Crop Scene Investigation - 58: What made Steve's soybeans turn yellow?

Monday, December 7, 2015

by BERNARD TOBIN

A tough growing season like 2015 keeps agronomists on their toes and also on their phones.

This year, many farmers who walked their fields after planting found trouble in their corn and soybean crops, and the problems often ended up with agronomists like Alan Broad and Dale Cowan, who work for AGRIS Co-operative and joint venture partner Haggerty Creek Ltd.

One of those calls came on July 1 from Steve, a Lambton County farmer. "He was pretty concerned," recalls Cowan. "The farmer explained that his new growth soybeans had turned a bright yellow. We suspected the problem was probably manganese deficiency, but I thought there was more to the story, so I headed over to take a look."

As Cowan walked the soybean field, he noted that, while the yellowing plants could be seen across the field, Steve's manganese diagnosis wasn't adding up. "The damage was indeed on the new growth, but it didn't look like manganese deficiency. It didn't have the classic green veins. It was a general overall yellowing that just isn't characteristic."

The symptoms were most pronounced on the headlands, field corners, heavy crop residue areas and some lower slope positions, but not all of them. The field had good soil structure and lots of moisture. "That's not where we would expect to see it," says Cowan. He also noted the field had no history of manganese deficiency and soil tests proved it.

Cowan now had a bit of a mystery on his hands. "There are a whole pile of other things that can produce early-season yellowing in soybean plants. There's soil compaction, poor root development and nodulation, nitrogen deficiency or simply just wet feet – and we had had plenty of rainfall." He also listed magnesium and potash deficiency among other possible causes.

"When we dug up some plant roots, we observed good root structure, and good nodulation. They were all pink inside, so I ruled out nitrogen deficiency." His attention then turned to Steve's herbicide program. The grower had used glyphosate and Dupont Classic herbicide according to the label and everything appeared to be in order.

It was then time for a chat with Steve. Cowan wanted to know what the conditions were like when the herbicide was applied. Steve said the crop was sprayed 10 days earlier on June 10, just as the now-yellow trifoliates were unfolding.

It was cool and wet with little sunshine and the weeds were starting to compete with the crop, Steve recalled. With more rain in the forecast, he had to make a choice. He could either park the sprayer and wait a week, as the weeds continued to grow, and hold out for more favourable conditions or roll on and apply the weed control before the rains came. His decision? He gave the sprayer operator the green light to get it done before the rains came.

For Cowan, the yellowing of the new leaves now made sense. He assured Steve there was little to worry about. "I told him go back and take another look at the field in a couple of weeks. Time and sunshine would likely fix the problem."

Do you know what was causing Steve's soybeans to turn yellow? 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 solution, along with the reasoning followed to reach it, will appear in the next issue of Better Farming. BF