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.

亚临床乳腺炎（SCM）通过减少牛奶产量并导致临床乳腺炎和过早淘汰的较高发生率，给乳品生产者造成经济损失。泌乳早期，初生母牛的SCM患病率最高。这项研究的目的是估计在第一胎的荷斯坦牛泌乳初期SCM的遗传参数。每月从参与加拿大牛乳腺炎研究网络的国家奶牛场的91个加拿大牛群中收集体细胞计数测试日记录。仅考虑在5到30天之间的牛奶中的第一个测试日记录进行分析。最终数据集包含来自第一个哺乳期荷斯坦小母牛的8,518条记录。使用SCC的不同截断值（范围从150,000到400）定义了六个替代性状作为SCM的指标，000细胞/ mL。使用线性和阈值动物模型。使用6个特质的SCM总体患病率在13％至24％之间。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）。我们发现其他SCC性状之间的遗传相关性很强（标准误），范围从0.90至0.99（0.013至0.069），表明这6个性状在遗传上相似。尽管遗传力较低，但根据这两个模型预测的估计育种值（EBV），我们注意到了父系中可利用的遗传变异。具有较高SCM抗性的EBV对应于没有SCM的子代具有较高百分比的父亲。基于线性模型（所有6个特征），患有SCM的女儿的百分比从69％的父亲的前10％和最差的10％的5％到13％和19％到33％不等，至少有5个畜群中有至少20个女儿。从线性模型（从0.75到0.95）和阈值模型（从0.74到0.95）预测的父系EBV之间的Spearman等级相关性分别为中度到较高。根据线性和阈值模型预测的相同性状的EBV之间具有很高的等级相关性（0.98至0.99），表明基于所使用的模型对父系进行重新排名很少。总之，尽管遗传力较低，但我们在小母牛的早期泌乳中发现了可利用的遗传变异。因此，认为在小母牛的早期哺乳中进行遗传选择以提高对SCM的遗传抗性是可能的。从线性模型（从0.75到0.95）和阈值模型（从0.74到0.95）预测的父系EBV之间的Spearman等级相关性分别为中度到较高。根据线性和阈值模型预测的相同性状的EBV之间具有很高的等级相关性（0.98至0.99），表明基于所使用的模型对父系进行重新排名很少。总之，尽管遗传力较低，但我们在小母牛的早期泌乳中发现了可利用的遗传变异。因此，认为在小母牛的早期哺乳中进行遗传选择以提高对SCM的遗传抗性是可能的。从线性模型（从0.75到0.95）和阈值模型（从0.74到0.95）预测的父系EBV之间的Spearman等级相关性分别为中度到较高。根据线性和阈值模型预测的相同性状的EBV之间具有很高的等级相关性（0.98至0.99），表明基于所使用的模型对父系进行重新排名很少。总之，尽管遗传力较低，但我们在小母牛的早期泌乳中发现了可利用的遗传变异。因此，认为在小母牛的早期哺乳中进行遗传选择以提高对SCM的遗传抗性是可能的。根据线性模型和阈值模型预测的相同性状的EBV之间的差异（99）表明基于所使用模型的父系重新排名很少。总之，尽管遗传力较低，但我们在小母牛的早期泌乳中发现了可利用的遗传变异。因此，认为在小母牛的早期哺乳中进行遗传选择以提高对SCM的遗传抗性是可能的。根据线性模型和阈值模型预测的相同性状的EBV之间的差异（99）表明基于所用模型对父系进行重新排序的可能性很小。总之，尽管遗传力较低，但我们在小母牛的早期泌乳中发现了可利用的遗传变异。因此，认为在小母牛的早期哺乳中进行遗传选择以提高对SCM的遗传抗性是可能的。