Seasonal changes in tissue weights in Scottish Blackface ewes over multiple production cycles

Hill ewes undergo large changes in body fat and muscle weight throughout the annual production cycle as they contend with the pressures of reproduction and lactation, as well as harsh environmental conditions. This study modelled seasonal changes in fat and muscle weights in Scottish Blackface hill ewes throughout their productive lifetime using random regression statistical techniques. Scottish Blackface ewes (no. = 308) were scanned using computed tomography (CT) four times per year, from 2 until 5 years old. Heritabilities of tissue weights were estimated at 2-weekly intervals throughout the productive life of the ewe. Genetic correlations between tissue weights at the same point in the production cycle at different ages, and between tissue weights at different events within each annual production cycle were predicted. Animal solutions from random regression analyses were used to estimate tissue weights, from pre-mating at 2 years old to weaning at 5 years old. The effects of litter size in the current and previous production years on fat and muscle weights were investigated. Correlations between CT tissue weights and those predicted by a sin/cos random regression model were 0.87, 0.84, 0.88 for carcass fat, internal fat and muscle respectively. Heritabilities ranged from 0.31 to 0.90 for carcass fat weight, 0.21 to 0.68 for internal fat weight and 0.26 to 0.57 for muscle weight, throughout the productive lifetime of the ewe. Heritabilities were highest during mating for fat weights, and during the dry period and lambing time for muscle weights. Heritabilities of tissue weights in 3-year-old ewes were higher than in other age groups. Genetic correlations were 1.00 between tissue weights at the same scanning event at different ages, but ranged from close to zero to 0.97 between scanning events within age groups. Clearly environmental variation across time was large. The number of lambs produced in both the current and the previous year influenced tissue levels. Ewes that did not produce lambs (barren) in a given year carried more muscle during that year than ewes producing lambs. As ewes aged, barren ewes carried increasingly more carcass fat and muscle than ewes with lambs. Barren ewes also had significantly more muscle during the following year than ewes that had weaned lambs. Ewes that reared twins had significantly less carcass fat the following year than singleton-bearing or barren ewes. These effects of previous litter size increased significantly with age.