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Early Spraying: Why Do It?

Early in-crop weed control will yield more than late control in almost all instances. To spray early is considered the safest economic approach, and then spraying a second time if necessary.

In terms of cost, early spraying costs the same as late spraying, assuming that rates and tank mixes are the same, but the yield benefit of early spraying can make a big difference to the return on herbicide investment. Spraying early when weeds are smaller can also provide effective control at lower label rates, making spraying early the economic winner.

There are a variety of Studies available to support this early spraying recommendation, a summary of which can be found on the Canola Council website.

How Early is too Early?

Are tiny weeds in the early growth stages too small to spray? No. Broadleaf weeds sprayed at the cotyledon stage with canola herbicides, including Liberty, Roundup, and the Clearfield products, should be well controlled. The key is coverage. Large droplets and low water volumes could mean that small weeds do not get sprayed. When using coarse sprays, it is recommended to also use higher water volumes to improve coverage.

Grassy weeds, such as wild oats, may not be controlled as well during the coleoptile stage as the true leaves will be protected from exposure to the herbicides. The other challenge is that for weeds such as wild oats, the natural leaf surface is hard to wet making spraying herbicide on the leaf surface difficult. If the only weeds in your field are grassy weeds, that wait until the earliest weeds are at the 2-3 leaf stage. If both broadleaf and grassy weeds are present, early application to control the broadleaf weeds is still recommended. If necessary, you can spray again to control the grassy weeds, using a grassy weed herbicide alone or in a tank mix with the partner product.

How Late is too Late?

With any late applications, always follow label instructions for rate, time, and pre-harvest intervals. Applications made after the label window can lead to reduced weed control due to advanced weed staging and reduced herbicide contact with the weeds. Late applications can also cause canola buds to abort, increasing the potential for permanent yield loss, and also increase the risk of elevated residues in the marketable grain.

Weed Scouting Tips

Thorough scouting for weeds is essential to making informed herbicide and spraying decisions. Weeds that occur along the field margin or near the approach may not occur throughout the rest of the field. Similarly, the most economically threatening weeds may not be present or in great numbers in the easy-to-scout areas of the field. Scouting should always be done across the entire field to gain an accurate idea of the distribution, staging, and species of weeds present. Checking a minimum of 20 one-square-metre areas per field is recommended to identify the weeds present and how big they are.

Waiting for more weeds to establish rarely pays off. These weeds become larger and have much more time to get established making them more difficult to control. 

Information taken from Spraying Early: Economics of Early Weed Control

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