176 Luteolytic efficiency of a single or double dose of cloprostenol sodium in a 5-day artificial insemination protocol in dairy cows

This study aimed to compare induction of luteolysis using two preparations of cloprostenol sodium (PGF) in a single or double dose during presynchronization or 5d-Ovsynch. Lactating dairy cows with a body condition score of 2.7 ± 0.2 were submitted to a presynchronization treatment (Day −29 and −18) and a 5-day timed AI protocol (GnRH Day −8; PGF Day −3; GnRH Day −1 and AI Day −0). In Experiment 1, cows received as the first injection of presynchronization protocol (Day −29) 500 µg of PGF, IM, of a non-aqueous solution associated with fatty acids (PGF LA, n = 8; BioRelease Technologies) or a commercial product (control, n = 9; Estron, Agener União Saúde Animal). Cows were eligible to be treated either with PGF LA or PGF when one or more corpus lutea (CL) were detected by ultrasound (US) on Day −29. Blood samples were collected 0, 6, 12, 24, 36, and 48 h after treatment. Plasma samples were stored at −20°C until they were assayed for P₄ concentration by chemiluminescence-immunoassay with an analytical assay sensitivity of 0.2 ng mL⁻¹. Successful luteolysis was assumed to have occurred when pretreatment P₄ concentrations were >1 ng mL⁻¹ before treatment and then decreased to <1 ng mL⁻¹ by 48 h. Preovulatory follicle diameter was measured by US at 48 h and oestrous behaviour observed between 48 and 96 h. In Experiment 2, cows were enrolled (PGF LA, n = 9 and control, n = 9) to receive a twice-luteolytic dose (1000 µg) as one (PGF LA, IM, Day −3) or two injections of PGF, IM, (Day −3 and −2) 5 days after GnRH of the synchronization protocol. Ultrasound examinations and blood collections were done as described in Experiment 1 and volumes of CL at 0 and 48 h were calculated (V = 4/3 × R³ × π). The unpaired Student t-test was used to examine if luteal volume, number of accessory CL, P₄ concentration, and follicular diameter differed between treatments, and chi-square was used to compare oestrus manifestation and complete luteolysis. In both experiments, differences with P ≤ 0.05 were considered significant and 0.05 < P ≤ 0.1 were considered as tendencies. In Experiment 1, no differences were detected on P₄ concentrations at 0 h (4.2 ± 4.3 vs 4.8 ± 2.9 ng mL⁻¹) and 48 h (1.0 ± 0.7 vs 1.0 ± 1.2 ng mL⁻¹) and complete luteolysis (62.5% vs 66.6%) between PGF LA and control treatment, respectively. Cows treated with PGF LA had more oestrus manifestation (62.5%) than control cows (44.4%), whereas dominant follicle diameters were similar (PGF LA: 14.2 ± 3.1 vs control: 13.4 ± 5.3 mm) at 48 h. In Experiment 2, PGF LA cows had a greater incidence of accessory CL (44.4 vs 22.2%) resulting in a greater CL volume at 0 h (PGF LA: 10.4 ± 5.5 vs control: 4.6 ± 2.4 cm³) and at 48 h after PGF treatment (PGF LA: 5.7 ± 4.2 vs control: 2.3 ± 1.3 cm³). The P₄ concentrations tended to be higher in PGF LA cows at 0 h (6.4 ± 3.3 ng mL⁻¹) compared with control cows (3.8 ± 2.0 ng mL⁻¹) but were similar at 48 h (PGF LA: 1.2 ± 0.7 vs control: 0.9 ± 0.6 ng mL⁻¹). Complete luteolysis tended to be lower in PGF LA (55.5%) compared with control cows (77%), whereas dominant follicle diameters did not differ between treatments (PGF LA: 14.2 ± 1.7 vs control: 13.8 ± 2.1 mm). Data indicate that regression of newly formed CL resultant of ovulation to the first GnRH treatment was improved when twice the luteolytic dose of PGF was divided into two injections compared with one injection of PGF LA in dairy cows.