Erin Burns, MSU weed control specialist, discussed weed control strategies for problem species during Ag Action Day last week. Some species require new approaches as they are beginning to be resistant to available pesticides.
Johnsongrass, pigweed species and marestail or horseweed are major invaders that growers in southwest Michigan have been monitoring for several years. An emerging problem with annual bluegrass is another species to add to the watch list.
Johnsongrass is a perennial which grows from rhizomes, making the species almost impossible to eradicate. Seedlings closely resemble corn making early identification challenging. The best control, according to Burns, is to keep the weed from invading by keeping fence rows controlled and avoiding mechanical movement of the rhizomes within a field or from field to field. In some cases, that means cleaning equipment between fields or even within the same field.
Mechanical control by fall plowing can be an effective control as long as the winter is cold enough to kill the rhizomes.
Chemical applications control only seedlings, leaving active rhizomes untouched. An herbicide rotation is recommended year to year. Mature plants produce as many as 80,000 seeds per plant each year, a great motivation for species control.
Pigweed species include water hemp and Palmer amaranth, are two aggressive glyphosate resistant species that can overtake fields in only a few growing seasons without control. Identification and control as seedlings is key to management. In addition to glyphosate resistance, some Palmer amaranth plants are now resistant to ALS inhibitor action herbicide. In Barry County, certain plants had a three-way herbicide resistance that included Atrazine, ALS inhibitor and glyphosate.
Corn fields infested with Palmer amaranth will require early growing season herbicide applications. Liberty Link seed is a choice to consider for heavy infestations in soybean field. Burns says a clean burndown coupled with a residual herbicide will allow beans to develop a canopy before the invaders compete for sunlight and water. Herbicide control after the seedling reaches three inches is ineffective so timely applications are critical.
An application of Gramoxone between cuttings in alfalfa fields provides control, however there are harvest restrictions to follow if used.
The massive amount of seed the plant produces requires eliminating the species from a field, even if it means pulling plants manually before it seeds. Palmer amaranth has been identified in Allegan, Cass, Berrien and Kalamazoo counties of southwestern Michigan.
Marestail (horseweed) is a late maturing pest that competes with crops throughout the growing season. Plants produce 200,000 seeds each that have a 60-90 percent overwinter survival rate. Liberty Link is a control recommendation along with tillage and strict burndown and residual chemical controls in both fall and spring.
Annual bluegrass is an emerging problem, primarily in turf grass but is appearing in vegetable and field crops. The cool season grass has a shallow root system that grows in clumps to a height of eight inches and thrives in irrigated systems.
The most current copy of weed control measures for Michigan is available through the MSU Weed Control Guide for Field Crops either online or at local Extension offices www.msuweeds.com/publications/weed-control-guide/.