Effect of progesterone supplementation in repeat-breeder cattle on conception and plasma progesterone.

Reposital-type progesterone (75 mg/d for cows, 40 mg/d for heifers) or saline were administered to 24 Holstein cattle to assess the effects of exogenous progesterone (P) on fertility in repeat-breeders. Treatments were administered daily from day 6 to 10 after fourth, fifth and sixth insemination. Cumulative conception rate (57.1%) for the fourth through sixth insemination was affected by lactation number, service number, plasma P (4.67 ng/ml for pregnant cows vs 4.06 ng/ml for nonpregnant cows) and corpus luteum size on days 5 and 15. Plasma P concentrations were affected by day of cycle and size of corpus luteum on both ovaries on day 15. A treatment by day interaction accounted for higher plasma P in P-treated cattle than saline-treated cattle from day 5 through 15. Administration of exogenous P during early life of the corpus luteum in the repeat-breeder increased plasma P. Higher plasma P was associated with insemination success.