Influence of nutrition and genetic selection for puberty on the reproductive response of Nelore heifers submitted to fixed-time AI and oocyte recovery with in vitro fertilization

This study evaluated the effects of nutrition and genetic selection for early puberty on the reproductive response of Nelore heifers submitted to FTAI and oocyte pick-up with in vitro embryo production. Prepubertal heifers ( n  = 198; 13.8 ± 0.07 months) were randomly assigned to a 2 × 2 factorial design with two nutritional treatments (improved nutrition, IN; moderate nutrition, MN) and two genetic groups for puberty (high genetic, HG, early puberty; low genetic, LG, normal puberty). Nutritional treatments lasted 90 days (Day -60 to 34) and body weight (BW) was recorded at regular intervals. On day 0 (beginning of 1 st FTAI protocol), a subset of heifers underwent ovum pick-up (OPU; n  = 39) and blood sampling ( n  = 81) for IGF-I, insulin, and glucose analysis. At the same time, subcutaneous fat thickness and gynecological examination were performed. All heifers then underwent an estradiol/progesterone estrus synchronization protocol twice for FTAI (24-day interval between FTAI). Data were analyzed by the GLIMMIX procedure of SAS 9.4. There was no interaction ( P >0.10) between nutrition and genetics for BW, subcutaneous rump fat thickness (RFAT), subcutaneous back fat thickness (BFAT), diameter of the largest follicle (LF), uterus diameter (UT), glucose, IGF-I, P/AI, and embryo production. Regardless of genetics, BW was greater for heifers on IN than MN on Day -30 ( P < 0.01) and 0 ( P < 0.01). Heifers on IN had greater RFAT ( P < 0.01), BFAT ( P < 0.01), LF ( P  = 0.02) and UT ( P < 0.01). Glucose and IGF-I were greater in heifers on IN ( P < 0.01). The pregnancy rate at 1 st FTAI ( P  = 0.02) and cumulative pregnancy rate were greater ( P  = 0.07) for heifers on IN compared to MN. The number of viable blastocysts per OPU were greater ( P  = 0.05) for heifers on IN than MN. Regardless of nutrition, HG heifers had greater RFAT ( P  = 0.03), BFAT ( P < 0.01), IGF-I ( P  = 0.04), LF ( P < 0.01) and UT ( P  = 0.02) than LG heifers. No effects of genetics were observed for pregnancy rate. HG heifers had a greater number of blastocysts ( P  = 0.05) and viable blastocysts ( P < 0.01) per OPU, and a greater viable blastocyst rate ( P  = 0.01) compared with LG heifers. There was an interaction nutrition*genetics ( P  = 0.03) for insulin concentration, with insulin being lower in the MN-LG group. The findings indicate that both nutrition and genetics can impact productive and reproductive performance in Nelore heifers.