Those Wily Weeds

Tammy Jones B.Sc., P.Ag

Tammy Jones completed her B.Sc. in crop protection at the University of Manitoba. She has more than 15 years of experience in the crops industry in Manitoba and Alberta, with a focus on agronomy. Tammy lives near Carman, Man., and spends her time scouting for weeds and working with cattle at the family farm in Napinka.

Post-Harvest Weed Control

“Cut thistles in May, they’ll grow again some day. Cut thistles in June, that will be too soon. Cut thistles in July, they’ll lay down and die.”

This piece of practical advice, or perhaps it is folklore, was part of a conversation with Mike Cowbrough, weed specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs regarding tough-to-control weeds. The saying highlights the importance of weed biology in determining the most effective timing for weed control efforts. Spring weed control is especially effective for annual weeds and protecting crop yields. Pre-harvest weed control can be a good opportunity for optimizing harvest efficiency by reducing green plant material. But limitations on suitable crops, suitable herbicides, and issues such as wheel tracks in the field, complicate and possibly reduce the effectiveness of the pre-harvest options. There is a strong argument to be made for post-harvest weed control even though it too has challenges.

Post-harvest timing can help control perennial weeds, minimize the impact of annuals’ re-growth, and address any newly emerged winter annuals. It’s also a great tool for optimizing weed control when the next crop has limited in-crop herbicide options. The main challenge for post-harvest weed control, is completing harvest before winter sets in. There are years where harvest doesn’t end until the next spring, but in years where there is an open fall and the opportunity, here are reasons to consider post-harvest weed control:

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1. Scouting is easier than for pre-harvest. No one wants to walk through a heavily knitted crop at maturity or try to see what weeds lurk under the crop canopy. Weeds are more evident in the stubble, so patch management is easily done and travelling across a field will not damage crops. Perennial weed control is optimized. While annual weeds are essentially killed when they are young (spring), perennial weeds require a systemic herbicide to be translocated into the roots. The ideal timing for this to occur is once the weed is replenishing the root reserves (late summer or fall).

2.Soil moisture and nutrient management can be addressed. In dry years, soil moisture conservation is essential, to reduce impact on subsequent crop yields. One example is a study from

Washington State where Russian thistle used more than 25 gallons of water between an early August harvest and a killing frost in late October. That was more moisture than the weed had used while growing with the wheat crop. In another study conducted in Kansas, post-harvest weed control conducted immediately post-harvest conserved significantly more soil moisture resulting in 15 bushels more corn in the following year.

If soil moisture is not limiting, then weed growth can result in uptake of available nutrients, minimizing leaching or other potential losses.

3.Minimizing the weed seed bank. Annual weeds that are cut off at harvest may regrow and often set seed (see picture). In a study conducted in Montana, kochia were able to set more than 4,000 seeds per plant after being cut off at harvest.

While that is less than the entire plant would have produced, it does increase the weed pressure in future years. Controlling those weeds will help minimize future weed issues.

4.Flexibility to select a weed control option. Options that could not be considered when there is a crop present are now viable. Post-harvest weed control could be a flail mower, shallow or deep tillage, a herbicide application or a cover crop. Soil moisture would help decide which of these options is most feasible. In a dry year, the flail mower or shallow tillage will conserve more soil moisture. The flail mower helps minimize re-growth and seed production of annual weeds and keep tumbleweeds from spreading into neighbouring fields.

Shallow tillage will uproot newly sprouted winter annuals and certain perennial weeds while stimulating the germination of non-dormant annual weeds (volunteer canola), which would then be killed in the winter to minimize weed pressure in the spring. Deep tillage and cover crops are likely better suited to times when moisture levels are less limiting. Deep tillage may bury weed seeds, which may in turn result in them degrading in the soil or being consumed by insects. While deep tillage may also reduce some perennials weeds, there is the danger of spreading root fragments, so patch tillage is an option. Cover crops, like fall rye, may provide many advantages, suppressing weeds by crop competition, shading or allelopathy and preventing soil erosion on delicate soils. Fall applications of residual herbicides are also effective in controlling early emerging spring weeds.

5.Time management. Completing field activities in the fall can mean less stress on resources in the spring.

There are a number of fitness and sports quotes, business quotes, and so many other sayings that begin with “There is no finish line…” Farming is no exception, although harvesting a crop may feel like the end of the cycle, another cycle is just beginning, especially when it comes to weed control. Post-harvest weed control can be tricky for timing and effectiveness, however it can be the key to next year’s successful crop.

Annual weeds like lamb’s quarters, yellow foxtail, and wild buckwheat can set seed after being cut off at harvest.