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Purdue University, West Lafayette, IN 479073
Abstract
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<.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<.01) from AH and HH dams in percentage of assisted births and DS. The AH and HH dams were not different (P>.01) in percentage of assisted births, DS or birth weight of calf. C-sired calves had higher birth weights (P<.01) and their dams had higher DS (P<.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<.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<.01) DS and percentage of assisted births than did dams in all other age groups. Age of dam influenced birth weight (P<.01), with paired comparisons of age classes showing differences between all age groups except 4- and 
5-yr-old cows. Year of birth was a source of variation (P<.05) for all traits measured. In 1972, there was a higher mean DS and a higher percentage of assisted births (P<.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<.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<.01) at birth, but dystocia tended to decrease. Condition entering the breeding pasture influenced (P<.05) pregnancy rate among 2- to 7-yr-old cows. Thin (score 2) 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<.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%.
1 Journal Paper No. 8140, Purdue Univ. Agr. Exp. Sta. A contribution from the Dept. of Anim. Sci.
2 The authors express appreciation to David A. Huber and personnel at the Miller-Purdue Agr. Center for management of cattle and assistance in data collection and to T. S. Stewart for assistance with statistical analyses.
3 This research was supported in part by a cooperative agreement with the American-International Charolais Association, Houston, TX.
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  B. J. Renquist, J. W. Oltjen, R. D. Sainz, and C. C. Calvert Effects of age on body condition and production parameters of multiparous beef cows J Anim Sci, July 1, 2006; 84(7): 1890 - 1895. [Abstract] [Full Text] [PDF]
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