Genetic trends and trade-offs between growth and reproductive traits in a Nellore herd

The knowledge of genetic trends and trade-offs between growth and reproductive traits might be useful to understand the evolution of these traits in livestock and natural populations of animals. We estimated the genetic trends and trade-offs between pre-weaning growth and calving intervals of Nellore animals from a commercial farm. Two-trait animal models were used to estimate covariance components and breeding values (EBV) for direct and maternal genetic effects of pre-weaning growth and direct genetic effects of calving intervals. Regression analyses were performed to examine the relationship between direct and maternal EBV of pre-weaning growth and direct EBV of calving intervals (dependent variables) and the coefficient of generation of each animal (independent variable). We also performed regression analyses to examine the relationship between direct EBV of calving intervals (dependent variables) and direct and maternal EBV of pre-weaning growth (independent variables). The genetic trends for direct and maternal genetic effect for pre-weaning growth were significant and presented genetic evolution in the studied Nellore herd. The genetic trends for the reproductive traits were also significant but indicated genetic changes in an unfavorable way. The genetic correlations between direct effects of pre-weaning growth and calving intervals traits and the genetic correlations between maternal effects of pre-weaning growth traits and direct effects of calving interval traits were not different from zero. The presence of trade-offs between the direct effects of growth and reproductive traits were confirmed through regression from direct EBV of calving intervals over EBV of pre-weaning growth traits. In addition, regression analyses showed that selection to increase pre-weaning growth also increased calving intervals. Our results showed that pre-weaning growth and calving intervals are increasing over generations and that trade-offs occurred between those traits in the studied Nellore herd.解析生长与繁殖性状间的遗传趋势及权衡关系，可为理解家畜与动物自然种群中这些性状的演化历程提供理论支撑。本研究针对某商业养殖场的内洛尔牛（Nellore）群体，估算了其断奶前生长性状与产犊间隔间的遗传趋势及权衡关系。本研究采用两性状动物模型，估算断奶前生长性状的直接遗传效应与母体遗传效应、以及产犊间隔的直接遗传效应的协方差组分与估计育种值（EBV）。通过回归分析，探究了以各动物的世代系数为自变量时，断奶前生长性状的直接与母体估计育种值、以及产犊间隔的直接估计育种值作为因变量的相关关系。此外，本研究还开展了另一组回归分析，以断奶前生长性状的直接与母体估计育种值为自变量，探究其与产犊间隔的直接估计育种值（因变量）间的相关关系。断奶前生长性状的直接遗传效应与母体遗传效应的遗传趋势均显著，表明所研究的内洛尔牛群体存在遗传演化。繁殖性状的遗传趋势同样显著，但呈现出不利方向的遗传变化。断奶前生长性状直接效应与产犊间隔性状间的遗传相关，以及断奶前生长性状母体效应与产犊间隔直接效应间的遗传相关，均与零无显著差异。通过以产犊间隔直接估计育种值为因变量、断奶前生长性状估计育种值为自变量的回归分析，证实了生长性状直接效应与繁殖性状间存在权衡关系。此外，回归分析结果显示，针对断奶前生长性状的选择会同时延长产犊间隔。本研究结果表明，随着世代更迭，内洛尔牛群体的断奶前生长性状与产犊间隔均呈提升趋势，且两类性状间确实存在权衡关系。