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Johnsongrass Control in Pastures and Hayfields

A common sight right now is thick stands of what might be confused for early corn growing on roadsides, pastures, and hayfields.  What you’re seeing is most likely Johnsongrass (Sorghum halepense), a plant that is enjoying our moist weather and really taking off in growth.  The question most often is what good is this plant?  Is it a beneficial forage plant or merely a persistent weed? Johnsongrass is a summer perennial grass that belongs to the sorghum family and can serve as a good forage crop in our pastures and hayfields.  Livestock will graze young Johnsongrass plants if given the chance, as it is relatively high in crude protein and is highly digestible.    The issues with Johnsongrass arise from its persistent growth and potential toxicity issues for livestock.   Johnsongrass grows from a very thick set of fibrous roots and rhizomes (underground root nodules that form new plants) that make the plant more difficult to kill because it can “fall back” on energy stores in these rhizomes whenever the plant is stressed, whether by grazing, mowing, or herbicides.  These rhizomes can also form new plants if disturbed or cut (by plowing or by leaving part of the root in the ground). These rhizomes over‐winter and send out new shoots in the spring and early summer.  Johnsongrass also reproduces by seed, with a single plant producing 80,000 seeds per year.  Because of these tendencies, Johnsongrass can be very persistent in a field if not controlled early and often.  In many states, Johnsongrass is considered a noxious weed and eradication programs are in place to help control the plant. Although Johnsongrass can be a forage crop, its downside is its potential toxic components.  When Johnsongrass is stressed, whether by drought or frost damage, the plant produces hydrocyanic acid (a derivative of cyanide). Also known as prussic acid, this compound can be very toxic and even lethal to cattle by interfering with the oxygen carrying capacity of red blood cells causing suffocation.  Other symptoms include staggering, excessive salivation, rapid breathing, and spasms.  Unlike nitrate poisoning, prussic acid can dissipate over time within the forage.  If a large field of Johnsongrass is cut for hay, the hay should be dried to a safe baling content (15 to 18%) to ensure the prussic acid content has dissipated.  Young plants, plants killed after frost, or plants growing after a long drought are the most susceptible to high prussic acid levels.  Grazing of Johnsongrass under these conditions should be safe after one week.  Ensiling of Johnsongrass should reduce prussic acid levels after three weeks. Control of this plant is difficult if it’s allowed to take control of a field in large areas.  Treatment of plants with glyphosate (Roundup) when they are in seedling stage will allow for translocation to the root system.  Tillage is not recommended as it will most likely make the problem worst by distributing more rhizomes.  Pulling up of plants is possible, but making sure that all the root is dug up is important.  Applications of Pastora early in the spring or after a hay cutting in bermudagrass fields can help with grasses such as Johnsongrass.  Pre-emergent applications of Prowl H20 prior to green-up will help prevent any new seed germination in the fields.  Mowing or grazing to prevent seed head production will help keep the plant at bay and prevent more seeds causing future problems. There’s definitely plenty of Johnsongrass out there right now, and it’s up the individual producer if it’s a plant that is welcome on their property.  If so, caution is warranted.  If not, vigilance will be needed.