186 the Effect of Breeding Program on Cow Maintenance Requirement and Efficiency of Calf Growth

With genetic tools available today, it is possible to select for traits within breed to achieve similar production phenotypes. Our hypothesis was that biological efficiency of preweaning calf growth would be improved with crossbreeding when dams were selected for similar mature frame size, weight, growth, and milk production. The objective of this study was to determine maintenance energy requirements and efficiency of calf growth through weaning for Angus and Hereford x Angus crossbred cows selected for similar mature BW, growth, and milk production potential. Fifty-nine Angus (n = 32) and Hereford x Angus (n = 27) cow/ calf pairs were assigned to four pen replicates per breed. Cows and calves were limit-fed for a 21-d adaptation period followed by an 84-d experimental period to achieve BW and BCS stasis. Calves did not have access to cow’s feed and daily calf feed allotment was adjusted biweekly to provide 1.25% of previous week’s mean BW. There were no differences due to breed in cow hip height, BW, milk yield (P > 0.31),or cow maintenance energy requirement (P = 0.54). Milk from Hereford-sired cows was slightly more concentrated in energy (P = 0.05) and Hereford-sired cows had greater BCS and ultrasound rib fat and rump fat (P < 0.05) throughout the experiment. Calves from Angus dams were heavier (P 0.01) at the initiation of the experiment, although there were no differences in ADG, final BW, or adjusted weaning BW (P ≥ 0.24). Efficiency of calf growth was not different when expressed as calf BW gain to calf feed and milk energy consumed or as cow/calf pair feed intake or energy intake per unit of calf BW gain (P ≥ 0.31). While the crossbreeding system did not improve biological efficiency of calf growth, more research is necessary to determine if increased cow BCS is advantageous