Marestail has become more problematic in New York’s northern areas than elsewhere in the state, but it’s making inroads statewide, according to Mike Hunter, regional field crops specialist for CCE’s North Country Regional Ag Team. He presented “Effective Programs for Controlling Marestail in Soybeans” at the recent Soybean & Small Grains Congress in Seneca Falls.

Marestail (also known as horseweed) began appearing in northern New York in 2019. The weed’s growing cycle makes it especially challenging to suppress. Marestail emerges in April through June and then again in late summer through autumn. Its early emergence means that few plants exist to shade it out.

Since the plant can survive winter, it can keep growing in spring. At that point, marestail can be very difficult to control. Each stem can bear up to 200,000 seeds which are wind-borne, making proliferation across a farm and to neighboring fields rapid.

Hunter encouraged farmers to “start clean and stay clean” to prevent marestail from spreading. Although no-till and low-till management have helped improve soil health, it appears to encourage weeds like marestail.

“Not as much tillage is why we’re seeing more marestail,” Hunter said. “We want to catch it before four inches.”

Dealing with marestail

Marestail can start early among a crop. Photo courtesy of Mike Hunter

In three years of trials in northern New York, spraying with Liberty, Enlist One and Enlist Duo showed very effective result (ranging from 88.5% to 99%) in Enlist E3 soybeans. Spray may be applied from emergence through R1 (beginning flower). But where Enlist One and Enlist Duo may be applied varies by county.

Enlist One may be applied to all E3 soybeans across New York; Enlist Duo may not be applied in Genesee, Seneca and Wayne counties. Enlist applicator training is not required. Hunter advised applying spray when no rain is expected for 48 hours.

He also advised that when managing no-till burndown in soybeans with glyphosate or glufosinate plus Sharpen and metribuzin, Paraquat plus 2,4-D LVE and metribuzin or Sharpen plus 2,4-D plus metribuzin (no grass control) to “use MSO with it – that or a pre-mix. Keep those rates to a pint rate to come back into plant in seven days.”

Since there are no effective post-emergence herbicides to control multiple resistant marestail in glyphosate-tolerant or conventional soybeans, Hunter advised considering planting Xtend, XtendFlex, Enlist E3 or Liberty Link soybeans “to allow for effective post-emergence control options, if necessary.” In trials, it appeared that fine droplets helped with coverage.

In addition, growers should avoid planting into actively growing weeds. Hunter suggested using intensive tillage prior to planting.

“Apply residual with a pre-plant burndown program or separate in no-till soybeans,” Hunter said. “Treat marestail when it is four inches tall or less. Be timely; get it when it’s small, pre-seeding. In the future, we’re going to be looking at two-pass systems with marestail.”

By tackling marestail when it is at four inches or less in height, operators can kill it before it is well-established and before it goes to seed.

by Deborah Jeanne Sergeant