Assess frosted crops before spraying herbicide

Some Manitoba crops were damaged by frost May 29 and early indications suggest most will recover, but where there has been damage farmers should hold off applying herbicides.

Farmers considering spraying should wait 24 to 48 hours to avoid further injury, says Nasir Shaikh, a weed specialist with Manitoba Agriculture, Food and Rural Initiatives (MAFRI) at Carman.

"Cold weather slows the growth of both crops and weeds," he wrote in an email May 31. "This can mean that the amount of herbicide absorbed and translocated is reduced resulting in less than the required lethal dose reaching the target site.

“This will set back the weeds for the while, but there is a good chance that these weeds will come back. Also, the crop is still recovering from the cold shock and has been under stress. Any additional stress due to herbicide/fungicide applications will further set back the crop and delay its recovery for active growth."

Where there has been relatively little crop or weed damage, no more frost is expected and daytime temperatures are in the high teens or low 20s, use the following recommendations for improved weed control and crop safety:

  • Wait at least 48 hours before spraying in-crop herbicides such as group 1 graminicides (Achieve, Horizon, Assure, Axial, Centurion, Post, Puma, et cetera) and all group 2 products (Absolute, Assert, Everest, Frontline, Odyssey, Pursuit, Solo, et cetera) to ensure that plants have resumed active growth.
  • Wait at least 24 hours to apply Glyphosate products (Roundup Ready canola, soybean and corn). For corn, post-emergent applications should be evaluated carefully until corn begins to demonstrate "normal" growth and development. However, if weed pressure is high, and if weeds are large relative to the optimum size for control, consider application once weeds have resumed active growth.
  • Wait at least 24 hours to apply Liberty (Liberty canola), if applying both Liberty and Centurion/Select tank-mixed, wait for 48 hours to ensure the group 1 product will perform optimally.
  • Use extreme caution if spraying corn with growth regulators such as 2,4-D. The corn must be actively growing and not any larger than the four-leaf stage or crop injury will result. Considering the current staging of the 2012 crop before frost May 19, 2,4-D might not be a viable option.
  • For product-specific questions, call the herbicide manufacturer.

Some canola was set back by the frost, but MAFRI oilseed specialist Anastasia Kubinec says it appears in most cases the growing point is still alive and plants should recover. There could be exceptions, especially in areas of the northern Interlake and southeastern Manitoba where it was colder for a longer period.

If canola was severely damaged by frost it should be visible by the end of today, she added. Lighter damage may take a few more days to show up.

The minimum canola plant population required for a decent yield is four per square foot. That stand is a better option than ripping it up and reseeding, Kubinec said given that tomorrow is June 1. Canola is a crop that can fill in and make up for lost plants.

Some corn was also damaged, but MAFRI’s cereal specialist Pam de Rocquigny doesn’t think the growing point was injured. That means most corn plants will recover, she said. Most corn is still small enough that its growing point is below the soil surface, she said.

"It might not look very pretty for a while, but it should come back with good growing conditions," de Rocquigny said.
Dennis Lange, a MAFRI farm production specialist who specializes in pulse crops is also optimistic about how soybean and edible bean crops will fare.

"So far so good from what I’ve been seeing," he said. "The growing point still looks OK in many of the plants (where) I’ve looked. I don’t think we were far away from some losses."

As of yesterday not many farmers had registered insurance claims due to frost damage with the insurance division of the Manitoba Agricultural Services Corp., said claim services manager David Van Deynze. That could be in part because it takes a few days for damage to show up and there’s the possibility plants will recover.

According to MAFRI, temperatures across much of southern Manitoba early May 29 ranged from 0 to 1 C, as seen in the attached PDF map.

— Allan Dawson is a reporter with the Manitoba Co-operator at Miami, Man.

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