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Beef Cattle INTRODUCTION Beef cattle are kept in many climatic regions and, except for some intensive production systems, are largely exposed to naturally occurring climatic con- ditions. In intensive beef production systems such as feedlots with shelters or confinement barns, there may be some modulation and protection from cli- mate, but other stress factors, such as gaseous contaminants, dust, mud, or crowding, may be generated that have a detrimental influence on animal per- formance. The following presentation on adjustment factors for the feeding of beef cattle is based on the Fifth Revised Edition of Nutrient Requirements of Beef Cattle (NRC, 1976a) and the adjustment factors pertain to the values and ta- bles presented in that publication. Some modification may be necessary to adapt the suggested adjustment factors to other feeding systems. The adjust- ment factors relate primarily to the effects of thermal stress, both hot and cold, on energy requirement and utilization by beef cattle. Where informa- tion is available, consideration is given to nonthermal stress factors as well as to protein and other dietary components. In developing adjustment fac- tors, it has been necessary to assume that the recommendations contained in Nutrient Requirements of Beef Cattle are for cattle exposed to conditions rel- atively free of thermal stress or an ambient temperature range of about IS to 25°C. The midpoint, 20°C, has been adopted as a reference point for thermal adjustments. The reference temperature is reasonable since it represents ther- mal conditions where beef cattle of various ages and physiological states should exhibit little or no thermal discomfort (Johnson, 1976; Monteith, 59
60 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT 1974; Robertshaw, 1974~. Of course, some data forming the basis of Nutri- ent Requirements of Beef Cattle are from field trials where temperatures were outside the 15 to 25°C range. Unfortunately, for many of these studies a de- scription of the thermal environment is not available. The complexity of climatic factors would preferably be expressed in the composite unit of effective ambient temperature (EAT; see page 61. However, sufficient information is not presently available for estimation of EAT for beef cattle under practical commercial conditions. Instead, mean air temperature will be used with the understanding that while it is not the ideal description of environmental conditions, it is usually available and does provide a rea- sonable index. The reader should use the best-available information and should not refrain from making nutrient adjustments through the lack of a precise measure of EAT. Extremes in ambient temperature influence the behavior, function, and productivity of animals by complex and involved processes (see Part I). To develop adjustment factors for beef cattle, simplification is necessary and three areas of influence are identified. These are: (1) voluntary feed and wa- ter intake, (2) nutrient value of ingested feedstuffs, and (3) the maintenance energy requirement of the animal. The last is really two components, one as- sociated with longer-termed acclimatization to thermal stress and the other TABLE 12 Summary of Voluntary Food Intake of Beef Cattle in Different Thermal Environments Thermal Environment Intakes Relative to Values Tabulated in Nutrient Requirements of Beef Cattle ~ 35°C 25' to 35°C 15 to 25°C 5 to 15°C -5 to 5°C Marked depression in intake, especially with high humidity and/or solar radiation and where there is little night cooling. Cattle on full feed-10 to 35 percent depression. Cattle near maintenance 5 to 20 percent depression. Intakes depressed less when shade or cooling available and with low fiber diets. Intakes depressed 3 to 10 percent. Preferred values as tabulated in Nutrient Requirements of Beef Cattle. Intakes stimulated 2 to 5 percent. Intakes stimulated 3 to 8 percent. Sudden cold snap or storm may result in digestive disturbances in young stock. - 15 to - 5°C Intakes stimulated 5 to 10 percent. < - 15°C Intakes stimulated 8 to 25 percent. Intakes during extreme cold (< - 25°C) or during blizzards and storms may be temporarily depressed. Intake of high roughage feeds may be limited by bulk.
Beef Cattle 61 TABLE 13 Summary of Voluntary Food Intake of Beef Cattle Exposed to Nonthermal Stress Stressor Estimated Effect on Intake and Adjustment Values Tabulated in Nutrient Requirements of Beef Cattle Rain Temporary depression of 10 to 30 percent. Mud Mild, 10 to 20 cm deep Intakes depressed 5 to 15 percent. Severe, 30 to 60 cm deep Intakes depressed 15 to 30 percent. Mud effects are greatest when access to feed is limited and when there is a lack of a suitable bedded area. Illness or disease Usually accompanied with severe depression in voluntary intake. (nonspecific) with acute metabolic responses associated with an immediate hot or cold stress. Until more research data are available, rectilinear responses have been assumed, and, therefore, caution should be exercised in utilizing the sug- gested adjustment factors in extreme conditions. VOLUNTARY FOOD INTAKE In general, voluntary intakes of food tend to decrease as ambient temperature increases and increase when ambient temperatures decrease. Tables 12 and 13 provide summaries of changes in voluntary food and water intake due to various stressors and are presented relative to the estimated dry matter in- takes tabulated in Nutrient Requirements of Beef Cattle. Estimates of feed in- take both within and among animals become more variable and less predicta- ble as ambient temperatures vary further from the 20°C reference temperature. Furthermore, a predictable response in an animal's food intake is not always possible following a sudden or abrupt change in the environ- ment, especially where the change is beyond the animal's adaptive range. In practice, a predicted intake based upon the general state of the environment, for example, on mean weekly or monthly temperature, is generally more use- ful than one based on daily or within-day variations. However, in diurnally fluctuating conditions where, for example, night cooling may relieve an ani- mal of the severity of hot daytime temperatures, both voluntary intake and performance may be higher than would be predicted from the mean daily or weekly temperature. WATER NEEDS Water is consumed by cattle as free water and water with feed. Total water requirement varies with live weight, feed intake, physiological state, and en
62 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT TABLE 14 Water Requirements of Beef Cattle in Different Thermal Environments Thermal Environment Water Requirements > 35°C 25 to 35°C 15 to 25°C -5 to 15°C < - 5°C 8 to 15 kg water per kg DM intake. 4 to 10 kg water per kg DM intake. 3 to 5 kg water per kg DM intake. Young and lactating animals require 1~50 percent more water. 2 to 4 kg water per kg DM intake. 2 to 3 kg water per kg DM intake. Increases of 5~100 percent occur with a rise in ambient temperature following a period of very cold temperature, e.g., a rise from - 20 to 0°C. vironmental temperature. The need for water increases with increased intakes of protein or salt, and in lactating cows. Relationships between ambient tem- perature and water requirement of cattle are detailed on page 39 and sum- marized in Table 14. Water quality is important to cattle, especially with re- spect to the content of salts and toxic compounds (NRC, 19741. NUTRIENT VALUES OF FEEDSTUFFS It is evident from data presented in Table 2 that the ability of ruminants to digest roughage diets depends upon the thermal environment. Roughages tend to be more highly digested during warm conditions than when the same diet is fed to cattle exposed to cold temperatures. While research data are still rather limited on the physiological mechanism of this influence of the environment, the effect appears to be associated with rate of passage of di- gesta, metabolic acclimation, and thyroid hormone activity. Until more re- search data are available, a rectilinear adjustment is suggested that can be ap- plied directly to roughage feeds in Table 11 of the Feed Composition Table in Nutrient Requirements of Beef Cattle (NRC, 1976a). Adjustments for ther- mal effect on digestibility can be made to diet component values for feeds or diets by the following general formula: where A = B + B[Cs (T- 20)], A = value adjusted for environment, B = diet component value from NRC feed composition table, Cf = correction factor (see below), and T = effective ambient temperature (°C).
Beef Cattle Correction Factor for Effect of Temperature on Diet Digestibility (Cf) . . Diet Component Dry matter Energy components (ME, NE, TON) Acid detergent fiber Nitrogen (crude protein) 63 0 0016 0.0010 0.0037 0.0011 Table 15 provides examples of the effect of the thermal environment on the estimated nutrient value of alfalfa hay. Evidence from sheep (see Table 2) indicates that the above adjustment for the effect of temperature on diet digestibility is of more importance for roughage than for concentrate diets. Presently, no adjustment is recom- mended for concentrate diets fed to beef cattle. ADJUSTMENTS TO MAINTENANCE ENERGY REQUIREMENT Thermal environment influences maintenance energy requirements of cattle two ways: first, that due to acclimatization as a consequence of prolonged exposure to a thermal environment, and second, that due to an immediate in- crease in heat production necessary for maintenance of homeothermy when the animal is exposed to an acute heat or cold stress. The acclimatization component is associated with hormonal and metabolic functional changes that develop as a consequence of prolonged exposure and, therefore, is more associated with seasonal changes in the thermal environment than with daily or short-term weather fluctuations. In making adjustments for metabolic ac- climatization, mean monthly temperatures would be an appropriate basis for adjustment. On the other hand, acute response to heat or cold as a conse- quence of exposure to extreme temperatures requires an immediate response, TABLE 15 Example of Adjustment to the Feeding Value of Alfalfa Hay for Feeding to Beef Cattle Exposed to Warm, Thermoneutral, and Cold En- vironmental Conditions Environmental Temperature (~C) 30 20 -s ME (Meal/kg) 1.94 1.92a 1.87 NEm (Meal/kg) 1.14 1.13 1.11 NEg (Meal/kg) 0.40 0.40 0.39 TON (%) 53.5 53.0 51.7 Digestible protein (Jo) 11.5 11.4 11.1 a From Table 1 1, NRC, 1 976a.
64 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT and, therefore, daily weather fluctuations would be the more appropriate basis for adjusting beef cattle diets. When considering any feeding adjust- ment that requires a change in diet type, caution should be exercised to avoid possible disturbance to rumen microbial functions. Adjustment to Maintenance Energy Requirements for Metabolic Acclimatization This adjustment is necessary because the basal or resting metabolism of ani- mals is to some extent dependent upon the thermal environment to which they are partially or fully acclimatized (see page 22~. Basal metabolic rates tend to be lower in animals with prior exposure to warm and elevated in ani- mals with prior exposure to cold conditions. The continually variable sea- sonal conditions of the natural environment result in continuous changes in the state of animal acclimatization. In beef cattle, adjustments based on sea- sonal changes in the thermal environment (mean monthly temperature) would be most appropriate, although it is likely that cattle never fully accli- matize to the extremes in regions where there are marked seasonal fluctua- tions in ambient temperature. In Nutrient Requirements of Beef Cattle (NRC, 1976a), the estimates for net energy requirement for maintenance (NEm) is based on the relationship NEm = aWD 75, where NEm = net energy for maintenance (Meal/day), a = 0.077 for TNZ, W = live weight (kg). By assuming a rectilinear relationship between basal or resting metabolism and temperature of prior exposure, the above equation can be modified to ac- count for metabolic changes associated with acclimatization. For each °C prior exposure to ambient temperatures above or below 20°C, 0.0007 should be subtracted or added respectively to a in the above equation. Thus, for cat- tle with prior exposure to temperatures of 30, 20, 10, and 0°C, the value a becomes 0.070, 0.077, 0.084, and 0.091, respectively. An alternative exists to adjusting maintenance requirement by the above equation and then having to calculate adjusted NEm values for tables in Nutri- ent Requirements of Beef Cattle. Adjustments can be made directly to the NEm and maintenance energy requirement values expressed in terms of ME or TDN by decreasing (for heat) or increasing (for cold) the tabulated values by 0.91 percent for each °C the cattle have had prior extended exposure to conditions above or below 20°C.
Beef Cattle Adjustment in Maintenance Energy Requirements for Immediate Heat or Cold Stress 65 Heat Stress Adjustments to maintenance energy requirement for heat stress should be based on the severity of heat stress, which can vary consid- erably among animals depending upon acclimatization, diet, level of pro- ductivity, and diurnal fluctuations in radiant heat load. During severe heat stress maintenance requirements increase through the increased cost of pant- ing and alterations in tissue metabolism because of increased tissue tempera- tures (Q,0 law). The type and intensity of panting can provide an index for an appropriate adjustment in maintenance requirements. The percentage in- crease in oxygen consumption, and thus maintenance requirement, increases about 7 percent while the animal is in first-phase panting, i.e., rapid shallow panting, but 11 to 25 percent during second-phase open-mouth panting (Hales, 1973; Hales and Findlay, 1968; Kibler and Brody, 1951~. However, with severe heat stress, appetite is usually lowered, which results in reduced productivity and metabolic heat production. Cold Stress Adjustments for acute cold stress apply when cattle are ex- posed to thermal environments below their lower critical temperature (LCT). The LCT tends to be lower in beef cattle compared with other domestic ani- mals (Table 1), and direct cold stress is generally not a practical nutritional problem except in areas with extremely cold winters or through the chilling effects of moisture and wind on young stock or animals with poor resistance to cold. The overall cold hardiness of beef cattle is a consequence of their large size, their usually effective thermal insulation, and the relatively large amounts of heat that arise from normal digestion and metabolic processes (heat increments). Often more important than the immediate increase in en- ergy requirements during a severe cold stress, such as from a winter storm or blizzard, can be the challenge to animal survival. The severity of the chal- lenge to survival is not only dependent upon the environmental conditions, but also upon the animal's level of acclimatization to cold. Cold acclimatized cattle not only have an increased metabolic rate, but also an enhanced capac- ity to increase their rate of metabolic heat production (summit metabolism) to prevent hypothermia when severely cold stressed, i.e., acclimatized cattle may therefore survive in situations where nonacclimatized cattle may suc- cumb. increased heat production during cold stress requires an immediate utiliza- tion of energy substrates from either the diet or from tissue reserves. Nutri- tional adjustments can be estimated to maintain animal productivity. It is then a relatively simple step to estimate the increase in maintenance energy requirements for exposure to temperatures below the animal's lower critical temperature (LCT). But, to calculate such adjustments it is necessary to first establish the LCT of the animal.
66 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT LCT of cattle can be estimated from the equation: LCT = Tc - l (HE - He)' where LCT = lower critical temperature (°C), Tc = core temperature (°C) (39°C satisfactory assumption), I = total insulation, i.e., tissue plus external, see Table 16 (0C/Mcal/m2/day), HE = heat production (Mcal/m2/day), and He = heat of evaporation (Mcal/m2/day). For beef cattle the value HE is conveniently estimated from values in Nutri- ent Requirements of Beef Cattle as the metabolizable energy intake (ME) and the net energy required for production (NEp), as shown below: HE = (ME-NEp~lA, where HE = heat production (Mcal/m2/day), ME = metabolizable energy intake (Meal/day), NEp = net energy for production (Meal/day), and A = surface area (my. NOTE: Surface area may be calculated from body weight according to the general formula A(m2) = 0.09 kg075 The increased energy requirement to maintain productivity in an environ- ment colder than the animal's LCT is given by the formula: ME* = A(LCT- Till, where ME* = increase in maintenance energy requirement (Meal/day), A = surface area (m2), LCT = lower critical temperature (°C), T = effective ambient temperature (°C), and = total insulation, i.e., tissue plus external, see Table 16 (0C/Mcal/m2/day) .
Beef Cattle TABLE 16 Estimates of Tissue and External Insulation for Beef Cattle 67 Tissue Insulation (I,)a 1, (°C/Mcal/m2/day) Newborn calf Month-old calf Yearling Adult cattle External Insulation (Ie)b 2.5 6.5 5.5 to 8.0 6.0 to 12.0 le (0C/Meal/m2/day) in Relation to Coat Depth Wind Speed (mph) <5 mm 10 mm20 mm 30 mm < 1 7 1 114 17 4 5 7.510 13.5 8 4 5.58 9 16 3 45 6.5 a Cattle in thin or emaciated body conditions and breeds with thin skins (dairy breeds and Bos indices) have lower values than fat stock and breeds with thick skins (e.g., Hereford). With ae elimation to cold I, increases. b Presence of moisture, wet snow, or mud in the coal could reduce le in affected areas by 50 to 80 percent. SOURCE: Composite of data from Webster (1974, 1976) and Blaxter (1977). Typical LCT values for cattle and estimated increase in feed energy require- ments as a consequence of exposure to subcritical temperatures are shown in Table 17. Evident from these examples are the low LCT values for cattle in dry conditions free from wind and the impact of mud or moisture on Ie. In cold climates wind is sometimes a problem, but because of the intense cold there is usually little moisture. At these cold temperatures snow tends to re- main dry and powdery and does not reduce the insulative value of the hair coat as much as wet snow or rain. From the estimates in Table 17 there are obvious advantages for providing cattle with protection from wind and, in wet cold climates, overhead shelter and dry bedding. The economics of pro- viding such protection must be calculated for each specific situation. ADJUSTMENTS TO NONENERGY COMPONENTS While environmental stress has direct consequences on the dietary energy re- quirements of cattle, there is at present considerable uncertainty as to desir- able adjustments for the nonenergy components of diets. As an example, the dietary energy requirements of cattle increase to compensate for the in- creased heat production during cold exposure, but do protein, mineral, and vitamin needs increase proportionately? There is some evidence that vitamin
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Beef Cattle 69 A requirements may increase in cattle during cold exposure (Hidiroglou and Lessard, 1971; Jones et al.,- 1962~. A recent report by Ames et al. (1980) in- dicates that protein as a percentage of the diet can be reduced during winter without affecting the growth rate of feedlot cattle. Any decrease in protein in the ration should only be proportioned to the increase in food intake suchrthat the absolute amount of protein intake is maintained. Any adjustment in ration composition should consider economics and the possible effect of feeding excesses of some ration components. Most protein supplements fed in excess of immediate animal needs are catabolized and utilized as an energy source. Increasing percentage roughage in cattle diets results in a slight increase in heat production because of increased heat increment of feeding. Under hot environmental conditions this increased heat production can result in reduced voluntary food intake. It is therefore advantageous to feed-diets of low roughage content during hot weather. On the other hand, for restricted fed cattle in a cold climate an increase in roughage may at times~be advanta- geous. However, if feed is not restricted in cold conditions substituting roughage for concentrate feeds may limit the total available energy intake and reduce rate of productivity. There is clearly an urgent need for further research into the interaction be- tween protein, mineral, and vitamin needs of cattle and the array of environ- mental stressors encountered in commercial beef cattle systems. SUMMARY OF ADJUSTMENTS FOR ENVIRONMENTAL STRESS FOR BEEF CATTLE 1. Voluntary Feed Intake a. Adjust values tabulated in Nutrient Requirements of Beef Cattle (1976a) in accord with adjustment factors indicated in Tables 12 and 13. 2. Nutrient Value of Feedstuffs a. Adjust the nutrient value of roughage feeds for the ambient temper- ature to which the consuming animal is exposed, see page 62. 3. Maintenance Energy Requirement a. Adjust by either the NEm equation or the percentage factor method the maintenance energy requirement estimates for animals tabulated in Nutrient Requirements of Beef Cattle downward 0.91 percent for each °C cattle have prior exposure to seasonal temperatures above 20°C and upward 0.91 percent for each °C below 20°C, see page 64. b. (i) Heat stress: Using type and intensity of panting as an index, in- crease the estimated maintenance energy requirements by up to 7
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Beef Cattle 71 percent for rapid shallow breathing and 11 to 25 percent for deep open-mouth panting, see page 65. (ii) Cold stress: The adjustment for direct cold stress applies only if animals are exposed to temperatures below their LCT. If a direct cold stress is~suspected, first determine the thermal insulation and LCT values of the cattle, then apply an adjustment to the mainte- nance energy requirements of the cattle for each °C the effective ambient temperature is below the LCT of the cattle, see page 66. EXAMPLES OF ENVIRONMENTAL INFLUENCES ON NUTRITION OF BEEF CATTLE The following examples illustrate the use of the above suggested adjustments and are based on formulation of an adequate ration as presented in Nutrient Requirements of Beef Cattle (NRC, 1976a). Example I Evaluation for a 300-kg finishing steer (Charolais-Angus cross) with antici- pated gain of 1.1 kg per day. In this example a simple diet formulation from corn silage, ground ear corn, and a supplement is used as presented in For- mulating Diets (page 21 and Table 6 of Nutrient Requirements of Beef Cat- tle, NRC, 1976a). Four environmental situations have been considered: a. Nonstressful conditions. b. A hot dry environment with an average ambient temperature of 30°C, but where for a short duration daily mean temperatures rise to 35°C, re- sulting in rapid shallow breathing. c. A cold wet environment with an average temperature of 0°C and with the presence of mud and wet snow. There is also a lack of a suitable bedding area. d. A cold dry environment with an average ambient temperature of -15°C and where there is a dry bedding area and effective protection from wind. Estimated NEm and NEg values for the diet are presented in Table 18. Table 19 summarizes estimates of the adequacy of the diet and expected perfor- mance of the steer in the four environments. Example 2 Evaluation for a 500-kg dry pregnant mature cow (Hereford breed) in the last third of pregnancy on a diet of brome hay (1-00-890~. Four environmental situations have been considered:
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Beef Cattle 73 w. Nonstressful conditions. x. A hot dry environment with an average ambient temperature of 30°C, but where for a short duration daily mean temperatures rise to 35°C re- sulting in rapid shallow breathing. y. A cold dry environment with an average ambient temperature of -25°C and where there is a dry bedding area and effective protection from wind. A cold dry environment with seasonal temperatures of usually-15°C, but estimates are required for a period of several days of a winter storm with air temperatures of-25°C, 10 mph winds, drifting snow, and lack of suitable bedding or shelter. Estimates of daily maintenance requirement in terms of ME and kilograms of hay are shown in Table 20 (p. 741. z.
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