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Nutritional Err. ITIClenCy Efficiency is the comparison of output to input. There are numerous schemes for measuring efficiency of converting ingested foodstuffs to animal product. For livestock, total (gross) efficiency is calculated as follows: Total efficiency = Total output x 100 Total input The most common "feed efficiency" term used by stockmen relates weight of feed ingested to weight of animal product output (feed to gain ratio) and may at times be confusing because it relates weights that in essence are not really comparable. For example, dry matter content of both feedstuffs and animal product are highly variable, which may result in from 3 to 25 units of feed by weight required to produce a unit of body weight gain. However, feed to gain ratio is an accepted way of describing total efficiency and is a useful term in practical situations since ultimately it is weight of feed that must be grown or purchased and fed and weight of product sold. It is more meaningful when relating environment and efficiency to calcu- late total efficiency in caloric terms to obtain an energetic efficiency term as shown below: . . Total energy gain (RE) Total energetic efficiency = Total energy intake (lE) x 100. Partial efficiency is defined as the observed change in gain for a given change in feed intake expressed as a percentage: 51
52 FARM ANIMALS AND THE ENVIRONMENT Partial efficiency = /`Energy gain (RE) X 100 Synergy intake (lE) Since by definition, gains do not occur at or below maintenance, the main- tenance level of feeding can be used as a baseline from which to calculate partial efficiency: Partial efficiency =. Gain (RE) X 100. Intake (TE) - Maintenance (HE) Partial efficiency is simply the ability to convert the energy surplus above maintenance to stored chemical energy in terms of growth or product. Ani- mal gains can vary considerably depending on the fat:protein ratio; thus effi- ciency of energy retention (partial efficiency) will vary with composition of gain since the cost of fat synthesis is different than protein. When the effi- ciency of producing product from energy surplus to maintenance (partial effi- ciency) is altered, then total efficiency is also affected. Some data suggest a lipogenic effect of cold (Fuller and Boyne, 1971), while other studies (Hacker et al., 1973) suggest leaner carcasses during cold exposure. Magni- tude of cold and availability of food are major determinants of composition of growth for animals. exposed to thermal stress. Seasonal variation in com- position of expelled product such as butterfat content of milk are well known. However, when all factors are considered, environment has little ef fect on partial efficiency. Changes in total efficiency do not infer changes in partial efficiency. Gen- erally, environment influences total efficiency by affecting rates of intake and maintenance energy requirement. Adverse environments alter the effi- ciency of converting foodstuffs to animal product and therefore are economi- cally important to study. The fact that climatic environment alters the rela- tionship of output per unit input has led to varied studies designed to describe effect of environment or environmental modification on efficiency of con- verting feed to product. Henderson and Geasler (1969) summarized several studies comparing the value of modifying both summer and winter environ- ments with natural environments for the beef cattle studies. In general, for the locations included in the studies, environmental modification resulted in improved efficiency of feed conversion although economic advantage was not always positively correlated with efficiency. For specific effects of climatic factors several workers have reported de- creased feed efficiency of swine exposed to either heat (Hazen and Mangold, 1960; Jensen, 1971) or cold (Fuller and Boyne, 1971; Mangold et al., 1967), although there is some evidence that finishing hogs exhibiting compensatory
Nutritional Efficiency 53 growth after removal from heat stress are as efficient as nonstressed animals (Hahn et-al., 1975~. Ames and~Brink (1977) reported reduced feed efficiency for lambs.exposed to either'heat or cold when compared with thermoneutral- ity for temperatures ranging from - 5 to 35°C when estimated lower critical temperature was 159C. Again both heat and cold result in reduced efficiency of milk production ('McDowell et al., 1976) when compared as kilograms of milk per megacalorie feed-energy. Under heat stress, feed efficiency (mega- calories of ME per kilogram of milk) declines rapidly above 27°C (Moody et al., 19671. An example of an environmental effect on rate of performance and ener- getic efficiency of food animals is illustrated by data shown in Table 11 col- lected from similar swine grown in temperatures ranging from cold stress (0°C) to heat stress (35°C). The energetic efficiency was reduced during both cold and heat stress and was highest during the TNZ. While the temperature and efficiency values may differ for animals with different insulation, diets, etc., or for different species and products, the same general pattern of re- duced energetic efficiency is consistent among animals exposed to stressful environments. This reduced energetic efficiency, in turn, causes an eco- nomic loss. Livestock producers are usually willing to incorporate manage- ment systems to improve energetic efficiency when that is advantageous eco- nomically. There are some reports (Holme and Coey, 1967; Sugahara et al., 1970) of improved efficiency of swine during mild cold compared with thermoneutral conditions. Figure 16 suggests this may happen if rate of increased voluntary intake is more rapid than rate of increased energy requirement for heat pro- duction during cold. When exposed to heat, the combination of reduced in TABLE 11 Effect of Temperature on Intake, Growth Rate, and Efficiency of Energy Conversion for Swine (70 to 100 kg) Temperature Caloric Intake Growth Rate Product Caloric ( C) (kcal DE/day) (kg/day) (kcal GE)a Efficiency (%)b 0 15,377 0.54 2,991 19.4 5 11,404 0.53 2,936 25.7 10 10,616 0.80 4,432 41.7 15 9,554 0.79 4,376 45.8 20 9,766 0.85 4,709 48.2 25 7,976 0.72 3,988 50.1 30 6,703 0.45 2,493 37.1 35 4,579 0.31 1,717 37.4 a Estimated caloric value of gain for an 80-kg pig is 5.54 kcal GE/g (Thorbeck, 1975). b Calculated: (kcal GE in product . kcal DE intake) x 100. SOURCE: Ames, 1980.
54 FARM ANIMALS AND THE ENVIRONMENT LOT UCT \ \ - I - UJ at - RE Cold TNZ >( Heat EFFECTIVE AMBIENT TEMPERATURE (EAT) FIGURE 16. Schematic relationship of heat production (HE), intake en- ergy (lE), and energy for production (RE = lE - HE) with temperature zones. take and increased heat production result in reduced efficiency for growing animals. In contrast to most data for growing animals, total energetic efficiency of producing expelled products such as eggs and milk have been reported to be improved during heat by Davis et al. (1972) and Johnson (1965), respec- tively. This apparent difference in efficiency of an expelled product com- pared with tissue growth is explained by catabolism of body stores to meet energy demands during heat when intake is reduced. (See page 112 for a de- tailed description of energetic efficiency of egg production). Consequently, caloric efficiency of producing an expelled product is improved primarily be- cause of mobilization of tissue reserves while only ingested energy is used to calculate caloric efficiency. Of course, original costs of depositing depot fat are not considered, and, therefore, caloric efficiency of producing an ex- pelled product such as the egg during heat must be taken within the context of short-term utilization of stored energy. Conversely, catabolism of tissue stores in animals where growth is the end point of production is self- defeating and results in lowered efficiency, although a lower percent carcass fat may be considered a desirable result. There may, however, be instances of management systems that rely on the ability of animals to withstand per- iods of reduced feed quantity or quality and then recover tissue stores through compensatory growth when the diet is more favorable. There appear to be two additional reasons for improved efficiency of producing eggs dur- ing heat. First, efficiency of using body fat as an energy source is high com- pared with using energy from feed sources, which results in higher efficiency of egg production during heat exposure. And second, van Kampen ( 1974) re
Nutritional Efficiency 55 ported lowered HeE, while McDonald (1978) reported lowered "existence energy" during heat as a result of lowered intake and reduced activity of lay- ing hens. Reports of energetic efficiency for animals exposed to fluctuating environ- ments compared with constant temperature are conflicting. Bond et al. (1963) found that pigs exposed to a constant 21°C environment had higher total efficiency than pigs exposed to cyclic 10 to 32°C or 4 to 38°C environ- ments. Giacomini (1979), working with lambs, and Sorensen and Moust- gaard (1961) and Hahn et al. (1975), with pigs, found no difference in effi- ciency of growth when cyclic environments were compared with constant temperature environments of the same mean temperature. The variety of dif- ferences in fluctuating environments (i.e., duration, magnitude of change, etc.) will require much more data before conclusions can be drawn. Numerous studies suggest that specific climatic variables change total effi- ciency. Morrison et al. (1966) found no effect of 45, 70, and 95 percent rela- tive humidity on hogs when temperature was considered optimum, but a sig- nificant effect was noted when studied during heat (Morrison et al., 1969~. Berry et al. (1964) reported declines in milk production with an increasing TH! value. Morrison et al. (1971) indicated lower total efficiency of gain for cattle exposed to rain but improved total efficiency with wind during mild winter conditions in California. Further examples of effects of specific cli- matic variables only substantiate the need to use effective ambient tempera- ture when relating effects of the climatic environment to performance. Measurement of efficiency in the short term can lead to erroneous conclu- sions because of differences in an animal's previous nutritional background. For example, animals that have received restricted intake will compensate with improved feed efficiency when allowed ad libitum intake. Hens for- merly on restricted diets gradually renew their lipid reserves (Polin and Wolford, 1972) by depositing lipid in the carcass more efficiently than non- restricted birds. Allden (1968) reported that feed consumption and feed utili- zation were not affected in the long term following a period of nutrient re- striction although compensation was observed early in the recovery period. Searle and Graham (1975) reported no difference in body composition of ani- mals held at constant weight for up to 6 months by restricted feeding and then fed ad libitum compared with lambs fed ad libitum. Short-term changes in efficiency during heat stress may occur, but studies show (Hahn et al., 1975) that animals convert feed more efficiently after relief from heat stress compared with unstressed animals subjected to restricted nutrition and tend to equalize in the long run with little effect on long-term efficiency. Degen and Young (1980) reported that rapid changes in live weight during and fol- lowing cold exposure can be largely attributed to losing and gaining of body fluids.
56 FARM ANIMALS AND THE ENVIRONMENT The impact of climatic environment on energy flow in terms of both en- ergy intake and that available for growth may directly affect the utilization of other nutrients because in many cases nutrient requirement is a function of available energy. For example, protein efficiency ratio (grams of gain per gram of cP) is lowered during both heat and cold stress in sheep (Ames and Brink, 1977), and Fuller and Boyne (1971) and Roy et al. (1969) have re- ported lowered nitrogen retention during thermal stress in swine and cattle, respectively. These examples emphasize the need to consider available en- ergy in the light of environmental stress and to adjust rations to enhance effi- cient utilization of all nutrients.