Johnsongrass is a coarse aggressive non-native perennial grass that grows 3 to 6 feet tall and spreads from seeds and underground roots. It was introduced to the United States in the 1830's and began arriving in Texas around the 1880's. It is invasive in cropland fields and shows up readily with disturbed rangeland. It is very palatable to cattle and will not persist with moderate grazing. It provides good forage for all livestock during the growing season and can be baled for hay. Due to the potential for prussic acid poisoning, animals should not be allowed on Johnsongrass for a few weeks after green growth resumes following a drought or frost. Johnson grass baled during such time periods may retain prussic acid for a short time if not cured properly.
(From Range Plants of North Central Texas - A Land user's Guide to Their Identification ,Value and Managment, 2014, by Ricky J. Linex, USDA-Natural Resources Conservation Service.)
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