Beef x beef and dairy x beef females mated to Angus and Charolais sires. I. Pregnancy rate, dystocia and birth weight.

Pregnancy rate of 310 females and birth weight, dystocia score and percentage of assisted births among 910 calves born from 1972 through 1977 were examined. Dams were 2 to 7 yr old and of Angus x Hereford (AH), Charolais x Hereford (CH), straightbred Hereford (HH) and Brown Swiss x Hereford (SH) breed types. Females were exposed to A and C bulls. Breed of dam affected (P less than .01) dystocia score (DS), percentage of assisted births and birth weight. When adjusted for birth weight of calf and dam's postcalving weight, CH and SH dams generally differed (P less than .01) from AH and HH dams in percentage of assisted births and DS. The AH and HH dams were not different (P greater than .01) in percentage of assisted births, DS or birth weight of calf. C-sired calves had higher birth weights (P less than .01) and their dams had higher DS (P less than .05). However, there was no difference between C- and A-sired calves in percentage of assisted births when data were adjusted for effects of birth weight of calf and weight of dam. Calf sex affected birth weight (P less than .01) but had little influence on DS or percentage of assisted births when birth weight was held constant. Male calves were 1.7 kg heavier at birth than females. Two-year-old dams had higher (P less than .01) DS and percentage of assisted births than did dams in all other age groups. Age of dam influenced birth weight (P less than .01), with paired comparisons of age classes showing differences between all age groups except 4- and greater than or equal to 5-yr-old cows. Year of birth was a source of variation (P less than .05) for all traits measured. In 1972, there was a higher mean DS and a higher percentage of assisted births (P less than .05) than in any other year. However, all calves born in 1972 were from 2-yr-old cows. Percentage difficulty and DS increased linearly (P less than .01) with birth weight. Each 1-kg increment in birth weight increased percentage difficulty by 2.6 percentage points and DS by .04 units. Dam's with heavier weights had calves that were heavier (P less than .01) at birth, but dystocia tended to decrease. Condition entering the breeding pasture influenced (P less than .05) pregnancy rate among 2- to 7-yr-old cows. Thin (score w) cows had a lower conception rate than average flesh (score 3) or fat (score 4) cows. Date of calving prior to the breeding season also affected (P less than .05) pregnancy rate. Each 10 d later calving resulted in a 1.2% reduction in conception rate. No significant sources of variation were observed for pregnancy rate among yearling heifers. Pregnancy rates ranged from 97.1% for CH and SH heifers to 86.9% for HH females. Yearling AH females were intermediate at 93.5%.