By Harry Brook
Spring is here and it's time to hit the fields. One of the first activities on fields is applying glyphosate and other herbicides as a pre-seed burndown to remove the weeds. If you can get your crop to germinate before weeds do, you drastically reduce the effect of weed competition on crop yield. On the other hand, weeds starting before the crop emerges can be very costly to yield. Every plant has a critical period where competition from weeds will adversely affect yield. In almost all cases, that is very early on in the plant’s growth.
In most crops, such as wheat, oats and barley, yield potential is set very early on in the season. As stem elongation begins, the total number of seeds in the head will be set. Up until stem elongation is the crucial time for wheat. This also applies to other cereals, as well. Canola yield is set around bolting, and flax at flowering. Keeping yield potential high means controlling weeds well into spring.
Tillage or pre-seed spraying are both good ways to control weeds and allow your crop to get the jump on the competition. There are other, cultural ways to improve the odds and reduce weed competition. Light is one of the most common triggers to start weed seedling growth. Heavier seeding, up to 40 plants/square foot can provide crop competition. The object here is to establish total ground cover as quickly as possible. Once ground is covered, weed seeds stop germinating and competition for nutrients and moisture are at an end, at least until leaf drop in the crop.
Other techniques to reduce weed competition include early seeding. This doesn’t work for all crops but recent research by Agriculture and Agrifood Canada has shown very early seeding of wheat, at soil temperatures as low as one degree C, can provide greater consistency in yield as crop certainly gets ahead of the weeds. Part of this process also requires the seed to receive seed treatments as it may sit in the soil for some time prior to germination.
Winter annual weeds are a problem in the spring. These weeds germinate in the fall, overwinter as a rosette, then green up early and go to seed before anything else emerges, thereby increasing the weed seed bank. Some of the worst ones are narrow-leaved hawk’s-beard, scentless chamomile, downy bromegrass and common groundsel. Late in the fall is a good time to control these plants. At that early stage, it doesn’t take much to kill them off.
All weed control needs certain basic conditions to be successfully controlled. The plant has to be actively growing, otherwise the herbicide or control method will not stop them. Weeds need to be able to absorb herbicide and translocate it to where it is active. If temperatures are too cool, herbicides won’t be absorbed, translocated or active. Air temperatures should be in the low to mid-teens Celsius to work. Prior weather conditions are also important. If a frost has occurred in the last couple of days, it may take the plants a few hours, if not a couple of days, to become actively growing.
On top of this, you need to be aware of the potential for herbicide resistance creeping into your fields. Good field records and a proper herbicide rotation is necessary to keep the tools we have effective. Using more than one herbicide group in the application can assist in control and delaying resistance from showing up.
There is no, one, silver bullet to control weeds early. Remember, every weed out there is competing for water, nutrients and light. The sooner the weeds are controlled, the better off the crop will be. The phrase, “Get them while they’re young!” takes on new meaning when applied to weeds.