Protein in Corn Silage

¹ Faculty of Agricultural Sciences, University of British Columbia, Vancouver, British Columbia and ² Abbotsford Veterinary Clinic, Abbotsford, British Columbia

Sources of protein: microbial and rumen-bypass

All animals need to consume protein to supply the building blocks, called amino acids, that are used to build the proteins in muscle, membranes, enzymes and milk. Ruminant animals are different from non-ruminants in how they obtain amino acids. Non-ruminant animals derive all their amino acids directly from the protein in their feed. Ruminants derive amino acids from two sources: from the microbes that grow in the rumen and are then digested in the intestines, and from the feed protein that passes through the rumen and is then digested in the intestines (Fig. 1). This protein is commonly referred to as bypass protein.

Figure 1. Schematic representation of protein digestion and utilization in the cow.

Microbes in the rumen require protein (nitrogen) and energy to grow and multiply. In order to acquire nitrogen for growth, these microbes break down feed protein into both amino acids and non-protein nitrogen compounds such as ammonia. Microbes derive energy from the carbohydrates (sugars, starches, cellulose and hemicellulose) found in grains and forages. Microbes pass out of the rumen and are digested in the small intestine providing 40 to 80% of the ruminant’s requirement for metabolizable protein, depending on stage of growth or lactation.

Rumen microbes grow best when the supply of energy and nitrogen is in synchrony. Slowly digesting carbohydrates such as cellulose are most compatible with protein sources having slow rates of degradation (breakdown) that provide a steady supply of nitrogen. Frequent meals also help to provide a steady supply of nitrogen for the microbes.

The proteins in corn silage and other forages contain both degradable and undegradable fractions (Fig. 2). The overall degradability of protein in the rumen is determined by two factors: the portion of protein that is digested by microbes in the rumen and the speed of digestion in the rumen relative to rate of passage out of the rumen. If the rate of passage out of the rumen is high (high feed intake), then the microbes will not have a chance to degrade the feed protein. Hence, rumen protein degradability will be decreased. The degradable protein fraction can be subdivided into those proteins that degrade rapidly and those that degrade over a longer period of time.

Figure 2. Disappearance of protein fractions in forages as a function of time.

Protein in silage corn

Although corn silage is low in crude protein (CP) content, a ration where corn silage is the main source of forage can contribute up to 25% of the crude protein requirement of a high producing dairy cow. Of interest is the proportion of corn silage crude protein that is soluble (assumed to be readily degradable) in the rumen and that which is slowly degradable (bypass protein) as shown in Table 1.

Microbes can use soluble protein if sufficient carbohydrate is supplied in the diet. Excess soluble protein will be absorbed through the rumen wall and excreted through milk (Milk Urea Nitrogen) or urine. As shown in Table 1, there is a large range in soluble crude protein values in corn silage. Soluble crude protein is increased by the ensiling process, hence samples of “green” or uncured silage will contain less crude protein in the soluble form. Similarly, there is a large variation in the amount of crude protein in corn silage.

Table 1. Crude protein degradation characteristics of corn silage collected from farms in coastal British Columbia




Soluble CP (% of CP)




Bypass CP (% of CP)




Rate of Ruminal CP Degradation (% per h)




From ML Swift, 2003.  PhD Thesis, U British Columbia, Vancouver, BC.


1. Swift, M.L. 2003. Ph.D. Thesis. University of British Columbia.