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The performance was compared of cows sired by Holstein, Jersey or Brown Swiss bulls out of Holstein-Friesian x Gir dams of 1/2 to 3/4 Holstein-Friesian content. The animals were kept in a single herd under the same management. The data were analysed by least-squares techniques under a model that included the fixed effects of breed of sire. Bos taurus fraction of the dam, parity, year and season of calving, and a random cow effect. Based on 480 observations, the milk yields per lactation for the Holstein, Jersey and Brown Swiss sired groups were 2,821 +/- 163, 2,320 +/- 61 and 2,418 +/- 119 kg, respectively. The corresponding means for fat yield per lactation were 96.9 +/- 6.6, 86.8 +/- 2.5 and 92.8 +/- 4.8 kg; for protein yield per lactation were 85.3 +/- 5.1, 71.3+/-1.9 and 76.3 +/- 3.7 kg; for lactation length, 339 +/- 18, 283 +/- 7 and 313 +/- 14 days for fat percentage. 3.37 +/- 0.10, 3.73 +/- 0.04 and 3.77 +/- 0.07%; and for protein percentage, 3.02 +/- 0.05, 3.10 +/- 0.02 and 3.16 +/- 0.04%. The respective calving intervals were 487 +/- 24, 408 +/- 11 and 461 +/- 245 days. The yields of milk and protein per day of calving interval were similar in the Jersey and Holstein sired groups, while the former had higher yields of fat, implying that production economics might favour the smaller Jersey crosses in production systems in which the males were not reared. Crossing with Brown Swiss did not improve performance over the Holstein sired crosses.

The effects of procedures generally used to reduce variation of lactation length on the efficiency of selection for milk yield are examined applying existing theory to a set of average literature estimates of heritabilities and correlations between lactation yield and length. Adjustment of milk yield for lactation length should be expected to remove more genetic than phenotypic variation, thus reducing selection efficiency in relation to unadjusted yield. Selecting individuals on an optimum index of lactation yield and length would be more efficient for improving yield than selecting on yield alone, while both criteria would have practically the same efficiency for selection on progeny test. This result could be applied to reduce milk recording frequency without losing selection accuracy. Culling on lactation length before selecting on yield would have little effect on individual selection efficiency. However, excluding short lactation records should be expected to reduce both selection accuracy of the progeny test and genetic variation in yield.