Can Forage Quality Be Too Good for Cows?

High quality forages are the backbone of every feeding program. For years, dairy producers have been cutting forages early to maximize protein and energy content. Too often though, these forages do not result in the milk production one might expect from the feeding of high quality forage. What has gone wrong?

An important function of forages is to provide the fibre necessary for rumen function. Cutting forage early improves quality (protein, digestibility) but decreases the fibre content. Some top quality legumes may have ADF and NDF values less than 30 and 40%, respectively. It becomes difficult to balance a ration for energy while still maintaining sufficient fibre in the diet. The solution is to feed a blend of the top quality forage and 4 to 5 pounds of lower quality , higher fibre forage. In this manner, cows benefit from the high quality forage while still receiving sufficient fibre for the rumen.

Of equal concern is the effect that excessively high protein levels may have on production. Early cut alfalfa hay can have a crude protein level in excess of 20% and a soluble protein (SP) of over 30%. If these forages are ensiled, the SP can rise to over 60%. Soluble protein is the portion of the total protein which is rapidly converted to ammonia by rumen microbes. If energy is not lacking, the microbes are able to further convert the ammonia to microbial protein. Problems arise when the conversion of ammonia to microbial protein cannot keep pace with the amount of soluble protein entering the rumen. The excess ammonia is absorbed into the blood and passes to the liver where it is converted to urea. Most of the excess urea is excreted in the urine and some passes into the milk.

The conversion of ammonia to urea diverts feed energy which would normally be used for milk production. These losses can be significant. It has been estimated that diets high in soluble protein can result in seven pounds less milk as energy is diverted from milk production to urea synthesis.

High blood and milk urea nitrogen (BUN, MUN) levels have also been associated with decreased reproduction. The exact mechanism by which this occurs is unknown but several theories have been proposed. The energy cost of removing excess nitrogen from the body may lead to a negative energy balance in early lactation cows thus reducing hormone levels and fertility. Excess BUN may be toxic to ova, sperm or developing embryos, may produce a toxic uterine environment and/or negatively affect hormones needed to ensure pregnancy.

Proper Feeding Management

Proper feeding management can minimize the potential effects of feeding forages with high levels of soluble protein.

     1. Analyze forages for % soluble protein. It is particularly important that silages and haylages be
        analyzed.

     2. Blend high SP forages with forages low in SP. Dry hay has a significantly lower soluble protein
        value than silage.

     3. Ensure there are no other NPN sources in the diet (ex, ammoniated forage, urea-containing
        supplements).

     4. Feed a rapidly degradable energy source at the same time as high SP forages. This allows
        rumen microbes to convert more of the ammonia to microbial protein. Barley, wheat and high
        moisture corn are more rapidly degraded than dry corn. Increasing the fineness of grind will
        increase energy availability.

     5. Balance high SP forages with protein supplements low in soluble protein (and high in bypass
        protein). Examples of feeds low in SP are dried distillers grains, roasted soybeans, blood meal,
        corn gluten meal.

By Karen Dupchak (Farm Production Extension, Animal Nutritionist)
Manitoba Agriculture Food, and Rural Initiatives