Post-emergent herbicides more important in a late year

We have just come through a cold and windy stretch of weather that has not only resulted in slow weed emergence but also a significant reduction in the amount of pre-burn herbicides that have been applied.


Farmers either opted to skip the burn-down entirely or were shut out by the weather once they decided to go ahead. 


Whatever the circumstances, where the burn-off was missed, farmers now face a battle to control large winter and spring annual weeds that are ahead of the crop.


This is putting increased pressure on post-emergent herbicides to deal with the weed populations. Larger weeds and those that have undergone stress conditions tend to be more difficult to control than normal weeds. 


In a research study conducted by Neil Harker of Agriculture Canada at Lacombe, Alta., early weed removal provided greater yield increases than waiting for later emerging weeds before applying herbicides. This is especially important if a pre-seed burn-down was missed.


Timing of weed emergence is important, and yield reductions from crop competition are even higher in a year when weeds emerge before the crop. 


Herbicides vary as to the stages at which they can be applied and the stage they control weeds. It is important to select a product that will allow you to get in early to control weeds.


The choice is easy in herbicide-tolerant canola systems because all systems have excellent crop safety at early leaf stages: glyphosate on Roundup Ready canola from the cotyledon to six-leaf stage; Liberty on Liberty Link canola from the cotyledon to early bolting stage; and Odyssey on Clearfield canola from the two- to six-leaf stage.


Good herbicide choices are also available in pulse crops, which allow early application. For example, Odyssey and Solo can be used in peas and Clearfield peas from the one- to six-leaf stage.


In cereals, Group 1 herbicides, such as Axial, Horizon, Puma 120 Super and Achieve, can be applied as early as the one or two leaf stage of the crop. Group 2 products such as Assert, Everest and Velocity M3 can be applied at the one leaf stage.


Broadleaf weeds in cereals are a little more complicated.


Application of products containing high rates of 2,4-D, such as Attain, Adrenalin SC and Dichlorprop, must be delayed because applying them before tiller initiation can result in yield losses up to 25 percent. This usually corresponds with the four-leaf stage.


MCPA tends to be softer on most cereal crops, which means it or products containing MCPA can often be applied as early as the two- or three-leaf stage, depending on the product. Some tank-mix products include MCPA at low rates, and these can be safely used at early leaf stages as well. Check label details with these products for the appropriate timing.


Strong choices for broadleaf weed control in cereals before the three leaf stage include bromoxynil products, such as Buctril M and Logic M (bromoxynil and MCPA ester) and Target, sulfonylurea products such as Refine SG and Triton C, Frontline XL, Pulsar, Spectrum and Infinity. 


Another group of products, including MCPA and dicamba based products and new chemistry such as Stellar, can be applied at the three-leaf stage. Check labels for complete recommendations.


Farmers should consider Harker’s conclusions from his studies:


  • Growers should not wait for second flushes of weeds before applying herbicides. 

  • The initial application should be made as early as possible to protect high yields. 

  • Late emerging weeds may require a second application. 

  • Early emerging weeds are the greatest threat to crop yields.

Thom Weir is an agronomist with Farmer’s Edge. He can be reached by emailing thom.weir@farmersedge.ca.