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Effect of environment on heritability estimates for body weight in the domestic fowl Paulson, Stewart George

Abstract

Male progeny of 15 sires each mated to 2 dams from the University of British Columbia's New Hampshire flock were reared in 2 environmental treatments, an environment chamber or a standard floor pen. Birds reared in the chamber were transferred to the floor at 3 weeks of age where they were reared with the birds of the floor treatment to 7 weeks of age. Six experiments were conducted involving a total of 339 birds. Individual body weights were recorded for the progeny at hatch and at weekly intervals until 7 weeks of age. Chamber temperatures were started at 95°F for experiment 1 and 90°F for experiments 2-6 and reduced 3°F every 3 days to 3 weeks. Relative humidity was held constant at 50%. Birds on the chamber treatment were heavier at 7 weeks of age than birds raised on the floor from hatch to 7 weeks. This difference was attributed to the high 1-3 week growth rate of chamber reared birds. Using multiple linear regression techniques the 1-3 and 3-7 week growth periods were found to be the important periods in accounting for the variability in 7-week weight for the floor and chamber birds respectively. Heritability estimates of body weight based on σ²s for floor and chamber birds were found to be high (0.82) and relatively the same between the 2 environmental treatments from hatch to 2 weeks of age. They diverged to 0 and 1 by the 5th week for the floor and chamber, respectively. The high heritability based on σ²s for chamber reared birds at the time of transfer was attributed to an increase in selectable genetic variance due to stress. The effect of stress after transfer was reflected in low growth rates during the 3-4 week period. Average 1-week weight, average 1-3 week growth rate and average 3-7 week growth rate were regressed on the heritability (based on σ²s) of 7-week body weight. For floor reared birds the early growth period was the important period in accounting for variability in heritability of 7-week weight while for chamber birds the 3-7 week growth period was the most meaningful period. It was concluded that the environment controls the variability of weekly growth rate and changes the importance of these growth periods in accounting for the genotypic variability in 7-week weight. In a theoretical selection program based on 7-week weight the percentage of progeny each sire contributed to the selected population depended upon the environment in which the progeny were reared. However a small number of sires were able to maintain their percentage contribution in both environments. For the floor treatment more selected birds had an above average 1-3 week growth rate than 3-7 week growth rate. For the chamber treatment more selected individuals had an above average 3-7 week growth rate than 1-3 week growth rate. Although growth rate from 1-3 weeks of age raised the mean 7-week weight of chamber birds, it was the individual’s ability to grow under the stress conditions which placed it in the selected population. Birds which were below average with respect to 1 or 2 of the traits, 1-week body weight, 1-3 week growth rate and 3-7 week growth rate were present in the population of birds which had been selected on the basis of 7-week body weight. The percentage of selected individuals carrying any particular combination of these traits depended upon the environment in which the birds were reared. It was concluded that selection based on individuals which were above average with respect to all 3 of the aforementioned traits as well as 7-week weight would improve the selection response when compared to selection based upon 7-week weight alone.

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