Loker S, Miglior F, Bohmanova J, Schaeffer L R, Jamrozik J, Kistemaker G
Centre for Genetic Improvement of Livestock, University of Guelph, Guelph, Ontario, Canada, N1G 2W1.
J Dairy Sci. 2009 May;92(5):2270-5. doi: 10.3168/jds.2008-1806.
Preadjustment of phenotypic records is an alternative to accounting for the effect of pregnancy within the genetic evaluation model. Variance components used in the Canadian Test-Day Model may need to be re-estimated after preadjusting for pregnancy. The objective of this study was to assess the effect of preadjusting test-day yields on variance components and estimated breeding values using a random regression test-day model in a random sample of Ayrshire cows. A random sample of 981 Canadian Ayrshire cows from 18 complete herds (average of 54.5 cows/herd) was analyzed. Two data sets were created using the same animals, one with unadjusted milk, fat, and protein yields, and one data set with test-day records adjusted for pregnancy effects. Pregnancy effect estimates from a previous study were used for additive preadjustment of records. Variance components were estimated using both data sets. Results were very similar between the 2 data sets for all estimated genetic parameters (heritabilities, genetic, and permanent environmental correlations). The relative squared differences were very small: 0.05% for heritabilities, 0.20% for genetic correlations, and 0.18% for permanent environmental correlations. Furthermore, paired Student's t-tests showed that the differences between the genetic parameters of data sets adjusted and unadjusted for pregnancy effect were not significantly different from 0. Results from this study show that preadjusting data for pregnancy did not yield changes in covariance component estimates, thus suggesting that preadjusting test-day records could be a feasible solution to account for pregnancy in the Canadian Test-Day Model without changing the current model. Estimated breeding values (EBV) were calculated for both data sets to observe the impact of preadjusting for pregnancy. Overall, the largest changes in EBV seen when preadjusting for pregnancy (compared with unadjusted records) occurred for nonpregnant elite cows, whose EBV declined. Preadjusting for pregnancy before genetic evaluations improves the estimation of breeding values by adding the negative impact of pregnancy back onto pregnant cow test-day records, causing an increase in their production EBV.
对表型记录进行预调整是在遗传评估模型中考虑怀孕影响的一种替代方法。在对怀孕进行预调整后，加拿大测定日模型中使用的方差分量可能需要重新估计。本研究的目的是在艾尔夏奶牛的随机样本中，使用随机回归测定日模型评估对测定日产奶量进行预调整对方差分量和估计育种值的影响。分析了来自18个完整牛群（平均每群54.5头奶牛）的981头加拿大艾尔夏奶牛的随机样本。使用相同的动物创建了两个数据集，一个包含未调整的牛奶、脂肪和蛋白质产量，另一个数据集的测定日记录针对怀孕影响进行了调整。来自先前研究的怀孕影响估计值用于记录的加法预调整。使用这两个数据集估计方差分量。对于所有估计的遗传参数（遗传力、遗传相关性和永久环境相关性），两个数据集的结果非常相似。相对平方差非常小：遗传力为0.05%，遗传相关性为0.20%，永久环境相关性为0.18%。此外，配对学生t检验表明，针对怀孕影响进行调整和未调整的数据集的遗传参数之间的差异与0没有显著差异。本研究结果表明，对怀孕数据进行预调整不会导致协方差分量估计值发生变化，因此表明在加拿大测定日模型中对测定日记录进行预调整可能是一种可行的方法，无需改变当前模型即可考虑怀孕因素。为两个数据集计算了估计育种值（EBV），以观察对怀孕进行预调整的影响。总体而言，在对怀孕进行预调整时（与未调整的记录相比），EBV变化最大的是非怀孕的优秀奶牛，其EBV下降。在遗传评估前对怀孕进行预调整，通过将怀孕的负面影响重新加到怀孕奶牛的测定日记录上，从而提高了育种值的估计，导致其生产EBV增加。
