A comparison of growth rate in twin cattle of eight breeds and crosses

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Abstract
SUMMARY 1. Rates of live-weight gain from 2 to 96 weeks of age are compared in twin cattle of 8 breeds and crosses. The 258 animals in the trial consisted of 77 pairs of one-egg (MZ) and 52 pairs of two-egg (DZ) twins. All were females. 2. Between collection during the first week after birth, and 36 weeks of age, treatment of animals was uniform. After this some animals were grazed and others remained housed, depending on their date of birth. Members of each twin pair were, however, always treated alike. Between 36 and 96 weeks of age grazed stock gained 62 lb. more weight than housed stock. This difference was allowed for in making breed comparisons. 3. The trial period was divided into six stages. The most rapid gains were made between 24 and 36 weeks of age (1·43 lb. per day) and between 36 and 52 weeks (1·48 lb. per day). Earlier and later growth rates were about 30% less than these. 4. Rates of gain varied from 1·10 lb. per day in Ayrshires to 1·38 lb. per day in Friesian × Shorthorn crosses. The Friesian and its crosses with Shorthorn generally made fastest gains throughout the trial. The Ayrshire was slower than the rest. The Shorthorn and its crosses with Hereford and Ayrshire were intermediate in performance. 5. Ratios of within-pair variance in MZ and DZ twins indicate that differences in performance are most strongly dependent on genetic differences-at 12 to 24 and 52 to 72 weeks of age.