Abstract

This thesis summarises and discusses results of studies concerning the ovarian and endocrine responses of anestrous ewes to the ram effect, the relation between the use of progestogen primings and the reproductive response of anestrous and cyclic ewes to the ram effect, and the endocrine and testicular changes in rams used to stimulate anestrous ewes. Ovarian responses to the ram effect (monitored ultrasonographically) were highly variable. Ewes responded with two luteal phases (short and normal, respectively), delayed ovulations (Days 5-7) followed by normal or short luteal phases, luteinization of non-ovulatory follicles, luteinized follicular cyst and no luteal phase. There was no relation between the growth status of the largest follicle present when rams were introduced and the ovarian responding pattern of anoestrous ewes to the ram effect. Similar results in estrus incidence and fertility were obtained after using intravaginal sponges containing medroxyprogesterone acetate (MAP) for 6, 9, or 13 days. Priming with sponges containing 20, 40, or 60 mg of MAP for 6 days gave similar reproductive results. Six-day primings with sponges impregnated with MAP or fluorogestone acetate, or intravaginal devices containing progesterone (CIDR) were equally effective in improving the response to the ram effect. A single administration of 2.5 mg of MAP 1, 3, or 5 days before the introduction of the rams concentrated estrus in ewes 17 to 20 days later. In MAP-primed ewes, the endocrine pattern of the induced follicular phase of ewes that came into estrus was similar to a normal follicular phase, in ewes that ovulated without expressing estrus no significant increase in estradiol-17b or decrease in FSH was observed. The ram effect does not seem to affect the ovarian response of cyclic ewes during the midbreeding season. Rams used to stimulate anestrous ewes show an increase in LH and testosterone concentrations beginning at 12 h after joining, and high concentrations are maintained while estrous ewes are present and mating takes place in the flock. The response of rams to estrous ewes may be at least part of the mechanism by which anestrous ewes submitted to the ram effect express maximum reproductive response when ewes in estrus are introduced together with the rams.