 5.3 percent, but with decreased digestibility the net increase in digestible en- ergy was only 2.7 percent. Christopherson (1976), who conducted extensive studies of digestibility during prolonged cold with both sheep and cattle, also found a temperature effect independent of level of intake. He reported that dry matter digestibility had a 0.31, 0.21, and 0.08 percent decline per °C cold stress for sheep, calves, and steers, respectively. Kennedy and Milligan (1978) suggest that cold effect on digestibility is greater with higher levels of food intake. ENERGY REQUIREMENT DURING COLD STRESS When exposed to effective ambient temperatures (EAT) below the cool zone, energy expenditure must compensate for increased energy loss. Increased heat production during cold stress has been measured by several authors (Alexander, 1962; Bennett, 1972; Blaxter et al., 1966; Graham et al., 1959~. Wind (Joyce and Blaxter, 1964), rain (Panaretto et al., 1968), and wind and rain combinations (Blaxter et al., 1966) have been shown to increase further the rate of heat loss of sheep exposed to cold. These findings suggest an ap- parent reduction in external insulation and result in increased rate of sensible heat loss when measured at a given thermal gradient. Use of effective ambi- ent temperature measures such as wind-chill temperature (Ames and Insley, 1975) and insulatory value of the wetted fleece should provide similar esti- mates of heat loss.
Sheep 89 When sheep are exposed to temperatures below the LCT, there are two fac- tors that determine rate of heat loss: (1) thermal gradient between core tem- perature and ambient temperature, and (2) amount of insulation provided by tissue, fleece, and air. These factors can be used to estimate rate of heat loss, and therefore maintenance energy requirement for animals exposed to cold, by the following equation: MEm = a\;V1~75 + b l- where MEm = metabolizable energy for maintenance corrected for effec- tive temperature (kcal ME/day), T = magnitude of cold (°C), i.e., difference between animal's lower critical temperatures and effective temperature, I = total insulation (0C/kcal/m2/day), a = coefficient of maintenance requirement for animal in zone of thermoneutrality (kcal ME/day), b = surface area of animal (my. NOTE: Coefficient a is 127.8 kcal ME/day for sheep. Table 24 shows the percentage increase in maintenance energy require- ments at seven levels of insulation for five different weights. The system to estimate the listed values does have shortcomings and cannot be calculated without some error. First, the insulative value of the animal is not uniform over its entire surface. Indeed, small errors in estimating insulative value will lead to relatively large differences in energy required during cold. For exam- ple, Blaxter et al. (1959) reported conductive value of fleece (in similar sheep) ranges from 122 to 149 kcal/m2/24 h/°C/cm. That variation would result in a 111 percent error for a 50-kg lamb with 0.04°C/kcal/m2/day total insulation when estimating additional heat loss during cold. Second, the ani- mal's ability to shunt blood to and from specific areas of the body reduces the accuracy of the estimates of insulation. Third, calculations assume that the animal is a sphere with no facing surfaces. When the system presented in Table 24 is compared to observed increases in heat production for rather well defined sheep, the utility of the system can be assessed (Table 251. Variations of up to 19.7 percent are noted; however, in each case insulative value was estimated, and increased metabolic heat production during cold exposure was measured for a relatively short period. While more work is obviously needed to improve estimates of insulation, in a general sense these data support increasing energy requirements of sheep based on estimated insulation and body weight.
90 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT TABLE 24 Percent Increase in Maintenance Energy Cost per Degree Centigrade Below Lower Critical Temperature Sheep Weight (kg) Insulation (0C/kcal/m2/day)a 40 50 60 70 80 0.01 5.8 5.4 5.2 4.7 4.6 0.02 2.9 2.7 2.6 2.4 2.3 0.03 1.9 1.8 1.7 1.6 1.5 0.04 1.4 1.3 1.3 1.2 1.2 0.05 1.2 1.1 1.0 0.9 0.9 0.06 1.0 0.9 0.8 0.8 0.7 0.07 0.8 0.7 0.7 0.7 0.6 a Wool provides about 0.007°C/kcal/m2/day per cm depth (Blaxter et al., 19S9). The options for increasing energy intake for animals fed ad libitum are more limiting than for those where adjustment can be accomplished by sim- ply providing more feed. In this situation increasing caloric density by addi- tion of fat or increasing relative amounts of feedstuffs with higher caloric densities (i.e., replacing roughages with concentrates) are possibilities. The former approach, however, is contrary to the notion of increasing heat incre- ment during cold since fat is lower in heat increment compared with most feedstuffs. However, its high caloric density may still be used advanta- geously. The value of heat increment during cold has been mentioned previ- ously, and, while this may prove valuable, it should be understood that heat increment for each unit of weight of roughages and concentrates is similar. However, heat increment is a higher percentage of digestible energy for roughages as compared with concentrates, so that calories supplied by the heat increment of a roughage may be less expensive than those from a con TABLE 25 Comparison of Measured and Estimated Heat Loss (kcal/m2/ day/°C) During Cold Difference Measured Estimated (%) Data Sourcea 115 96 + 11.9 Graham et al., 1959 77 96 - 19.7 Webster et al., 1969 84.2 75 + 11.2 Webster and Blaxter, 1966 a Description of sheep used in these studies were used to obtain estimated values from Table 24.
Sheep 91 centrate source. Consequently, from an economic standpoint energy costs may be reduced during cold by increasing roughages at the expense of con- centrates. Moose et al. (1969) reported that feeds with high heat increments fed during cold have a sparing effect on net energy for production, thus al- lowing their use for gain. Brokken (1971) developed a model using the data of Lofgreen and Garrett (1968) to blend rations to improve performance dur- ing cold stress. This model is based on maximum use of heat increment and concludes that economic advantages do exist for altering rations during cold. The magnitude of this advantage is dependent upon relative ingredient prices, magnitude of cold, and the effect of cold on intake. ENERGY REQUIREMENT DURING HEAT STRESS Expected increases in heat production during heat exposure of sheep have been reported, but little quantification of increased energy needs is availa- ble. During heat exposure, energy requirement increases because of energy expended during panting (Kibler, 1957), sweat gland activity (Macfarlane, 1964), and the calorigenic effect of hormones (Whittow and Findlay, 1968~. An additional factor when core temperature rises is the Q,O effect (Schmidt- Nielsen et al., 19674. Whittow and Findlay (1968) calculated that the effect of increased rectal temperature accounted for as much as 62 percent of in- creased O2 consumption in cattle, and Ames et al. (1971) measured a 41 per- cent increase in O2 consumption of sheep assuming Q,O = 2.0. It is apparent that increases during heat are nonlinear (Graham et al., 1959) as opposed to linear increases during cold. Nonlinearity is to be expected because of the decreased efficiency of evaporative mechanisms and increased Q,O effect as heat stress becomes more severe (Ames et al., 1971~. More precise estimates of increased energy requirement during heat are further confounded by de pressed appetite. The adjustment of diets during heat stress can be a practical approach to- ward minimizing the effect of heat even though increased maintenance re- quirement during heat is difficult to estimate accurately. McDowell (1972) and Brink and Ames (1978a) have reported nonlinear increases in mainte- nance energy during heat when maintenance is calculated as the difference between intake and gain. Lofgreen (1974) confirmed the validity of formulating diets for relief of heat stress. His adjustment involved lowering heat increment of the diets while keeping net energy constant. These adjustments were accomplished by reducing the roughage content of the diets, adding dried beet pulp, and in- creasing fat content. Moose et al. (1969) found that low-concentrate diets (35 percent) had lowered heat increment than higher-concentrate diets (70 percent) when fed to lambs and reported that at temperatures above 25°C high heat increment can seriously impair the efficiency of diets containing higher percentages of roughage. Rea and Ross (1961), in a growing trial with
92 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT lambs, concluded that lambs gain more rapidly in warm temperatures when given diets of 60 percent concentrate as compared to 40 percent concentrate. THERMAL EFFECTS ON PROTEIN REQUIREMENT OF SHEEP Protein requirement includes both that necessary to maintain nitrogen equi- librium (maintenance protein) and that needed for productive function. Ide- ally, dietary protein exceeding that needed for maintenance is used only for production (growth, wool, or milk); however, growth and other productive functions may be limited by available energy because of increased energy for maintenance during thermal stress. When energy is limiting, protein may then be catabolized and serve as an energy source (Crampton and Harris, 1969). Because of the relationship between energy and protein requirement, the direct effect of climate on energy requirement has a subsequent effect on pro- tein required for growth or production. Data suggest no effect of thermal en- vironment on protein required to maintain nitrogen equilibrium (Brink and Ames, 1978b). The frequently used protein-to-energy ratio for formulation of animal diets is not appropriate for describing diets during thermal stress when maintenance energy requirement and intake vary unless only protein and calorie values surplus to maintenance are used to calculate a ratio. In- stead, when formulating diets with respect to the thermal environment, both energy and protein should be included to meet requirements for each nutrient separately, and the protein-to-energy ratio of the diet should be ignored. When energy requirement for maintenance increases (i.e., cold stress), less energy is available for production, and.con-sequently the protein-to- calorie ratio above maintenance levels increases. Ames and Brink (1977) have reported reduced protein efficiency ratio of lambs~during both cold and heat compared to TNZ. Ames et al. (1980) have suggested a system for ad- justing protein above maintenance to match expected growth rate of lambs exposed to thermal stress. When protein is adjusted, growth rate during ther- mal stress is not altered, but protein efficiency ratio is improved. When pro- tein is a more expensive nutrient than energy, cost of gain is decreased. Ta- ble 26 indicates protein adjustments for a 27 kg lamb receiving a diet that is expected to result in 272-g ADG. ENVIRONMENTAL EFFECTS ON LAMB PERFORMANCE Exposure of sheep to thermal stress affects voluntary food intake and mainte- nance requirement as discussed previously. Obviously, average daily gain and feed required per unit of gain are also affected by thermal stress. Few studies have reported the effect of temperature on performance of growing
Sheep TABLE 26 Protein Adjustment for Growing Lambs 93 Deviation from Expected ADO Maintenance Protein for cP Critical Temperature (g)a Protein (g)b Growth (g) in Ration (%)c 20 54 33.2 17.0 2.4 15 132 33.2 41.0 5.8 10 195 33.2 60.8 8.6 5 236 33.2 73.6 10.4 Critical temperature 272 33.2 84.9 12.0 - 5 222 33.2 69.3 9.8 - 10 181 33.2 56.6 8.0 - 15 136 33.2 42.5 6.0 - 20 95 33.2 29.7 4.2 a Ames et al., 1975. bPreston, 1966. c Constant intake. lambs, but Ames and Brink (1977) conducted growth and efficiency studies for shorn lambs exposed to a wide range ~ - 5 to 35°C) of ambient tempera- tures. These lambs received a 50 percent concentrate (grain sorghum), 50 percent roughage (alfalfa) diet. Table 27 relates temperature with ADG and feed efficiency. Daily gain for lambs receiving a grain sorghum (50 percent), alfalfa (50 percent) ration can be predicted for the range of temperatures above by the equation: ADO(g) = 129.94 + 9.27 T- 0.35 T7, where T = temperature in °C. This equation suggests 13°C as the temperature for maximum ADG of shorn lambs. For lambs with fleece, temperature for maximum growth and effi- ciency would be lowered. MISCELLANEOUS ENVIRONMENTAL FACTORS AFFECTING NUTRITION OF SHEEP Knowlton et al. (1969) studied effects of high carbon dioxide levels on nutri- tion of sheep. They reported reduced feed intake when exposed to 4 percent
94 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT TABLE 27 ADG and Feed Efficiency of Lambs Grown at Different Ambient Temperatures and Fed Ad Libitum Feed Efficiency Temperature (°C) ADG (g) (gain/feed) 5 73 0.04 0 1 30 0.08 5 170 0.11 10 192 0.15 15 197 0.14 20 184 0.13 30 107 0.08 35 41 0.04 SOURCE: Ames and Brink, 1977. CO2, but intake was inversely related to CO, concentration at exposure levels of 8, 12, 16, and 18 percent. Nutrient digestibility was not influenced by CO2 levels up to 16 percent, but variable decreases were measured at 16 and 18 percent CO2. Blaxter (1978) studied the effect of simulated altitude on sheep and found no difference in heat production or heat increment values when oxygen con- centration in air was 150 ml/liter compared to 200 ml/liter. Confinement rearing of sheep has been estimated to reduce energy require- ments by 30 percent (Parker, 19761. Direct measurement of heat production in commercial confinement management systems has not been done, but Young and Corbett (1972) reported that maintenance requirements were gen- erally 6~70 percent greater for grazing sheep compared to housed animals. The need for a more precise estimate of effect of confinement on energy re- quired for maintenance is needed. Harbers et al. (1975) reported that sheep acclimate to sound of 100 dB or less; however, intermittent noise was shown to increase nutrient digestibility during short-term exposure. Allden (1968) reported that feed consumption and utilization for weight gain were not affected in the long term by prolonged periods of undernutri- tion. They reported that lambs subjected to restricted feed levels during the first 6 months of life failed to fully recover (in terms of weight) compared with lambs receiving adequate nutrition. However, when unlimited feed was available, when lambs were 6-12 months old, they completely recovered earlier weight loss. Graham and Searle (1975) held 4-month-old Tethers at 20-kg live weight for 4 or 6 months then fed ad libitum to recover weight for age. They reported increased voluntary food intake during rehabilitation
Sheep 95 compared to controls. While heat production per unit metabolic size fell dur- ing weight stasis, it rose in the first month of recovery but remained less than controls. Gross efficiency was higher during the first week of recovery then returned to control values. Except for acute changes in intake and consequent efficiency during the first week of recovery following undernutrition, there appear to be no long-lasting nutritional differences in sheep receiving re- stricted levels of nutrition. SUMMARY Sheep are probably most tolerant of environmental extremes compared with swine, cattle, and poultry. They are unique because of the potentially large insulatory value of the fleece. Responses of sheep to thermal stress in terms of intake, maintenance energy requirement, and rate of performance are typi- cal. Nutrient adjustments for changes in voluntary intake, energy adjust- ments for cold stress, and protein adjustments for differences in thermally in- duced changes in rate of growth are presented but are based on limited research data. More basic information is needed to accurately predict the im- pact of environment on nutrient requirements of sheep.