THE INFLUENCE OF MONENSIN AND LASALOCID ON THE BREEDING PERFORMANCE OF EWES AND THE ENDOCRINE STATUS OF EWE LAMBS

Two experiments were performed, in December and August, to determine the effects of dietary ionophores on the breeding performance of ewes and the endocrine status of ewe lambs. Experiment 1 employed 127 mixed-parity ewes fed diets containing 33 ppm monensin (M, n = 43), 45 ppm lasalocid (L, n = 43) or no additive (C, n = 41). Estrus was synchronized with progesterone sponges and the ewes were allowed to breed. The proportion of ewes conceiving at their first estrus was higher (P < 0.05) for control ewes than for L ewes (70.7 vs. 51.2%) M ewes being intermediate (53.5%). Litter size of ewes conceiving at first estrus was not affected by treatment. Experiment 2 employed 57 ewe lambs allocated in a 3 x 2 factorial arrangement to be fed rations containing 33 ppm M, 45 ppm L or no additive for 26 d and to receive, or not, an injection of 50-mu-g GnRH at 21 d. For lambs receiving GnRH, blood samples were taken every 15 min for 2 h from 30 min pre-injection. At 72 h after GnRH injection, all lambs received an injection of 50-mu-g estradiol-17-beta. Further blood samples were taken from all lambs at 4-h intervals for 48 h. Serum LH and FSH concentrations increased from pre-injection means of 1.6 +/- 0.2 and 7.8 +/- 1.2 ng mL-1 to 20.9 +/- 1.9 and 13.4 +/- 1.4 ng mL-1, respectively, by 15 min after GnRH injection. These responses were not affected by either ionophore. The estradiol-induced LH surge height was reduced (P < 0.01) in lambs that had received GnRH. These data indicate that at the dietary inclusion levels employed, ionophores may be contra-indicated in the breeding ration.