Influência da estimulação do crescimento folicular na fertilidade de vacas de corte tratadas com eCG ou FSH em protocolos de IATF

Abstract

Background: The pregnancy rate in fixed-time insemination (FTAI) programs in beef cattle is affected by several factors, including the size of ovulatory follicle (OF). Induced ovulation of small OF (≤11 mm) reduces the chance of pregnancy after FTAI when compared to ovulation of larger follicles (≥12 mm). Hormonal treatments to enhance the follicle growth have been widely used in comercial beef cattle systems, aiming to improve the development of a dominant follicle. The objective of this experiment was to evaluate the effect of treatment with eCG or FSH at the time of progesterone implant removal on the diameter of OF, final follicle growth and pregnancy rate in beef cows. Materials, Methods & Results: Three hundred and seven suckled beef cows with body condition score 2.80 ± 0.24 (1 to 5 scale) and 45 to 70 days postpartum were synchronized to FTAI. On day 0, intravaginal implants containing 1 g of progesterone, were inserted and administered 2 mg of estradiol benzoate (EB), i.m. On day 7, intravaginal implants were removed and 150 μg of D-cloprostenol were administered i.m. At this time, three groups were assigned according to the treatments: eCG (400 UI eCG, i.m.), FSH (25 UI FSH, i.m.) and Control (1 mL Saline, i.m.). On day 8, all cows were injected with 1mg EB i.m. and were inseminated 52 to 56 h after the implant removal. Transrectal ultrasonography exams of ovaries were performed at the time of implant removal and before FTAI to determine the diameter of the largest follicle and the follicular daily growth. Pregnancy exams were conducted by transretal ultrasonography 30 days after FTAI. Statistical analyzes were performed by Chi-square test for pregnancy rate. Ovarian diameters and follicular daily growth were analyzed by ANOVA with means compared by Tukey test considering significant P value < 0.05. The diameter of the largest follicle measured on day 7 (10.07, 9.64 and 10.36 mm) and on day 9 (12.73, 11.53 and 12.57 mm), to eCG, FSH and Control respectively, did not differ within treatments (P > 0.05). Final follicular growth was greater (P < 0.05) on eCG group (1.33 mm/day) compared to Control (1.1 mm/day) and FSH (0.94 mm/day). The pregnancy rates of FTAI were 56.6% (60/106), 52% (51/98) and 50.4% (52/103) to eCG, FSH and Control, respectively, with no statistic difference (P > 0.05). Discussion: Suckled beef cows managed under grazing systems frequently have reduced follicular development and longer postpartum anestrous. In this situation, protocols to FTAI combining progesterone, estradiol and eCG or FSH at the time of progesterone device removal are encouraged to improve final follicular growth, ovulation and to increase conception rate. The hypotesis of similar follicular growth and pregnancy rate in suckled beef cows treated with eCG or FSH was not confirmed in the field trial. eCG treated cows had greater follicular growth, follicular diameter at day 9 and higher pregnancy rate compared to FSH and Control cows. FSH produced similar follicular growth rates to Controls and both were significantly smaller than observed in cows receiving eCG. Also FSH supplementation failed to enhance pregnancy rate compared to Control cows. The longer half-life of eCG and its greater LH-like effects could explain the difference in final follicular stimulation when compared to FSH in FTAI protocols. Thus, administration of a single dose of FSH at progesterone device removal failed to be an alternative to eCG in FTAI synchronization programs.