From time to time, I’ll have questions about a good forage grass that we love to hate: Johnson grass. 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 the dryer, grain-producing areas, you may be surprised to learn this robust grain is related to the (often cussed) Johnson grass.
Over my years in this job, 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 sometimes, locally called “bull grass”.
From a distance, you can easily recognize Johnson grass with its upright, airy seed head with pink-ish color.
Look up images of both on a search engine. You’ll quickly notice the difference. While much less prominent, the typical location for Johnson grass in our area is roadsides where management and input differ greatly.
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, as I’ve learned, 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.
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.
What may not be known is that Johnson grass can be a great forage. It’s protein and digestibility are up there with Coastal bermuda grass, and the productivity (tons per acre) is right up there as well. Johnson grass in its younger stages of growth is so desirable that it can be grazed out of a pasture if not stocked correctly.
Now that we know more about each of these grasses, 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 hydrocyanic acid (a cyanide based compound), develops in Sudan grass or sorghum grasses which are severely stressed. One of these stressors is frost. The hydrocyanic acid develops within only a few hours after the frost and should dissipate within a few days
As of this printing in our part of the world in east Texas, we have yet to have our first frost.
According to my favorite pasture 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, you will ensure that the prussic acid levels have subsided to safe levels.
Often, I’ll find myself recommending waiting 14 days to give producers and myself an added margin of safety.
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 should be doing 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.
