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Post-Harvest Weed Control

By: John Ippolito, P.Ag., Regional Crops Specialist, Kindersley

September 2017

There are always a couple of weed problems that producers feel it may be necessary to control  after harvest if they were not able to do as a pre-harvest practice. This year is no different and two weeds that are definitely on producers’ minds are kochia and Canada thistle.

Kochia may not have been totally controlled through pre-harvest herbicide applications or a portion of the plant remains viable after harvest which may be the case in cereals and oilseeds. There has been research conducted in both Montana and Alberta that studied regrowth of kochia plants and their ability to produce viable seed. The research found that the portions of a kochia plant remaining in the wheat stubble will continue to mature and produce viable seed up until a hard frost occurs. Harvest operations after August 15 in Montana resulted in less viable seed being produced but there was still seed produced. The research in Alberta compared herbicide applications 1 week and 3 weeks after harvest and found that there was not a significant reduction in seed production compared to the untreated check.

These trial results would indicate that there is not a lot of merit for post-harvest herbicide applications specifically to prevent kochia from producing more seed after harvest. For producers wishing to use this practice in west central and southwest Saskatchewan it is important to note that a herbicide mixture including glyphosate and some other herbicide groups may be required.

Canada thistle control after harvest is always an option, but a couple of recommendations must be considered closely. The requirement for regrowth after harvest is likely the biggest concern in 2017. The plants should be allowed to regrow until there is at least 8 inches of new growth prior to application. Glyphosate rates required for post-harvest application are normally recommended to be in the range of 690 to 1,020 grams of active ingredient per acre, which, essentially, is double recommended pre-harvest rates. There is also a requirement for a frost free period after application.

Post-harvest weed control practices are often an option but in the case of 2017 and the weeds involved we may not achieve the desired results until we have a significant change in weather conditions.  For more information on post-harvest weed control operations contact your regional crop specialist or the Agriculture Knowledge Centre at 1-866-457-2377.

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