Producers generally wait to spray weeds — often too long.
If a herbicide is able to be used until the flag leaf of a cereal crop and the sixth leaf stage of a weed, producers tend to wait that long to control the most recently germinated weeds.
After all, who wants to spray more than once in a crop?
But what about the effect those weeds have on the crops they have been growing beside?
Once the cereal crop germinates, so too do the weeds.
While we wait to spray, the weeds are using up moisture, using up nutrients, competing for sunlight and creating shading.
This year, we have also just come through a cold and windy stretch of weather.
That resulted in slow weed emergence, and also significantly reduced the amount of pre-burn herbicides that have been applied.
Farmers either opted to skip the burn down entirely, or got shut out by the weather once they decided to go ahead with the burn down.
Where the burn off was missed, farmers face a battle to control large, winter and spring annual weeds that are further along in development than the crop. This puts more pressure on post-emergent herbicides to deal with the weeds.
Larger weeds and weeds that have undergone stress conditions tend to be more difficult to control than normal weeds.
Many studies have been conducted over the years comparing the timing of weed removal.
Recently, in a study conducted by Neil Harker of Agriculture Canada at Lacombe, Alta., early weed removal provided greater yield increases than did waiting for later emerging weeds before applying herbicides. This is especially important if a pre-seed burn down was missed.
Farmers should consider Harker’s conclusions:
- Growers should not wait for second flushes of weeds before applying herbicides.
- To protect high yields, the initial application should be made as early as possible.
- Late emerging weeds may require a second application.
- Early emerging weeds are the greatest threat to crop yields.
Harker found that all crops reacted in similar fashions to this weed competition.
His work found that delaying weed removal after the crops have emerged for two weeks can cause significant yield loss. As well, it showed that in some crops, such as peas, removal at the first week after emergence stage still caused yield reduction.
Early emerging weeds are the greatest threat to crop yield. Timing of weed emergence is important, and in a year when weeds emerge before the crop, yield reductions from competition are even higher.
Herbicides vary as to the stages at which they can be applied to the crop and the stage they control weeds. It is important to select a product that will allow early to control of weeds.
In herbicide-tolerant canola systems, that choice is easy, as 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 Ares on Clearfield canola from two- to seven-leaf stage.
There are also very good herbicide choices in pulse crops, which allow early application. Odyssey and Solo, for example can be used in peas and Clearfield peas from the one- to six-leaf stage.
In cereals, Group 1 herbicides, such as Axial, clodinafop, fenoxaprop and tralkoxydim 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 more complicated. Application of products containing high rates of 2, 4-D, such as Attain, Frontline D, dichlorprop, must be delayed be-cause applying them prior to 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 so it, or products containing MCPA, can often be applied as early as 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 Buctil M, bromoxynil and MCPA ester and sulfonylurea products like Refine SG or Triton C, Frontline XL, Pulsar, Spectrum and Infinity.
Another group of products including MCPA and dicamba and new chemistry such as Stellar can be applied at the three-leaf stage. Always check labels for complete recommendations.