Ontario Agri-Food Technologies – FAQs

Weeds are a major problem in agricultural crops. They reduce yield and can contaminate the crop with their seeds which may be toxic. This is especially a problem in a crop like canola.

Herbicides have been intensely used for a number of years as pre-planting, pre-emergence and post emergence sprays or as crop sprays that kill weeds in preference to the crop plants.

In the past, some of the pre-planting herbicides were a problem because some were quite unpleasant compounds and resided in the soil for long periods (e.g. atrazine) but were essential for weed control (e.g. in corn). They were also used as a matter of routine in anticipation of a weed problem that often did not occur.

Many of the common modern herbicides inhibit the formation of essential components required for the growth of the plant. The most commonly used herbicide is glyphosate (Roundup) which inhibits the formation of a class of amino acids called aromatic amino acids. When a plant is sprayed with glyphosate it literally starves because it cannot make these essential amino acids. These amino acids cannot be made by animals including humans. The herbicide glyphosate is therefore non-toxic to animals; we have to obtain our aromatic amino acids from our diet. Bacteria can also make aromatic amino acids but unlike plants they are insensitive to glyphosate. If the gene that allows bacteria to make aromatic amino acids is moved to plants, the plants, like the bacteria, can make aromatic amino acids in the presence of glyphosate and become resistant to the herbicide. Because bacteria are resistant to glyphosate, they readily break it down in the soil and its effect is lost within a few days of its application.

Herbicide resistant plants will allow the use of broad spectrum herbicides that tend to be readily broken down in the soil. These sprays can be used once the crop is established to eliminate all weeds with one application of herbicide instead of several that are often used now. Weeds are most problematic at this seedling stage when they compete with the crop plants. The early application of herbicides to herbicide resistant crops allows crops to fill in the space preventing light from penetrating and inhibiting weed growth. The same or less herbicide may ultimately be used but there is also the prospect of increased yields.

Herbicide resistant crops have encouraged the evolution of new types of farming that could prove to be very beneficial for the environment. In particular, the use of no-till agriculture is becoming very popular. This involves the direct seeding into the remains of the previous years crop without plowing or in some cases even tilling the soil. This reduces costs to the farmer but also helps reduce the erosion of the soil. This method of farming has dramatically increased with the advent of herbicide resistant crops since they allow the used of broad spectrum herbicides once the crop has become established as the most effective means of weed control.

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