Serum progesterone in heifers with different luteal activity treated with vaginal devices

The progesterone (P4) device is a very important step in the ovulation control in Timed Artificial Insemination (TAI) protocols. It represents about 43% of the hormone costs, thus it has been the subject of several studies on efficiency of the reused device as an alternative to reduce TAI costs. However, to our knowledge, profiles for P4 release in animals with different endogenous concentrations of P4 are not clearly described. This study aimed to evaluate serum concentration of P4 in females with different ovarian conditions - related to luteal activity - and treated with a new intravaginal device (1g of P4) for 8 days. Thirty normally cyclic crossbred heifers were divided into three groups: for G1 and G2, P4 device was inserted (D0) seven days after ovulation (7 day old CL). Additional PGF (0.15 mg of D-cloprostenol) was given three days later to promote luteolysis in the G2 group. For G3, CL was killed before P4 insertion and the exogenous progesterone was the only source of this hormone. Blood samples were collected on D0, D3, D5 and D8 and P4 concentration was measured by radioimmunoassay (RIA). Means for P4 concentration were compared among groups in each day and within the group among days using the Tukey test. Before P4 device insertion (D0), P4 levels were higher (P<0.05) in G1 and G2 when compared to G3 (5.3±3.1 and 5.3±1.4 vs. 0.6±0.3ng/mL, respectively). Three days later (D3), the same pattern was observed (5.7±2.6 and 5.4±2.0 and 3.6±0.8ng/mL, respectively for G1 and G2 vs. G3, P<0.05). Thirty-six hours (D5) after PGF injection (G2), P4 in G2 dropped to levels similar to the G3 group and both differed (P<0.05) from G1 (3.3±1.6 vs. 2.4±0.9 and 2.1±0.7ng/mL, G1 vs. G2 and G3, respectively). There were no differences (P>0.05) among groups on D8 (3.1±1.3, 1.8±0.8 and 1.6±0.6ng/mL, respectively, for G1, G2 and G3). Progesterone increase after P4 insertion (D3 - D0) was higher (P<0.05) in G3 compared to G1 and G2 (2.8±0.9 vs. 0.4±1.8 and 0.2±1.4ng/mL, respectively). The interpretation was that animals with higher endogenous P4 promote less release of the exogenous P4 from the device.Therefore, the remaining P4 levels from used progesterone devices depend on the physiological condition of the animal at the time of insertion and possible changes during the treatment.