Effects of Crossbreeding on Rate of Growth in Dairy Cattle

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Abstract
Body weights and 5 body measurements of 50 to 87 purebred Holsteins, purebred Guernseys, and reciprocal Holstein-Guernsey crosses were determined and recorded at 3, 6, 12, 18, 24, 30, 36, and 48 months of age. Purebreds and crossbreds were produced by alternate purebreeding and cross-breeding within a group of purebred Holstein and Guernsey foundation animals. The data were analyzed by the method of least squares to provide unbiased estimates of the effects of system of. mating, breed of dam, and breed of sire. The mean of the crossbreds exceeded that of the purebreds for all 6 measurements, at all ages, except for height at withers at 6 months of age. For body weight, heart girth, and paunch girth, which are largely measures of fleshiness, the differences between crossbreds and purebreds were significant at 18 and 24 months of age and also at 12 months of age for heart girth. For depth of chest, height at withers, and length of body, which are largely measures of skeletal development, the differences were not significant at any age. The significant effects associated with system of mating in the measurements of degree of fleshiness indicate a definite heterosis at the ages of 18 and 24 months. The data also suggest a differential action of the heterosis, such that increased heterozygosity apparently does not significantly increase the nonadditive genetic effects upon skeletal measurements, but does significantly increase rate of growth in measures of fleshiness to about 2 years of age. The breed of dam effect (the difference between progeny of Holstein dams and progeny of Guernsey dams) was significant at all ages for every measurement and was consistently larger than the breed of sire effect (the difference between progeny of Holstein sires and progeny of Guernsey sires). The latter effect became significant, generally, after about 12 months of age. This difference between the effects of breed of sire and breed of dam could be due to genetic differences between sires and dams of the same breed, but also suggests the possibility of an important prenatal environmental effect which persists from birth to maturity. The relative sizes of the effects of system of mating, breed of sire, and breed of dam indicate that, for all measurements, additive genetic factors are the predominant genetic influences upon rate of growth.