Genetic evolution of milk yield, udder morphology and behavior in Gir dairy cattle

ABSTRACT The objective of this study was to analyze the genetic evolution of milk production, udder morphology, and animal behavior of Gir dairy cattle. Data were obtained on the predicted breeding values for milk yield of 60,226 cows born from 1968 to 2015 and standardized predicted breeding values for udder conformation and milking behavior of 7,635 cows born from 1987 to 2015. The influence of tested bulls on genetic trends was discussed, with special focus on the period after 1993, the year when the predicted breeding values of the first group of tested bulls were released. The traits of interest were 305-d milk yield, fore udder attachment, rear udder height, rear udder width, teat length, teat thickness, ease of milking, and reactivity. Significant genetic evolution was observed for 305-d milk yield, with increasing annual growth observed from 1993, as the number of cows sired by the best proven bulls increased. Before 1993, no genetic evolution was observed for any of the studied traits. From 1993 onward, all traits showed genetic gains, except fore udder attachment. The use of genetically-tested bulls since 1993 has led to important genetic gains in the Brazilian Dairy Gir population. Before the identification of genetically superior bulls began, many traits showed no improvement. Selection for milk yield has modified the conformation traits of the Gir mammary system. The longevity of the mammary glands of high-yield cows may be impaired due to poor fore udder attachment. Regarding behavioral traits, this population has evolved to less-reactive animals that are easier to milk.

摘要
本研究旨在分析吉尔乳牛（Gir dairy cattle）的产奶性能、乳房形态与动物行为的遗传演化。本研究采集了1968年至2015年间出生的60226头奶牛的产奶量预测育种值，以及1987年至2015年间出生的7635头奶牛的乳房体型与挤奶行为标准化预测育种值。研究探讨了经后裔测定的种公牛对遗传趋势的影响，特别聚焦于1993年之后的阶段——该年首批经后裔测定的种公牛的预测育种值正式发布。

本研究关注的性状包括305天产奶量、前乳房附着强度、后乳房高度、后乳房宽度、乳头长度、乳头粗度、挤奶难易度与反应性。研究观测到305天产奶量存在显著的遗传演化，自1993年起年增幅持续提升，这与优秀经后裔测定种公牛的配种母牛数量增加密切相关。1993年之前，所有研究性状均未观测到遗传演化。1993年之后，除前乳房附着强度外，其余所有性状均实现了遗传增益。

自1993年起使用经过遗传测定的种公牛，推动巴西吉尔乳牛种群实现了显著的遗传增益。在遗传优势种公牛的筛选工作开展之前，多数性状均未得到改善。针对产奶量的选择已改变了吉尔乳牛的乳房系统体型性状。高产奶牛的乳房寿命可能因前乳房附着不良而受损。就行为性状而言，该种群已演化出反应性更低、更易于挤奶的个体。