Effect of Post Insemination Progesterone Supplement on Pregnancy Rates of Repeat Breeder Friesian Cows

Fifty repeat breeder (RB) Friesian cows were allocated to five groups of 10 cows each, to determine the effect of progesterone (P4) supplement on P4 concentrations and pregnancy rates during the periods of corpus luteum (CL) formation and development between days 2-7 and 7-12 following a spontaneous or $PGF_{2{\alpha}}$-induced estrus. Cows were artificially inseminated during $PGF_{2{\alpha}}$-induced (PGF-P4-d2 and PGF-P4-d7 groups) or spontaneous (S-P4-d2, S-P4-d7, and control groups) estrus. Progesterone-releasing intravaginal device (PRID) devoid of estrogen capsule were inserted either on d 2 (PGF-P4-d2 and S-P4-d2 groups) or d 7 (PGF-P4-d7 and S-P4-d7 groups) post-insemination and left in place for 5 days. Control cows did not receive any treatment. Blood samples were collected for progesterone analysis from all cows once daily for 4 days starting on the day of estrus (d 0) and once every 3 days thereafter until d 22. Progesterone treatment by day interaction accounted for higher plasma P4 in treated than non-treated control cows. Progesterone concentrations differed significantly (p<0.05) during metestrus (d 2 to d 7) but not during diestrus (d 7 to d 12). $PGF_{2{\alpha}}$ treatment, lactation number, service number or their interactions did not affect progesterone concentrations and pregnancy rates. Therefore, cows were grouped according to the day of P4 supplement irrespective of the $PGF_{2{\alpha}}$ treatment. Progesterone supplement on d 7 but not d 2 significantly increased (p<0.03) pregnancy rates in repeat breeding cows with four or more previous services but not in cows in their third service. In conclusion, post-insemination P4 supplement to repeat breeding cows with four or more previous services improved pregnancy rates and should be advocated when no specific reason for infertility is diagnosed. Further studies with larger numbers of repeat breeding cows under field conditions are needed to ascertain the findings of this study.