Relative Economic Importance of Milk and Milk Fat Percentage in Selecting Artificially Inseminated Proven Dairy Sires

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Abstract
The relative economic importance of the genetic changes in milk yield and milk fat percentage attributed to individual sires was investigated for artificially inseminated (AI) proven sires of the 5 major dairy breeds in the USA which were summarized with daughter-herd-mate comparisons in 1965. Twenty groups of sires were studied with multiple regression techniques which took into consideration represen-tative pricing systems, breed averages, and sire effects for milk yield and milk fat percentage expressed as differences from herdmates for their AI daughters. For the entire population of sires, and a represen-tative pricing system, variations in milk fat percentage were 30, 32, 43, 47, and 37 percent as important as were variations in milk yield for Ayrshires, Brown Swiss, Guernseys, Holsteins, and Jerseys, respectively. Within the top third of the sires on milk production only, variations in milk fat percentage were 54, 55, 89, 82, and 58 percent as important as were variations in milk yield for Ayrshires, Brown Swiss, Guernseys, Holsteins, and Jerseys, respectively. Sires of the Guernsey, Jersey, and Holstein breeds summarized for the first time in 1965 were similar to the entire population of sires in the relative importance of milk yield and milk fat percentage. Interrelated factors which tended to make variations in milk fat percentage have a relatively high economic importance were: a high level of production, low milk fat percentage, low blend price paid for the milk, high milk fat differen-tial, and basing the blend price on a high milk fat percentage.

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