The abnormally wet spring in the Eastern Corn Belt has not only hampered planting, but it also has prevented timely weed burndown applications, said a Purdue Extension weed scientist.
A common problem in Indiana has been yellow fields caused by cressleaf groundsel, commonly known as ragwort, senicio or butterweed. Rain kept farmers from controlling this weed with herbicides earlier this spring.
"Farmers have a lot of questions about controlling cressleaf groundsel because the excessively wet weather did not allow burndown applications to be made in late April," said Bill Johnson. "Now we have fields with groundsel, plus chickweed, henbit, deadnettle and winter annual grass at the seed set stage. Not to mention the summer annuals, such as giant foxtail, giant ragweed, common lambsquarters, black nightshade, pigweeds and waterhemp that have started to emerge."
Bolting horseweed, or marestail, emerged last fall, and seedling horseweed that emerged this spring also continues to be an issue. Since a fairly high proportion of marestail in Indiana is resistant to glyphosate, Johnson said it's best to get it under control before crops are put into the ground.
While it is later in the season, Johnson said there are still a couple of herbicide programs for farmers to consider.
"In all cases it is going to be best to add some 2,4-D or Sharpen to the mix to improve control of groundsel, bolting horseweed and the summer annual broadleaf weeds," he said. "The chickweed and other winter annuals won't be controlled well by anything since they are in the seed set stage, but they could be dried up more rapidly if a paraquat-based program is used.
"Control of groundsel also will be a challenge since it is large, flowering and many of the lower leaves have fallen off of the plant, so herbicide update is limited by lack of leaf area."
If farmers are battling flowering groundsel, Johnson said they could use glyphosate, plus 2,4-D or Sharpen; or they can use Sharpen or 2,4-D plus paraquat and either Senecor for soybeans or atrazine for corn.
With the glyphosate-based program, he recommended farmers use the 1.5 lb. ae/A rate with 1 pt/A of 2,4-D. Most 2,4-D labels require a seven-day waiting period before planting corn or soybeans at this application rate. Johnson said this is why many growers should consider switching 2,4-D for Sharpen, which does not require a preplant interval.
In the paraquat-based program, Johnson said growers should use the upper end of the rate range for more effective large weed control.
"In our research, these programs have usually provided about 85 to 90 percent control of large, flowering groundsel," he said. "However, if the weather stays cool and wet, expect some regrowth of groundsel with either herbicide program. The regrowth can be cleaned up with various postemergence treatments in corn or soybeans."
Once farmers have a good handle on the weeds in 2011, Johnson said it's a good time to think about ways to prevent a repeat of excessive groundsel in 2012.
Because groundsel is primarily a winter annual, he recommended farmers look at the benefits of a fall-applied herbicide program containing glyphosate and/or 2,4-D.
"The use of fall-applied herbicides is not the solution to all spring weed problems, particularly spring-emerging horseweed, but can be effective in reducing groundsel, chickweed, henbit and deadnettle," Johnson said.
Johnson's full report on controlling groundsel, titled "What Do We Do About the Yellow Fields?" is available on the Purdue Extension weed science website
under the "Featured Articles" heading on the left side of the page.