Interrelationships of heifer milk production and other biological traits with production efficiency to weaning.

Interrelationships among milk production, dam-calf feed efficiency to weaning, and other biological traits were evaluated on 411 first-calf heifers and their calves. Individual feed intakes were measured in a drylot environment for a 1-yr period until weaning of the first calf. Heifer breed types included crossbred Angus-Hereford, Simmental-Hereford, and Tarentaise-Hereford produced in two-breed rotational crossbreeding systems, F1 Salers-Hereford and straightbred Hereford. Production efficiency was defined as cumulative feed ME consumed by the dam-calf pair during the year divided by calf weaning weight. Milk production potential seemed to be unrelated to cow size, adjusted for breed-group effects. Lactational feed energy was increased for dams of increased milk production potential but not enough to offset the increased production of calf weaning weight in terms of production efficiency. Results indicated that increased levels of milk production of heifers were associated with improved production efficiency to weaning. However, the incremental improvement in efficiency per unit of increased milk was less for each additional unit of milk, within the range evaluated. These results are applicable to first-calf heifers that have not yet reached peak age for maximum milk production.