Department of Animal Nutrition and Production, School of Veterinary Medicine and Animal Sciences, University of São Paulo (USP), Pirassununga, São Paulo, 13635-900, Brazil.
Department of Animal Science, Macdonald Campus, McGill University, H9X 3V9, Quebec, Canada.
J Dairy Sci. 2018 May;101(5):4357-4366. doi: 10.3168/jds.2017-13286. Epub 2018 Feb 15.
The reduction of milk production caused by subclinical mastitis in dairy cows was evaluated through the regression of test-day milk yield on log-transformed somatic cell counts (LnSCC). Official test-day records (n = 1,688,054) of Holstein cows (n = 87,695) were obtained from 719 herds from January 2010 to December 2015. Editing was performed to ensure both reliability and consistency for the statistical analysis, and the final data set comprised 232,937 test-day records from 31,692 Holstein cows in 243 herds. A segmented regression was fitted to estimate the cutoff point in the LnSCC scale where milk yield started to be affected by mastitis. The statistical model used to explain daily milk yield included the effect of herd as a random effect and days in milk and LnSCC as fixed effects regressions, and analyses were performed by parity and stage of lactation. The cutoff point where milk yield starts to be affected by changes in LnSCC was estimated to be around 2.52 (the average of all estimates of approximately 12,400 cells/mL) for Holsteins cows from Brazilian herds. For first-lactation cows, milk losses per unit increase of LnSCC had estimates around 0.68 kg/d in the beginning of the lactation [5 to 19 d in milk (DIM)], 0.55 kg/d in mid-lactation (110 to 124 DIM), and 0.97 kg/d at the end of the lactation (289 to 304 DIM). For second-lactation cows, milk losses per unit increase of LnSCC had estimates around 1.47 kg/d in the beginning of the lactation (5 to 19 DIM), 1.09 kg/d in mid-lactation (110 to 124 DIM), and 2.45 kg/d at the end of the lactation (289 to 304 DIM). For third-lactation cows, milk losses per unit increase of LnSCC had estimates around 2.22 kg/d in the beginning of the lactation (5 to 19 DIM), 1.13 kg/d in mid-lactation (140 to 154 DIM), and 2.65 kg/d at the end of the lactation (289 to 304 DIM). Daily milk losses caused by increased LnSCC were dependent on parity and stage of lactation, and these factors should be considered when estimating losses associated with subclinical mastitis.
本研究通过对奶牛隐性乳腺炎测试日产奶量与体细胞计数（SCC）的对数转换（LnSCC）进行回归，评估了亚临床乳腺炎导致的产奶量减少。从 2010 年 1 月至 2015 年 12 月，从 719 个牧场获得了荷斯坦奶牛（n = 87695）的官方测试日记录（n = 1688054）。为了保证统计分析的可靠性和一致性，对数据进行了编辑，最终数据集包括了 232937 个来自 243 个牧场的 31692 头荷斯坦奶牛的测试日记录。使用分段回归来估计 LnSCC 水平上开始影响产奶量的临界点。用于解释每日产奶量的统计模型包括将牛群作为随机效应，以及产奶天数和 LnSCC 作为固定效应回归的影响，并且通过胎次和泌乳阶段进行分析。巴西牛群的荷斯坦奶牛的产奶量开始受到 LnSCC 变化影响的临界点估计约为 2.52（约 12400 个细胞/ml 的所有估计值的平均值）。对于初产奶牛，每增加单位 LnSCC，产奶量的损失估计在泌乳早期[5-19 天（DIM）]约为 0.68kg/d，泌乳中期[110-124 DIM]约为 0.55kg/d，泌乳后期[289-304 DIM]约为 0.97kg/d。对于二胎奶牛，每增加单位 LnSCC，产奶量的损失估计在泌乳早期[5-19 DIM]约为 1.47kg/d，泌乳中期[110-124 DIM]约为 1.09kg/d，泌乳后期[289-304 DIM]约为 2.45kg/d。对于三胎奶牛，每增加单位 LnSCC，产奶量的损失估计在泌乳早期[5-19 DIM]约为 2.22kg/d，泌乳中期[140-154 DIM]约为 1.13kg/d，泌乳后期[289-304 DIM]约为 2.65kg/d。由于 LnSCC 增加而导致的每日产奶量损失取决于胎次和泌乳阶段，在估计与亚临床乳腺炎相关的损失时应考虑这些因素。
