Regression Factors for Extending Part Lactation Milk Records

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Abstract
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 9036 Holstein cows in 374 herds in 5 New York counties. The best single months for estimating a complete lactation are the 4th, 5th, and 6th months. The correlation between the predicted record and the complete record is 0.85 for testing in the 4th, 5th or 6th months. If sequential monthly tests are used for prediction, the multiple correlation of the first 5 monthly tests with complete lactations is 0.92, for 6 months 0.94, and for 7 months 0.96. Factors developed from the cumulative test day production are only slightly less accurate. The correlation of the sumof the first 7 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 1st monthly records of a lactation are missing. Bimonthly tests when extended with multiple regression are highly correlated with total lactation yield. 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 2 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.