Influence of body composition on energy requirements of beef cows during winter.

Twenty Angus-Hereford and 20 Angus-Holstein cows were individually fed 12.9 or 18.0 Mcal metabolizable energy (ME)/head daily from November 28, 1979 through February 21, 1980. Energy retentions for the winter feeding period were calculated by determining body composition at the initiation and at the termination of the feeding period. After the experimental period, all cows were managed the same through weaning. Maintenance energy requirements (Kcal/d) estimated from linear regressions of energy retentions on ME intakes per unit body weight (BW).75 were 127.6 and 140.3 kcal/BW.75 for Angus-Hereford and Angus-Holstein cows, respectively. Estimates of maintenance energy requirements for thin and fat cows within each breed type indicated that fatter cows of Angus-Hereford breeding had 6.1% lower energy requirements than thinner cows. Opposite trends occurred with Angus-Holstein cows, where fat cows had 2.7% higher maintenance requirements. Estimated maintenance energy requirements were higher (P less than .01) for protein than fat tissue. Maintenance energy requirements of fat was -1.55 kcal ME/kg for Angus-Hereford cows, indicating that for cows of the same lean body mass, cows with more fat have a lower daily energy requirement during winter. Angus-Holstein cows had an estimated maintenance energy requirement of fat of 51.11 kcal ME/kg. Because cattle of Holstein breeding have less subcutaneous fat than cattle of the beef breeds, and less subcutaneous fat would provide less insulation, the estimated maintenance requirements of fat in Angus-Holstein cows may be an estimate of the true maintenance requirement of fat. Estimates of the partial efficiency of ME use for tissue gain and the ME sparing effect of body tissue loss were 78.8% and .70, respectively, for Angus-Hereford cows and 53.8% and .46, respectively, for Angus-Holstein cows. Regression of retained energy on cow BW.75, body fat and body protein calculated for Angus-Hereford and Angus-Holstein cows from within energy level indicated that BW.75 accounted for less variation in retained energy than weight of empty body fat or protein. Multiple regressions that contained all three variables accounted for 75% and 32% of the variation for Angus-Hereford and Angus-Holstein cows, respectively. Subsequent performance of the cow and calf was not affected by winter energy levels fed, body composition of cows before calving or body energy changes of cows during the winter.