SEASONALLY ANOVULATORY EWES FAIL TO RESPOND TO PROGESTOGEN TREATMENT IN THE ABSENCE OF GONADOTROPIN STIMULATION

The induction of synchronized estrus for June-July breeding of ewes tr eated with melengestrol acetate (MGA) or Norgestomet implants (SMB) wa s evaluated in relation to the ram effect and treatment with PG-600 at the time of progestogen withdrawal. Anovulatory Dorset, Suffolk, and Hampshire ewes (n=140) were randomly assigned within breed groups to 1 of 5 treatments: 1) control; 2) MGA only (MGA); 3) MGA + PG-600 (PMGA ); 4) SMB only (SMB); and 5) SMB + PG-600(PSMB). Beginning June 15, MG A- and PMGA-treated ewes were fed 0.3 mg MGA/d in a mixture of shelled corn and a commercially prepared pelleted supplement containing MGA. Concomitantly, the SMB- and PSMB-treated ewes were given a 3-mg SMB im plant inserted subcutaneously at the back of the ear. At the end of th e 10-d treatment period, MGA supplementation was discontinued, the SMB implants were removed, and both the PMGA- and PSMB-treated ewes were given a single intramuscular injection of PG-600 (400 IU PMSG + 200 IU hCG). One-half of the ewes in each treatment group were placed with f ertile rams equipped with marking harnesses on June 24 for a 30-d bree ding period, while the other ewes remained isolated from the rams. To monitor luteal activity of ewes exposed or not exposed to rams, blood samples for progesterone analysis were collected at 0, 4, 7 and 10 d a fter termination of the progestogen treatments. At least 79% of all th e ewes exposed to rams had luteal activity. In the absence of rams, on ly ewes treated with PG-600 had an increase (P < 0.0001) in luteal act ivity. Fertility for control and MGA- and SMB-treated ewes was not dif ferent, but the progestogen-treated ewes lambed an average of 12 d ear lier in the lambing period than the control ewes. These data indicate that ram exposure or administration of an exogenous source of gonadotr opin after progestogen withdrawal is necessary for the induction of ov ulation and subsequent corpus luteum formation for the June-July breed ing of anovulatory ewes.