Correlated Genetic Responses of Feed Efficiency, Growth, and Body Size in Cattle Selected for Milk Solids Yield

Abstract

Correlated genetic responses in feed consumption, feed efficiency, growth, and body size in Holstein and Ayrshire cattle selected for 180-day milk solids yield were estimated. The two breeds differed in correlated responses to selection. Growth from 180 to 240 days of age increased significantly in both breeds with increased feed efficiency of the Holsteins but increased feed consumption of the Ayrshires. Efficiency of total digestible nutrient utilization for production of fat-corrected milk improved in Holsteins but not in Ayrshires. This increase in efficiency of Holsteins indicated that efficiency of milk production may be improved by selection for yield.Heritabilities of gain, feed use, and body size during the growth period were from .31 to .81. Heritability of fat-corrected milk production was .63 for Holsteins and .69 for Ayrshires. Heritabilities of other measures during lactation were from .22 to .90 with differences between breeds in all measures. Phenotypic relationships indicated that the faster growing heifers were taller, lighter, and consumed more feed. Animals producing the most milk were heavier at calving, gained more weight during lactation, and consumed more feed.During the course of the selection experiment there was an indication of a genetic trend toward a presumably optimum body weight and wither height with the Holsteins decreasing and the Ayrshires increasing.