323 EFFICIENCY OF CLOPROSTENOL-INDUCED LUTEOLYSIS IN SUPEROVULATED COWS

Abstract

The induction of multiple ovulations is a key procedure for in vivo embryo production. Many corpora lutea (CL) are developed, resulting in abnormally high progesterone concentrations. Luteolysis induction by prostaglandin F_2α and its analogues is well described in cows bearing one or few, but not multiple, CL, as occurs after superovulation. The objective of the first experiment was to compare the efficacy of a single prostaglandin F_2α treatment on inducing luteolysis in embryo donors immediately after flushing (D7, N = 24) or 4 days later (D11, N = 22). Holstein cows were superovulated with 400 IU of FSH following standard procedures. Embryo flushing was performed 7 days after AI, and cows were randomly allocated into 2 groups to receive either a 0.5 mg of sodium cloprostenol IM treatment immediately after flushing (D7 group) or the same treatment 4 days later (D11 group). Occurrence of luteolysis was monitored by plasma progesterone concentrations (P4), measured by radioimmunoassay in blood samples taken at 4-h intervals. There was no difference in P4 before treatment between D7 and D11 groups (28.6 ± 5.2 v. 36.4 ± 7.4 ng mL^–1, respectively; P > 0.05). Although cloprostenol caused a remarkable decline in P4 48 h after treatment in both groups (1.8 ± 0.3 and 1.6 ± 0.4 ng mL^–1 for D7 and D11 groups, respectively; P < 0.05), P4 continued decreasing in D11 cows thereafter, remaining smaller than 1 ng mL^–1 up to 196 h after treatment, whereas in D7 cows, there was no further reduction in P4. Luteolysis (P4 <1 ng mL^–1) was observed in all D11 cows, but failed in 11 of 20 (55%) D7 cows, in which P4 increased after the initial cloprostenol-induced decrease. The second experiment compared luteolysis in superovulated v. nonsuperovulated cows. Non-superovulated (control group, CG, N = 8) and superovulated cows (SOV, N = 6) received a single dose of sodium cloprostenol IM (0.5 mg) on day 11 after oestrus. Morphological and functional luteolysis were monitored daily by ovarian ultrasonography and P4 analysis; also, plasma LH was measured in blood samples taken every 20 min for 1 h, during 5 days. Individual CL size was smaller (1.8 ± 0.1 v. 3.5 ± 0.4 cm²) but total luteal tissue was greater (29.8 ± 7.0 v. 3.5 ± 0.4 cm²; P < 0.05) in SOV than in CG. A considerable decrease in P4 occurred in both groups 24 h after treatment (from 51.1 ± 12.9 to 5.1 ± 0.9 ng mL^–1 in SOV and from 5.9 ± 0.6 to 1.1 ± 0.1 ng mL^–1 in CG); however, SOV cows did not reach P4 values similar to CG until 96 h after treatment (0.9 ± 0.3 v. 0.4 ± 0.2 ng mL^–1, respectively; P > 0.05). There was no difference in initial LH values between SOV and CG (1.5 ± 0.1 and 1.5 ± 0.1 ng mL^–1; P > 0.05), but the slower decrease in P4 in the SOV group prevented LH from increasing up to 96 h after luteolysis induction, whereas mean LH values increased (P < 0.05) in CG after 24 h. In conclusion, luteolysis failure may occur when cloprostenol is given at the day of flushing (7 days after AI) in superovulated cows. In addition, luteolysis induction on day 11 after SOV is efficient, but the initial high progesterone concentration results in a slower rate of P4 decrease to basal levels.