AbstractThe effects of herd, year, age, season, and their interactions on milk and fat yield, fat percent, days in milk, days dry, and days open for purebred and high grade Holsteins in a tropical environment were determined by analysis of variance using DHIA records from Puerto Rico. There were 33,950 records for calvings from 1967 to 1973 in 62 herds. Herd effects were significant (P < .05) for all traits, but seasonal effects were significant only for milk yield, fat yield, and days open. Variation among herds significantly influenced all traits except milk yield. Age of calving affected all traits except lactation length and days open. A number of the interactions were also significant. The statistical model explained 39, 40, 24, 17, 13 and 12% of the total variation in milk yield, fat yield, fat percent, lactation length, days dry, and days open, respectively. Total variances were 12,900,000 lb2 for milk yield and 15,000 lb2 for fat yield, which are comparable to those for temperate areas. It is concluded that the factors affecting the performance of Holsteins in Puerto Rico do not differ markedly from those in temperate countries. The total variation appears large enough to permit selection for higher milk and fat yields. As in temperate regions, length of lactation, length of dry period, and time of rebreeding (days open) are largely influenced by decisions made on the part of herd owners.
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