318 Award Talk: The Evaluation of source and level of rumen undegradable protein on growth performance, follicular parameters, and embryonic development in developing beef heifers.

Abstract Rumen undegradable protein (RUP) source can impact heifer reproductive parameters. The objective of study 1 was to evaluate the effect of source and level of RUP on growth and follicular parameters in beef heifers grazing native rangelands. Beef heifers (N = 32) were randomly assigned to one of four supplement treatments: 1) 36% plant sourced RUP (36P, n = 8), 2) 50% plant sourced RUP (50P, n = 8), 3) 36% animal sourced RUP (36A, n = 8), or 4) 50% animal sourced RUP (50A, n = 8). Heifers were individually fed three times a week for 110 d and offered treatments at a rate of 1.13 kg/hd/d. Oocyte pick-ups (OPUs) were conducted 75 d after supplementation began. All oocytes collected from OPUs underwent in-vitro embryo production. Average daily gain (ADG) and BW were analyzed as repeated measures using MIXED procedures in SAS. Embryo data was analyzed using MIXED PROC of SAS with the number of oocytes as a covariate. Source, level, and source × level did not influence heifer BW and ADG (P ≥ 0.25). Source, level, and source × level did not affect presumptive zygotes and cleaved embryos (P ≥ 0.27). Heifers offered the 50 level of RUP tended to have increased number of blastocysts from OPUs compared to heifers offered level 36 of RUP (P = 0.09). The objective of study 2 was to evaluate source and level of RUP on ovarian parameters in heifers developed in a dry-lot. Crossbred heifers (N = 20) were randomly assigned to one of four RUP supplements: 1) 36% plant sourced RUP (36P, n = 5), 2) 50% plant sourced RUP (50P, n = 5), 3) 36% animal sourced RUP (36A, n = 5), or 4) 50% animal sourced RUP (50A, n = 5). Heifers were individually supplemented once daily at a rate of 1.13 kg/hd/d. for 77 d utilizing Calan Broadbent feeding systems and were offered ad libitum Sudan hay. At the end of the supplementation period heifers underwent bilateral ovariectomies. Weight, length, and height of each ovary was recorded. Surface antral follicles were counted and recorded on each ovary. Ovarian parameters were analyzed utilizing PROC MIXED. Level, source, and their interaction did not impact surface antral follicle counts, ovarian weight, length, or height (P ≥ 0.15). In conclusion, heifers fed 50 level of RUP tended to have increased blastocyst development in-vitro in study 1, therefore it is plausible that increased levels of RUP may improve conception rates in heifers. In study 2, level and source of RUP did not influence ovarian parameters. It is reasonable that the level and source of RUP is not affecting the ovary but other reproductive tissues.