Effect of ewe birth litter size and estimation of genetic parameters on ewe reproductive life traits

Ewe lifetime productivity has economic implications for producers because shorter lifetime productivity results in less profit. Productive years of ewes from extensive, range-based systems of the United States West are generally less than ewes from more temperate regions of the United States. Accordingly, ewes from range-based systems, especially those employing shed-lambing strategies, have been selected for increased litter size to offset decreased lifetime productivity. However, the relationship of the ewe's birth litter size (ELS_B) has not been considered a potential contributor to lifetime productivity. Longevity (number of productive years, n = 1 per population) and stayability (probability to survive to the next age; ages 2-7 years, n = 6 per population) were investigated to understand ELS_B effects on productive life. Columbia, Polypay, Rambouillet, and Targhee breeds were used in this study. Across-breed (n = 11 550) and within-breed (Columbia, n = 4 398; Polypay, n = 4 534; Rambouillet, n = 5 922; Targhee, n = 6 482) analyses were used. Depending on the population, records spanned from 1950 to 2008, where ewe's birth year was included as a fixed effect in the animal model using restricted maximum likelihood estimation procedures. Fixed effects investigated included ELS_B (single-, twin-, or triplet-born) and ewe breed (across-breed analyses only). Regardless of trait or population used, heritability ranged from 0.06 ± 0.02 to 0.34 ± 0.03, where stayability at younger ages had the highest estimates. The breed effect was significant in all across-breed analyses (0.0001 ≤ P ≤ 0.038; n = 7), where Polypay, a breed selected for accelerated lambing and increased fertility, averaged shorter productive life or had a lower probability of survival to the next age compared with other breeds (longevity: 0.009 ≤ P ≤ 0.223; stayability: 0.000 ≤ P ≤ 0.842). The ELS_B was significant in 60% (n = 5) and 37% (n = 30) of longevity and stayability analyses, respectively. Except for Targhee, all analyses showed ewes born in smaller litter sizes were associated with longer productive lives or higher probability of surviving to the next age, particularly in across-breed analyses (e.g., longevity: single- vs twin-born ewes, P = 0.004; vs triplet-born ewes, P = 0.003). This study provides evidence that increasing prolificacy in ewes from extensive, range-based production systems may impact productive life. Due to the low heritability of these traits, additional investigation into modeling these traits with dominance effects and litter size needs to be conducted.