The influence of short-term nutrient changes on follicle growth and embryo production following superovulation in beef heifers

Abstract

Acute decreases in nutrient intake can improve embryo quality in sheep, although reductions in ovulation rate can also occur. In cattle, short-term nutrient restriction prior to ovulation has been shown to increase subsequent pregnancy rates. Thus, the objective was to determine the effect of a severe reduction in food intake on follicle growth and embryo quantity and quality in heifers superovulated with FSH. Beef heifers (n=61) were offered a diet of grass silage and concentrates (ratio of 5:1, on a fresh weight basis), which was adjusted to provide a predicted intake of 28.6 Mcal/kg ME/d (H) or 9.6 Mcal/kg MEld (L). Heifers were synchronized with a progesterone-releasing device for 7 d. They were allocated to oocyte recovery (n=16/treatment) after 3 (225 IU) or 8 (600 IU) injections of FSH given at 12-h intervals. Oocytes were matured, fertilized and cultured individually in vitro. The remaining heifers (n=14/treatment) were superovulated using FSH (600 IU), and embryos were recovered 7 d after breeding. The embryos were morphologically graded and subsequently cultured for 24 h before differential staining to determine inner cell mass and trophectoderm cell numbers. Follicle numbers increased following 8 (16.6±2.0) compared with 3 (6.7±0.6) injections of FSH (P<0.0001). Heifers on the L diet had more follicles than those on the H diet (13.5±2.4 vs 9.6±1.2; P<0.06), which was predominantly due to an increase in the number of 7- to 10-mm follicles. However, this effect was only evident after 8 injections of FSH. There was no nutritional effect on cleavage rates in vitro (55.6±8.1 vs 53.8±9.0 for H vs L diets, respectively). However, cleavage rates were lower in oocytes collected after 8 than after 3 injections of FSH (31.3 vs 69.2%; P<0.0001). There was no significant effect of nutrition on ovulation rate after FSH (14.4±1.9 vs 16.3±3.0 for H vs L diet, respectively). The number of embryos recovered was not different between heifers on H (10.4±1.3) and L (11.3±2.4) diets. Following culture for 24 h, a significantly higher proportion of embryos from heifers on the L diet developed to the blastocyst stage (72.9 vs 41.5%; P<0.01). Total cell numbers on Day 8 were greater in embryos from heifers on the L diet (98.3 vs 75.4; P< 0.0001); yet the inner cell mass as a percentage of total cells was not different (21 vs 20%). These data indicate that low energy intake prior to and during superovulation resulted in more follicles and in improved embryo quality, as evident from the increased number of blastocysts formed and higher cell numbers.