Department of Production Animal Health, Faculty of Veterinary Medicine, University of Calgary, Calgary, AB, Canada, T2N 1N4; Centre for Genetic Improvement of Livestock, Department of Animal Biosciences, University of Guelph, Guelph, ON, Canada, N1G 2W1; Canadian Bovine Mastitis and Milk Quality Research Network, St-Hyacinthe, QC, J2S 7C6, Canada.
Centre for Genetic Improvement of Livestock, Department of Animal Biosciences, University of Guelph, Guelph, ON, Canada, N1G 2W1; Canadian Bovine Mastitis and Milk Quality Research Network, St-Hyacinthe, QC, J2S 7C6, Canada; Canadian Dairy Network, Guelph, ON, Canada, N1K 1E5.
J Dairy Sci. 2018 Dec;101(12):11120-11131. doi: 10.3168/jds.2018-15126. Epub 2018 Oct 11.
Subclinical mastitis (SCM) causes economic losses for dairy producers by reducing milk production and leading to higher incidence of clinical mastitis and premature culling. The prevalence of SCM in first-lactation heifers is highest during early lactation. The objective of this study was to estimate genetic parameters for SCM in early lactation in first-parity Holsteins. Somatic cell count test-day records were collected monthly in 91 Canadian herds participating in the National Cohort of Dairy Farms of the Canadian Bovine Mastitis Research Network. Only the first test-day record available between 5 and 30 d in milk was considered for analysis. The final data set contained 8,518 records from first lactation Holstein heifers. Six alternative traits were defined as indicators of SCM, using various cutoff values of SCC, ranging from 150,000 to 400,000 cells/mL. Both linear and threshold animal models were used. Overall prevalence of SCM using the 6 traits ranged from 13 to 24%. Heritability estimates (standard error) from linear and threshold models ranged from 0.037 to 0.057 (0.015 to 0.018) and from 0.040 to 0.051 (0.017 to 0.020), respectively. We found strong genetic correlations (standard error) among alternative SCC traits, ranging from 0.90 to 0.99 (0.013 to 0.069), indicating that these 6 traits were genetically similar. Despite low heritability, based on estimated breeding values (EBV) predicted from both models, we noted exploitable genetic variation among sires. Higher EBV of SCM resistance corresponded to sires with a higher percentage of daughters without SCM. Based on a linear model (all 6 traits), percentage of daughters with SCM ranged from 5 to 13% and from 19 to 33% for the top 10% and worst 10% of 69 sires with minimum 20 daughters in at least 5 herds, respectively. Spearman's rank correlations among EBV of sires predicted from linear (from 0.75 to 0.95) and threshold (from 0.74 to 0.95) models were moderate to high, respectively. Very high rank correlations (0.98 to 0.99) between EBV predicted for the same trait from linear and threshold model indicated that reranking of sires based on model used was minimal. In conclusion, despite low heritability, we found utilizable genetic variation in early lactation of heifers. Hence, genetic selection to improve genetic resistance to SCM in early lactation of heifers was deemed possible.
隐性乳房炎（SCM）通过降低产奶量并导致更高的临床乳腺炎发病率和提前淘汰，给奶农造成经济损失。初产荷斯坦奶牛在泌乳早期的 SCM 患病率最高。本研究的目的是估计初产荷斯坦奶牛泌乳早期 SCM 的遗传参数。在加拿大奶牛乳房炎研究网络的国家奶牛队列中，有 91 个加拿大牧场每月收集体细胞计数测试日记录。仅考虑在产奶 5 至 30 天之间获得的第一个测试日记录进行分析。最终数据集包含 8518 份来自初产荷斯坦奶牛的记录。使用 SCC 的各种截断值（150,000 至 400,000 个细胞/ml）定义了 6 种替代特征作为 SCM 的指标。使用线性和阈值动物模型。使用 6 种性状的 SCM 总患病率为 13%至 24%。线性和阈值模型的遗传力估计值（标准误差）分别为 0.037 至 0.057（0.015 至 0.018）和 0.040 至 0.051（0.017 至 0.020）。我们发现替代 SCC 性状之间具有很强的遗传相关性（标准误差），范围为 0.90 至 0.99（0.013 至 0.069），表明这些 6 种性状在遗传上相似。尽管遗传力较低，但基于两种模型预测的估计育种值（EBV），我们注意到在雄性之间存在可利用的遗传变异。对 SCM 抗性的 EBV 越高，没有 SCM 的女儿比例就越高。基于线性模型（所有 6 种性状），在 69 头至少有 20 头女儿的雄性中，SCM 阳性女儿的比例从顶部 10%的 5%到最差 10%的 13%不等，在至少 5 个牧场中，SCM 阳性女儿的比例从顶部 10%的 19%到最差 10%的 33%不等。线性（0.75 至 0.95）和阈值（0.74 至 0.95）模型预测的雄性 EBV 之间的斯皮尔曼等级相关系数为中度至高度相关。基于线性和阈值模型预测同一性状的 EBV 之间非常高的等级相关系数（0.98 至 0.99）表明，基于使用的模型对雄性进行重新排序的程度最小。总之，尽管遗传力较低，但我们发现初产奶牛泌乳早期存在可利用的遗传变异。因此，认为可以通过遗传选择来提高初产奶牛泌乳早期对 SCM 的遗传抗性。
