180 Effect of treatment with a long-acting cloprostenol sodium formulation in timed AI protocol in beef cattle

Luteolysis is a key event for the success of timed AI (TAI) protocols in beef cattle. An incomplete corpus luteum (CL) regression may determine a failure in decreasing progesterone (P4) concentrations to basal levels (under 0.5 ng/mL) at TAI, resulting in lower pregnancy rates. The aim of this study was to determine whether an experimental formula of a nonaqueous solution of cloprostenol sodium associated with fatty acids and solvents (PGF LA, BioRelease Technology) would induce complete luteolysis and increase oestrus manifestation and subsequent P/AI compared with the use of a commercial standard cloprostenol sodium product (PGF, Estron, Agener União Saúde Animal) in an oestradiol/progesterone-based TAI synchronisation protocol in multiparous and nulliparous beef females. Transrectal ultrasonography exams were performed on Day 0 to select females with detectable CL, on Day 9 to measure ovulatory follicle (OF) diameters, and 28–32 days after TAI for pregnancy diagnosis. 261 and 194 cross-bred cycling Angus cows and heifers with a body condition score (1–5 scale) of 2.9 ± 0.3 and 3.1 ± 0.2, respectively, received a 1 g P4 intravaginal device and 2 mg oestradiol benzoate, i.m. On Day 7, P4 devices were removed, and 1 mg oestradiol cypionate and either 500 mg PGF LA (n = 112 cows, n = 100 heifers) or PGF (n = 149 cows, n = 94 heifers) were given via i.m. Sacral and tail head regions were painted with marking paint to evaluate oestrus manifestation. Oestrus was determined by removal of 50% or more of tail paint between device removal and the moment of TAI, 48 h later. Blood samples (n = 35) were taken by coccygeal venipuncture before TAI to determine serum P4 concentrations by RIA. Continuous dependent variables were submitted to one-way ANOVA and predicted probability of pregnancy was analysed using a logistic regression model including parity, treatment, oestrus behaviour, and interactions. Intervals from luteolytic drug injection to oestrus (40.8 ± 8.6 h vs41.7 ± 7.8 h), oestrus manifestation (89% vs 85%), and OF diameter (11.1 ± 2 vs10.8 ± 1.9 mm) were similar (P > 0.05) between PGF LA and PGF treatment groups, respectively. Serum P4 concentrations at TAI were lower (P < 0.05) in the PGF LA (0.23 ± 0.20 ng/mL) compared with the PGF (0.65 ± 0.30 ng/mL) group. Pregnancy rate was higher (P < 0.05) in cows treated with PGF LA (63.3%) than in the PGF group (47.6%), being similar in heifers treated either with PGF LA (49%) or PGF (53.1%). Cows treated with PGF LA were 2.17 times (OR, P = 0.04) more likely to become pregnant after TAI than the PGF group, with oestrus manifestation and OF size not being influenced by the treatments (P > 0.05). In summary, PGF LA treatment was more effective to induce complete luteal regression, determining lower P4 concentrations at TAI and a higher pregnancy rate per AI in beef cows.