| Sumario: | Reducing the number of days from birth to slaughter is one strategy to improve animal feed and environmental efficiency. The export market use yearling sheep weighing 22–30 kg and usually purchases from large and small-scale traders at an early age (lambs weighing ≥ 15 kg) for mutton production. Hence, this study aimed to derive a new trait phenotype, which helps to reduce sheep market age without adverse effects on the market weight of Menz sheep and to evaluate the extent of exploitable genetic variation in this new trait. To this end, 11,258 lambs weighing ≥ 15 kg between 66 and 395 days of age were considered in this study. Co (variance) components and heritability estimates for novel traits were estimated using the average information restricted maximum likelihood method in WOMBAT fitting the animal model. The best-fitted model was selected from six models based on likelihood ratio test and Akaike's information criterion. The days-to-market weight of 16.3% of the animals was shorter by 48.2 days (with an estimated breeding value of −26.2 days) compared to the mean of the contemporary groups. The market weight of 15.5% of the animals was higher by 1.73 kg compared to the mean of the contemporary groups. There was a phenotypic variability of deviation in age at market weight (DAMW) and deviation in weight at market age (DWMA) for the sheep population in Molalie village compared to other villages. Likewise, the genetic standard deviation for DAMW and DWMA was 25 days and 0.79 kg, respectively. Based on the best-fitted model, the direct heritability estimate for DAMW and DWMA was 0.65 and 0.57, respectively. In addition, the maternal genetic effect explains 28% of the phenotypic variation in DAMW and 26% of the phenotypic variation in DWMA. The DAMW of Menz sheep in Dargegn and Molalie villages decreased significantly by 2.113 and 1.192 days year−1, respectively. The observed additive genetic variance for DAMW suggests further scope for genetic improvement in the flock to reduce the days-to-market weight of Menz sheep. Including this novel trait in a breeding objective could shorten days to market weight without necessarily reducing the genetic merit of the live weight included in the breeding objective.
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