Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/24/2002
Publication Date: 10/1/2002
Citation: Freetly, H.C., Nienaber, J.A., Brown Brandl, T.M. 2002. Relationship between aging and nutritional controlled growth rate on heat production of ewe lambs. Journal of Animal Science. 80:2759-2763. Interpretive Summary: The cost of feed is one of the largest financial inputs in a sheep production system. Managing feed resources to optimize biological and economical returns requires the ability to predict animal performance based on feed inputs. Current feeding systems assume that energy requirements are solely a function of an animal's body weight. However, this study supports that a feeding system that accounts for body weight, age, breed, and previous nutritional history is required to accurately predict energy requirements.
Technical Abstract: The objective of this study was to determine how reducing growth rate nutritionally alters the relationship between heat production per unit body weight and aging. Fasting heat production of 12 Dorset ewe lambs at 114 ± 2 d of age was determined, and ewes were assigned to treatments. Treatments consisted of two different feeding levels of the same diet (ME = 2.5 Mcal/kg DM and 16.6% CP). The High treatment was offered 4.5% of their weekly BW, and the Low treatment was offered 2.5% of their weekly BW. Each treatment consisted of six animals that remained within treatment for the remainder of the study. Calorimetry measurements were repeated every 6 wk. Treatments differed in both the linear and quadratic term for fasted BW on age (P < 0.001). The rate of BW gain decreased as ewes aged in the High treatment, and the rate of BW gain increased as ewes aged in the Low treatment. The heat production:BW (HP:BW) ratio decreased in the High treatment as ewes aged and was described well by a previously reported prediction equation, but the ratio in the Low treatment was not described by this same equation. Describing the HP:BW on age response with treatment specific decay functions fits the data better than the pooled treatment function. The HP:BW ratio decreased rapidly in the Low treatment following feed restriction, but remained elevated compared to the High treatment as animals aged. After excluding the initial measurements in the Low treatment that were taken before nutritional treatments were imposed, the HP:BW ratio was best described by a linear decrease. In conclusion, this study suggests that a model taking into account proportion of mature body size is a reasonable predictor for heat production across breeds of sheep growing in nutritionally adequate environments; however, it cannot be extended to sheep that are proportionally smaller in their mature BW due to nutritional restriction.