Exploring the Genetic Variation of Fleece and Crutch Shedding in Sheep, Recorded at Different Times Within and Across Age Classes

The global value of wool from coarse woolled sheep continues to decline, prompting producers to seek alternatives that reduce or eliminate the need for shearing. Several sheep breeds naturally shed their fleece in response to photoperiod, and these breeds are being crossed with populations that have high genetic merit for traits such as reproduction, growth and carcase quality. Breeding programmes globally are now aiming to improve the shedding ability of these crosses. To achieve this, producers need to understand the optimal timing and age to record shedding in their flocks in order to maximise genetic progress. This study aimed to investigate the genetic merit of visually scored fleece and crutch shedding traits across different ages and throughout the shedding season, which spans from the winter solstice (June) to the summer solstice (December) when fleece begins to regrow, in Australian maternal composites. Shedding was assessed as a visually scored trait (1 = completely shed, 5 = fully woolly) over 10 years from three age classes: lambs, yearlings and adults. Records were grouped into three periods within the shedding season: early (June-September), peak (October-December) and late (January-April). Variance components and heritabilities were estimated using univariate and bivariate single-step animal models. The genetic correlation between fleece and crutch shedding was estimated within season and across ages, as well as between fleece and crutch shedding. Results showed that fleece and crutch shedding heritabilities were highest during the peak and late periods (0.56-0.66 for fleece; 0.38-0.57 for crutch) and lower during the early period for fleece shedding (~0.40). Genetic correlations between adjacent records within age stages were strong (Peak-Late or Early-Peak, ranging between 0.78 and 0.91) but slightly lower for records taken further apart (Early-Late ~0.74). Correlations between yearling and adult traits were more similar (range 0.62 to 0.91), while the correlations between lamb and adult records were lower (0.57 to 0.78). Fleece and crutch shedding recorded within the same period were highly correlated (> 0.75), and trait repeatabilities within age classes ranged from 0.41 to 0.60. These results suggest that scoring shedding during the peak or late periods captures the greatest phenotypic expression and genetic variation. Selection based solely on lamb shedding may not accurately predict adult performance, especially for breeding programmes where complete shedding in adults is the objective.