Regression Factors for Extending Part Lactation Milk Records

Abstract

Summary Regression factors for extending part lactation milk records are developed from the within herd analysis of age-at-calving and season-of-calving corrected monthly test-day records of 9,036 Holstein cows in 374 herds in five New York counties. The best single months for estimating a complete lactation are the 4th, 5th, and 6th mo. The correlation between the predicted record and the complete record is 0.85 for testing in the 4th, 5th, or 6th mo. If sequential monthly tests are used for prediction, the multiple correlation of the first five monthly tests with complete lactations is 0.92, for 6 mo. 0.94, and for 7 mo. 0.96. Factors developed from the cumulative test-day production are only slightly less accurate. The correlation of the sum of the first seven monthly tests, with total production, is 0.95. Regression factors are also developed for extending monthly records by the use of multiple factors and cumulative months when the first monthly records of a lactation are missing. Bimonthly tests when extended with multiple regression factors are highly correlated with total lactation yield. The multiple correlations for both bimonthly testing possibilities are 0.98. Estimates based on cumulative bimonthly test records are also highly correlated (0.98) with complete yield. The best two trimonthly testing possibilities have multiple correlations of 0.96, with complete lactations. The correlations between cumulative trimonthly tests and complete yield ranged from 0.92 to 0.96. Bimonthly testing appears to be satisfactory for use, since the accuracy of prediction is extremely high and since the cost of a bimonthly testing program should be much lower than the existing once-a-month programs.