12. Building a Better Ewe: Lifetime Productivity of Polypay Sheep at the U.S. Sheep Experiment Station.

Abstract A primary determinant of the profitability of a sheep operation is the ability to maintain productive ewes in the flock beyond what is required to cover annual ewe costs. Measuring a ewe’s productivity is not simply a measure of how many years she remains in the flock but is a complex interplay of number and weight of lambs produced over a lifetime. The objective was to evaluate the current lifetime ewe productivity of the U.S. Sheep Experiment Station (USSES) Polypay prior to developing a genetic evaluation for ewe longevity. Records from USSES were used to assess lifetime productivity traits for ewes born from 1995 to 2016, such that they had an opportunity to complete their life cycle. There were 10,094 records from 2,941 ewes that either lambed as a 1-yr-old (n = 2,399) or as a 2-yr-old (n = 542). The ewes represented 342 sires and 1,631 dams. The influence of age at first lambing (AFL) and prolificacy at first lambing (AFL_NLB) on lifetime productivity were of primary interest due to concerns that high prolificacy of a still-growing yearling could be detrimental in the long-term and thereby shorten her productive life. Ewes remained in the flock from 1 to 8 yr, with an average of 3.02 ± 1.75 yr, and an average annual replacement rate of 30.9 ± 9.7 %. Ewes who lambed as a 1-yr-old had significantly higher lifetime lambs born (LLB), lifetime lambs weaned (LLW), and lifetime kilograms weaned at 120 days (LKg120d) than ewes who first lambed as 2-yr-olds (Table 1), but a significantly lower maternal rearing ability (MRA; LLW/LLB). Within the AFL_NLB categories, the highest LLB were observed by ewes who lambed as 1-yr-olds with twins (1_2), 1-yr-olds with multiples (1_3), and 2-yr-olds with multiples (2_3) with the lowest LLB from ewes who lambed as 2-yr-olds with a single (2_1). The highest LLW and LKg120d were from 1_2 and 1_3 ewes. Stayability (STAY i|j) and marginal stayability (MARG i|j) were defined as the probability of a ewe to survive in the flock until i years old, given that she already survived until j years old. Ewes who lambed as 2-yr-olds had significantly higher STAY32, but ewes who lambed as 1-yr-olds had significantly higher STAY72, indicating greater longevity of the early prolific ewes. Similarly, the MARG stayability traits were higher for ewes who lambed as 1-yr-olds, but only significantly so for MARG43. Within the AFL_NLB categories, STAY and MARG traits were generally not significantly different except for STAY62 where 1_1 and 1_2 ewes had higher stayability than 2_1 ewes. Ewes who lamb as 1-yr-olds and who are prolific at their first lambing have a lifetime production advantage in this flock. Genetic evaluation to identify superior animals for ewe longevity is planned. For image description, please refer to the figure legend and surrounding text.