Can You Feed Too Much Protein?

The answer is yes! Almost everyone has had experience with cows scouring when placed on pasture or if fed a high protein forage. Of more importance, however, is the effect that high dietary protein levels can have on reproduction.

As we have discussed before, feeds differ in their proportions of soluble, degradable and undegradable protein. For example, alfalfa hay and haylage could both have 20% crude protein but differ considerably in the amount of soluble and degradable protein they contain. Ensiled forages and those cut at an early stage of maturity will contain a much higher level of soluble protein than dry and/or more mature forages.

The problem arises when too much of these highly soluble feeds are fed. Even though the crude protein requirement for early lactation rations may be met with minimal protein supplementation, there is an excess of soluble, readily degradable protein.

Some of the ammonia produced by the breakdown of degradable protein is used by rumen microbes to synthesize microbial protein. Some is carried to the liver via the bloodstream where it is detoxified and then excreted in the urine. Removing excess protein from the body requires energy. When there is an excess of degradable protein and/or a shortage of energy, these systems cannot keep up with the high levels of ammonia. The ammonia can then cause toxic effects in many parts of the body including the uterus. A toxic uterine environment is believed to impair fertilization and embryo development thus leading to lower conception rates. A reduction in available energy may also lead to rebreeding problems.

When balancing rations, attention should be given not only to meeting the % crude protein requirement but also to ensuring that the right proportions of soluble, degradable and undegradable protein are provided. Soluble protein values are normally provided in a routine feed analysis and should not exceed 30% of crude protein in the complete diet. Feeds high in undegradable protein and low in degradable and soluble protein will be needed to offset the high soluble protein levels found in some forages. On average, degradable and undegradable protein levels should be 60 and 40% of total protein, respectively.

Blood and milk urea levels appear to reflect a cow's protein status. An exciting new test for analyzing milk urea nitrogen (MUN) is presently being evaluated in parts of Canada and the U.S. This may offer producers a convenient way of determining the effectiveness of their feeding program.

For further information contact:

Karen Dupchak
Farm Production Extension, Animal Nutritionist
Manitoba Agriculture Food, and Rural Initiatives
204-545 University Crescent
Winnipeg, MB R3T 5S6
Phone: 204-945-7668
Fax: 204-945-4327