Abstract #314

# 314
Effects of supplementing excess amounts of MP from a moderately abundant RUP source on ovarian function of beef cows consuming low quality forage.
Taylor C. Geppert*1, George A. Perry2, Patrick J. Gunn1, 1Department of Animal Science, Iowa State University, Ames, IA, 2Department of Animal Sciences, South Dakota State University, Brookings, SD.

The objective of this experiment was to determine the effects of excess MP supplementation from a moderately abundant RUP source on ovarian function in beef cows consuming low quality forage. Non-pregnant, nonlactating mature beef cows (n = 16) were allocated by age, BW and BCS to 1 of 2 isocaloric supplements designed to maintain BW for 60 d. All cows were offered ad-libitum access to corn stalks and individually offered a corn gluten meal-based supplement at (1) 125% (MP125) or (2) 150% (MP150) of NRC MP requirements. After a 20-d supplement adaptation period, cows were synchronized for ovulation using the 5-d CO-Synch + CIDR protocol. Ten days after synchronization, 100 ┬ÁL of GnRH was administered to reset follicular growth. Starting at GnRH administration and daily thereafter, transrectal ultrasonography was performed to diagram follicular waves, and blood samples were collected for hormone and metabolite analyses. Seven days after observation of estrus, corpus luteum (CL) size was determined via ultrasound and supplementation was terminated. Data were analyzed using the MIXED procedures of SAS. There were no differences (P ≥ 0.21) between BW and BCS. Preovulatory ovarian follicle characteristics including size at dominance, duration of dominance, post-dominance growth, size at spontaneous luteolysis, post-luteolysis growth, length of proestrus, and wavelength were not different between treatments (P ≥ 0.11). However, ovulatory follicles were larger (P = 0.04) and average antral follicle count was greater (P = 0.01) in the MP150 than MP125 treatment. Estradiol concentrations and the ratio of estradiol to ovulatory follicle volume was not affected (P ≥ 0.51) by treatment. Although CL volume 7 d post-estrus was greater (P < 0.01) in MP150 than MP125, circulating progesterone 7 d post-estrus and the ratio of progesterone to CL volume was not different (P ≥ 0.21). In conclusion, supplementation of CP at 150% of NRC MP requirements from a moderately undegradable protein source may enhance growth of the ovulatory follicle and subsequent CL compared with MP supplementation at 125% of NRC requirements.

Key Words: crude protein, follicle, ovulation