ABSTRACT In New Zealand cows are usually milked twice a day (TAD), however milking cows once-a-day (OAD) for the entire lactation is steadily increasing. This study aimed to estimate genetic correlations (r g) for milk production traits across TAD and OAD milking frequency (MF) environments from first lactation cows. Lactation yields of milk (MY), fat (FY) and protein (PY), and somatic cell score (SCS) of progeny from 1126 sires that were herd-tested during 2008–2012 were analysed. The r g estimates of the same trait expressed in the two MF were lower than unity but greater than 0.80. Genetic correlation between MY and PY across MF environments was high (0.79), moderate between MY and PY with FY (0.43–0.51), and low when the estimation involved SCS (<0.15). The results show that genetic evaluations made in TAD should include the G×MF effects, and although these effects are not expected to cause significant reductions in the efficiency of selection programmes in OAD systems for milk production traits in New Zealand dairy cattle. These estimates presented in this study can be used to predict genetic progress for the dairy population milked OAD when sires proven with daughters milked TAD are used in OAD herds.

中文翻译：

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新西兰第一次泌乳产奶性状跨挤奶频率的遗传参数

摘要 在新西兰，奶牛通常每天挤奶两次 (TAD)，但是在整个泌乳期间每天挤奶一次 (OAD) 的奶牛正在稳步增加。本研究旨在估计第一次泌乳奶牛在 TAD 和 OAD 挤奶频率 (MF) 环境中产奶性状的遗传相关性 (rg)。分析了 2008-2012 年间进行畜群测试的 1126 头公牛的泌乳量 (MY)、脂肪 (FY) 和蛋白质 (PY) 以及后代的体细胞评分 (SCS)。在两个 MF 中表达的相同性状的 rg 估计值低于 1 但大于 0.80。MF 环境中 MY 和 PY 之间的遗传相关性高 (0.79)，MY 和 PY 与 FY 之间的遗传相关性中等 (0.43–0.51)，当估计涉及 SCS 时则低 (<0.15)。结果表明，在 TAD 中进行的遗传评估应包括 G×MF 效应，尽管预计这些影响不会显着降低新西兰奶牛产奶性状的 OAD 系统选择程序的效率。本研究中提供的这些估计值可用于预测在 OAD 牛群中使用已证明用 TAD 的女儿挤奶的公牛时，挤奶 OAD 的奶牛种群的遗传进展。