Scrotal circumference (SC) and live weight (LW) were collected over 6yr (1982-1987) on 541 spring-bom Rambouillet ram lambs (109 sires, 307 dams). Scrotal circumference and LW were predicted at four ages ( 90, 120, 150 and 180 d) for each lamb. Factors affecting predicted SC growth between all possible ages and estimates of heritability were determined. Two models were used. Model 1 included year and type of birth as fixed effects; day of birth nested within year, live weight change (ALW) and (ALW)2 as covariates and sire nested within year and residual error as random effects. Model 2 was similar to model 1 but with ALW and (ALW)2 excluded. The relationship matrix among sires was taken into account in both models. Year effects on SC growth were significant (P<.05) for the majority of the SC growth periods. Type of birth effects were significant (P<05) for different SC growth periods but not for total SC growth (90 - 180 d). Regression coefficients for ALW, (ALW)2 and day of birth were, in general, significantly different from zero. Heritabilities for SC growth ranged from .17 to .60. Adjustments for ALW led to a 0 to 35% decrease in heritability of SC growth.

C. AP Matos, D. L Thomas

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XVI. Poultry, fish and horse genetics and breeding, growth and reproduction, immune response and disease resistance., , 374–377, 1990
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