Genetic and Environmental Influences upon Solids-Not-Fat Content of Cow's Milk

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Abstract
Monthly composite milk samples from the Holstein and Guernsey herds of the New Jersey Agricultural Experiment Station were taken from July, 1953, to February, 1958. Solids-not-fat (SNF) content was determined by formulas based on specific gravity (Watson Pattern Lactometer) and fat percentage (Babcock fat test). A study of 4772 samples from 222 cows revealed no specific yearly trends in SNF content, although 1954 was significantly lower with Holsteins, and 1956 significantly higher with Guernseys, than the averages of the remaining years. Monthly effects were significant and similar for both breeds. SNF content was high in December and January and low in June and July. Although stage-of-lactation effects were more extreme on Holsteins, SNF content of both, breeds was high shortly after parturition, dropped to a lactation low at 40 to 60 days, increased very slowly to 6 months, and then increased rapidly to 10 months. Pregnancy effects were noticeable at the 4th or 5th month of gestation, and accounted for most of the SNF increase in the latter stages of lactation. Two types of lactation averages were calculated: (1) based on all tests from parturition to 305 days, and (2) based on the 2d through 6th tests. Records from 182 cows by 50 sires were studied. SNF content of 310 Holstein records averaged (1) 8.48 and (2) 8.42%; 110 Guernsey records averaged (1) 8.95 and (2) 8.93%, respectively. SNF content decreased with age in a curvilinear manner. Intra-cow repeatabilities were (1) 0.61 and (2) 0.56, showing the 305-day record (1) to be slightly more desirable. Heritability estimates calculated from paternal half-sib correlations were 0.93 and 0.97. Doubling the intra-breed dam-daughter regression coefficient obtained from 87 dam-daughter pairs resulted in a heritability estimate of 0.57.