Estimation of genetic parameters for development of alternative selection criteria for increased litter size in Rambouillet sheep
Waldron, Daniel Fred
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Permalink
https://hdl.handle.net/2142/21480
Description
Title
Estimation of genetic parameters for development of alternative selection criteria for increased litter size in Rambouillet sheep
Author(s)
Waldron, Daniel Fred
Issue Date
1990
Doctoral Committee Chair(s)
Thomas, David L.
Department of Study
Animal Sciences
Discipline
Animal Sciences
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Genetics
Agriculture, Animal Culture and Nutrition
Language
eng
Abstract
The variance and covariance components needed to estimate heritabilities of and genetic correlations among litter size, ovulation rate, scrotal circumference and body weight in Rambouillet sheep were estimated using REML (Restricted Maximum Likelihood) via an EM (Expectation-Maximization) type algorithm. Data were obtained from the University of Illinois Rambouillet flock located at the Dixon Springs Agricultural Center near Simpson, Illinois. There were 437 ram lambs with scrotal circumference records, 552 ewes with lambing records, 610 ewes with ovulation rate records and 998 lambs with body weight records. A bivariate animal model which included year of birth as a fixed effect and additive genetic value of animal and residual as random effects was employed. The inverse of the numerator relationship matrix was included in the mixed model equations. The heritability estimates from bivariate analyses were:.18,.20,.23,.34 and.10 for litter size, ovulation rate, scrotal circumference, 180 day body weight of females and 180 day body weight of males, respectively. The heritability estimate of litter size from a univariate analysis was 14%. Bias due to natural selection is suggested as the reason for the difference between the univariate and bivariate estimates. The genetic correlation between litter size and ovulation rate was near unity. Body weight in ewes had a moderate genetic correlation with both litter size (.22) and ovulation rate (.20), and a low residual correlation with both litter size (.03) and ovulation rate (.09). The genetic correlation between body weight in rams and scrotal circumference was zero, while the residual correlation was.71. The genetic correlations of scrotal circumference with litter size and ovulation rate were $-$.25 and +.20, respectively.
Comparison of direct, indirect and combined selection indices showed that, of the traits studied, ovulation rate was the most useful indirect selection criterion for genetic improvement of litter size. Expected response in litter size to indirect selection on ovulation rate was 93% as large as the expected response to direct selection on litter size. Selection based on an index of litter size and ovulation rate is expected to produce 132% as much response as selection on litter size.
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