| The hypothesis that concentration of progesterone influences number of follicles that mature during estrus, and thus ovulation rate, was tested in cycling ewes. Ewes were assigned at random to three treatments, and bred at estrus after treatment. Ewes in low (LP) and high (HP) groups received exogenous progesterone from day 4 to 14; control ewes (MP) were untreated. The LP ewes were given PGF2alpha on day 6 to regress corpora lutea (CL). Numbers of follicles and CL were observed by transrectal ultrasonography. Fewer follicles disappeared at estrus in ewes on HP than in MP or LP. Ewes on LP had a greater number of CL compared with MP or HP, which did not differ. Number of CL increased, but lambs born per CL decreased linearly with decreasing concentrations of progesterone; therefore litter size was not changed. In conclusion, LP increased ovulation rates, but did not increase prolificacy. |
