Johnson grass · Sorghum halepense (L.) Pers · is a perennial, noxious weed native to the Mediterranean region and was first identified in Kansas in 1880.

It has moderate forage value, however hydrocyanic acid formed in the plant under frost or drought stress renders it poisonous to livestock.

It spreads rapidly by seed or its vigorous rhizome structure. A single mature plant may produce over 80,000 seeds and 200 feet of rhizomes. The seed can remain viable in the soil for up to 25 years and will begin producing lateral rhizomes 6-9 weeks after germination.

Crop Losses

Johnson Grass is one of the most costly weeds with which farmers must contend. It costs them millions of dollars each year in lost crops, poor quality grain and lower crop yields. Five Johnson Grass stems per .0001 acre reduces soybean yields by 4.2%; 50 stems reduce yields by 23% and 340 stems, 88%. One Johnson Grass head per 3.3 feet of grain sorghum row reduces yield by 52 pounds per acre while 50 heads per 3.3 feet reduce yields by 50%. A single plant at maturity may produce over 80,000 seeds and more than 212 feet of rhizomes. Johnson Grass seed can remain viable in the soil for up to 25 years.

Prevention of Spread

Preventing seed production and its spread is of primary importance. New infestations of Johnson Grass may be reduced by planting Johnson Grass-free seed, using livestock feed that is free of Johnson Grass seed, and cleaning machinery before leaving infested fields.

Specific Controls

Control of Johnson Grass shall mean preventing the production of viable seed and destroying the plant's ability to reproduce by vegetative means. Control may be achieved by chemical, cultural or mechanical means, or by combinations of these methods.

Cultivation may begin any time during the growing season and shall cut off all the weed plant at each operation (use duckfoot or blade type implement). Cultivations shall be 3 to 5 inches deep at intervals of 14 to 18 days. When the plants have so weakened that they emerge more slowly, the cultivation intervals may be extended to such time as will permit the plants to grow not more than ten days after each emergence of the first plants, but not to exceed intervals of three weeks. Cultivation shall be continued until the weeds have been eradicated or suppressed to such an extent that remaining plants may be more economically destroyed by application of approved chemicals to individual plants or by hand cultivation.

In yards, flower gardens, lawns, and around trees and shrubbery, hoeing and other effective means of thoroughly cutting the weeds at regular intervals, not to exceed 14 days during the growing season, shall be construed as intensive cultivation.

Close grazing or mowing at 2 or 3 weeks intervals through the growing season and followed by late fall plowing to expose the root stalks through the winter is a accepted control practice.