Common milkweed, asclepias syriaca, can become a serious problem over time in no-till fields and hay and pasture fields where glyphosate-resistance in the crop is not an option. This weed has an extensive and deep root system and is tolerant to many common herbicides. Multiple herbicide applications are often required.
Insulating winter snow and moist summer conditions favor the vegetative and seed-born spread of milkweed patches. Seed from milkweed plants is very viable. In addition, the seeds have a pappus, or “parachute,” enabling them to travel relatively long distances by wind. It takes only three weeks for a newly germinated milkweed plant to become “perennial,” able to reproduce from underground roots. New plants growing from established roots start early in spring and generally grow faster than the intended spring-seeded crop. It’s best to deal with milkweed before it spreads over a large area. Individual plants and small patches are cheaper and easier to treat than entire fields.
In glyphosate-resistant crops, milkweed control is not difficult to control. Glyphosate, when applied at the proper rate and timing, will give good control. In glyphosate-resistant corn and soybeans, milkweed should be treated with glyphosate at 0.75 lbs a.e./acre glyphosate to control or suppress milkweed. It is always recommended to include 17 lbs spray-grade ammonium sulfate per 100 gallons of water. Late, post-emergent applications when plants are in the bloom stage will be most effective in killing roots. There are several herbicides listed for tank-mixes with glyphosate for corn and soybeans. Details can be found in the 2012 MSU Weed Control Guide for Field Crops.
Milkweed control in non-glyphosate-resistant crops is another matter. In fallow or non-crop areas, a stronger solution of glyphosate up to 2.25 a.e./acre can be used, or a mix of glyphosate at 1.5 a.e./acre plus 2,4-D ester at 0.75 lbs active ingredient/acre. In small grains, milkweed should be allowed to regrow to a mature stage after harvest or mowing, and then treated. In hay or pasture, milkweed can be spot-treated with glyphosate applied with a wipe-on applicator while the milkweed is taller than the crop, or spot-treated with a hand-sprayer. When these fields are rotated or renovated, that is the time to make your best effort to deal with milkweed aggressively. Fence rows, field borders and nearby, non-crop areas should be monitored and any milkweed found should be controlled.