Influence of different doses of progesterone treatments on ovarian follicle status in beef cows

To determine a dose of progesterone (P4) that allow ovarian follicular wave control, Aberdeen Angus cows were randomly assigned into four groups: T600 (n = 5), 600 mg of P4/day; T400 (n = 5), 400 mg of P4/day; T200 (n = 4), 200 mg of P4/day and Control (n = 4) (excipient only). Progesterone was injected from day 3 to 9 of estrous cycle. Ultrasonographies and blood sample collections were performed daily from day 2 to 10 and on day 15 of the estrous cycle. Additionally, an ultrasonographic study was conducted on day 13. Progesterone concentrations were different among all groups (P < 0.01). The diameter of the dominant follicle was greater for control than for T200, T400 and T600 groups (P < 0.01); there was no difference between T200 and T400 (P > 0.05), but they had a greater diameter follicle than the T600 group (P < 0.01). The growth rate of the dominant follicle between day 3 and 7 of estrous cycle was greater for control group (1.63 ± 0.3 mm day−1) than for T200 (0.56 ± 0.19 mm day−1, P < 0.05), T400 (0.6 ± 0.23 mm day−1, P < 0.05) and T600 (0.11 ± 0.13 mm day−1, P < 0.01) groups. The mean number of class I follicles (3–4 mm) per day for the entire experimental period was less for the control group than for T200 (P < 0.05), T400 and T600 (P < 0.01) groups (3.7 ± 1.3; 5.3 ± 1.3; 6.6 ± 1.8 and 8.1 ± 1.9, respectively). The mean number for the T200 group was less than for T600 (P < 0.05) and similar for T400 and T600 groups (P > 0.05). The number of class III follicles was greater for control group than for the other groups (P < 0.01). T200 and T400 groups had similar numbers of class III follicles (P > 0.05) and both had greater numbers of follicles than the T600 group (P < 0.05). The diameter of the corpus luteum of the T600 group (15.8 ± 1.6 mm) was less than for control (21.0 ± 2.5 mm, P < 0.01), T200 (19.3 ± 2.7 mm, P < 0.01) and T400 (20.0 ± 2.2 mm) groups (P < 0.05). The mean diameter of corpus luteum of T200 was similar to T400 (P > 0.05), but different from the control group (P < 0.05). In conclusion, the daily intramuscular administration of 200 mg or more of progesterone from day 3 to 9 of the estrous cycle indicates that plasma concentrations of progesterone can be used to modify the pattern of follicular development during the follicular wave. From day 5 of the estrous cycle, progesterone concentrations greater than 15 ng/ml (T600 group: 600 mg/day of progesterone from day 3 to 9 of the estrous cycle) inhibit dominant follicle development, increase the class I follicle populations (3–4 mm) and diminish the development of the corpus luteum.