The aim of this study is to investigate the effect of progesterone (P4) on the multiple follicular growth during superovulation treatment in dairy cows. Animals were divided into three groups as follows; 1) Growing CL + CIDR (G1): Cows (n=4) were received a total dose of 28 A.U. FSH through the first 4 days (twice daily) after natural ovulation (day 0=d0). CIDR was inserted on d1 and removed 12 h after the last dose of the FSH. 2) Growing CL (G2): Cows (n=4, 7 trials) were treated similar to G1 but without CIDR. 3) CL absence (G3): Cows (n=4, 7 trials) received PGF2α at 10 days after ovulation. After 36 h, all follicles (≥5 mm) were aspirated (d0). The FSH was given 24 h after aspiration as G1. The follicles were counted on d1, 3 and 5 in all groups. Blood was collected daily for 6 days. P4, E2, IGF-I and growth hormone (GH) were measured by EIA. While the P4 level was above 1 ng/ml from d2 to d5 in G1, it was under 0.5 ng/ml in the G3. P4 of G2 showed a gradual increase from 0.5 ng/ml at d1 to 2 ng/ml at d5. In G3, all cows showed an increase in E2 at d3 or d4 followed by an increase of IGF-1 within 24 h. GH increased concomitantly with the E2 increase in 4 trials out of 7. In contrast, the cows in G1 and G2 showed neither E2 nor IGF-1 increase. At d5, the number of follicles ≥8mm in G3 (22.8±2.0) was significantly (P<0.05) higher than G2 (9.2±2.9) and G3 (11.6±2.0). These data demonstrate that low P4 level during FSH treatment enhance multiple follicular growth and E2 secretion which followed by increase of IGF-1. Therefore, P4 may play a critical role in the superovulation response by controlling the multiple follicular growth.
