UBC Theses and Dissertations
Studies of milk production in crossbred dairy cattle Okumu Awili, Charles
The main objectives of this investigation were to determine class averages for crossbreds and for backcrosses, to study heterosis, results of polyallel analysis, sire combining ability and age of maximum milk production in crosses. Production records (60,931) of dairy cattle under test by the Dairy Herd Improvement Association in British Columbia for the years 1958-1961 inclusive were used. Out of these records, 56,765 were of purebreeds (including high grades), 3113 of cross breds and 1053 of backcrosses. To decide class averages for crossbreds, a t-test and F-test (using the average of milk and fat indexes for t-test and fat-corrected-milk values for F-test) were carried out. There were no significant differences between production records of various groups of crossbreds at 5% level of probability in both the tests. Therefore, these data indicate that a single set of class averages may be recommended for all crossbred dairy cows. Fat-corrected-milk values for purebreeds, crossbreds and backcrosses were compared. Nine out of 12 comparisons made indicated that backcrosses were closer in production to the parental breed used in backcrossing than to the crossbreds of the breeds concerned. Therefore, for the purposes of establishing base averages and of calculating indexes, it is considered that backcrosses should be classified with the parental breed used in backcrossing. Fat-corrected-milk values were also used for a comparison of purebreeds and their corresponding crossbreds to detect the presence or absence of heterosis. Hybrid vigour was demonstrated in all crossbreds except Holstein-Ayrshire. A polyallel analysis was applied to the data using the average of milk and fat indexes for the two-year-old and three-year-old production records of the crossbreds. It was found that the general combining ability and reciprical effect were highly significant. Specific combining ability was significant only at the 5% level. Heritability was calculated for fat-corrected-milk values and found to be 0.47. Purebreed and crossbreed daughter indexes were used from 10 artificial insemination sires for a within-breed-of-sire comparison. The analyses indicated that the best sire for purebreeding is not necessarily recommended for crossbreeding. The average production of crossbreds, grouped according to age, showed a sharp increase from 2 years to 4 years, followed by a relatively small increase from 4 years to 7 years. There was a gradual drop in production after the seventh year. Crossbreds reached their maximum milk production at 7 years of age.
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