THE EFFECTS OF DAYS DRY AND DAYS OPEN ON THE MILK AND BUTTERFAT PRODUCTION OF HOLSTEIN-FRIESIAN CATTLE

Abstract

The effect of days dry and days open on first, second and later lactation production was examined by intra-sire-herd-season regression. Neither variation in days dry nor days open accounted for more than 1% of the within-sire variance in fat test. Variation in preceding days dry accounted for 4.8% of the within-sire variance in milk production in second lactations but only 0.6% of the variance in later lactations. Differences in days open accounted for 4.5, 3.5, and 4.5% of the intra-sire variance in milk production in first, second, and later lactations, respectively. The effects of days dry and days open on fat production were, in all cases, slightly less than the effects on milk production. Heritabilities for both traits were low, indicating that the effects of both days dry and days open on production were largely of an environmental nature. Adjustment of milk yield for the number of days cows are open could lead to more accurate selection of dairy animals.