The paradigm shift going on in weed control

Tuesday, February 5, 2013

We should strive for those clean fields we had in the 1970s, on the grounds that it is easier to keep a field clean than it is to clean one up

by PAT LYNCH

A paradigm shift is a change from one way of thinking to another. It is a transformation, a sort of metamorphosis. It does not just happen, but rather is driven by agents of change. We have a paradigm shift going on now in weed control thinking and actions.

I used to think that if there were no weeds at harvest, we were paying too much for weed control. I now believe we should strive for those clean fields we had in the 1970s when grain prices were high and perfect weed control was achieved.

The advent of resistant weeds and weed misses now suggest we do not want these weeds to go to seed. A clean field at harvest is starting to make more sense. It is easier to keep a field clean than it is to clean one up.

Some grower expectations must change about the weed control they get when they apply an herbicide. You cannot get 100 per cent weed control with one pass. And seldom will you get 100 per cent weed control with two passes. When an herbicide is registered and given a "9" in Pub 75 "Guide To Weed Control," it means it gives 90 per cent control, 90 per cent of the time. If you have high weed pressure, 90 per cent control can still leave enough weeds to reduce yield. One-pass weed control is an oxymoron. To get the weed control you need, count on at least two passes.

Another factor in the paradigm shift happened in some fields over the last two years where moisture limited yields. We used to say that weeds emerging after the critical Weed Free Period did not affect yield. I think that during the past two years we saw that under dry conditions, late germinating weeds caused a yield loss. We do not have dry conditions every year on every acre, but we do have areas in most fields most years that suffer from lack of soil moisture.

These dry areas can be mapped and spot-sprayed. The advent of GPS equipment allows you to enter these areas and apply herbicides only to those areas. They include areas of high organic matter or areas that are light and where corn rows do not fill in. These are the areas where late germinating grasses take off. These grasses add to the weed bank and then spread to other areas. If you have started making weed maps, you can map these areas into your system to take the guesswork out of spot spraying.

Weed seed banks are a weed's resource to keep coming back. You know that every chance you get to deplete this bank is worthwhile. This year, growers who planted an oats cover crop after winter wheat noted how few weeds there were in acres planted to cover crops. In areas not seeded, weeds germinated and grew and added to the weed seed bank. In areas with cover crops, the weed seed germinated and died. Perennial forages are another good way to reduce weed seed banks.

More growers are using residual herbicides in glyphosate-tolerant (GT) crops. The evolution of GT crops started with growers just spraying glyphosate. It has evolved to over 90 per cent of GT corn acres and 25 per cent of GT soys receiving a residual herbicide. The weed pressure in GT soys at harvest last year suggests that there will be more than 25 per cent of GT soy acres sprayed with a residual herbicide in 2013.

This shift to using a residual herbicide in GT crops has occurred because of weed shifts to later germinating weeds like lamb's quarters, ragweed and nightshade. The fear of glyphosate-resistant weeds has also increased the number of acres sprayed with a residual herbicide. And growers want insurance that their fields will be clean. Like they used to be in the 1970s. BF

Consulting agronomist Pat Lynch, CCA (ON,) formerly worked with the Ontario agriculture ministry and with Cargill.