Johnsongrass: Friend or Foe?

Christine Gelley, Agriculture and Natural Resources Educator, Noble County OSU Extension

Like other members of the sorghum family, johnsongrass can cause nitrate or prussic acid toxicity.

Johnsongrass is easy to find in July in Ohio. It is a warm-season grass that is related to corn. Unlike it’s relatives- corn, sorghum, and sorghum-sudangrass, which are annual species commonly used for agronomic purposes, johnsongrass is a perennial that has naturalized itself in our environment. Johnsongrass begins actively growing when soil temperatures reach 70 degrees Fahrenheit, which is why it is more prevalent in Ohio in mid-summer to fall.

Whether we classify johnsongrass as a weed or as a forage could be debated, but it is formally listed as a noxious weed in Ohio and therefore the debate is resolved. Johnsongrass is a non-native, aggressive, naturalized weed that does provide some value as a forage, but by no means should be purposefully planted or propagated due to the threats it poses to our native species and agronomic cropping systems. It was initially introduced from the Mediterranean as a forage crop and then dispersed in an attempt to fight erosion in floodplains, which it can do effectively, but the problem is the ability it has to swallow up habitat for native plants that rely on those environments to persist.

Because johnsongrass is a grass, it can be very challenging to control with herbicides in pastures and hayfields that are predominately grass swards. There are a greater range of options for effective herbicide use in legume systems, which I will not elaborate on in this article. The best news for pasture and hayfield managers regarding this weed is that although johnsongrass is aggressive and spreads both by seeds and rhizomes, it is not tolerant of close grazing or mowing.

Continually using animals to graze down johnsongrass or defoliating it with machinery can prevent the development of seedheads and weaken the root system over time, while also providing a source of feed for livestock, with the exception of equine animals. Horses should not be introduced to areas where they would have the opportunity to graze johnsongrass or other sorghum type forages due to the risk of developing equine cystitis.

Alas, there are also other concerns with grazing johnsongrass which stem from how the plant responds to environmental stress. Like other members of the sorghum family, johnsongrass can cause nitrate or prussic acid toxicity if plants are overfertilized with nitrogen, subjected to drought, and/or frost. During times of stress nitrogen cannot be effectively metabolized by the plants and thus can accumulate at levels that are dangerous for animals to consume. In turn, prussic acid is produced by the plant in response to stress, which is also dangerous and can lead to animal distress or death.

Both nitrate and prussic acid levels will dissipate over time after the stress event and can be fed again safely, but managers must be observant and responsive when growing conditions inhibited by environmental stress. Nitrates and prussic acids do not dissipate to safe levels with the act of harvesting or ensiling. Therefore, managers cannot speed up the time to safe consumption with mechanical harvest. Patience is the tool that is needed.

Additional information about the forage value of johnsongrass and options for control are available by request from local service agencies and educational fact sheets that elaborate on the complexities of managing johnsongrass. Testing services are also available for managers who have concerns about nitrate levels in sorghum crops. If you want to learn more about johnsongrass management or nitrate testing, connect with your local Extension Educator.

Photo: Oklahoma State University