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Title:   Author  item
                                            BROWN, J - Wingate Universityitem
                                            LEWIS, GREGORY - Retired ARS Employeeitem
                                            Taylor, Joshua

                                                         - *Bret*
  Submitted to: Sheep and Goat Research Journal
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 2/1/2016
                                    Publication  Date: 4/1/2016
                                    Citation: Brown, J., Lewis, G.S., Taylor, J.B. 2016. Sex of littermate twin affects lifetime ewe productivity. Sheep and Goat Research Journal. 31:1-8.Interpretive Summary: Ewe productivity, defined as litter weight weaned per ewe exposed to rams for breeding, is considered the key determinant of net economic return per ewe. Based on the results of this study, ewes that were born twin to a male sibling were likely to be more productive over their lifetime than ewes that were born twin to a female sibling. We do not have a mechanistic explanation for the environmental “benefit” to a ewe born twin to a male sibling and, thus, suggest that cause-and-effect studies are needed to explore this phenomenon in depth. Also, we believe that the effect of sex of the sibling twin should be further evaluated 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.
                                Technical Abstract: 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 reproductive performance of twinborn ewes. U.S. Sheep Experiment Station 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 recorded weaning weights, cumulative weight of lambs weaned, lifetime count of lambing events, and age at first lambing (1, 2, 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 further evaluated 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.


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