Harvest of winter wheat in Indiana has officially started in the southern regions with about 8% harvested last week and we expect that number to increase rapidly in the next couple of weeks.
This past week I witnessed something that mildly bothered myself as a weed scientist. The efficiency of today's agriculture machinery was on display as two combines finished off a wheat field with a 24 row soybean planter pulling into the field to quickly get the ground back into production.
The thing that bothered me was the amount of living, green, marestail and ragweed plants sticking up in the wheat stubble, getting ready to take full advantage of the newly opened canopy and much younger soybean seedlings soon to emerge.
Now this is all under the assumption that there wasn’t a sprayer down the road filling up with herbicide getting ready to do a burndown application following the planter.
This may not be the case in all wheat fields, but producers need to be aware of summer annual weeds hiding under the mature wheat canopy, and be prepared to control these weeds early.
Weed scientists at Purdue strongly recommend starting with a weed-free seedbed regardless if we are growing full-season or double crop soybean. It is especially important to control marestail and giant ragweed prior to soybean emergence.
Another issue that we have received calls on in the past couple of weeks has been uncontrolled winter annual grass, such as annual ryegrass in winter wheat. The following will cover some recommendations on how these weeds should be handled going into double crop soybeans.
Marestail and Giant Ragweed
These are two weeds that can be difficult to control with postemerge soybean herbicides, especially if the population is resistant to glyphosate. So it is important that producers recognize if these weed species exist as weeds following wheat harvest, the density of the weeds, and past history of glyphosate resistance in the field.
Whether the population is resistant or susceptible, producers should still plan on doing a burndown on fields with high densities of these two weeds. A recommended burndown that would not require a planting restriction would be glyphosate plus sharpen. Unfortunately, there are not any residuals for marestail and giant ragweed that are safe to rotate to corn in 10 months, besides metribuzin (Sencor). Metribuzin will provide residual activity on marestail, but not much on giant ragweed.
It will be much easier to control these weed species prior to soybean emergence, rather than waiting to control 12-inch-plus plus weeds post-emerge with the few options of glyphosate or PPO-inhibiting herbicides or Liberty in Liberty Link soybeans.
Winter Annual Grasses
This is likely much less of problem in much of Indiana, but I know I have gotten a couple of calls and will pass on the recommendation that were given to those producers. The specific issues I have encountered were annual ryegrass and little barley that were not controlled in the wheat crop.
Both of these species are known winter annuals, but can also persist and/or emerge in double crop soybeans. At this point in the season, it is obviously too late to keep these species from adding seed to the seed bank.
Although steps can be taken to keep these species from competing into the soybean crop with a burndown using high rates of glyphosate (≥1.25 lb ae/A) to control any grass that has not yet matured, plus a grass residual such as Dual II Magnum or Outlook to keep any additional plants from emerging in the soybean crop.