11 Early Induction of Luteolysis in Fixed-Time Artificial Insemination Protocols Increases Fertility in Beef Cows

Abstract

Complete luteolysis is essential for pregnancy success in beef cows submitted to fixed-time AI (FTAI) protocols. The objective of this study was to evaluate the effect of prostaglandin F (PGF) administration on Day 7 compared with Day 8 of an FTAI protocol on serum progesterone (P4) concentration, oestrus behaviour, ovulatory follicle (OF) diameter, and pregnancy per AI (P/AI). A total of 469 nonlactating Angus cows [with corpus luteum (CL), n = 359 or without CL, n = 110], body condition score 2.9 ± 0.2 (1 to 5 scale), maintained on pasture were used. Cows received a 1-g progesterone-releasing intravaginal device (Reproneo; GlobalGen, Jaboticabal, Brazil) and 2 mg of oestradiol benzoate i.m. (Syncrogen; GlobalGen) on Day 0. On Day 7, animals were randomly allocated into 2 groups. Cows were treated with 500 μg of sodium cloprostenol, IM (Inducio; GlobalGen), on Day 7 (PGF7, n = 238) or Day 8 (PGF8, n = 231). Then, P4 devices were removed and 0.5 mg of oestradiol cypionate, IM (Cipion; GlobalGen) was injected in all females on Day 8. The FTAI was performed 48 to 52 h after P4 device removal. Females had sacral region painted with appropriate paint (TELL TAIL, GEA, Hamilton New Zealand) on Day 8 to detect expression of oestrus. Transrectal ultrasonography (7.5-MHz linear transducer, MediSono P3V, Wilmington, DE, USA) was performed on Day 0 to detect presence of CL, on Day 10 to measure OF diameter, and on Day 40 to diagnose pregnancy. Blood samples were collected on Days 8 and 10 to measure P4 serum concentration by radioimmunoassay. Oestrus behaviour and pregnancy rate were analysed as binary outcomes using logistic regression (Proc GLIMMIX, SAS; SAS Institute Inc., Cary, NC, USA). OF diameter and P4 concentration were submitted to analysis of variance (Proc GLIMMIX, SAS). The oestrus manifestation was greater (odds ratio = 2.9; P = 0.0002) in females of PGF7 group (91.6%; 218/238) than PGF8 group (78.8%; 182/231). The P/AI of cows that exhibited oestrus was 60.2% (241/400) v. 39.1% (27/69), resulting in higher chance (odds ratio = 2.4) of pregnancy associated with oestrus behaviour (P = 0.0014). Diameter of the OF did not differ (P = 0.0881) between PGF7 (11.7 mm) and PGF8 (11.3 mm). The P/AI was higher (P = 0.0034) for PGF7 (63.9%, 152/238) v. PGF8 (50.2%, 116/231). In group PGF7, P4 on Day 8 did not differ between pregnant (1.7 ng mL⁻¹) and nonpregnant (1.9 ng mL⁻¹) females. However, group PGF8 pregnant females had lower P4 concentration (2.6 ng mL⁻¹) on Day 8 (P = 0.0005) than nonpregnant (3.4 ng mL⁻¹) females. On Day 10, P4 did not differ between treatments (PGF7 = 0.11 ng mL⁻¹ v. PGF8 = 0.09 ng mL⁻¹) and did not affect fertility (P = 0.2515). According to results, OF diameter and P4 concentration on Day 10 were not influenced by the day of PGF administration; however, earlier PGF injection resulted in higher oestrus behaviour and increased fertility.