Pre-harvest perennial weed control

The later stages of the growing season are a very effective time for controlling perennial weeds such as Canada thistle, quackgrass, and dandelion.

“At this time of the year, perennial weeds are preparing for winter by putting energy reserves into their root systems and the application of a glyphosate product now generally provides very good control of these weeds,” says Mark Cutts,

crop specialist at the Alberta Ag-Info Centre. “Glyphosate is a systemic herbicide that moves through the foliage into the root system and results in the death of the entire plant. Applying glyphosate to a standing crop is referred to as pre-harvest and has become a very common management practice.”

“It should be noted that in order for the glyphosate application to be effective, the weeds need to be actively growing,”

Cutts adds. “With drier conditions in areas of the province this growing season, an evaluation of these weeds should be made before a herbicide application.”

Cutts says that producers considering pre-harvest as a management tool for their crops should consider several factors.

Registered crops

Pre-harvest glyphosate is registered on a number of crops including cereals (wheat, barley and oats), a number of pulse crops, canola, and flax.

“Producers growing barley for malt should contact their malt buyer to ensure a pre-harvest application is acceptable. It should also be noted that a preharvest application shouldn’t be applied to a crop that’s being grown for seed due to potential reductions in germination and vigour levels.”

Timing of application

For registered crops, pre-harvest applications should be done when seed moisture is less than 30 per cent. Says Cutts, “This moisture content can be assessed by using the thumbnail test or visual test. For example, in registered cereal crops, 30 per cent seed moisture correlates to the hard dough stage of the grain. At this stage, a thumbnail impression remains on the seed. In a field pea crop, a visual test is used.

At 30 per cent seed moisture, the majority of the pods (75-80 per cent) are brown. For visual symptoms for remaining registered crops, you can refer to the  Crop Protection 2018 publication.”

Rate of application

The standard rate for glyphosate in a pre-harvest application is 360 grams active ingredient per acre. “Due to variations in the concentration of glyphosate products, producers will need to ensure that the glyphosate product is being applied at the proper rate. For example, the application rate will vary from 0.67 to 1.00 litres per acre depending upon the product being used.”

Time required

A minimum of three days is required after the application of the glyphosate before a crop can be swathed or harvested. “If weather conditions aren’t favourable after the glyphosate application, waiting a few extra days may benefit weed control,” says Cutts.

For more info: 310- FARM (3276).



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