Heterosis and breed additive effects for Hereford, Tarentaise, and the reciprocal crosses for calf traits.

Records from 595 straightbred Hereford (HH), straightbred Tarentaise (TT), and reciprocal-cross (HT) females, randomly mated to HH, TT, or HT bulls, were analyzed for estimates of heterosis and breed additive effects for calf traits that included birth weight (BWT), calving difficulty (DIFF), prebreeding (PRE) and postbreeding (PST) weight, weaning weight (WWT), weaning condition score (WCS), weaning hip height (WHH), and preweaning average daily gain (ADG). The statistical model included year, age of dam, sex, regression on age at time of measurement, and regressions for the genetic effects of breed individual, breed maternal, breed grandmaternal, individual heterosis, and maternal heterosis. Breed effects were coded to reflect TT- HH differences. Age at the time of measurement, year, age of dam, and sex were significant for most traits. Individual breed effects were important (P < .05) for BWT, PRE, PST, WCS, and WHH but not for WWT, resulting in lower weights, less condition, and taller animals for TT. Maternal breed effects did not influence BWT, but they were important ( P < .05) for PRE, PST, WWT, WCS, WHH, and ADG. Grandmaternal effects were only important for BWT and ADG. Individual and maternal heterosis were important (P < .05) for most traits measured, resulting in increased calf size, weight, and body condition.