Title: The evaluation of ruminal undegradable protein supplementation level on growth performance and ovarian development in beef heifers grazing native rangelands.

Increased supply of rumen undegradable protein (RUP) in beef heifers improved pregnancy rates, however physiological mechanisms underlying enhanced conception rates remain unclear. The objective of this study was to determine the effects of the level of RUP on growth performance, ovarian morphometrics, and ovarian development in beef heifers grazing native rangelands. Beef heifers (N = 30) were randomly assigned to one of two diets: 1) 36 % crude protein (CP) containing 36 % RUP (36RUP, n = 15) or 2) 36 % CP containing 50 % RUP (50RUP, n = 15). Heifers were individually fed treatments three times a week for 90 d at a rate of 1.14 kg/hd/d and bilateral ovariectomies were conducted at the end of the feeding period. Diets did not influence heifer growth (P = 0.55) or average daily gain [ADG: (P = 0.17)]. Antral follicle counts and ovarian histology did not differ among treatments (P ≥ 0.40). Ovarian morphometrics, size of the largest preovulatory follicle, surface antral follicle counts, and corpus luteum weight were not impacted by diets (P ≥ 0.31). Circulating urea nitrogen concentrations were increased in 36RUP heifers compared to 50RUP heifers (P < 0.01). Estradiol and progesterone follicular fluid concentrations were not influenced by treatments (P ≥ 0.19), however 36RUP heifers tended to have elevated urea nitrogen follicular fluid concentrations compared to 50RUP heifers (P = 0.06). In conclusion, feeding an increased level of RUP did not improve growth, ovarian morphometrics, and ovarian histology in heifers grazing dormant forage.