Coproducts provide producers a concentrated package of crude protein (CP) and energy, which can be paired with low quality forages to make economical, nutrient rich diets that meet nutrient requirements of beef cows. This pairing is common in beef cow wintering diets, which provides the potential for excess CP supplementation to occur during the cold winter months.

However, the effects of excessive CP (specifically excess metabolizable protein [MP] from protein sources differing in degradability) on beef cow reproduction have not been made clear.

Therefore, the effects of diets containing 150% of MP requirements from a moderately low (soybean meal) or moderately high (corn gluten meal) rumen undegradable protein (RUP) source on ovarian function around the time of ovulation were evaluated using 18 non-pregnant, non-lactating Angus-Simmental beef cows.

Cows were allocated by age, BCS, and BW, and assigned to 1 of 2 isocaloric, isonitrogenous diets for 60 days: either 1) soybean meal (SBM) or 2) corn gluten (CGM).

We observed that excess dietary RUP via CGM enhanced ovulatory parameters of ovulatory follicle size, while excess SBM improved circulating progesterone concentration post-ovulation.

Based on these data, the source of CP when fed in excess may have differential impacts on ovarian function in mature beef cows. However, the ultimate impacts of excess dietary CP from different feedstuffs on fertility are still widely unknown at this time and warrant further research in a future study.