Age-related estimates of genetic parameters for growth and ultrasonic M. longissimus thoracis et lumborum muscle traits in Eşme lambs

The growth performance and ultrasonic M. longissimus thoracis et lumborum (LTL) muscle traits of 2397 Eşme lambs raised in the Eşme district of Uşak province, an important lamb production area in Western Anatolia, were evaluated at 3 months (n = 1690) and 5 months (n = 707) of age. The average age of the lambs at the time of the first measurement (at weaning) was 100.15 days, and 152.37 days at the second. At 3 months of age, the average live weight (LW) was 29.34 kg and the average daily gain (ADG) was 248.35 g/day. By month 5, the LW had increased to 38.53 kg, while the ADG had decreased slightly to 222.62 g/day. Environmental factors such as year, farm, birth type, and sex significantly affected most traits (p <0.01). At month 3, heritability estimates for LW and ADG were 0.44 and 0.46, while ultrasonic traits of the LTL muscle—muscle depth (MD), backfat thickness (BFT), and skin + backfat thickness (SBFT)—showed heritabilities of 0.33, 0.47, and 0.61, respectively. At month 5, the heritability of ADG rose to 0.80, with values of 0.66, 0.27, 0.37, and 0.60 for LW, MD, BFT, and SBFT. Genetic correlations between ADG and LW were very high and positive (rG ≈ 0.97–0.98) across the ages. Growth and carcass-related ultrasound traits showed moderate to high heritability, suggesting that they may be successfully improved by selection. Since lambs are generally marketed or slaughtered at 3–3.5 months of age due to breeder and consumer demands, evaluation and selection at this age are particularly critical. The results demonstrate that genetic progress can be achieved at weaning age, confirming the suitability of current marketing practices and supporting the use of early selection in Eşme sheep breeding programs .