Reproductive performance of ewe lambs from ewes from different selection practices with or without induced estrus

Three groups of ewe lambs born in May (experiment 1; n = 211) or April (exp eriment 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 ex periment with induction of estrus as the main effect. In early December, Ma y-born Targhee (n = 82) and Rambouillet x Targhee (n = 129) ewes were rando mly assigned within body weight to one of two treatment groups: control or induction of estrus. Experiment 2 was designed in a 2 x 2 factorial array w ith the main effects of induction of estrus or selection line. In early Nov ember, 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 an d a line that had bren selected since 1976, based on the weight of lamb wea ned. Ewes from each line were randomly assigned within body weight to one o f the treatment groups. In experiments 1 and 2, estrus was induced using MA P pessaries. Pessaries were inserted for 12 days. At the time of pessary re moval, ewe lambs received 400IU eCG i.m. All ewe lambs were bred in multi-s ire pens. Pregnancy rate and fetal numbers were determined either by lambin g data or real-time ultrasound. Body weight, lambing date and fetal numbers were analyzed by GLM, and remaining variables were analyzed by CAT-MOD. Fo r experiment 1, estrus induction increased (P < 0.01) pregnancy rates (61 v ersus 31%) and number of fetuses estimated by real-time ultrasound (79 vers us 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 experi ment 2, estrus induction tended to increase (P <less than> 0.07) pregnancy rate, and pregnancy rate differed (P < 0.01) between selection lines. Estru s 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 earlie r in both e experiments for the estrus-induced ewe lambs compared to contro ls. These results indicate that induction of estrus can be recommended if i ncreased reproduction is desired for ewe lambs.. Published by Elsevier Scie nce B.V.