Johnsongrass: Friend or Foe
Plus Master Cattleman Program in Dalton this Fall
By: Eddie Ayers, County Extension Agent
A common sight right now is thick stands of what might be confused for corn growing on roadsides, pastures, and hayfields. What you’re seeing is most likely Johnsongrass (Sorghum halepense.) Two important questions often asked are “what good is this plant” and “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 highly digestible. The issues with Johnsongrass arise from its persistent growth and potential toxicity issues for livestock. When stressed by drought or frost damage, the plant produces hydrocyanic acid which is a derivative of cyanide, also known as prussic acid. This compound can be very toxic and even lethal to cattle.
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 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.
Even though the symptoms of poisoning from Johnsongrass look like nitrate poisoning, the 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.
Control of this plant is difficult if it’s allowed to take control of a field in large areas. Tillage is not recommended as it will most likely make the problem worse by distributing more rhizomes. Pulling up of plants is possible, but making sure that all the root is dug up is important. Mowing or grazing to prevent seed head production will help keep the plant at bay, but it will not remove the plant from the field.
There are some herbicides available to control Johnsongrass, but most of them cannot be used in tall fescue, which is the major part of our hayfields and pastures in Gilmer County. Treatment of plants with glyphosate (Roundup) will allow for translocation of the product into the root system. One good option to get the glysophate to the Johnsongrass and not harm the desirable forage is through a wick applicator. Fortunately in our area the Limestone Valley Soil and Water Conservation District has one that can be rented. It is housed at Hinton Milling Company in Jasper. Contact them at 706-692-3626 to schedule a time to use it. You can also make or buy a wick applicator to try and control this (and other) pesky weeds.
I also want to mention that the UGA Extension office in Whitfield County is hosting a Master Cattleman Program this fall in Dalton from September 4th through October 23rd from 6:30 – 8:30 pm, which is every Tuesday for 8 weeks. Paid registration before August 17th entitles participants to one free forage sample analysis; sample must be submitted no later than September 11th. Registration is $85 per person and includes a dinner on the final night. Pre-Registration deadline is August 24th and can be done on-line at: https://nwgeorgiacattle2018.eventbrite.com or for more information, contact me in the Gilmer County UGA Extension office.
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