Estimation of Genetic Parameters Associated with Ewe Reproductive Life and Lamb Mortality in Northwestern United States Sheep
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Ewe reproductive life traits such as longevity and stayability (the probability that a ewe will avoid removal from the flock due to reproductive performance or health) are economically important because of the relationship of removal rates with the value of the cull ewe and cost of replacements. Ewe reproductive life was evaluated as: 1) ewe longevity, 2) ewe stayability to six different ages, and 3) ewe survival. The litter size of a ewe at her birth and her litter size at rearing were investigated for potential effects on these traits. Ewe litter size at rearing was not significant in any analyses (P > 0.15), but the ewe’s litter size at birth was a significant effect in most analyses. Smaller litter sizes at birth, particularly ewes born as singles, were more often associated with greater mean longevity and stayability to specific ages (P < 0.05). Ewes born as singles had survivor functions characterized by higher probability of survival to older ages (P < 0.05). Polypay ewes had lower longevity and stayability to different ages than other breeds, as well as a survival function with lower probabilities than other breeds for survival to most older ages. Genetic parameters for these measures were consistently low for longevity (estimates of heritability ranged from 0.06 ± 0.022 in the within breed analysis of Columbia to 0.16 ± 0.024 in within breed analysis of Rambouillet). Estimates of heritability for stayability in general were somewhat higher than those for ewe longevity (ranged from 0.08 ± 0.061 for stayability to 6 yr in Columbia to 0.34 ± 0.027 for stayability to 2 yr in the across breed analyses). Lamb mortality is important to the sheep industry. Mortality was evaluated as: 1) mortality due to any reason, 2) mortality associated with birth (or at birth), and 3) mortality due to pneumonia. Estimates of heritability ranged from 0.05 ± 0.022 in the analysis of overall mortality in Polypay to 0.47 ± 0.032 in the analysis of birth mortality in Polypay. Some of these estimates of heritability suggest that selection programs could be effective for these traits.
Holland, Paul Wesley (2018). Estimation of Genetic Parameters Associated with Ewe Reproductive Life and Lamb Mortality in Northwestern United States Sheep. Doctoral dissertation, Texas A & M University. Available electronically from