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Timing of Weed Control

By: Sherri Roberts AAg, Regional Crops Specialist, Weyburn

Corn seedlings
Corn seedlings
Can you afford to lose yield by improperly timing your herbicide applications? Scientific studies have shown that improperly timing herbicide applications can cost you as much as 95 per cent of your corn yield and 70 per cent of your soybean yield.

A three-year research project at the Agriculture and Agri-Food Canada Research Centre in Harrow, Ont. demonstrated a range of yield losses in corn and soybean from green foxtail, lamb’s quarters and common ragweed. In corn, lamb’s-quarters populations of 20 plants/m² reduced corn yields by 40 to 80 per cent, common ragweed populations of 5 to 60 plants/m² reduced yields by 15 to 60 per cent and green foxtail populations of 120 plants/m² reduced yields by 26 per cent. In soybeans, meanwhile, lamb’s-quarters populations of 5 to 35 plants/m² resulted in 55 to 60 per cent yield loss, common ragweed plant populations of as little as 10 plants/m² resulted in a 35 to 50 per cent yield loss and green foxtail populations of 50 plants/m² resulted in yield losses of 20 per cent.

Paul Johnson, South Dakota State University Extension weed scientist, states that weeds can start stealing yield as soon as two weeks after corn emerges and three weeks after soybeans emerge.

Studies conducted from 2012 to 2014 at the University of Minnesota demonstrated that corn yields were reduced by between 12 and 13 bu./acre within the first week and between 27 and29 bu./acre within the second week if weeds were allowed to remain in the field after they reached four inches in height.

Soybean seedlings
Soybean seedlings growing in
a field with high weed pressure
Research from the University of Nebraska pegs the critical period for weed control in corn at 20 to 39 days after crop emergence, says Steven Knezevic, University of Nebraska weed scientist. Delaying weed removal after the start of this critical period will cost you about two per cent in yield per corn-leaf-stage of delay, up until the canopy closes.

In soybeans, the critical weed-free period occurs between the vegetative second-leaf (V2) and V4 stages of growth.

To assist you with staging various pulses, visit the SaskPulse website to view excellent diagrams.

Also check out the article in this newsletter Soybean Growth and Development for a closer look at soybean growth staging.

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