Crop Scene Investigation - 22: What's the culprit in Bill's sprayer tank?

Sunday, January 10, 2010

by BERNARD TOBIN

Herbicides are great tools for growers to use in defending crops against yield-robbing weeds. But when crop injury occurs after spraying, the herbicide and seed technology is often at the top of the suspect list.

In some cases, however, the evidence just doesn't add up and that's when Ontario Ministry of Agriculture Food and Rural Affairs' Weed Management Lead, Mike Cowbrough, gets a call.

One such case involved a corn field where the grower and several crop
specialists believed glyphosate-tolerant corn had been damaged by an application of glyphosate.

"The corn field was about 40 acres and every 45 feet there was a relatively healthy row of corn that was obviously missed by the sprayer. The rest of the plants were dramatically more stunted. The newer plant tissue was yellowish-brown and there were some plants that were almost brown in the middle," recalls Cowbrough.

"I talked with Bill, the grower, and he was convinced it was a case of glyphosate injury. He said, 'Why else would there be healthy plants only in the rows that were not sprayed?' It was a good theory, but it just didn't make sense. We checked the seed tags and the corn company said it was indeed a Roundup Ready hybrid and it had performed well in herbicide tolerance tests."

The glyphosate product that Bill was using also checked out. It was safe for in-crop application and there were no issues with the formulation. "Quite simply, there had to be something else in the tank," says Cowbrough. "As I scanned the field, I noticed the odd healthy pigweed and foxtail. Their presence convinced me that whatever was in the tank had no residual protection.

"I then asked Bill what he was spraying before he moved to the corn field. He told me he had been using a broad-spectrum herbicide, Buctril-M, in a neighbouring wheat field. He noted there may have been 20 or 30 litres left in the tank when he switched to glyphosate, but we both agreed that wasn't an issue because Buctril-M is registered in both wheat and corn and should not be a problem.

"It truly was a head-scratcher. While we stood there searching for answers, I was staring into Bill's wheat field. It was a really nice field with great weed control. As I scanned the field, I noticed a couple of patches of dying wild oats. How was this grass killed so effectively?

"I turned to Bill and asked him whether he added something to the Buctril-M to control the wild oats. He thought for a second and then told me that he had added Achieve Liquid to the tank to give him the grass control he needed in some of his wheat fields. He quickly pointed out that corn is also a grass. He didn't think a little Achieve left in the tank would hurt his corn."

That was also a good theory, but was it correct? Do you know why Bill's corn field was struggling to survive? Send your solution to Better Farming at: rirwin@betterfarming.com or by fax to: 613-678-5993.

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. BF