Calving probability in the first and second reproductive years of beef heifers that reached the recommended body weight at first breeding season

ABSTRACT The objective was to evaluate factors that influence calving probability in the first and second reproductive years of beef heifers that reached the minimum recommended body weight (BW) in the first breeding season. Two hundred twenty-seven 24-month-old Charolais × Nellore heifers that mated between 2003 and 2012 were analyzed. The parameters evaluated were: year effect; individual and maternal heterozygosis; percentage of Charolais genotype; average daily gain (ADG) from weaning to the end of the first breeding season; BW at 18 and 24 months of age, end of breeding season, parturition, and weaning; Julian date of calving; and adjusted calf weight at weaning. These variables were subjected to logistic regression. Calving rates in the first and second reproductive years were 58.1 and 49.5%, respectively. Performance until the end of the first breeding season, BW before the first breeding season, and individual heterozygosis affected the calving probability in the first breeding season. In the second breeding season, BW variation from the first breeding season and calving, Julian date of calving, and BW at the end of the second breeding season influenced calving probability. Until the end of the first breeding season, ADG of beef heifers responded positively to the calving probability, even after reaching the usually recommended BW for the first breeding season. Primiparous cows calving at the beginning of the breeding season and gaining more BW between the first and second breeding season are more likely to give birth in the second breeding season.

摘要
本研究旨在评估在首个配种季节达到推荐最低体重（body weight, BW）的肉用后备母牛，其首、次两个繁殖年度内产犊概率的影响因素。本研究选取2003至2012年间参与配种的227头24月龄夏洛莱牛（Charolais）×内洛尔牛（Nellore）杂交后备母牛进行分析。本次评估的参数包括：年度效应、个体杂合性与母本杂合性、夏洛莱基因型占比、从断奶至首个配种季节结束的平均日增重（average daily gain, ADG）、18与24月龄体重、配种季节结束时体重、产犊时体重、断奶时体重、产犊儒略日，以及断奶校正犊牛体重。对上述变量开展了逻辑回归分析。
首、次两个繁殖年度的产犊率分别为58.1%与49.5%。截至首个配种季节结束的生长性能、首个配种季节前的体重以及个体杂合性，均会影响首个配种季节的产犊概率。在第二个配种季节中，从首个配种季节至产犊的体重变化、产犊儒略日，以及第二个配种季节结束时的体重，均对产犊概率存在影响。即便肉用后备母牛已达到首个配种季节的推荐最低体重，其截至首个配种季节结束的平均日增重仍对产犊概率具有正向作用。在配种季节初期产犊且在首、次配种季节间体重增幅更高的初产母牛，在第二个配种季节产犊的概率更高。