Marestail, or horseweed, an annual weed that affects both no-tillage and tilled fields, emerges from late March through June, and again in late summer through fall. The plant starts out as a rosette, flowers in July, and sets and disperses seed via the wind from August through October. Individual plants can produce up to 200,000 seeds. Approximately 80 percent of these seeds can germinate immediately, with the remainder contributing to the soil seed bank to create future weed control challenges.

Because marestail does not fully mature until late summer or early fall, it interferes with harvest and reduces the crop by competing with soybean growth over the course of the growing season. Management of marestail when it is small and in the rosette stage is imperative, because control becomes more difficult in early summer and beyond as the plant grows.

Furthering the marestail problem, glyphosate- and ALS-resistant marestail is now prevalent in various areas of the U.S. According to the Purdue University Extension, glyphosate-resistant marestail was first discovered in Delaware in 2000 where continuous soybean fields had been treated with only glyphosate for three years. As the species presents one of the largest glyphosate-resistant weed management problems across the Corn Belt, researchers recommend marestail be managed with a preplant or pre-emergence burndown application, in combination with a soil residual herbicide effective for control of marestail.Post-emergence herbicide options for control of glyphosate and ALS-resistant marestail in soybeansare limited, which make burndown and residual control essential.

For marestail control, a residual or burndown treatment controlling the growth of any new weeds can provide additional control until a post-emergence treatment can be applied. Herbicide effectiveness depends on the stage of growth and size of the plants, and herbicides should be applied before marestail bolts or shoots a main stem in the spring, according to the Kansas State University Extension.

Syngenta solution

To help eliminate marestail before it wreaks havoc on your yield, Syngenta offers Gramoxone® SL 2.0 herbicide as a versatile burndown and preplant solution for soybeans. Gramoxone SL 2.0 delivers a convenient, flexible and dependable burndown of emerged grass and broadleaf weeds, such as marestail, in as little as 48 hours. Gramoxone SL 2.0 can be used in combination with 2,4-D or a dicamba herbicide to provide effective burndown of emerged tough weeds such as marestail.

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