Relation of Lactation Milk Production in Dairy Cows to Maximum Initial Milk Yield and Persistency of Lactation

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Abstract
A correlation study was performed on "total", "among-lactations," and "among-cows-in-lactation" bases, with 139 lactation records from 93 Holstein cows and 57 lactation records from 34 Guernsey cows, to determine the effects of maximum initial milk yield and persistency of lactation on lactation milk production. Since the relationships among these factors were similar in the 2 breeds, combined correlations were obtained on an "among-lactations-in-breed" and "among-cows-in-lactation-breed" basis. Maximum initial milk yield and persistency were highly correlated with lactation milk production (0.81 and 0.32, respectively) on an "among-cows-in-lactation-in-breed" basis; whereas, they had no correlation with each other (0.04), suggesting that maximum initial milk yield and persistency were independent factors which collectively influence lactation milk production in a positive way (R = 0.86). On an "among-lactations-in-breed" basis, lactation milk production and persistency were negatively correlated (-0.72), which expresses the observed effects of age of maturity on this relationship. In an analysis of the variation association with lactation milk production on an "among-cows-in-lactation-in-breed" basis, the effects associated with the correlation of maximum initial milk yield accounted for 65.6% of the total variation; whereas, the additional effects due to persistency contributed 8.5% to the total variation. Similarly, on an "among-lactations-in-breed" basis, the effects associated with the correlation of maximum initial milk yield with lactation milk production accounted for 92.1% of the total variation, and the effects due to persistency contributed an additional 0.9% to the total variation.