Variation in Milk Fat Production in State-Owned Herds in Iowa1,2

Abstract

Summary This investigation was of 12,623 records made by 4,487 cows from 1940 through 1956 in 12 state-owned herds in Iowa. Herd differences and herd-by-year interactions were responsible for about 6 and 10% of the total variance, respectively. Years, seasons, and the other interactions among these three mainly environmental factors were of minor importance individually; together they accounted for 4% of the total variance. The records were corrected for the major environmental effects by taking each record as a deviation from the average of all records made in that herd in that same year. Then, for obtaining sire and dam components, etc., only the cows whose sires and dams each had at least two daughters in the herd were used. This analysis involved 7,003 records by 2,398 daughters of 295 sires and was made in three different ways: (1) hierarchal classification of sires, dams within sires, etc.; (2) full sister analysis; and (3) cross classification by sires and by dams. Four times the sire component in the first method gave 0.16 as the intraherd heritability of differences in butterfat production. By the second method, twice the full sister component gave 0.12 as the estimate. By the first method 1.6% of the total variance was attributed to dominance deviations and other things, such as maternal effects, that make full sisters resemble each other more than twice as much as paternal half-sisters. In the cross classification, the component for sire-by-dam interaction was 1.8%. These results suggest that deviations from the additive scheme of inheritance are unimportant, but these estimates of nonadditive genetic effects were not precise.