Date of Award
Master of Science
Benny R. Bell
J. B. McLaren, J. C. Waller
Weaning records of 10,979 calves from the Hereford and Angus herds of the University of Tennessee were analyzed to compare different selection methods to increase weaning weight of beef cattle. Records were collected over a 25-year period from 1959 to 1984. Variables recorded were date of birth, sex, birth weight, breed of dam and sire, weaning age and weaning weight. Weaning weights were adjusted for sex, age of calf at weaning, and age of dam. Selection decisions were made on a within-herd-year basis. Three methods of selection were compared. Objectives were to determine the effects on weaning weights when selection of replacement females was based on (1) age of her dam, (2) average daily gain as a calf, and (3) most probable producing ability of her dam and granddam. Linear regression of progeny weaning weights on age of dam yielded substantial positive results (b=0.69). Negative estimates, however, resulted when progeny weaning weight was regressed on the age of granddam when the dam was born. Least-square means indicated that selecting females from immature dams was more effective in increasing weaning weights than selection from other age groups. Similar inverse dam-offspring relationships were observed when regression of progeny weights on MPPA were utilized. Due to the negative estimates being relatively small, selection of replacement females from dams with lower MPPA values did not maximize weaning weights. Weaning weights declined dramatically, however, from one generation to another. Comparison of least-square means of quartiles when selection was based on the average daily gain, likewise, yielded similar results to the other two selection systems. Selection of females from dams with the highest average daily gain significantly increased weaning weights. Females selected from dams of lower average daily gain quartiles weaned calves at weights lower than when no selection was practiced. Analysis revealed the existence of a possible negative genetic and/or environmental relationships between preweaning growth and subsequent maternal effects. With the exception of selection from immature dams, the magnitude of this antagonistic effect did not seem to overwhelm the direct genetic component for growth.
Morris, Michael Hiram, "Comparison of methods of selection to increase weaning weight of beef cattle. " Master's Thesis, University of Tennessee, 1988.