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Pre-harvest herbicides: desiccation or weed control?

By Brent Flaten, P.Ag. CCA, Crops Extension Specialist, Moose Jaw

When thinking of pre-harvest herbicide management, it is important to understand the difference between spraying a crop desiccant versus a pre-harvest application of glyphosate.

Both of the above can be used to help dry down crop and weeds but to different degrees. They are applied when a crop is physiologically mature and is in the process of naturally drying down. Product labels will describe that stage. Desiccants dry down the above-ground growth of crops and weeds quickly, usually within days. However, since desiccants do not translocate into plant roots, perennial weeds aren't controlled and late-season moisture can cause annual plants to regrow from their roots.

Two of the most common desiccants are diquat and saflufenacil (Heat). Pre-harvest application of glyphosate can technically be considered a type of harvest aid by drying down crops and weeds, but it is extremely slow and therefore isn't considered a desiccant. Pre-harvest glyphosate must also be applied when the labelled crop is physiologically mature to avoid higher residues in the seed, potential yield loss and lower-quality grains.

The better use of pre-harvest glyphosate is for controlling perennial weeds in a physiologically mature crop. Glyphosate's systemic action allows it to go into the roots of perennial weeds such as Canada thistle, toadflax and dandelions, making it a valuable weed control tool. However, the maximum pre-harvest rate is only 360 grams active per acre. Not all glyphosate brands have the same labelled pre-harvest uses, so refer to each brand's label. Beyond making sure you are only applying pre-harvest glyphosate on labelled crops, grain buyers may have further restrictions in this regard, depending on their markets.

Maximum residue limits (MRLs) are the legally allowed limits of pesticide residue allowed in harvested grain. MRLs are determined during the registration process and are set well below the limits of residue that could result in health concerns for humans or animals. Although there are widely recognized international standards for pesticide MRLs, individual countries can establish their own MRLs. Failure to be under the MRLs of countries importing our grains can result in our grain being turned away. To avoid market disruption, MRLs should be taken into consideration when choosing crop protection products and application timing. Speak with your grain buyer to ensure that the target market, whether domestic or foreign, accepts the use of desiccants or pre-harvest glyphosate. Be aware that if some importing countries have set significantly lower MRLs than those in Canada, it may impact whether you can use a desiccant or pre-harvest glyphosate in crops destined for that country.

For more information, contact a Crops Extension Specialist at a Regional Office near you or call the Agriculture Knowledge Centre’s general inquiry line toll-free at 1-866-457-2377.

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