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

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 five major dairy breeds in the United States, smnmarized with daughter-herdmate comparisons in 1965. Twenty groups of sires were studied with multiple regression techniques which took into consideration representative 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 representative pricing system, variations in milk fat percentage were 30, 32, 43, 47, and 37% 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% 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 1) a high level of production, 2) low milk fat percentage, 3) low blend price paid for the milk, 4) high milk fat differential, and 5) basing the blend price on a high milk fat percentage The present United States Department of Agriculture (USDA) sire evaluation program is based on herdmate differences in milk and milk fat yield. Because milk yield is believed to be the most important economic trait for most dairymen and the heritability of milk yield is high enough to make progress by selection