Evaluation of models to induce low progesterone during the early luteal phase in cattle.

Two experiments were designed to evaluate models for generation of low circulating progesterone concentrations during early pregnancy in cattle. In Experiment 1, 17 crossbred heifers (Bos taurus) were assigned to either prostaglandin F(2alpha) (PGF(2alpha)) administration on Days 3, 3.5, and 4 (PG3; n=9) or to control (n=8). Blood samples were collected from heifers from Days 1 to 9 for progesterone assay. Progesterone concentrations were decreased (P<0.03) between 18 and 48h after first PGF(2alpha) treatment in heifers assigned to PG3 compared with that of controls. In Experiment 2, 39 crossbred heifers detected in estrus were inseminated (Day 0) and assigned to either (1) PGF(2alpha) administration on Days 3, 3.5, and 4 (PG3; n=10), (2) PGF(2alpha) administration on Days 3, 3.5, 4, and 4.5 (PG4; n=10), (3) Progesterone Releasing Intravaginal Device (PRID) insertion on Day 4.5 with PGF(2alpha) administration on Days 5 and 6 (PRID+PGF(2alpha); n=10), or (4) control (n=9). Blood samples were collected daily until Day 15, and conceptus survival rate was determined at slaughter on Day 16. Progesterone concentrations during the sampling period in the PG3 and PG4 groups did not differ but were less than that of controls (P<0.01). After an initial peak, progesterone concentrations in the PRID+PGF(2alpha) group were similar to that of controls. More heifers in the PG4 group (6 of 10) had complete luteal regression than did those in the PG3 group (3 of 10). Conceptus survival rate on Day 16 did not differ between groups. There was a significant correlation between progesterone concentration on Days 5 and 6 and conceptus size on Day 16. In summary, treatment with PGF(2alpha) on Days 3, 3.5, and 4 postestrus appeared to provide the best model to induce reduced circulating progesterone concentrations during the early luteal phase in cattle.