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OSU Extension

College of Food, Agricultural, and Environmental Sciences

October 19, 2016 - 8:33am -- Anonymous

Forage Management Considerations After Frost

            We have had a few, light scattered frosts in the area that have generated some questions about forage use after a frost.  The two most common questions concern the use of warm season grasses in the sorghum family and grazing alfalfa.  The issue with grasses in the sorghum family, which includes sorghum-sudangrass hybrids, sudangrass and Johnsongrass in addition to sorghum, is that they contain cyanogenic glycosides and enzymes that convert those compounds to free cyanide (sometimes called Prussic acid) within their cells.  Prussic acid or cyanide is a lethal toxin. 

Under normal circumstances the cyanogenic glycosides and the enzymes are held in different locations within the plant cell and don’t come into contact with each other.  However, when plant cells are ruptured after being frozen, chopped, wilted or crushed, those cell barriers are broken and cyanide can rapidly form.  Cyanide is a gas and it will volatilize and leave the plant tissue but it takes some time, thus the recommendation is do not allow livestock to graze frost damaged forages until several days (3-4) have passed. Generally this refers to a hard frost. In the case of light frosts where the temperature is greater than 28 F, there are publications that say to wait 2 weeks until grazing.  The highest concentration of prussic acid is found in the leaves of immature plants (less than 18-24 inches tall) while stalks of mature plants (greater than 30 inches tall) contain the lowest concentration. 

Probably the safest and least risk practice of utilizing sorghum species forages after frost is as a dry hay or ensiled forage.  By the time the plants are dry enough to bale the cyanide gas will have volatilized and dissipated from the plant, so there is no feeding risk.  In the case of an ensiled forage or wet wrapped baleage, the cyanide concentration is greatly reduced during the ensiling process.  The general recommendation is not to feed these ensiled or baleage forages until at least 4-6 weeks after ensiling or wrapping.

Occasionally there are questions about grazing alfalfa after a frost.  Anytime a pure or very high percentage legume is grazed, the livestock owner should take precautions to prevent bloating, but in the case of alfalfa, the risk of bloating is increased for a few days after the plants have been exposed to a hard frost of 25 F or lower.  Once those plants start to wilt (in the case of a hard killing frost) or several days have passed, the risk of bloat decreases.

For those livestock owners with tall fescue pasture, frost is actually good news because the sugar content within fescue increases.  It is part of the reason that tall fescue works well for stockpiled late fall and winter grazing.


Recommended TMR Sampling Protocol

            Total mixed rations (TMR) are commonly used on dairy farms to deliver an all-in-one feedstuff to dairy cows.  Sampling is done to determine if the formulated ration matches what ends up in the feed bunk in front of the cows.  In the September/October issue of the Buckeye Dairy News, Dr. Bill Weiss, Extension Dairy Nutrition Specialist wrote an article entitled “Does TMR Sampling Provide Useful Nutrient Composition Data?”  The article discusses the potential benefits of TMR sampling but notes that accurate values can be difficult to come by due to sampling variation and most often, sampling error.  Research done by Dr. Weiss resulted in the following TMR sampling protocol recommendation:

1.      As you walk the feed bunk carrying a clean container such as a 5 gal bucket, take a handful of TMR approximately every 10 to 30 ft. and place it into the bucket. For shorter bunks sample at 10 ft. intervals but for very long bunks sample at 30 ft. intervals. You want to have at least 10 handfuls by the time you reach the end of the bunk.

2.      Alternate samples so that the top, middle and bottom third of the TMR is sampled.

3.      When taking the handful, ensure that your palm is facing up to avoid dropping small particles.

4.      After you have walked the entire feed bunk, mix the contents of the bucket and then dump the contents onto a clean floor or large piece of plastic.

5.      Spread the contents out into a circle, divide the circle into quarters and then using a scoop to ensure you get all the particles, place one of the quarters into a sampling bag and send to the lab. The sample should be larger than a softball but smaller than a volley ball.

Using a simple, yet good sampling technique for obtaining TMR samples was generally accurate for DM and CP; however, using results from a single sample had a high risk of being wrong (>10% different) with respect to NDF and minerals. Taking duplicate samples and averaging NDF values reduced the risk of being wrong to an acceptable level. Sampling TMR did not accurately assess mineral delivery and should not be used for this purpose.

The entire article is available on-line at: .  Contact the Wayne County Extension office at 330-264-8722 if you need a hard copy of the article.