Sex of Littermate Twin Affects Lifetime Ewe Productivity3

Ewe productivity is synonymous with annual litter-weight weaned (LWW) per ewe exposed to rams for breeding, and LWW is largely a function of number of lambs born (NLB) and weaned (NLW). Selecting for LWW should increase litter size and numbers of ewe-ram co-twins. Thus, we used historical records to determine whether sex of co-twin affected lifetime productivity of twinborn ewes. United States Sheep Experiment Station (USSES) lambing records (n = 8,650) from 1991 through 1997 were queried to identify twinborn ewes that were reared with their biological dams and retained in the breeding flock (n = 1,628; Columbia, 383; Polypay, 536; Rambouillet, 383; and Targhee, 326). Corresponding records for lifetime-cumulative counts of lambs born (stillborn and live-born) with recorded birth weights and weaned with recordedweaning weights, cumulative weight of lambs weaned, lifetime count of lambing events, and age at first lambing (1 yr, 2 yr, or 3 yr) were evaluated using PROC GLM and PROC MIXED methods. Alpha was set at 0.10. Only the main effects of sex of co-twin, ewe-weaning weight, ewe breed, and ewe-birth year were significant. Per-ewe exposed to rams, but not per-ewe lambing, cumulative-lifetime weaning weight (P = 0.03) and numbers of lambs born (P = 0.07) and weaned (P = 0.04) were greater for ewes with a ram co-twin than for ewes with a ewe co-twin. Sex of co-twin did not affect number of lifetime-lambing events or age at first lambing for ewes exposed (P = 0.14 and P = 0.59, respectively) or ewes lambing (P = 0.67 and P = 0.27, respectively). Ewe-weaning weight affected cumulative-lifetime weaning weights (P = 0.0003), lifetime numbers of lambs born (P = 0.001) and weaned (P = 0.02), and lifetime-lambing events (P = 0.001), but not age at first lambing (P = 0.44) per-ewe exposed. Productivity of Polypay and Rambouillet ewes generally exceeded that of Columbia and Targhee ewes, although breed ranking was not constant among productivity traits of twinborn ewes. Based on the data, we concluded that ewes born co-twin to a ram had an advantage over ewes born co-twin to a ewe. This advantage amounted to 15.55 kg in lifetime litter-weight weaned per-ewe exposed. We believe that sex of co-twin should be evaluated further to determine whether it is a useful environmental adjustment, beyond lamb sex and type of birth and rearing, for lamb weights and traits related to ewe productivity.