Tiezzi Francesco, Valente Bruno D, Cassandro Martino, Maltecca Christian
Department of Animal Science, North Carolina State University, Raleigh, NC, 27695, USA.
Department of Animal Science, University of Wisconsin, Madison, WI, 53706, USA.
Genet Sel Evol. 2015 May 13;47(1):45. doi: 10.1186/s12711-015-0123-7.
Recently, selection for milk technological traits was initiated in the Italian dairy cattle industry based on direct measures of milk coagulation properties (MCP) such as rennet coagulation time (RCT) and curd firmness 30 min after rennet addition (a30) and on some traditional milk quality traits that are used as predictors, such as somatic cell score (SCS) and casein percentage (CAS). The aim of this study was to shed light on the causal relationships between traditional milk quality traits and MCP. Different structural equation models that included causal effects of SCS and CAS on RCT and a30 and of RCT on a30 were implemented in a Bayesian framework.
Our results indicate a non-zero magnitude of the causal relationships between the traits studied. Causal effects of SCS and CAS on RCT and a30 were observed, which suggests that the relationship between milk coagulation ability and traditional milk quality traits depends more on phenotypic causal pathways than directly on common genetic influence. While RCT does not seem to be largely controlled by SCS and CAS, some of the variation in a30 depends on the phenotypes of these traits. However, a30 depends heavily on coagulation time. Our results also indicate that, when direct effects of SCS, CAS and RCT are considered simultaneously, most of the overall genetic variability of a30 is mediated by other traits.
This study suggests that selection for RCT and a30 should not be performed on correlated traits such as SCS or CAS but on direct measures because the ability of milk to coagulate is improved through the causal effect that the former play on the latter, rather than from a common source of genetic variation. Breaking the causal link (e.g. standardizing SCS or CAS before the milk is processed into cheese) would reduce the impact of the improvement due to selective breeding. Since a30 depends heavily on RCT, the relative emphasis that is put on this trait should be reconsidered and weighted for the fact that the pure measure of a30 almost double-counts RCT.
最近，意大利奶牛养殖业开始基于对牛奶凝固特性（MCP）的直接测量来选择牛奶技术特性，如凝乳酶凝固时间（RCT）和添加凝乳酶30分钟后的凝乳硬度（a30），以及一些用作预测指标的传统牛奶质量特性，如体细胞评分（SCS）和酪蛋白百分比（CAS）。本研究的目的是阐明传统牛奶质量特性与MCP之间的因果关系。在贝叶斯框架下实施了不同的结构方程模型，这些模型包括SCS和CAS对RCT和a30的因果效应以及RCT对a30的因果效应。
我们的结果表明所研究的性状之间存在非零程度的因果关系。观察到SCS和CAS对RCT和a30的因果效应，这表明牛奶凝固能力与传统牛奶质量特性之间的关系更多地取决于表型因果途径，而不是直接取决于共同的遗传影响。虽然RCT似乎在很大程度上不受SCS和CAS的控制，但a30的一些变异取决于这些性状的表型。然而，a30在很大程度上取决于凝固时间。我们的结果还表明，当同时考虑SCS、CAS和RCT的直接效应时，a30的大部分总体遗传变异性由其他性状介导。
本研究表明，不应基于SCS或CAS等相关性状对RCT和a30进行选择，而应基于直接测量进行选择，因为牛奶的凝固能力是通过前者对后者的因果效应而提高的，而不是来自共同的遗传变异源。打破因果联系（例如在将牛奶加工成奶酪之前对SCS或CAS进行标准化）将减少选择性育种带来的改善的影响。由于a30在很大程度上取决于RCT，因此应重新考虑对该性状的相对重视程度，并考虑到a30的纯测量几乎重复计算了RCT这一事实进行加权。
