EFFECT OF COBALT SUPPLEMENTATION AND ESTROUS SYNCRONZATION ON BODY WEIGHT GAIN AND CONCEPTION RATES IN YEARLING EWES AND PERIPUBERTAL EWE LAMBS

Conception rates of peripubertal ewe lambs during the fall following their birth and yearling ewes influences flock productivity and economic returns. Our objectives were to evaluate the influence of cobalt (Co) supplementation and estrus synchronization on growth performance and conception rates. Yearling ewes (n = 50) and peripubertal ewe lambs (n = 63) were sorted by breed (Suffolk, Hampshire, Rambouillet, Columbia), allocated to 1of 8 pens (2 pens/breed), and fed control supplement (0.12 kg distillers grain/h/d) or control supplement containing Co (10 mg) for 28 d before and 14 d into the breeding season. All animals received 0.91 kg/d alfalfa hay and 0.91 kg/d grain-based supplement (15% CP, DM basis). On d 1 of the breeding season, estrous cycles of one-half of the ewe lambs from each age group in each pen were synchronized by treatment with GnRH (d 1; 100 μg) and PGF2α (d 7; 12.5 mg). Fertile rams fitted with marking harnesses were placed with all ewes on d 1 for a 35 d breeding period. Pregnancy status of all animals was confirmed by ultrasound 60 d after ram removal. Supplementation with Co did not influence (P = 0.49) ADG or d to estrus (P = 0.76). Synchronization tended to increase (P = 0.07) the proportion of ewes in estrus during the 10 d interval following PGF2α administration from 49.4% to 62.0%. However, synchronization did not (P = 0.84) influence overall conception rates. Breed (82.3, 93.8, 75.0, and 90.0% for Columbia, Hampshire, Rambouillet and Suffolk, respectively; P = 0.10) and age (90.1 vs. 80.2% for yearling and peripurberal ewe lambs, respectively; P = 0.09) tended to influence conception rates. Inclusion of supplemental Co in diets of yearling ewes or peripubertal ewe lambs prior to, and early in the breeding season did not influence ADG, d to estrus, or conception rates. Synchronization of estrus with the GnRH and PGF2α did not influence overall conception rates but tended to increase the proportion of ewes mated by the 10th day of the breeding season to values that would be predicted (62.5%) if all ewes were exhibiting estrous cycles.