Johnsongrass; Feed or Weed?

Sep 30, 2022

By Jordan Penrose

Recently Christine Gelley wrote an article “Johnsongrass: Friend or Foe?”, it was an excellent article, and if you haven’t read it, I highly recommend that you do so. But, I bet that many of you like me have noticed johnsongrass showing up in pasture and hay fields a lot more over the past few years and especially this year. Let me start by giving some history on johnsongrass.

Johnsongrass (Sorghum halepense) is a competitive perennial warm-season grass that is native to the Mediterranean region. Johnsongrass seed was exported around the world to be primarily used to control erosion. It got its common name here in the United States from an Alabama plantation owner by the name of William Johnson, who used the seed in the 1840’s to plant on his river-bottom farm as a forage alternative and to help control water erosion.

Today, johnsongrass to many is now considered a weed and in many states is considered a noxious weed. In an article by Oklahoma State University “Johnsongrass in Pastures: Weed or Forage?” johnsongrass is known as the weed that we love to hate and hate to love. The reason it is a weed to many is that it reduces the yield and quality for crops that it grows in. But it also has some upsides to it as a forage because it has a high yield and can have good palatability and quality.

When identifying johnsongrass, it can resemble a few different plants at its seedling stage, such as corn and sorghum, but once it is past that stage the plant becomes more distinct. At the seedling stage, the easiest way to identify johnsongrass is that it has more narrow stems and leaves than corn and sorghum and has a distinct and prominent white mid-vein. Some of the ways to identify a mature plant; is that it can range from 2 to 8 feet tall, the stems are more of a pale yellow green color, and can be up to 0.8 inches in diameter. The lower part of the leaf that encloses the stem are flattened, hairy, opened, ribbed, and slightly toothed. When you pull the plant out by the roots you will notice that it also has rhizomes. The rhizomes are a good indicator of johnsongrass because most other plants that resemble it do not have them. The rhizomes are white with red and purple spots and are long.

Managing johnsongrass can be difficult because it can reproduce new plants by seed or by rhizomes. The way to control the rhizomes is to keep the plant from producing new ones. Most production of rhizomes happens when the plant exceeds 2 feet in height and begins producing a seed head. The most effective way to reduce rhizome production is to keep the plants under a foot tall by closely grazing or mowing, which can work when the grass is in pasture field, but more of a challenge in hayfields.

Even though johnsongrass is considered a weed, it has some desirable forage traits to it. Johnsongrass has relatively high quality and can have high yields, making it quite comparable to other forages like Sudangrass. Johnsongrass is very palatable before it hits its reproductive growth stage, then the quality and palatability go down and then livestock like cattle will avoid it. A word of caution is johnsongrass will produce prussic acid and can be lethal to livestock.  According to Dr. Mark Sulc’s article “Precautions for Feeding Frosted and Drought-Stressed Forages,” do not graze after a killing frost until plants are dry, which usually takes 5 to 7 days. And after a non-killing frost, do not allow animals to graze for two weeks because the plants usually contain high concentrations of prussic acid.

If you are considering herbicides to control johnsongrass make sure to read the label and know the type of herbicide you are using and how you are going to use it. A herbicide to kill johnsongrass will likely kill other grasses. Another option that has been used with some success is a rope wick applicator, with a non-selective herbicide when the johnsongrass is taller than the other desirable forages. If you have enough legumes and are willing to eliminate all grasses, a selective grass herbicide can work. If you want to replant a field, I would consider a no-till seeding after a non-selective herbicide, as tilling a field and reseeding could leave viable rhizomes, allowing for rapid reintroduction of johnsongrass. A long-term option to reduce johnsongrass is to fence in the field and graze livestock during the growing season. On my family’s farm we have two fields that johnsongrass is in, both are permanent hayfields, the other fields we take a cutting or two of hay off, then rotationally graze and there is no johnsongrass in any of those fields.  We usually don’t see the johnsongrass show up until we start our second cutting hay, and it is still young enough that it has that positive upside for hay unless it gets too tall. Johnsongrass can be managed, but it will be more of a challenge if its in a permanent hayfield.

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