Climatological and Genetic Effects on Milk Composition and Yield

Objectives were to quantify variations in milk from genetic and climatic effects in a subtropical climate. Monthly test day samples of milk from 1959 to 1977 from the Florida Agricultural Experiment Station were characterized for 17 variables and subjected to least squares analysis of variance. Maximum temperature, minimum temperature, minimum relative humidity, and solar radiation as a set accounted for significant portions of variability in all traits, from 1.2 to 14.5% with Holsteins and .5 to 18.1% with Jerseys. Yields were affected more by climate with Holsteins, whereas percentages were more affected with Jerseys. Heritabilities from variances of paternal half sisters for a mean of 10 monthly measurements ranged from .10 to .23 for Holsteins and .23 to .27 for Jerseys for yields, and .27 to .90 for Holsteins and .18 to .59 for Jerseys for percentages. Genetic correlations between yields ranged from .07 to .96 for Holsteins and .69 to .98 for Jerseys. Genetic correlation between milk yield and chloride percent was .51, – .82 between milk yield and feulgen-deoxyribonucleic acid reflectance percent (an inverse measure of somatic cell count), and –.88 between chloride percent and reflectance percent with Holsteins. Ratios of solids-not-fat and protein to fat were correlated positively with yields and negatively with percentages. Phenotypic correlations of reflectance percent with yields were positive and low, consistent in sign but variable in magnitude with percentages. Selection for milk yield should cause correlated increases in other yield traits and decreases in percentages.