The aim of this study was to estimate genetic associations between body weight (BW), scrotal circumference (SC), and age at first calving (AFC) in Guzera cattle using random regression models. The analyses used 159,366 and 23,780 records of BW and SC obtained between 335 and 724 days of age and divided into classes at intervals of 10 days, as well as 63,596 records of AFC. A multitrait animal model via Gibbs sampling was adopted that included random direct additive genetic, permanent environmental (only for BW and SC), and residual effects. The fixed effects considered were the contemporary group, age of cow at calving (linear and quadratic effect), and the fixed curve to model the average trend of the population on the age classes of the animals. For BW and SC, four models were compared using the deviance information criterion, with Legendre polynomials ranging from one to four coefficients (intercept, linear, quadratic, and cubic) for additive genetic and permanent environmental effects. The model with three coefficients (quadratic) was considered the most appropriate to describe the (co)variance structure of the traits analyzed. The mean heritability estimates ranged from 0.17 to 0.31 for BW and from 0.19 to 0.28 for SC across age classes. The mean heritability for AFC was 0.07. The genetic correlations between BW and SC across age classes were always positive and favorable, ranging from 0.33 to 0.39. These findings indicate that selection for higher BW should result in animals with greater SC. The genetic correlations between BW at different ages and AFC ranged from -0.56 to -0.38, while those between SC at different ages and AFC ranged from -0.55 to 0.08. Except for some genetic correlations close to zero and positive between SC close to 365 days of age and AFC, in general the results showed that the selection for either higher BW or greater SC should result in a reduction of AFC in Guzera cattle. Calculation of the relative selection efficiency indicated a greater correlated response in AFC by selection for SC, especially after 634 days of age, compared to direct selection for AFC.

本研究的目的是使用随机回归模型估计 Guzera 牛体重 (BW)、阴囊周长 (SC) 和首次产犊年龄 (AFC) 之间的遗传关联。分析使用了在 335 到 724 天之间获得的 159,366 和 23,780 条 BW 和 SC 记录，并以 10 天的间隔分组，以及 63,596 条 AFC 记录。采用吉布斯抽样的多性状动物模型，包括随机直接加性遗传、永久环境（仅适用于 BW 和 SC）和残留效应。所考虑的固定效应是当代组、产犊时的母牛年龄（线性和二次效应），以及用于模拟动物年龄等级的种群平均趋势的固定曲线。对于 BW 和 SC，使用偏差信息标准比较了四个模型，勒让德多项式的范围从一到四个系数（截距、线性、二次和三次）用于加性遗传和永久环境影响。具有三个系数（二次）的模型被认为最适合描述所分析性状的（协）方差结构。BW 的平均遗传力估计值范围为 0.17 至 0.31，SC 各年龄段的平均遗传力估计值范围为 0.19 至 0.28。AFC 的平均遗传率为 0.07。不同年龄段的 BW 和 SC 之间的遗传相关性始终是积极的和有利的，范围从 0.33 到 0.39。这些发现表明，选择更高的 BW 应该会导致动物具有更大的 SC。不同年龄BW与AFC之间的遗传相关范围为-0.56至-0.38，而不同年龄SC与AFC之间的遗传相关范围为-0.55至0.08。除了接近 365 日龄的 SC 与 AFC 之间的某些遗传相关性接近于零和正相关外，一般而言，结果表明，选择更高的 BW 或更大的 SC 应该会导致 Guzera 牛的 AFC 减少。相对选择效率的计算表明，与直接选择 AFC 相比，通过选择 SC，尤其是在 634 日龄后，AFC 中的相关反应更大。