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Factors Affecting Differences Between Milk Fat Tests Obtained in the Production Testing Programs and from Semimonthly Composited Samples^1,2,

Department of Veterinary and Animal Sciences, University of Massachusetts, Amherst 01002

ABSTRACT

Differences between the herd average Production Testing Program milk fat percentages (Official Dairy Herd Improvement, Dairy Herd Improvement Registry, and Owner-Sampler) and corresponding dealers' composite milk fat percentages were studied on 257 Holstein herds for 19 months. The milk fat test comparisons were: a) monthly Production Testing Program test versus composite test closest to Production Testing Program test day; b) Production Testing Program test versus weighted (by length of compositing period) composite for the month. Mean differences between the two sources of tests were 0.12 and 0.15%, respectively, with the Production Testing Program test being the higher in each case. Range tests were run on the least-squares means. Significant differences were found for: Official Dairy Herd Improvement and Owner-Sampler herds over Dairy Herd Improvement Registry herds; contract testing labs (smaller plants) over plant testing labs (larger plants); summer and early winter over late winter and spring; smaller herds over larger herds. The results indicated that real and important differences existed between these two sources of tests, with the Production Testing Program tests being higher in all comparisons. Absolute biases eould not be determined. However, the consistent direction of bias and the magnitude of biases associated with some of the sources of variation indicated that critical study of absolute biases in each source of test is needed.

¹ This study was made possible through the cooperation of the Federal Market Milk Administration, Boston, Massachusetts, Richard D. Aplin, Administrator, and Herbert E. Shepard and Richard F. Murphy, Assistants, who supplied data on semimonthly composite milk fat tests. The identity of all data used in this study was coded by the Federal Market Milk Administration to retain anonymity in accord with regulations.

² Results of this study were submitted by the senior author to the Graduate School, University of Massachusetts, in partial fulfillment of the requirements for the Master of Science degree.

³ Present address: Graduate Research Assistant, Department of Animal Science, North Carolina State University, Raleigh 27607.

⁴ Present address: Leader, Dairy Herd Improvement Investigations, Building 263, Agricultural Research Center, USDA, Beltsville, Maryland 20705.