A multibreed cattle experiment involving 25 British breeds was set up to study genetic variation between breeds and genetic inter-breed relationships for a wide spectrum of traits in order to examine the problems of between-breed testing and selection. The experimental design is described and results on between-breed variation are presented for four traits.All animals were housed indoors and from 12 weeks of age were given a single complete pelleted diet ad libitum through a system of Calan-Broadbent electronic gates. Females were mated to produce one purebred and three crossbred calves, which were reared to slaughter in order to measure the efficiency of the cow-calf unit of production.Results based on a total of 292 animals, with an average of 12 per breed, are presented for body weight, cumulated voluntary food intake, daily weight gain and daily food intake over the age range from 12 to 72 weeks. The 25 breed-mean curves for body weight and cumulated food intake displayed a remarkably uniform pattern of rankings at all ages and the rankings were very similar for both traits.The multibreed design used was effective in estimating between-breed variation as a proportion of total variation for the four traits examined. After approximately 1 year of age, the proportion of variation between breeds was approximately 0·70 for body weight and 0·60 for cumulated voluntary food intake. Changes in these traits could therefore be brought about more effectively by selection between breeds rather than within breeds. For average daily weight gain measured over 12-week intervals, between-breed selection was estimated to be most effective in the period of maximum growth rate between 6 and 9 months of age, when between-breed variation was 0·52 of the total. For average daily food intake, measured over 12-week intervals, between-breed selection was likely to be effective beyond 6 months of age, when the proportion of between-breed variation plateaued at 0·48.At all ages, the coefficient of genetic variation between breeds was approximately 0·14 for body weight and daily gain, and remarkably constant at approximately 0·12 for both daily and cumulated food intake. It is suggested that, for growth and intake traits, the genetic variances within and between breeds remain proportional to each other at all ages.
SUMMARYIn a long-term experiment, the performance of British Friesian, Ayrshire and Jersey cattle and their first and second crosses was evaluated over 526 first and 477 second lactations. Live weight at 18 mo of age and the ratios of lactation yield to live weight were examined as well as milk yield and composition of milk.Percentage heterosis in each combination of two pure breeds was estimated. For the three first crosses combined, heterosis for 305-day milk yield averaged 6·4% in first and 3·7% in second lactations. Heterosis in live weight at 18 mo averaged 2·9%; none was found in the Ayrshire × British Friesian crosses, but the others averaged 4·7%. Second crosses overall had an average live weight near to that of first crosses, but their milk yields were lower and, in the combined lactations, approximately equalled the purebreds' total.Components of variance due to breed and environmental factors were calculated. Breed accounted for 50% of variance in live weight at 18 mo but less than 10% of variance in milk yield.The influence of fertility on heterosis is discussed with reference to earlier findings, and economic implications are considered.
SUMMARY1. 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.
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