With the cool spring, there has been a delay in plant growth including weeds. However, Del Voight commented during last week’s Field Crop News phone call that no-till and forage fields in southeast Pennsylvania have lots of corn speedwell and dandelion in bloom, so not everything is much behind. We have two studies at our Landisville research farm this spring focused on horseweed or marestail control. During a visit late last week, Dwight observed a fair amount of marestail winterkill in our plots, which is bad news for our research, but could be good news for you.

Marestail is an annual weed that can have both fall and spring emergence, so even with winterkill, don’t count it out completely. Also, spring emerging seedlings start off very slow and are not easy to see, so be sure to carefully scout soybean ground and be prepared to control emerge plants and potentially use residual herbicides. Pay particular attention to selecting “effective burndown herbicides”. Here is a summary of suggested management options compliments of Mark Loux (OSU) and Bill Johnson (Purdue).

Apply effective burndown herbicides in spring. Do not plant soybean into existing stands of marestail. Start weed-free at the time of planting by using one of the following preplant herbicide treatments, applied when the marestail plants are less than 4 inches tall.

  • 2,4-D ester plus glyphosate (1.5 lb ae/A).
  • 2,4-D ester plus Gramoxone 2.0 (3 to 4 pts/A) plus metribuzin-containing herbicide.
  • Liberty (29 to 36 fl oz/A) or Liberty plus a metribuzin-containing herbicide
  • Saflufenacil product (Sharpen or Verdict) plus MSO (1% v/v) plus either glyphosate or Liberty. You can apply 1 fl oz of Sharpen PRE or Preplant (or 5 fl oz Verdict), 1.5 fl oz Sharpen 14 days before planting or 2 fl oz 30 days or more before planting. Application restrictions increase for coarse soils with less than 2% organic matter. Tank mixing with other Group 14 herbicides (e.g. Valor and Authority) can also increase the delay between application and planting.
  • A mixture of glyphosate and 2,4-D ester has become more variable for control of marestail over time in some fields. Plants should be in the rosette stage at the time of application for best results. In fields where this mixture has previously failed to provide effective control, use one of the other burndown treatments listed (2,4-D ester can be added to any of these).
  • Use the highest rate of a 2,4-D ester product that is allowed, based on the interval between application and soybean planting. For all 2,4-D ester products, rates up to 0.5 lb active ingredient/A (usually 1 pint) must be applied at least 7 days before planting. Rates between 0.5 and 1 lb/A should be applied at least 30 days before planting, with the exception of several products (E-99, Salvo, and Weedone 650) that allow 1 lb/A to be applied 15 days before planting.
  • In ALS-sensitive populations, the activity of any of the above can be improved with the addition of an herbicide that contains chlorimuron (Canopy, Cloak, Fallout, Valor XLT, Envive, Authority XL) or cloransulam (Gangster, Sonic, Authority First). The addition of metribuzin to any burndown treatment can also improve control of emerged marestail. A number of additional residual herbicides can be added to the burndown for control of later emerging seedlings.