Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/11/2003
Publication Date: 7/1/2003
Citation: FREETLY, H.C., NIENABER, J.A., BROWN BRANDL, T.M. RELATIONSHIP BETWEEN AGING AND NUTRITIONALLY CONTROLLED GROWTH RATE ON HEAT PRODUCTION OF HEIFERS. JOURNAL OF ANIMAL SCIENCE. 2003. v. 81. p. 1847-1852. Interpretive Summary: The single largest cost associated with beef production is feed. In order to increase efficiency of feed utilization, we need to be able to predict the requirements of animals as they age. The ability to predict feed utilization of heifers has been limited by an absence of information describing nutrient usage as they age. It has long been recognized that previous nutritional history affects nutrient usage, but the relationship between age and previous nutrition has not been fully documented. Heat produced by heifers represents energy that is not retained by the animal. Our research describes that as heifers age, the amount of heat produced per unit of weight decreases. This decrease in heat production was described by a model we developed in sheep. Heifers whose growth has been slowed by nutrient restriction and then were fed to grow rapidly temporally have a higher heat production than heifers that are allowed a continuous supply of food. The time required for heat production to return to that of continuously fed heifers can require many weeks. These findings demonstrate that heat production changes with age and that applying single age estimates of heat production across ages will result in incorrect predictions of nutrients lost as heat. These findings also show that experimental designs that involve switches between feed levels need to allow extended lengths of time for growing animals to adapt to new levels of nutrition.
Technical Abstract: The first objective of this study was to test how well a function that was developed to describe heat production (HP) in growing ewes fit heat production data in growing heifers. The second objective was to determine the pattern of adaptation of HP to feed restriction and subsequent realimentation of nutrients. At 234.5 ± 0.5 d of age, heat production was determined by indirect-calorimetry on 32 MARC III heifers. Following the first calorimetry measurement, heifers on the High-High (HH) treatment continued to receive ad libitum access to feed, and daily feed offered to the Low-High (LH) heifers was set at 157 Mcal ME/(kg BW)0.75. Feed restriction of LH heifers continued for 84 d. After 84 d of feed restriction, LH heifers were allowed ad libitum access to feed. Heat production was determined 4 and 11 wk following feed restriction and 2, 5, 12, and 18 wk following realimentation. There was no residual bias when heat production in ad libitum fed heifers was estimated with an equation form developed in growing ewes [(kcal/d) = f(BW, matBW) = BW (Ae(k(BW/mature BW)))], nor was there a residual bias when heat production was predicted with an allometric equation [(kcal/d) = f(BW) = A(BW k)]. However, there were residual biases when heat production was estimated with an allometric equation that set the exponent to 0.75. Heat production per unit of BW of LH heifers was lower than that of HH heifers at 4 wk of feed restriction (P < 0.001), but HP did not differ between treatments at 11 wk of feed restriction (P = 0.87). At 2 (P = 0.002) and 5 wk (P < 0.001) following the increase in feed offered, HP per unit BW of the LH heifers was greater than the HH heifers. Heat production did not differ between treatments at 12 and 18 weeks following refeeding (P < 0.17). Our findings show that adaptation to nutrient restriction and realimentation are dynamic and adaptation to new nutrient status occurs over extended periods. These dynamic changes suggest that single time-point estimates of HP in animals adapting to new nutrient levels will lead to erroneous conclusions when these values are applied over an extended feeding period.