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Feed intake of sheep as affected by live weight, breed, sex and feed composition

R. M. Lewis^*, and G. C. Emmans^,

^* Department of Animal and Poultry Sciences (0306), Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 USA , Sustainable Livestock Systems Group, Scottish Agricultural College, West Mains Road, Edinburgh EH9 3JG UK Animal Nutrition and Health Department, Scottish Agricultural College, West Mains Road, Edinburgh EH9 3JG UK

rmlewis{at}vt.edu

Abstract

The hypotheses tested were that genetic size-scaling for mature BW (A, kg) would reduce variation in intake between kinds of sheep, and that quadratic polynomials on u = BW/A with zero intercept would provide good descriptions of the relationship between scaled intake (SI, g/A^0.73 d) and degree of maturity in BW (u) across feeds of differing quality. Both sexes of Suffolk sheep from two experimental lines (n 225), and from three breed types (Suffolk, Scottish Blackface and their cross; n 149), were recorded weekly for ad libitum feed intake and BW; recording of intake was from weaning through, in some cases, near maturity. Six diets of different quality were fed ad libitum. The relationship between intake and BW on a given feed varied considerably between kinds of sheep. Much, but not all, of that variation was removed by genetic size-scaling. In males, the maximum value of SI was higher than in females (P < 0.07), and was greater in Suffolk than in Scottish Blackface, with the cross intermediate (P = 0.025); there was no difference between the two Suffolk lines used (P = 0.106). The quadratic polynomial model, through the origin, was compared with a split-line (spline) regression for describing how SI varied with u. For the spline model, the intercept was not different from zero in any case (P > 0.05). The values of u at which SI achieved its maximum value (u* and SI*) were calculated. Both models fit the data well; the quadratic was preferred because it predicted that SI* would be achieved within the range of the long run data, as was observed. On a high quality food, for the spline regression, u* varied little around 0.434 (SD 0.020) for the ten different kinds of sheep used. For the quadratic, the mean value of 0.643 (SD 0.066) was more variable, but there were no consistent effects of kind of sheep. The values of u* and SI* estimated using the quadratic model varied among the six feeds: 0.643 and 78.5 on high quality; 0.760 and 79.6 on medium protein content; 0.859 and 73.3 on low protein content; 0.756 and 112 on a low energy content feed; 0.937 and 107 on Ryegrass; and, 1 (forced, as the fitted value of 1.11 was infeasible) and 135 on Lucerne. The value of u* tended to increase as feed digestibility decreased. We conclude that genetic size-scaling of intake is useful, and that a quadratic polynomial with zero intercept provides a good description of the relationship between SI and u for different kinds of sheep on feeds of different quality. Up to u 0.45, intake was directly proportional to BW.

Key Words: Feed composition • Feed intake • Genetic size-scaling • Selection • Sex • Sheep