Factors affecting secondary sex ratio in Iranian Holsteins.

The objective of this study was to evaluate the factors affecting secondary sex ratio (SSR) in Iranian Holsteins. Data of 942,941 Holstein calving events from the Animal Breeding Center of Iran, recorded between January 1996 and December 2007, were used in the analysis. A multivariable logistic regression model was used to model the logit of the probability of a male calf being born. Male births accounted for 49.6% of the total observations. The ratio of males to females varied from 52.5:47.5 in calving year 1996-1999 (odds ratio (OR)=1.18; P<0.0001), to 48.5:51.5 in calving year 2004-2007. The greatest occurrence of male births was observed in spring (OR=1.02; P<0.0001), and the lowest incidence of male births was for summer or fall calvings. Also, the frequency of male births decreased from parity 1 to parity 4 and beyond (P<0.0001; OR=1.11). The greatest number of sires had the SSR equal to 0.5 with a minimum SSR of 32% while the maximum was 97%. Among cows that had a male birth, the chance of delivering a male calf again was 25.5% when cows had delivered a male once (OR=1.14; P<0.0001), and 12.7% if a male calf was delivered twice by a cow. This indicated that characteristics peculiar to the dam influence the sex of her offspring and suggests some degree of repeatability of calf sex within cows.