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Managing Johnsongrass May Require Chemicals, Nitrate Testing
Missouri Ag Connection - 06/14/2018

Johnsongrass is listed as a noxious weed by the Missouri Department of Agriculture, which requires landowners to take steps toward controlling the weed on property they owned or have under their control.

"Grass weeds are difficult to control in pasture, but there are a few things that can be done to manage this weed," said Sarah Kenyon, agronomy specialist, University of Missouri Extension.

Chemical control of Johnsongrass includes chemicals with the active ingredient sulfosulfuron (trade name: Maverick, Outrider, or Oust XP), and spot treatment with glyphosate (Round Up) or paraquat (Gramoxone) can be used.

An alternative control technique would be to graze or hay the Johnsongrass-infested pasture. Johnsongrass is closely related to sorghum, and can be high quality, nutritious forage when managed correctly.

"Johnsongrass can accumulate toxic levels of prussic acid or nitrates, but with careful management, these anti-quality factors can be managed," said Kenyon.

Prussic acid, or cyanide poisoning, can occur in Johnsongrass or sorghum-sudangrass plants when injured or under drought or frost-damage stress. Prussic acid is readily absorbed into the bloodstream and causes toxicity by blocking normal cellular respiration in the animal.

"Nitrates can accumulate in Johnsongrass and other forages as a response to stress," said Kenyon.

Drought, unusually cold or cloudy weather, inadequate rest following grazing, and excessive nitrogen fertility beyond the crop's ability to assimilate the nitrogen, are examples of "stress" situations that can cause the plant to have potentially toxic nitrate levels.

"If Johnsongrass or sorghum-sudangrass is under stress, avoid grazing until the plants have recovered and exhibit at least 24 inches of regrowth," said Kenyon.

Following a severe frost, avoid grazing for 14 days or until the leaves turn brown, whichever is longer. Also, prussic acid and nitrate levels are highest in young, leafy tissue, whether it is initial growth after planting or regrowth after clipping.

Since it is the young, fast-growing tissue that contains dangerous levels, avoid grazing until the plant reaches a height of at least 24 inches.

"Prussic acid disappears during the hay curing or ensiling process; nitrates do not, forage samples should be conducted on suspected hay," said Kenyon.

University of Missouri Extension offices offer testing to determine if nitrates are present. Individuals wishing to have this test need to bring plant samples to an office by appointment.

Nitrates are usually higher in the lower portion of the plant which is why plants sampled need to be clipped close to the ground. Plant specimens should be collected randomly from multiple locations in the field that is to be grazed.

If the test indicates the presence of nitrates in the feedstuff, the plant material will need to be submitted to a testing lab to determine the total amount of nitrates present.

"This quick test does not test for prussic acid, but the conditions that lead to nitrate toxicity also lead to prussic acid," said Kenyon.

For more information, contact any of these MU Extension agronomy specialists in southwest Missouri: Tim Schnakenberg in Stone County, (417) 357-6812; Jill Scheidt in Barton County, (417) 682-3579; or Sarah Kenyon in Howell County, (417) 256-2391.

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