Identifying Johnson Grass and Worrying About Prussic Acid

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This past week we had our first frost. And it was right on time. According to historical data, the middle of November is our average date.

Since Wednesday’s cold snap, I’ve had a number of questions about properly identifying Johnson grass. I grew up with lots of Johnson grass in our pastures in Johnson County, just south of Fort Worth. Here in east Texas the predominant large grassy weed is Vasey grass, which is often locally called “bull-grass”.

The two grasses are not much alike, really.

Johnson grass is a member of the sorghum family. Its scientific name is Sorghum halepense. If you’ve ever seen the plump yellow to orange heads of grain sorghum being grown in dryer, grain producing areas, you may be surprised to learn this robust grain is related to Johnson grass.

From a distance, you can easily recognize Johnson grass with its upright, airy seed head with pink-ish color.

Vasey grass (Paspalum urvillei) is related to Bahiagrass. I have a book entitled Common Texas Grasses and it eight species of Paspalum grasses listed in it. Even crabgrass is in the Paspalum family.

Vasey grass is very prominent in local pastures and, I am convinced, is often mistaken for Johnson grass.

Vasey’s seed head has little color, being greenish-grey with the seed head clumped together, often leaning over a bit.

Look up images of both of them on a search engine. You’ll quickly notice the difference. While much harder to find, the last Johnson grass I saw was by the railroad tracks off Lufkin Avenue in front of the Georgia-Pacific Plant.

The good news is that Vasey grass does not have the prussic acid issues. And Vasey grass is by far the more common vegetation of the two that you may find.

With those two identified, let’s talk more about the potential for Prussic acid and what we can do as stockmen to safeguard against it.

Prussic acid, more correctly called hydrocyannic acid (a cyanide based compound) develops in sudangrass or sorghum grasses which are severely stressed. One of these stressors is frost. The hydrocyannic acid develops within only a few hours after the frost and should dissipates within a few days

According to my favorite reference book, Southern Forages, stockmen should wait seven days after a frost event before retuning livestock to graze on pastures with Johnson grass. By waiting seven days, you will ensure that the prussic acid levels have subsided to safe levels.

If you bale a hay meadow with Johnson grass shortly after a frost, you only have to wait until the hay has properly cured (as you would do anyways) to be free of worry.

However if you are one that is working with the new ensiled hay bales (wrapped in plastic) you’ll need to wait much, much longer. Research results vary, but studies from Iowa State University and others strongly suggest you delay feeding silage for eight weeks after ensiling. If the forage likely contained high prussic acid levels at time of harvest, hazardous levels of prussic acid might remain and the silage should be analyzed before feeding.

With those scary worries out of the way, stockmen in the surrounding area should feel confident in traditional hay baling and, come Wednesday, should feel confident turning cattle in on fields with Johnson grass.

Looking ahead to winter, let’s carefully evaluate the hay on hand, the grasses available to graze, and the number of livestock we can expect to carry thru the winter.

Author

Cary Sims
Cary Sims
Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu

Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.
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