Factors affecting the length of productive life in U.S. Katahdin ewes

Authors: PINTO, LUIS - Federal University Of Bahia (UFBA); LEWIS, RONALD - University Of Nebraska; ROCHA, ARTUR - Purdue University; Freking, Bradley - Brad; Murphy, Thomas - Tom; Wilson, Carrie - Welsh; NILSON, SARA - University Of Nebraska; Burke, Joan; BRITO, LUIZ - Purdue University

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Length of ewe productive life (LPL) is a key indicator of a sustainable sheep production system. However, genetic improvement in this trait is limited by the phenotype being measured and expressed late in the life of an individual ewe. Key factors influencing length of productive life in Katahdin ewes were determined from datasets provided by the National Sheep Improvement Program (NSIP) and recorded by U.S. Katahdin producers during routine husbandry activities. This is the first study using LPL as an indicator of ewe survival or longevity in U.S Katahdin sheep. A relatively low median LPL was estimated at between two and three years of age, but this result must be evaluated with caution because the strong association between the average lamb weight and LPL suggests breeders are selecting and voluntarily culling ewes at an early age for performance traits. In general, Katahdin ewes themselves born in multiple litters, and have gone on to produce litters of lambs with body weights around 4 to 5 kg at lambing and 20-25 kg at weaning (over their lifespan) were associated with a better survival probability and longer LPL. Future efforts are needed to quantify the involuntary culling rate in Katahdin ewes to identify whether longevity-related traits should be included in the overall breeding objective for Katahdin sheep.

Technical Abstract: The length of ewe productive life (LPL), defined as the number of days between the first and last lambing, is a key indicator of ewe longevity and is directly related to the sustainability of the sheep industry. Therefore, the primary objective of this study was to investigate systematic effects influencing LPL in Katahdin sheep. We analyzed the LPL of 10,474 Katahdin ewes (69.5% with uncensored and 30.5% with right-censored observations) born between 1992 and 2021 in 58 flocks located across the United States. The Kaplan-Meier (K-M) and Cox Proportional Hazard (Cox PH) methods were used to estimate survival probability. Four Cox PH models were evaluated. Model 1 included contemporary group (CG; flock-year-season of ewe birth) as a random effect and the ewe’s dam’s age (EDA), ewe’s own birth-rearing type (BR; 1/1, 2/1, 2/2, 3/2, 3/3, with the digit-3 including lamb counts>/=3), and age at first lambing (AFL) as fixed effects. Models 2 to 4 were an extension of model 1. Model 2 also included average lamb birth weight (ABW) per ewe lifetime, while model 3 included average lamb weaning weight (AWW) per ewe lifetime. Both ABW and AWW were fitted as fixed effects. Model 4 fitted all previous effects together. The factors CG, BR, ABW, and AWW affected LPL (P < 0.05) in all models in which these effects were fitted. The EDA effect only influenced LPL (P < 0.05) in model 1, while AFL had no effect (P > 0.05) in any model. The median LPL ranged from approximately 2 to 3 yr, depending on the risk factors analyzed. In general, Katahdin ewes themselves born in multiple litters, and that produced lambs weighing ~5 kg at lambing and 20 to 25 kg at weaning (over their lifespan) had better survival probability. The LPL of Katahdin sheep, although low, appears to be a consequence of voluntary culling due to the association of LPL with both ABW and AWW. Future studies should quantify the rate of involuntary culling in Katahdin ewes to identify whether longevity indicator traits should be included in more comprehensive breeding objectives.