Crop Scene Investigation - 38: What's behind the weed escape in Scott's soybeans?

Monday, March 5, 2012

by BERNARD TOBIN

Every year, growers call agronomist Pat Lynch to seek his advice in solving crop performance problems in their fields. One of Lynch's most memorable calls came from Scott, a Huron County grower who was really disappointed with the weed control in one of his soybean fields.

"He's a very good farmer, who typically has clean fields. But, this year, his post-emergent soybean herbicide program had performed poorly. He paid good money for it and was expecting better weed control," recalls Lynch, who notes that, other than custom combining, Scott does all his own fieldwork and dutifully maintains a soy-corn-wheat rotation.

Scott told Lynch that he had weed escapes throughout his soybean field and the escapes appeared to be more severe in different parts of the field. The herbicide he used has a high rating for the weeds in his field and Scott wondered whether there was an issue with the spray timing.

"We want to get those post-emergent herbicides on early because there is both residual and knockdown activity" explains Lynch. "When you apply your post-emergent weed control later, you sometimes take the risk that you waited too long and there is going to be escapes."

Lynch thought Scott's application timing was a tad late, but it still should have controlled the weeds in the field, as well as other grasses listed on the label.

Scott was puzzled and Lynch agreed to pay him a visit to inspect the field. When he arrived at the farm, Lynch made a mental note of Scott's poultry barns and the fact that he used chicken manure on the fields. "He was wondering whether he was getting so many weed seeds in the manure that it was overwhelming the herbicide," recalls Lynch.

Since they first talked on the phone, Scott had come up with some other theories to share with Lynch. Had there been enough moisture to activate the herbicide? Had a new grassy weed taken root in his field that his herbicide didn't control?

Lynch knew the answer to Scott's questions would likely be found in the field, so they walked over to look at his soybeans. When he saw the field, he immediately thought there were too many weeds, given the herbicide that had been applied.

Lynch quickly located the unsprayed check strip to get a good look at the actual weed pressure in the field. In the check strip, he noted the heavy broadleaf weed and grass pressure, including the weeds that Scott had mentioned. He then began to walk the sprayed area of the field, which was clean, except for one weed species (foxtail) escape.

As Lynch took a closer look at the dead and dying weeds that had been controlled by the herbicide, the reason for Scott's escapes was obvious to him. (See photo.)

Can you spot the clue that solved the mystery? What was the cause of Scott's foxtail escapes? Send your solution to Better Farming at: rirwin@betterfarming.com or by fax to: 613-678-5993. Be sure to provide your phone number and mailing address.

Correct answers will be pooled and one winner will be drawn for a chance to win a Wireless Weather Station. The correct answer, along with the reasoning followed to reach it, will appear in the next issue of Better Farming.

This completes our current CSI series. A new set of challenges will return this fall. BF