Effect of Age and Month-of-Calving on Solids-Not-Fat and Protein Yield for Five Dairy Breeds

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Abstract
Lactation records of solids-not-fat or protein from herds enrolled in the National Cooperative Dairy Herd Improvement Program from 1966-1975 were used to study the effect of age and month-of-calving on milk, fat, solids-not-fat and protein yields. Solids-not-fat data were in 183,282 records from 106,411 cows in 2215 herds in 41 states. Averages for Ayrshire, Brown Swiss, Guernsey, Holstein and Jersey breeds were 8.62, 9.04, 9.13, 8.54 and 9.36% solids-not-fat. Standard deviations among cows were .45, .43, .51, .45 and .64. Averages in 21,534 lactations were 3.37, 3.54, 3.64, 3.22 and 3.89% protein, and standard deviations were .20, .26, .27, .23 and .28. Effects of herd-years accounted for a similar percentage of variation in 305 day milk, fat and solids-not-fat yields ranging from 23-45% in the 5 breeds. Removal of effects of age raised multiple correlation squared from 5-18% and month-of-calving effects from 2-4% beyond that for herd-year. Interaction of age by month-of-calving for Guernseys, Holsteins, and Jerseys only raised multiple correlation squared from .2-.5%, nil in Ayrshires and Brown Swiss. Because of the limited numbers of records in some breeds, the interaction of age and month-of-calving probably should be ignored to allow more precision in adjusting main effects. Adjustment factors are given for solids-not-fat for 4 breeds.

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