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

Saturday, January 9, 2016

by BERNARD TOBIN

Steve's yellow soybeans were suffering from herbicide injury, not manganese deficiency as he had first suspected.

What happened in Steve's field was quite common across Ontario in 2015, says AGRIS Co-operative agronomist Dale Cowan. When Steve looked at his soybeans on July 1, he immediately linked the yellowing on the new leaves with manganese deficiency. But, as Cowan explains, the problem can be traced to three weeks earlier when Steve applied herbicide in less than optimum spray conditions.

"It's a decision that many growers were forced to make this spring," says Cowan, who worked with agronomist Alan Broad to solve the puzzle. In this situation, the weather had been cool and wet, the forecast was calling for more moisture and the weeds were growing fast. Rather than risk being shut out of the field and allowing weeds to gain the upper hand, Steve decided to spray the crop with glyphosate and Classic herbicide.

Applying herbicide under those conditions did stress the crop and caused the yellowing of new leaves, but the soybeans started to turn green once the weather warmed up and the plants were able to metabolize the herbicide. High residue and low slope areas of the field were also slow to drain. These areas remained cool and plant growth was sluggish. An added stress was the need for plants to metabolize the herbicide program's two modes of action.

"It was a good herbicide program and there was no yield loss," says Cowan. "It's important to remember that soybeans make their yield in the last half of the year. The setback came early in the season, the plant outgrew it and no flowers were lost."

Cowan notes that Steve, like many growers, faced difficult environmental conditions this spring when applying herbicide. "You really have to weigh the consequences. If you apply herbicide in tough conditions, you do stress the crop. But if you wait too long, weed competition can cut into your yield. In this situation, it worked out." Congratulations to Kevin Sluys, Chepstow, for his correct answer. BF