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

Authors: HANNA, LAUREN - North Dakota State University; Taylor, Joshua - Bret; HOLLAND, P - North Dakota State University; VONNAHME, KIM - North Dakota State University; REYNOLDS, LAWRENCE - North Dakota State University; RILEY, DAVID - North Dakota State University

Submitted to: Animal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2023
Publication Date: 6/28/2023
Citation: Hanna, L.L., Taylor, J.B., Holland, P.W., Vonnahme, K.A., Reynolds, L.P., Riley, D.G. 2023. Effect of ewe birth litter size and estimation of genetic parameters on ewe reproductive life traits. Animal. Volume 17(8). Article 100900. https://doi.org/10.1016/j.animal.2023.100900.
DOI: https://doi.org/10.1016/j.animal.2023.100900

Interpretive Summary: 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 are less than ewes from more temperate regions of the U.S. Accordingly, ewes from range-based systems have been selected for increased litter size to offset decreased lifetime productivity. Results from this study show that a ewe's birth litter size influences her productive lifespan, where twin- and triplet-born ewes generally live shorter productive lives than single-born ewes. Mechanisms of how birth litter size is associated with productive life needs to be explored further for more accurate selection decisions without compromising other production characteristics.

Technical Abstract: 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 U.S. West are generally less than ewes from more temperate regions of the U.S. Accordingly, sheep 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 (ELSB) on productive life has not been considered as 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 to 7 yr, n = 6 per population) were investigated to understand ELSB effects on productive life. Breeds utilized in this study were Columbia, Polypay, Rambouillet, and Targhee. 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 population, records spanned from 1950 to 2008, where ewe birth year was included as a fixed effect in the animal model using restricted maximum likelihood estimation procedures. Fixed effects investigated included ELSB (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 (0.000 = P = 0.038; n = 7), where Polypay, a breed selected for accelerated lambing and increased fertility, averaged shorter productive life or had 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 ELSB 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 low heritability of these traits, additional investigation into modeling these traits with dominance effects and litter size needs to be conducted.