Rain or shine, the weeds still grow

Because of cold and wet conditions, many acres in Manitoba remain unseeded and have not received a pre-seed burn-off herbicide. The weeds continue to grow and are reaching the stage where the herbicide efficacy may become compromised.

Here are a few things to consider with late weed control scenarios.

Pre-seed options:

For a majority of farmers, the first option is still to get the crop planted. However, if the fields are still wet and a pre-seed burnoff application is possible, a tank mix of glyphosate (Roundup WeatherMax) and Heat herbicide can get rid of the weeds that are already up. This tank mix provides faster burn-off compared to glyphosate alone. Please refer to the label of Heat for the restriction on crops to be planted after application.

If the weeds are too big, one option is to apply a higher rate of glyphosate, especially for winter annuals and perennial control. 

For pre-seed control, there are several pre-seed tank mix options available. Applying a tank-mix of glyphosate and another registered herbicide may help to achieve maximum efficacy and efficiency. Please refer to the Guide to Crop Protection 2011 for the different tank-mix combinations in different crops.

In-crop options:

For Roundup Ready (RR) crops, tank-mixes with glyphosate can be valuable to control tougher weed species. For example, in RR canola you can use Lontrel, in RR soybeans Assure II, Venture L or Pursuit can be tank-mixed and in RR corn, Atrazine, 2,4-D or Dyvel Dsp can be added. 

Use caution with herbicide choices in corn. 2,4-D applications to corn may result in serious injury to some corn hybrids. Consult your seed representative for varietal tolerance to 2,4-D applications. Apply prior to four-leaf stage of corn.

In recently seeded crops, be especially cautious. Careful selection of a pre-emergence or early post-emergence herbicide is necessary. Using a non-selective herbicide (as a burnoff) just before emergence can cause injury. 

General considerations:

Make sure to add the necessary adjuvants or surfactants in the spray solution for better burnoff control.

If there is a chance of rain, remember to check the rainfast period of the herbicide. Most herbicides require a rain-free period of at least two to four hours or more.

Do not use a nitrogen fertilizer as a herbicide carrier, as it may increase the chances of crop injury.

Because of the cool weather, the weeds may be growing slowly, and the activity of the herbicides may be even slower. But please be calm and have patience.

— Nasir Shaikh is a weed specialist with Manitoba Agriculture, Food and Rural Initiatives at Carman, Man.

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