By Jamie Martin
Protein supplementation can play an important role in supporting growth when cattle are fed corn silage-based diets.
According to Erin Laborie, a Nebraska Extension educator at the University of Nebraska–Lincoln, corn silage generally contains between 6.5% and 8.5% crude protein. Most of that protein is rumen degradable protein (RDP), which is used by microorganisms in the rumen.
Protein that escapes breakdown in the rumen is known as rumen undegradable protein, or RUP. It moves to the small intestine, where it is digested and absorbed.
Metabolizable protein (MP) comes from microbial protein produced in the rumen as well as RUP and supports cattle maintenance, growth and other metabolic functions. When the diet fails to provide adequate MP, animal growth can suffer.
The protein in corn silage is relatively low in RUP. Data from the 2022 Nebraska Beef Cattle Report indicate that approximately 16% of corn silage crude protein is RUP.
Distillers grains are commonly used to improve the protein supply because roughly 63% of their crude protein is RUP. Research reported in the 2011 Nebraska Beef Cattle Report found that adding distillers grains to silage-based diets could improve performance in growing calves.
Soybeans could serve as another protein source when distillers grains are expensive or difficult to obtain. They contain about 40% crude protein and can contribute fat to the ration. The amount of protein that escapes rumen degradation depends on processing.
Raw whole soybeans contain roughly 20% RUP, while roasting can raise that figure to at least 50%.
A recent University of Nebraska–Lincoln trial compared modified distillers grains with raw whole and roasted soybeans in diets based on corn silage.
Each protein source was fed at 7%, 14% or 21% of diet dry matter for 84 days, with the rations designed to meet or exceed the MP needs of growing heifers.
Modified distillers grains produced the greatest feed intake and weight gain. Roasted soybeans, however, resulted in better feed efficiency. Increasing the inclusion rate to 21% generally improved gains and feed conversion compared with the 14% level.
Researchers suggested the response may have been related to additional dietary energy because protein requirements were already satisfied.
Overall, the findings suggest that both raw and roasted soybeans can be practical alternatives to distillers grains. Roasted soybeans may provide a performance benefit because of their greater RUP concentration, while the choice between protein sources can depend on market price and availability.
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