Abstract
Length of productive life (LPL) of a ewe is a trait of considerable economic importance in sheep farming. Records of 66,050 Awassi and 12,010 Najdi ewes belong to 28 flocks in the northern part of the Arabian Peninsula were used to investigate factors influencing their LPL under intensive production systems at arid regions. Ewes with first lambing January 1, 2003 through December 31, 2009 were subjected to survival data analysis. Weibull distribution model was used to explore different explanatory variables affecting LPL Parity, breed, lambing interval, age at first lambing, flock effect and type of lambing were the variables with the largest contribution to the likelihood function for LPL Average age at first lambing was 16.5 +/- 1.18 and 17.7 +/- 1.76 months for Awassi and Najdi ewes, respectively. Average LPL was 2204 +/- 8.1 and 1635 +/- 11.0 days for Awassi and Najdi ewes, respectively. Within analyzed LPL. Awassi ewes had an average of 4.1 +/- 0.27 parities compared to 3.3 +/- 0.34 for Najdi ones. Awassi ewes were more adaptable to intensive production system than Najdi ewes under arid conditions. Najdi ewe was 1.8 +/- 0.32 more likely to be culled than Awassi ewe under the same conditions. The most important factor affecting LPL in the overall risk analysis was parity number. The relative culling risk was decreasing continuously in both breeds from the first parity (Awassi = 3.5, SE = 0.31: Najdi = 5.17, SE = 0.41) to the last one (Awassi = 0.45, SE = 0.14; Najdi = 0.63, SE = 0.42). The most appropriate lambing intervals (LI) were 221-340 days for Awassi and 251-310 days for Najdi ewes. LI below or above these ranges were significantly increased the risk of culling. Ewes lambed for the first time at <= 13 months were under considerable culling risk (Awassi = 1.42, SE = 0.19: Najdi = 2.73, SE = 0.33) compared to ewes lambed at older ages. The research showed that the optimum ages at first lambing were 17-19 months for Awassi and 20-22 months for Najdi. Ewes lambed triplets or more were under higher culling risk (Awassi = 1.72, SE = 0.10; Najdi = 1.97, SE = 0.19) than ewes lambed twins or singles. Interaction analysis showed that ewes lambed triplets or more during parity #1 were 3.42 +/- 0.50 more likely to be culled than ewes lambed singles during the same parity. LPL could be increased by allowing optimum lambing intervals, mating for the first time at maturity stage, and avoiding enhancement of large litter sizes particularly during early and late parities. Different management and housing systems applied in different flocks showed significant effects on culling risk. Considerable variations in calculated LPL were due to breed and flock effects. This can be utilized by sheep breeders to increase LPL through genetic and environmental manipulations. Genetic evaluation of longevity trait in Awassi and Najdi breeds should be conducted, which will be of importance for sheep breeding programs at arid regions.