Genetic analysis of weight, fat and muscle depth in growing lambs using random regression models

Genetic parameters were estimated using uni- and bi-variate random regression models for weight, eye-muscle depth and fat depth measures between 60 and 360 days of age. Each trait was measured up to five times in 50-day intervals following weaning on approximately 4000 Australian Poll Dorset Sheep. The model accounted for rearing type, dam age, management group and age of recording. The model used for analysing weight included quadratic, orthogonal polynomials for direct genetic and environmental effects, a linear polynomial for maternal genetic effects and heterogeneous error variance across ages. The fat and muscle analysis used linear orthogonal polynomials for direct genetic and environmental effects and heterogeneous error variance.Throughout the 300-day trajectory heritability for weight traits ranged from 0(.)20 to 0(.)31, while heritability for fat depth ranged from 0(.)24 to 0(.)34 and heritability for eye-muscle depth ranged from 0(.)24 to 0(.)40. Genetic correlations between repeated measures of the same trait at different ages were positive and declined as the age interval increased, to minimum values of 0(.)60, 0(.)31 and 0(.)50 for weight, fat and muscle respectively between 60 and 360 days of age. Genetic correlations between weight and fat and weight and eye muscle were moderate to high (0(.)6 to 0(.)8) and positive but decreased slightly with age. The genetic correlations between fat and muscle were moderate to high (0(.)5 to 0(.)7) throughout the 300-day trajectory. In all cases, the estimates produced in this study were reasonably consistent with the limited number of studies that exist in the reported literature. This study demonstrated the relationships that exist between repeated measures of weight, fat and muscle measures over time, which is of interest to prime lamb producers looking to select for specific breeding objectives or market end points requiring precise weight, fat and muscle combinations at certain ages.