The three seasons of weed control

Friday, June 6, 2014

The most important of these is efficacy. Make sure you scout your fields properly, and that means walking them. Then report the problems and take remedial action in a timely manner

by PAT LYNCH

There are three seasons to weed control. The first is pre-planting, when many growers look at the cost of a weed control program. During this season, cost is the driver.

The second season begins after crop emergence. During this season, crop injury is the driver. No grower wants to see any sign of crop damage from the program they chose.

The final season is efficacy. How well did the program work? You want an economical program and you don't want crop injury, but mainly you want good weed control.

There are some factors you should know about. First, all herbicides affect a crop. It takes energy for a crop to break down a herbicide. This is a stress to the plant. In most research trials, the "hoed check" plot is generally the highest yielding. Part of the reason is because there are no weeds and there is some yield advantage from aerating the soil. But, in the words of Dr. Clarence Swanton, professor in the Department of Plant Agriculture at the University of Guelph, "all herbicides need to be broken down by the crop. This extra stress can reduce yields."

The amount of damage a crop suffers is dependent on other stress factors, such as compaction, weather or low soil fertility. An aspect of crop injury is "injury without yield loss." In season, I prefer to refer to herbicide-suspected damage as herbicide "effect." It is not herbicide damage until there is a yield loss. Often, a herbicide can affect a crop without causing a yield loss. And you need to take suspected herbicide damage to harvest to see if there is a yield loss.

From my experience, the third season of weed control is the most important. Generally, you do not pay attention to this unless you can see weed escapes from the road. This is a mistake. If you are not paying someone to scout your field shortly after emergence, you must do it yourself. If the growers who now have glyphosate-resistant weeds had scouted their fields or paid someone to do it, they might not have had the problem they do now.

When you scout your fields, you have to walk the field. You cannot see weeds that are one inch high from the road. And if you can see weeds either alive or dead, you have already lost yield.

When scouting your field, know that every field has a check strip. With the use of guidance systems, check strips are harder to find than they were 10 years ago. But most fields have them. They are often on headlands when the sprayer was not turned on fast enough, or in a gore or a guess row. They can also be beside a water course. See what weeds are there and compare to the rest of the field when assessing weed control or suspected damage.

If you have problems with weed control, you must report them as soon as you see them. Sometimes you can take remedial action. Too often, weed problems are not acted upon soon enough. Report them, take remedial action and then solve them later. And, too often, weeds are left to grow when someone is trying to decide why they are there. During that time, the weeds get bigger and hard or impossible to control. Research has shown that a weed can grow from one to two inches a day under the right conditions.

Many companies have programs to support weed control of some of their products, but these must be reported in a timely manner. Do not put it off. And start next year's weed control program based on notes taken this year. BF

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