PRODUCTION STUDIES ON EXPERIMENTAL FARMS DAIRY HERDS: I. AGE CORRECTION OF 180-DAY MILK YIELD

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Abstract
Regressions of 180-day yield on age, measured in months at freshening, were fitted to the data from each of 13 herds representing three breeds. The regressions were significantly different among herds. The use of the separate herd regression for establishing age-correction factors would result in an estimated 1.3 per cent increase in efficiency over the use of the single, average within-herd regression.In one herd, where both Holsteins and Ayrshires were maintained under similar conditions, it appeared that the Ayrshires were quicker maturing than the Holsteins. The study incidentally indicates that persistency of lactation decreases with increase in age at freshening.Seasonal variations in age-corrected 180-day yield indicated that comparisons among contemporary cows producing within 6-month periods are likely to eliminate the misleading effects of season of freshening on yield from estimates of breeding values.

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