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Environment Nutrition Interactions Conventionally, measures of energy have been the basis of most animal feed- ing systems, feed composition tables, and nutrient recommendations for live- stock. Because energy in the form of heat is intimately involved with thermal balance, it is convenient to use energy as the common denominator when de- scribing the interaction between animals and the environment. Nutrient requirement tables list values for animals in conditions presumed to be relatively free of environmental stress and where animals are expected to perform near their genetic potential. In practice, environmental conditions are not always ideal and as a result animal performance often falls short of genetic potential. Contributing factors responsible for reduced productivity include naturally occurring climatic factors as well as those attributable to man (managerial). The latter arise largely through the confinement of ani- mals in intensive production systems. Of the many stresses affecting rate and efficiency of animal productivity, more is known of the consequences of the thermal environment and associated factors of humidity, radiation, and air movement than factors such as altitude, sound, animal density, confinement, chemical or biological contamination, etc. Individual stressors may indepen- dently reduce animal performance, or may interact with other factors creat- ing complex stressful situations whose origin may be at times difficult to as- sess. Animal shelters and housing are intended to eliminate or moderate the impact of the macroenvironment, but, simultaneously, may create a new ar- ray of microenvironmental stresses with which the animal must contend. 13
14 FARM ANIMALS AND THE ENVIRONMENT PARTITIONING OF FEED ENERGY Figure 3 illustrates schematically the partitioning of feed energy within ani- mals and is a useful basis for identifying the modes of influence of the envi- ronment on the nutrient requirements of animals. Where possible the termi- nology of the report Nutritional Energetics of Domestic Animals (NRC, 1981) has been used. Intake Energy ( I E ) Digestible Energy (DE Metabolizable Energy (M E) _ - Feces (FE) Combusti ble Gases (G E ) Urine (UK) Basal Metabolism Activity and a/ Obtaining Nutrientsa 4\ Combating . / External Stress _ Energy Available for Production ~ > Heat of Production > Expelled Products :~ (eggs, conceptus, milk, pelage) ( Retained Products (tissue) / ~ (RE) '' FIGURE 3. Partition of feed energy within the animal (after Young, 1975c).a Heat increments of voluntary activity, fermentation, digestion, absorption, and nutrient metabolism for maintenance and productive func- tions contribute to body heat and in cold environments aid in maintenance of body temperature. However, in hot conditions these heat increments may be a liability to the thermal balance of the animal.
Environment-Nutrition interactions 15 Intake energy (IE) iS the combustible energy ingested per day and is deter- mined from the combustible energy density of the feed, its opportunity for ingestion, and the appetite of the animal. Feed is not completely digested or absorbed. The nonabsorbed fraction is voided as feces and its combustible energy is referred to as fecal energy (FE). Digestible energy (DE) may be cal- culated as lE - FE. However, as feces also contain endogenous material, not all of the combustible energy of feces arises directly from the nonabsorbed fraction of feed. Because of the endogenous component the calculated value (lE-FE) iS more correctly termed the apparent digestible energy. Similarly, metabolizable energy (ME) intake may be calculated by subtracting from the intake energy the energy losses occurring in feces, urine (UK) and the gaseous products of digestion (GE), ViZ., ME = {E - ~ - UE - GE. Therefore, by definition, the metabolizable energy intake is that which is available to an animal for maintenance and productive functions. Maintenance functions involve the utilization and oxidation of metaboIiz- able energy for (1) basal metabolism that is represented by the heat energy evolved in sustaining body integrity by the vital life processes, (2) voluntary activity and obtaining nutrients including the muscular activity of seeking and obtaining food, the processes of digestion, absorption, conversion of food into metabolizable forms, and the formation and excretion of waste products, and (3) combating of external stressors related to an immediate and direct imposition of stress or stresses on the animal. With respect to the lat- ter, animals are consistently faced with various types and magnitudes of stress to which they must continually adjust both behaviorally and physiolog- ically. Although the physiology of stress is still poorly understood (Stott, 1981), some stressors, such as exposure to a cold environment, are known to increase the rate of oxidation of feed or body energy to produce heat. The en- ergetic costs of stressors such as parasites or pathogens are recognized but not well defined. The ME oxidized for the various maintenance processes is released in the animal as heat (maintenance heat) and is ultimately disposed to the environment through physical avenues of heat exchange. Metabolizable energy for production is available after the maintenance needs of the animal are met. Because of the inefficiencies of product syn- thesis (heat of production), energy available for production is not entirely in- corporated into animal products, be it retained in tissue growth or fattening, or expelled in a product, such as milk, pelage, eggs, or offspring. The latter includes inefficiencies of product synthesis as well as the costs of retaining or expelling the product. Typically, animals retain energy as glycogen, lipids, and (or) protein when metabolizable energy intake exceeds immediate needs. Likewise, re- tained energy is mobilized when the animal's demand is greater than the en- ergy available from feed. For example, dairymen allow their cows to accu
16 FARM ANIMALS AND THE ENVIRONMENT mutate body fat (energy) when not lactating, expecting it to be mobilized and utilized during peak lactation when maximum intake may be insufficient to meet the cow's immediate needs for both maintenance and maximum levels of lactation. In summary, Figure 3 represents the intake of feed energy and its partition through the major routes of energy disposed of as wastes, as expelled prod- ucts, and as heat or retained as tissue. Heat is dissipated via several pathways under the control of thermoregulatory mechanisms to prevent a rise or de- cline in body temperature. During cold stress, heat from maintenance and productive processes may be of immediate value to the animal in maintaining body temperature, reducing the need of the animal to produce extra body heat by shivering or other cold-induced thermogenic processes. On the other hand, during heat stress thermoregulatory mechanisms are activated to dissi- pate excess heat from the body to maintain homeothermy. Thus heat that may be beneficial during cold exposure may be a burden to the animal during heat stress. For example, heat evolved during productive functions effec- tively lowers the thermoneutral zone resulting in a greater magnitude of heat stress at a given temperature for producing compared with nonproducing ani- mals. Behavioral and physiological adjustments by the animal arising from ex- ternal stressors affect energy intake and its partition within the animal, the amount of energy available for production, the level of productivity, and the efficiency of utilization of feed. The influences of the environment are there- fore much broader in scope than simply implied in the single component of "combating external stress" in Figure 3. DIGESTIBILITY AND METABOLIZABILITY Digestibility and metabolizability are biological measures of energy or nutri- ent value assigned to feeds and depend not only on the physical and chemical nature of the feed itself but also on the animal ingesting the feed, the physio- logical state of the animal, and the amount of feed ingested (NRC, 1981~. Recognized differences, particularly in digestive processes, among species to which ingested feeds have different nutrient values have led to the devel- opment of somewhat independent feeding systems, including lists of feed composition tables for various species. Independent of any influence of the environment on plant growth and the composition or quality of animal feed per se there is a growing body of evi- dence indicating that the environment directly influences digestive and meta- bolic functions in animals. Although the extent and nature of the physiologi- cal changes in the animal are, as yet, not resolved, the possible consequences to applied animal nutrition are important. However, within temperate cli- matic zones, the ability of animals to digest roughages increases with warmer
Environment-Nutrition Interactions 17 temperatures and decreases with colder ambient temperatures, although in the severe heat stress of the tropics an animal's ability to digest feed may be depressed (Bhattacharya and Hussain, 1974; Sharma and Kehar, 1961~. Results summarized in Table 2 indicate the effect of ambient temperature on digestibility values. There has been hesitation in accepting the premise that the nutrient value of a feed could be influenced by the environment to which the animal is exposed (Fuller, 1965; Graham, 1965), because with sudden changes in the thermal environment there are transient changes in rate of pas- sage of digesta and volume of the gastrointestinal tract that are of sufficient magnitude to bias short-term estimates of apparent digestibility (Degen and Young, 1980; Graham, 1965~. Thus, caution needs to be exercised when in- terpreting feed digestibility estimates made during periods where there have been changes in the thermal environment or during constant heat or cold stress. Although possibly related to appetite changes occurring with exposure to hot or cold environments (see page 27), the observed changes in feed di- gestibility are not solely dependent on feed intake since the effects are also observed when feed intake is equalized, restricted, and controlled (Chris- topherson, 1976; Kennedy et al., 1977; Lippke, 1975; Warren et al., 1974~. Most data on effect of ambient temperature on the ability of animals to digest foodstuffs are for ruminant animals consuming roughages. In the few trials with sheep receiving concentrate (grain based) feeds, digestibility values have generally not been influenced by ambient temperature. Investigations of possible direct thermal effects on gastrointestinal tract temperature or on microbial populations have failed to show the significance of these routes of action (Cunningham et al., 1964~. The importance of rate of digesta passage in ruminants on diet digestibility is clearly evident (Belch, 1950; Blaxter etal., 1956; Mertens and Ely, 19791. Several recent studies indicate that ambient temperature may affect feed digestibility by altering the volume of the gastrointestinal tract and rate of digesta passage. During heat exposure, rumen motility of cattle decreases (Attebery and Johnson, 1969) and there is a concomitant increase in the re- tention time of digesta that should increase digestibility (Warren et al., 1974~. Virtually opposite responses have been reported for cold-exposed sheep and cattle, i.e., an increase in reticulorumen motility and rumination activity, an increase in rate of passage of digesta (decreased retention time), and a decrease in the apparent digestibility of feeds (Buckebusch and Mar- quet, 1964; Christopherson, 1976; Gonyou et al., 1979; Kennedy et al., 1977; Westra and Christopherson, 1976~. Studies on nonruminant animals have shown that the gastrointestinal tract motility is reduced by hypothyroidism and increased by the administration of thyroid hormones, as reviewed by Levin (19691. In 1974 Miller et al. re- ported that cows with damaged thyroid glands had a reduced rate of passage that could be restored to normal rate by feeding thyroprotein. Similarly,
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20 FARM ANIMALS AND THE ENVIRONMENT Kennedy et al. (1977) found thyroidectomized sheep had reduced rate of di- gesta passage that could be restored by thyroid therapy. That warm tempera- tures decrease thyroid activity and cold temperatures increase thyroid activity in animals is documented for several species (de Andrade et al., 1977; Gale, 1973; Johnson, 1976~. This suggests that the shift in thyroid activity in ani- mals because of exposure to different ambient temperatures may be associ- ated with both a change in gut motility and rate of digesta passage. The result is a shift in ration digestibility. More research is needed to more clearly un- derstand the sequence of physiological mechanisms involved and the impor- tance of diet type and level of feeding. Effects of the thermal environment on the dry matter and DE value of feed would also alter its ME value. However, the ME value of a feed is also depen- dent upon the losses of urinary energy and combustible gases from microbial fermentation. These losses, like fecal loss, are also dependent on the envi- ronment. There are during cold exposure increases in urinary energy and ni- trogen output, especially where there is substantial tissue protein degradation to provide substrate for thermogenesis (Blaxter and Wainman, 1961; Graham et al., 1959), which would lower calculated ME value (as defined). Such a bias is an ambiguity arising from the definition of metabolizable energy rather than a true penalty that should be placed against the energy value of the diet. However, the slight reduction in methane production observed in cold-exposed sheep (Kennedy and Milligan, 1978), probably reflecting a re- duction in microbial activity, would improve slightly the ME value of the diet consumed during periods of cold exposure. However, this improvement is insignificant relative to the reduction in digestibility observed during cold ex posure. The above evidence indicates an influence of the thermal environment on digestive function and suggests a need to develop appropriate adjustment fac- tors for roughage-based diets for ruminants. However, any adjustment fac- tors suggested at this stage can only be preliminary estimates that should be refined by further research. Although information on the thermal environ- ment is generally not available for the estimates of biological measures of feeds presently listed in feed composition tables, the data are generally as- sumed to be based on studies with animals in thermoneutral conditions. The effect of ambient temperature on digestion of foodstuffs by growing hogs has also indicated a decrease in energy and nitrogen digestibility when the animals are exposed to cold (Fuller, 1965; Fuller and Boyne, 1972; Phil- lips et al., 19791. Observed changes in digestibility values have ranged from 0.12 to 0.48 digestibility units per °C change in ambient temperature. At pre- sent there are, however, insufficient data for a recommendation to be made for adjusting swine diets for the effect of ambient temperature on feedstuff digestibility.
Environment-Nutrition interactions 21 Metabolizability rather than digestibility values of feeds are more readily measured in poultry. There is, however, no clearcut evidence that ambient temperature affects the ME value of diets fed to chickens. Swain and Farrell (1975) observed an increase in metabolizable energy value of diets when chickens were exposed to warmer temperatures, while other researchers (Matterson, 1970; Olson et al., 1972) have not detected a significant change. Davis et al. (1972) noted that ME declined during the first 3 weeks but re- turned to normal during the second 3 weeks in hens kept at 30°C.
