Avoiding herbicide mis-use

Herbicides have made it easier to feed the world, but beware of residuals and improper use

Tractor spraying soybean field

Herbicides are an integral and essential aspect of modern productive farming. Without our effective and efficient herbicides our dollar costs for food production would be double or triple what we now pay. Can your even visualize hand weeding agricultural and horticultural crops? As a youth I earned pocket money hand hoeing turnips and beets and pulling out weeds from a range of crops. It was slow backbreaking and labour-intensive work. Can you imagine this on a field scale with peas, wheat, lentils or canola? Herbicides have allowed modern agriculture to minimally or zero till land, giving it excellent protection from erosion and water loss by evaporation.

If you want to be an herbicide- or pesticide-free organic farmer, go ahead, but in order to earn a living you need premium prices at about three times that of standard commodity prices such as wheat, barley or flax.

I grew up on an “organic” farm in South Wales because we had few if any effective pesticides, particularly herbicides, as well not having the means of application. Putting food on the table 70 or 80 years ago took a much bigger part of a family’s income than it does at present in North America.

When I grew up in the ’40s and ’50s a good potato yield was five tons per acre and they had to be picked by hand. Potato quality was poor and they had to be hilled up frequently to control weeds. Now dryland potatoes hit 15 to 20 tons and irrigation potatoes range from 20 to 40 tons/acre virtually weed, insect and disease free. Seven to eight pounds of potatoes daily provide all the nutrient needs of a human being. So, if you got a yield of 40 tons to an acre on a quarter section of 160 acres, 150 irrigated, you could theoretically feed over 4,000 people for a year. No Irish famine here!

That 40 ton/acre potato crop can be picked, cleaned, hauled and stored by two to three workers. Now imagine in the late ’40s, say 1948, and you plant your potatoes organically as was the case then. If your planted 150 acres you would expect at best five tons an acre — enough to feed only 500 people for a year. Can you imagine the 100 or so people needed to hand pick these potatoes in 1948? The cost of this back breaking work? Your food cost? Don’t ever complain again about farm food prices.

Herbicides have taken out the onerous task of hand-weeding and depressed yields due to weed competition to become the right hand of economic farming.

Herbicides have become more selective than ever. Some varieties of crops such as corn, soybean and canola have herbicide resistance incorporated into their genetic make-up, giving highly effective weed control. The so-called GMO mantra is totally misunderstood and is used by many countries to restrict free crop trading.

Using herbicides

When you use herbicides your first move this winter will be to read all the information about the product or products that you intend to use. Read the herbicide label carefully. All agrochemical companies provide detailed information in regards to herbicide performance, precautions to take, optimum timing, soil types, soil pH that influence herbicide performance as well as frequency of use. As a full back-up, ensure that you have a copy of one of the three prairie provincial crop protection guides. (I am personally most acquainted with the Alberta “Crop Protection” guide since many years ago I was one of the editors.) These guides give you lots of details on pesticide performance along with the do’s and don’t’s and problems to avoid. All these guides are online but I prefer buying a hard copy for the $10 to $12 that each province charges. A book in hand is worth two on the net!

For example, with the herbicides flucarbazone/fluroxypyr it states that peas can be grown the year after the herbicide-treated wheat crop, provided that the soil pH is below 7.5, organic matter above four per cent and the previous year was normal rainfall and not a drought. Other crops such as barley and canola can be grown anyway but check your soil type to avoid a herbicide residue problem.

Clopyralid is an effective herbicide for weed control in small grain cereals, canola and flax. But do not seed the next year to peas, beans, lentils, potatoes or sunflowers — wait at least another year.

Imazamox and imazethapyr do a first-class job on recommended crops. Repeat application of this herbicide is only recommended for approved crops such as Clearfield canola but even then, avoid over use. Change your herbicide use since in low pH soils in particular this herbicide combination is very slow to break down and could potentially cause a crop failure unless you follow label guidelines. If the herbicide label says once in two years or once in four years on a particular soil type it means just that.

In the sale of herbicide products, no manufacturer has a crystal ball predicting a given year or years on herbicide performance. Unusually dry years may prolong herbicide persistence in the soil frequent rainstorms may reduce herbicide performance and too frequent use of an herbicide can lead to specific weed resistance.

Glyphosate does an excellent job and is seemingly without fault. It has, however, been shown to have a dark side. Over frequent use of this herbicide can cause a buildup in the soil especially in our colder climates. The glyphosate becomes fully inactive in soil, but if liberal applications of phosphate are applied to soils subject to frequent glyphosate application the applied phosphate fertilizer can displace the glyphosate from its bound iron and aluminum sites in the soil where its release may cause obvious crop damage. Several research publications have shown this in Denmark in particular.

Herbicide bioassays

An herbicide bioassay should be started in late summer or early fall especially in a dry season if you are worried about next year’s crop. If you have crop damage or even crop failure in a given year soil samples for bioassay in all instances should only be taken from zero to three inches (zero to six cm). most soil persistent herbicides remain in this zone.

The bioassay should tell you not only how active the herbicide is with potted susceptible crop seedlings but by its action on a range of crop types the probable identity of the herbicide. Chemical analysis is very expensive and on the other hand only gives the name and presence of the herbicide or herbicides but nothing about its effect on crop plants.

Five guidelines to using herbicides

  • DO NOT mix different herbicides unless recommended by the manufacturers.
  • DO NOT add other pesticides, nutrients or micronutrients to the herbicide unless specified by the manufacture.
  • NEVER spray in windy conditions or if rain is forecast.
  • Ensure that the herbicide is recommended for the specific crop.Did your herbicide freeze over winter or was it a non-labelled bargain purchase?
  • Timing of herbicide application is most important: not too early or too late, just right, when possible.

Rotate your herbicides to avoid possible soil built-up (residues), herbicide resistant weeds and if you don’t need one, don’t use one.

About the author


Dr. Ieuan Evans is a forensic plant pathologist based in Edmonton, Alta. He can be reached at [email protected]

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