Get The Most Out Of Glyphosate

Glyphosate is the most widely used herbicide in the world. Peter Sikkema, weed management specialist at University of Guelph, says, “Glyphosate is the best herbicide invented to date.” To make sure glyphosate keeps working, growers have to be aware of its limitations. Growers have a responsibility to tailor each application so the chemistry kills all the targeted weed every time.


Selecting the wrong rate of glyphosate is probably the most common mistake growers make. As of 2008, you have 18 glyphosate-based herbicide products labeled for use in Western Canada. These products differ in the salt ion included in the formulation of the glyphosate product, in the concentration of the salt and glyphosate acid, and in the adjuvant and other inert ingredients. As a result, the actual rate of product needed for the spectrum of weeds you want to control varies between products.

With three different salt formulations and with differences in the adjuvant added to various brands, not all glyphosate products are compatible for tank mixing. There have been reports of compatibility issues between various brands of glyphosate, so it is better to completely spray out one brand of glyphosate from the tank before changing to a different brand.

Sikkema also notes different weed species require different rates. No one rate fits all weeds. For example, pigweed is much easier to control with glyphosate than buckwheat is. As a general rule, glyphosate controls annual grasses much easier than broadleaf weeds, and both are controlled better than perennial weeds. Some weeds, such as yellow nutsedge, common milkweed and spotted spurge, have a natural tolerance to glyphosate. As well, as weeds get bigger, the rate of glyphosate needed to control them goes up. Always check the label of the glyphosate product you are using to ensure the correct rate is used for the weed spectrum and weed size in your field.

If you don’t, you won’t get a complete kill and you’ll be promoting a weed spectrum in your fields that glyphosate has trouble controlling.


Water quality can have a huge effect on glyphosate. Cations found in hard water will bind with the glyphosate molecule and significantly reduce weed control, Sikkema says. If you have to use hard water, it is important to add ammonium sulphate (AMS) to the spray water and thoroughly mix it in before adding the glyphosate. The sulfate ion in the AMS will bind with the cations in the hard water before the glyphosate molecule can. If the glyphosate

is added before the AMS, the glyphosate will be tied up already. Not only will weed control be reduced, but your investment in AMS will be wasted.

Some growers believe in adding AMS regardless of water quality. Tests have shown this practice may speed up the glyphosate initially, but after two months there is no measurable difference in weed control. If you have good quality water, Sikkema says increasing the rate of glyphosate by an amount equivalent to what the cost of AMS would be probably provides better value for your herbicide dollars.

Some growers have tried to substitute UAN liquid fertilizer for AMS, but UAN doesn’t do the job. It’s the sulphate ion found in AMS that mitigates hard water reactions.


Hard water is not the only water quality feature that reduces glyphosate performance. Glyphosate also readily binds with soil particles, so dirty water and water with high levels of dissolved solids will also result in reduced weed control.


In some cases, tank mixing glyphosates with other herbicides can enhance weed control and reduce the risk of weeds developing glyphosate resistance. But not all herbicides can be tank mixed with glyphosate. Sikkema warns that herbicides such as Sencor and Lorox, which are formulated on clay particles, will result in a loss of weed control. The clay carrier will tie up glyphosate molecules.

Some foliar fertilizers also antagonize glyphosate when tank mixed. Sikkema outlined one trial in which the glyphosate control of lamb’s quarters dropped from 75 per cent to only 15 per cent when one particular foliar fertilizer was tank mixed with glyphosate and applied in Roundup Ready corn. Furthermore, the corn yields dropped from 170 bushels per acre to 120 bushels per acre due to uncontrolled weeds left in the field.


Glyphosate responds positively to lower water rates. This may be due in part to a higher concentration of glyphosate in the spray solution and in part to reduced antagonism due to water quality. When given a range of water volumes, using the lowest labeled water volume usually provides the best results.

Some growers add a pH reducer in the belief this enhances the effectiveness of glyphosate. Research has shown a slight improvement in weed control from this practice if the water is slightly hard. In such cases, the glyphosate acid is released from the salt formulation so the impact of the cations in hard water is reduced. However, the glyphosate acid does not enter the plant tissue as readily as the salt formulation, so the overall impact of adding a pH reducer is likely less than adding AMS.

The adjuvant load also varies between glyphosate products and it is important growers follow label directions with respect to adding an adjuvant. Some glyphosates require the addition of an adjuvant. Other brands are fully loaded. If adding an adjuvant, make sure it is non-ionic and NOT oil soluble. Oil soluble adjuvants will antagonize glyphosate.


Environmental conditions also influence glyphosate effectiveness. Most growers have seen instances where the dust from sprayer wheels inactivates glyphosate, and poor weed control in the tracks is the result. Control can be less than expected alongside a well-traveled gravel road if dust has drifted from the road and coated the leaves of the weeds being sprayed. Dust from a summerfallow field nearby can do the same.

Temperature affects glyphosate activity. As temperatures increase, weed control improves as long as weeds are actively growing. High temperatures combined with drought will reduce your weed control. This combination causes the plants to form a thicker waxy cuticle layer in an attempt to reduce transpiration. As a result, the absorption of glyphosate is reduced.

High relative humidity results in a thinner waxy cuticle layer so glyphosate absorption increases, improving weed control. This waxy layer can take a while to form and a while to go away. Even if temperatures are warm and the RH is high the day of spraying, if it has been hot and dry for the week previously, weed control will still be reduced.

You’ll also find that cool, cloudy conditions result in much slower weed control. Weeds are not growing as fast and light intensity is lower.

Rainfall can wash glyphosate off the leaf before it is absorbed. The time needed between application and a rain event varies depending upon the glyphosate product and adjuvant in the spray solution. The effect of dew is questionable. While some growers say dew enhances weed control, others claim control is reduced. Sikkema says most research shows dew has little effect unless the dew is so heavy that when combined with the spray solution the glyphosate actually runs off the leaf.


The time of day you apply glyphosate can have a huge effect on performance, especially on weeds that exhibit a level of tolerance to glyphosate. Work by Dallas Peterson at Kansas State found that velvetleaf was almost completely controlled when sprayed at 10 a. m., 1:30 p. m., and 5 p. m. But when glyphosate was applied at 6 a. m. and 9 p. m., control dropped to 50 per cent.

The same effect was noted with Palmer amaranth, a weed similar to redroot pigweed. Daytime spraying gave excellent control but an evening, nighttime and early morning application resulted in control dropping to 80 per cent.

Bob Hartzler, a weed specialist at Iowa State University, says, “We know glyphosate is not as effective in late evening or at night as it is during the day. Spraying glyphosate in the evening or night can result in reduced control. Why this occurs is not completely understood. It may be the interaction of a number of factors. We know plant metabolism differs between day and night so it may be as much plant physiology as environmental.”

Hartzler says light intensity can also have an effect on herbicide performance. “Changes in light intensity can impact leaf cupping and leaf alignment, resulting in a decrease in the leaf area exposed to the herbicide application.”


Sikkema says the stage of the crop and weeds are the biggest factors in using glyphosate effectively. Small, actively growing weeds are much easier to control with glyphosate and killing these weeds early prevent the weeds from utilizing moisture and fertilizer needed by the crop. “There is a real temptation to delay glyphosate applications to control later flushes of weeds. This may work in fields with low weed density. However large yield losses may occur in fields with high weed populations.”

In one series of trials, Sikkema found delaying spraying past the second trifoliate leaf stage in soybeans in southern Ontario resulted in an average loss of one bushel per acre per day of soybean yield. A Wisconsin study found a one-day delay spraying glyphosate in Roundup Ready corn required an additional 10 kg per hectare (roughly 10 pounds per acre) of nitrogen ferilizer to maintain the same corn yield. “It is better to apply glyphosate too early rather than to late,” added Sikkema.

Gerald Pilger farms near Ohaton, Alta.

About the author

Gerald Pilger's recent articles



Stories from our other publications