Date of this Version
Published in BIOLOGY OF REPRODUCTION 47, 1009-1017 (1992).
Our working hypothesis was that the low concentrations of progesterone (P4) and synthetic progestins administered in hormonal regimens to control estrous cycles of cows would have similar effects on secretion of LH and 17β-estradiol (E2). In addition, we hypothesized that concentrations of exogenous P4 typical of the midluteal phase of the estrous cycle and the corpus luteum (CL) would have similar effects on LH and E2, and the effects would be different from those of synthetic progestins and low concentrations of P4. Cows (n = 29) were randomly assigned to one of five treatment groups: 1) one Progesterone Releasing Intravaginal Device (1PRID; n = 6); 2) two PRID5 (2PRID; n = 6); 3) norgestomet, as in Syncro-Mate-B® regimen (SMB; n = 6); 4) melengestrol acetate (MGA; 0.5 mg/day; n = 5); and 5) control (CONT; n = 6). Treatments were administered for 9 days (Day 0 =initiation of treatment). All cows from 1PRID, 2PBJD, SMB, and MGA groups were injected with prostaglandin F2α (PGF2α) on Days 2 and 5 of the treatment period to regress CL Cows in the 1PRID and SMB groups were also administered exogenous estrogen according to the respective estrous synchronization protocol for these products. Daily blood samples were collected from Day 0 to 35 to determine concentrations of P4. On Day 8, blood samples were collected at 15-mm intervals for 24 h to determine pattern of LH secretion. On Day 9, all treatments ceased and cows in the CONT group received injections of PGF2α. Blood samples were collected at 1-h intervals after cessation of treatments to determine time of the preovulatory surge of LH. During the treatment period, mean concentrations of P4 in 2PRID and CONT groups were greater (p < 0.05) than in cows from the other groups. Mean concentrations of E2 in cows from the SMB group were greater (p < 0.05) as compared to cows from the other groups during the treatment period. In addition, concentrations of E2 in cows from the 1PRID group were greater (p < 0.05) than in cows from the 2PRID and CONT groups. On Day 8 of the treatment period, frequency of LH pulses in cows treated with SMB was greater (p < 0.05) and mean concentrations of LH in cows from the CONT group were lower (p < 0.05) as compared to cows from the other groups. Amplitude of LH pulses was not different (p> 0.05) among cows receiving the various treatments. Initiation of the preovutatory surge of LH was 20 h earlier (p < 0.05) in cows from the SMB group and tended (p < 0.10) to be earlier in cows from the 1PRID group than cows in the 2PRID and CONT groups. Four of five cows treated with MGA failed to initiate a preovulatory surge of LH during the sampling period. Concentrations of P4 during the subsequent estrous cycle were greater (ı < 0.05) in cows from 2PRID and CONT groups than in cows from the other groups. We accept our working hypothesis that secretion of LH is similar in cows treated with the low dose of P4 and synthetic progestins. However, cows with the higher dose of P4 had a different profile of LH as compared to control cows with their corpus luteum in situ. Therefore, we reject this part of our working hypothesis.