Estimation of genetic and phenotypic trends for dairy traits of Gyr and Guzerá breeds

ABSTRACT. Milk production has high economic importance in dairy cattle production systems, which justifies its inclusion in genetic breeding programs. The objective of this study was to evaluate the genetic and phenotypic trends of dairy traits in Gyr (167 lactations) and Guzerá (148 lactations) breeds in the State of Rio Grande do Norte (RN). Milk, fat and protein production obtained from monthly dairy controls were the traits evaluated. The expected differences in progeny (DEPs) for these traits were estimated using an animal model. From the means of phenotypic values and DEPs, we estimated the genetic and phenotypic trends in linear regressions as a function of the year of calving of the animal. Positive genetic and phenotypic trends were observed for Gyr (26.8 and 88.7 kg year-1) and Guzerá (8.44 and 37.68 kg year-1). Gyr breed presented positive genetic trends for fat (1.08 kg year-1) and protein (0.45 kg year-1) production and the Guzerá breed showed positive genetic trends for fat (0.36 kg year-1) and protein (0.25 kg year-1) production. The selection applied in the evaluated herds has produced satisfactory genetic and phenotypic progress for milk, fat and protein production.摘要。牛奶产量在奶牛养殖体系中具有极高的经济价值，因此将其纳入遗传育种计划具备充分依据。本研究旨在评估巴西北里奥格兰德州（Rio Grande do Norte, RN）的吉尔牛（Gyr，167次泌乳记录）与古泽拉牛（Guzerá，148次泌乳记录）的产奶性状遗传趋势与表型趋势。本研究评估的性状为通过月度泌乳监测获得的产奶量、乳脂量与乳蛋白量。本研究采用动物模型（animal model）估计了上述性状的预期子代差异值（Expected Differences in Progeny, DEPs）。基于表型值与预期子代差异值的均值，本研究以动物产犊年份为自变量，通过线性回归分析估计了遗传趋势与表型趋势。吉尔牛与古泽拉牛均呈现正向遗传趋势与表型趋势，增速分别为26.8 kg·年⁻¹、88.7 kg·年⁻¹与8.44 kg·年⁻¹、37.68 kg·年⁻¹。吉尔牛的乳脂产量（1.08 kg·年⁻¹）与乳蛋白产量（0.45 kg·年⁻¹）均呈现正向遗传趋势，古泽拉牛的乳脂产量（0.36 kg·年⁻¹）与乳蛋白产量（0.25 kg·年⁻¹）亦呈现正向遗传趋势。受试牛群所实施的选择育种，在产奶量、乳脂产量与乳蛋白产量方面均取得了令人满意的遗传与表型进展。