Over the past two months Texas has seen some frosts, freezes and other inclement weather. And although Texans may never be fully prepared for cold weather, it arrived right on time. According to historical data, the middle of November is Texas’ average start date for freezing, undesirable weather.

Since November’s first cold snap, I have received a number of questions about properly identifying and managing Johnson grass during cold weather. 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”.

Although these grasses can be found in similar locations, Johnson grass and Vasey grass are actually two very different grasses.

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 has 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.

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

Since now we can identify the two grasses, let’s talk more about the potential for Prussic acid problems 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 when they are severely stressed. A prime example of a stressor is frost. The hydrocyannic acid develops within only a few hours after the frost and should dissipate 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, after a week, 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 in order to best care for our livestock and other farm and ranch management projects.


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 A&M AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, national origin, genetic information or veteran status.  The Texas A&M University System, U.S. Department of Agriculture, and the County Commissioners Courts of Texas Cooperating.