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Journal of Dairy Science Vol. 43 No. 7 951-957
© 1960 by American Dairy Science Association ®
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Errors in Estimation of Lactational Yields of Milk, Fat, and Solids-Not-Fat from Individual Cows

N. R. Thompson, W. K. Stone, G. C. Graf, C. Y. Kramer and R. J. Freund

Departments of Dairy Science and Statistics, Virginia Agricultural Experiment Station, Blacksburg

ABSTRACT

Individual milkings from 12 Holstein-Friesian cows were weighed and sampled daily through entire lactation periods. Samples from single milkings were combined to make one-day composite samples, which were analyzed for fat and solids-not-fat. Both milk yield and composition varied with stage of lactation. Percentages of fat and total solids in the composite samples were affected significantly by failure to take aliquot volumes, but percentage of solids-not-fat was not changed. Standard errors of estimate for the Babcock test and Watson lactometric procedure were ± 0.05% fat, ± 0.07% total solids, and ± 0.04% solids-not-fat. Bias for the Watson lactometric procedure, over whole lactations, was -0.02% total solids. This bias changed with stage of lactation. The large and small model Watson lactometers gave results that did not differ significantly.

Sampling errors for lactational yields of milk, fat, and solids-not-fat were 273, 13, and 26 lb., respectively. When expressed as coefficients of variability, the sampling errors for fat and solids-not-fat yields did not differ significantly. The monthly sampling procedure used in Dairy Herd Improvement Associations gave unbiased estimates for yields of milk, fat, and solids-not-fat. The final or total error of estimate consisted principally of sampling error.






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