The objective of this study was to determine the effects of feeding excess MP from feedstuffs differing in rumen degradability on ovarian function of beef cows. Non-pregnant, nonlactating beef cows (n = 18) were stratified by age, BCS, and BW to 1 of 2 isocaloric, isonitrogenous diets: (1) corn gluten meal (CGM) or (2) soybean meal (SBM), supplemented at 150% of MP requirements with ad libitum access to corn stalks for 58d. After a 20d supplement adaptation period, cows were synchronized for ovulation using the 5-d CO-Synch + CIDR protocol. Ten d after synchronization completion, 100 µg of GnRH was administered to reset follicular growth. Starting at GnRH administration and daily thereafter until spontaneous ovulation, transrectal ultrasonography was performed to diagram ovarian follicular growth, and blood samples were collected for hormone and metabolite analyses. Seven d after visual detection of estrus, corpus luteum (CL) size was determined and supplements were terminated. Data were analyzed using the MIXED procedures of SAS. Body weight and BCS, ovulatory follicular wavelength, antral follicle count, size of ovulatory follicle at dominance and duration of dominance were not different (P = 0.13) between treatments. Cows supplemented with CGM had greater (P < 0.01) post-dominance dominant follicle growth, larger (P = 0.03) dominant follicles at spontaneous luteolysis, shorter proestrus (36 vs. 68 h; P < 0.01) and larger (15.4 vs. 13.5 mm; P = 0.03) ovulatory follicles than SBM cows. No differences (P = 0.11) in estradiol, ratio of estradiol to ovulatory follicle volume, or plasma urea nitrogen concentrations were observed between treatments. Although CL volume and the ratio of progesterone to CL volume were not affected by treatment (P = 0.24), CGM treated cows tended to have lesser (4.7 vs. 5.7 ng/mL; P = 0.07) concentrations of circulating progesterone 7d post-estrus than SBM. In summary, these data illustrate that excess MP when supplemented to cows consuming a low quality forage may differentially affect ovarian function depending on rumen degradability of the protein source.