Reproductive performance of ewe lambs from ewes from different selection practices with or without induced estrus.
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Three groups of ewe lambs born in May (experiment 1; n=211) or April (experiment 2; n=174) were used to evaluate the effects of selection line and induction of estrus on pregnancy rate. Experiment 1 was a single factor experiment with induction of estrus as the main effect. In early December, May-born Targhee (n=82) and Rambouillet x Targhee (n=129) ewes were randomly assigned within body weight to one of two treatment groups: control or induction of estrus. Experiment 2 was designed in a 2x2 factorial array with the main effects of induction of estrus or selection line. In early November, April-born Targhee lambs (n=174) from two distinct selection lines were either treated as controls or received an estrus induction treatment. The two lines included an unselected control line of randomly bred ewes and a line that had been selected since 1976, based on the weight of lamb weaned. Ewes from each line were randomly assigned within body weight to one of the treatment groups. In experiments 1 and 2, estrus was induced using MAP pessaries. Pessaries were inserted for 12 days. At the time of pessary removal, ewe lambs received 400 IU eCG i.m. All ewe lambs were bred in multi-sire pens. Pregnancy rate and fetal numbers were determined either by lambing data or real-time ultrasound. Body weight, lambing date and fetal numbers were analyzed by GLM, and remaining variables were analyzed by CATMOD. For experiment 1, estrus induction increased (P<0.01) pregnancy rates (61 versus 31%) and number of fetuses estimated by real-time ultrasound (79 versus 35%) compared to control ewe lambs. Pregnancy rate and fetal number were increased (P<0.01) for the 1st year compared to the 2nd year. For experiment 2, estrus induction tended to increase (P<0.07) pregnancy rate, and pregnancy rate differed (P<0.01) between selection lines. Estrus induction increased (P<0.05) fetal numbers (0.96) compared to controls (0.77). Fetal numbers were greater (P<0.01) for the selected line (1.06) compared to random bred controls (0.67). Average date of lambing was earlier in both experiments for the estrus-induced ewe lambs compared to controls. These results indicate that induction of estrus can be recommended if increased reproduction is desired for ewe lambs.