By Harry Brook
People often get confused about what a pre-harvest application of glyphosate should do. You often hear people referring to glyphosate application as “desiccation”. Glyphosate is not registered for desiccation. It is a herbicide meant to kill plants and it does a good job at that.
Diquat, the active ingredient in Reglone©, is a desiccant. It dries down a crop rapidly. It is a contact product that does not translocate, whereas glyphosate translocates throughout the plant. This can cause issues with residues if you aren’t careful about crop stage when spraying. Glyphosate is registered for pre-harvest use as a weed control product. It works well for controlling perennial weeds at that time. If it is used as a crop dry down product, it fails miserably. In the fall, a crop sprayed with glyphosate can take 2 to 3 weeks to die and dry down. As a herbicide, it works best in an actively growing plant. When applied when temperatures are cold or droughty, it may not work at all as the plant has gone dormant.
To safely use glyphosate for pre-harvest weed control the crop should be down to 30% moisture, or less. Once the crop is that mature there is little chance of glyphosate residue being left in the grain. Due to some people spraying when there is more moisture in the crop, there have been glyphosate residues detected in a few crops sold overseas. This has led pulse buyers and malt barley buyers to insist that no glyphosate be used as a pre-harvest application. Some buyers are even requiring no herbicide be used at all to desiccate the crop. Very low residues in the grain can affect a crop’s marketability.
Glyphosate is one of the most widely used herbicides in the world because it is such a good product. It is used in conjunction with herbicide tolerant crops such as soybean, corn, cotton and canola. Due to long-term, multiple uses of glyphosate in a season, there have emerged weeds resistant to this herbicide. In some places it is so bad they can no longer use the herbicide. Resistance showed up several weeds in Ontario and recently was found in kochia in Alberta. There are patches of it, even in Flagstaff county.
Glyphosate is also useful in getting green feed harvested or used in making “yellow feed”. When glyphosate is applied to a green feed crop, at about the medium to hard dough stage, it causes the crop to stop maturing. Unlike a frost event, crop nutrient quality is not lost. There is about a 1 or 2% loss in protein but most of the feed value remains in the standing crop. This allows for livestock producers to preserve feed quality while leaving the crop standing. Then later, it can be swathed for swath grazing or baled as green feed.
Glyphosate is an amazing herbicide with a lot of valuable applications. Our big problem is that overuse of it could possibly see us lose this resource. Use it wisely and according to the label. After all, our entire zero-till farming system depends on this one herbicide.
Harry Brook is Flagstaff County’s Agricultural Fieldman. He can be reached via email at: hbrook@flagstaff.ab.ca or by phone at: 780-384-4138.