Factors influencing pregnancy rate and calving date in four breeds of beef cows on an Ozark mountain range

Abstract

Observations from 4928 matings made over 17 years were used to evaluate the influence of year, exposure class, cow weight, average daily gain (ADG) before and during the breeding season on pregnancy rate (PR) and calving date (CD) in four breeds of beef cows maintained on an Ozark mountain range. Pregnancy rate was the percentage of cows exposed to natural mating that were checked pregnant in the fall. Calving date was the number of days from a base date of 1 January. Pregnancy rate and CD were analyzed separately for each breed by least-squares analysis of variance. A model including the above factors accounted for 10 to 12% of the variation in PR and 3 to 23% of the variation in CD. Variation among years significantly affected PR and CD. Cows being bred for their first lactation had the lowest PR and the latest CD. Mature cows had the highest PR and the earliest CD. Partial regression coefficients of PR and CD on cow weight ranged from 2.9% and −3.2 days to 7.4% and 2.3 days/100 kg. Partial regression coefficients of PR and CD on ADG prior to the breeding season ranged from 3.4% and −14.5 days to 24.9% and −2.8 days/kg. Partial regression coefficients for PR and CD on ADG during the breeding season ranged from 10.4% and −18.2 days to 41.6% and −6.1 days/kg. A second analysis was performed on the adjusted data in which cows were nested within sires and offspring were nested within cows. The major source of variance for the two traits was among offspring within dams, which accounted for 78.7 to near 100% of the variation in PR and 50.6 to 84.6% of the variation in CD.