Nitrogen metabolism and fertility in cattle: II. Development of oocytes recovered from heifers offered diets differing in their rate of nitrogen release in the rumen

In vitro blastocyst production was determined for oocytes recovered postmor tem from 48 beef x dairy heifers offered low (Low NH3) or high (High NH3) p lasma ammonia-generating diets during the period of late antral follicle de velopment. Following the establishment of a reference estrus (d 0), the exp erimental diets were offered for an 18-d period starting on d 3 and during which a second estrus was induced (d 16) 4 d before the animals were slaugh tered. Blood samples collected at varying intervals were analyzed for ammon ia, urea, progesterone, and LH. Ovarian folliculogenesis was monitored dail y by transrectal ultrasonography. Ovaries were collected at slaughter and c umulus-oocyte complexes were aspirated from small (1 to 4 mm) and medium-si zed (> 4 to 8 mm) sized follicles. In vitro-matured and -fertilized putativ e d-1 zygotes were cultured for a further 7 d in vitro and embryo developme nt and metabolism were assessed. Relative to the low-NH3-generating diet, t he high-NH3-generating diet increased peak postprandial levels of plasma am monia (326.1 +/- 43.3 vs 52.1 +/- 7.4 mu mol/L; P < .001), mean levels of p lasma urea (7.0 vs 5.7 mmol/L SED = .2; P < .001), peak levels of plasma pr ogesterone prior to induced luteolysis (8.9 +/- .4 vs 6.8 +/- .3 mu g/L; P < .001), and follicular fluid levels of ammonia (267 +/- 18 vs 205 +/- 20 n mol/ml; P < .05) and progesterone (351 +/- 69 vs 199 +/- 26 ng/mL; P < .05) . The timing and level of the preovulatory LH surge was not affected by die tary treatment. Of oocytes cultured, cleavage (47.4 vs 62.4%; P = .02) and blastocyst production (10.9 vs 20.6%; P = .06) rates were reduced when the oocytes were derived from heifers offered the high- rather than the low-NH3 -generating diets. There were interactions between dietary treatment and fo llicle size class, which indicated that fewer blastocysts were produced fro m cleaved oocytes derived from medium-sized follicles of heifers offered th e high-NH3 treatment but that de novo protein synthesis was increased in su ch embryos. In conclusion, exposure to high levels of ammonia and(or) urea in vivo can significantly compromise the subsequent capacity of oocytes to develop to blastocysts in vitro, and oocytes recovered from medium-sized fo llicles are particularly sensitive to this effect.