Graduate Thesis Or Dissertation
 

Genetic and environmental variation in production components of purebred and crossbred ewes

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  • Coopworth sheep, a newly available genotype, were compared with other genotypes in two trials to assess cumulative lamb and wool production and contributing components related to survival, reproduction and growth, and for the effects of ewe body weight on reproductive traits. In a third trial, various genotypes generated in the process of upgrading native Kaghani sheep, were evaluated for lamb and wool production. In the first two trials six ewe genotypes, generated by mating Coopworth (C), Polypay (P) and Suffolk (S) rams to Polypay and Coopworth-type ewes, were exposed to Hampshire rams for spring lambing from 1986 through 1990. Overall conception rate averaged 96% and ranged from 93% for S X C ewes to 97% for P X C ewes. Sire breeds were not different in ovulation rate but daughters of Polypay dams averaged .18 higher ovulation rate than daughters of Coopworth dams (P<.05). Uterine efficiency for twin ovulators was .86 with little variation among sire breeds; however, daughters of Polypay ewes had a mean uterine efficiency of .93 compared to .78 for daughters of Coopworth ewes (P<.05). Mean litter size at birth averaged 1.63 and ranged from 1.45 for C ewes to 1.75 for S X P ewes. Ewes from Polypay dams had higher mean litter size (P<.01) than those from Coopworth dams (1.73 vs 1.54), but differences between sire breeds were not significant. Ewes from Suffolk sires weaned the heaviest lambs while ewes from Polypay sires weaned the greatest number of lambs, resulting in similar weight of lamb weaned per ewe mated. Adjustment of lamb production for ewe metabolic body size resulted in Coopworth-sired ewes being more efficient than the heavier Suffolk-sired ewes. Coopworth-sired ewes produced 32% more wool than ewes sired by the other two breeds. Combining lamb and wool production in an index resulted in a range of less than 2% among sire breeds for gross productivity per ewe mated. Productivity of Rambouillet ewes studied under Pakistani conditions declined significantly over time. Crossbred lambs were generally heavier at weaning than Rambouillets, while wool production was highest in purebred Rambouillet ewes followed by genotypes related closely to Rambouillets. In all trials crossbreds generally excelled purebreds in overall productivity; the degree of superiority depended on genotypes involved in the crosses.
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