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Linking soil nitrogen level to herbicide effectiveness

Many farmers know that crop-weed competition is strongly affected by fertilizer practices, so they manage their fertilizer rate, timing and application methods to give their crop the advantage. Now there is another angle on this issue that can add another tool to the farmer’s weed control toolbox; research results are showing that nitrogen fertilizer levels can influence the effectiveness of some types of herbicides.

“We had done quite a bit of work looking at how soil fertility affects weed growth and weed competition with crops. In some of that work, we were able to determine that both the timing and the method of fertilizer application – surface broadcast versus subsurface banding – made a difference to weed growth and weed competition. And we know from previous herbicide work by many people that for most of our newer herbicides, the better the weeds are growing, the easier they are to kill. This made me think that maybe we need to put those two pieces of information together and do some work on it,” explains Dr. Bob Blackshaw, a research scientist at the Agriculture and Agri-Food Canada Research Centre in Lethbridge, Alberta.

The link between weeds that are growing very well and increased herbicide effectiveness is due to the way many of the newer herbicides work. Blackshaw says, “Some of the older herbicides have a contact type of action; for that type of action it is less important if the weeds are actively growing. Also, some of the older herbicides are less specific in terms of how they kill the plant. Some of the newer ones are systemic and they act on a very specific enzyme in the plant and they have to move to a certain portion of the plant to exert their activity. So it’s more important for those types of herbicides that the weeds are actively growing and actively translocating those herbicides within the plant.”

To further explore this link, Blackshaw and his research team conducted field experiments at Lethbridge in 2004 and 2005. They assessed the influence of nitrogen level on the effectiveness of two herbicides on two weeds and on spring wheat.

They compared a low and a high level of nitrogen. The low level was simply the background amount of nitrogen in the soil, 30kg/ha. The high level was the background amount plus 80kg/ha of ammonium nitrate applied at seeding with mid-row banding, to make a total of 110kg/ha of nitrogen. The background amount of 30kg/ha of nitrogen is fairly typical in the Lethbridge area. The application of 80kg/ha of nitrogen is a little higher than the 50kg/ha to 60kg/ha normally used by most farmers in the area, but still within the range of what might be done on a farm.

The two herbicides used were flucarbazone (Everest) and sulfosulfuron (Sundance). Each herbicide was applied at five different rates, ranging from zero to the registered rate as the highest rate.

The two weeds seeded on the spring wheat plots were wild oats and wild mustard.

For wild oats control, the results showed that the low rates of Everest were more effective when the nitrogen level was high, but when Everest was applied at the recommended rate, soil nitrogen level did not affect the herbicide’s efficacy. Similarly, control of wild oats with Sundance was better at the high nitrogen level, especially when the herbicide was applied at the lower rates.

For wild mustard, a similar but smaller trend was seen with Everest. Sundance was so effective on wild
mustard that control was good even at the lower herbicide rates, no matter what the nitrogen level.

In terms of crop tolerance to herbicides, Blackshaw says, “We had an interesting finding with Everest. We
had a little more crop injury with the low soil fertility rate. That means the crop was not growing as quickly at the low fertility rate so it was not metabolising the herbicide into an inactive form as fast as it would if the nitrogen level had been higher.” Wheat tolerance to Sundance was not affected by nitrogen level.

Blackshaw explains that these results point to two key conclusions, “If the herbicide is very good for controlling a particular weed, then it probably doesn’t matter what the soil fertility is. But if the weed is more difficult to control, then improving soil fertility would become more important.” The second conclusion was that, “If the crop is very tolerant to the herbicide then soil fertility isn’t going to make any difference. If it has borderline tolerance, then improving soil fertility could improve the crop’s tolerance.”

The view from inside
Since completing the field experiments, Blackshaw has been co-operating with University of Guelph researchers to look at this issue from another perspective: inside the weed. They have examined how soil fertility level affects three aspects: the uptake of the herbicide into the plant, the translocation of the
herbicide within the plant, and the metabolism rate within the plant.

Blackshaw explains that the findings from this study show that “Uptake of the herbicide is about the same whether the nitrogen level is low or high. The metabolism rate is sometimes slower with a lower nitrogen level; a slower metabolism for the weed is okay because we actually get better weed control that way. However, translocation is reduced when the weed is not as actively growing with a lower nitrogen rate.”

So greater translocation is likely the most important factor causing the improvement in herbicide effectiveness at higher nitrogen levels.

Making the link on your farm
The link between nitrogen level and herbicide effectiveness adds another element for farmers to consider when deciding on crop, fertility and weed control options. Many farmers already make use of some of the interrelationships between these factors, like the fact that weed control is better when fertilizer is banded rather than broadcast, and when it is applied in the spring rather than the fall.

“The more we understand about what factors influence herbicide effectiveness, the better we are able to manipulate the crop management system so that the herbicide will work better. For example, we know that actively growing weeds are easier to control. The flip side of that is, if you were in a drought situation or had experienced some very cold weather or a late spring frost, and the weeds were not growing well, then you know this is a difficult situation to control the weeds. So you can look at what you can do to overcome those negative things. This would be a time to definitely use a full rate of the herbicide and maybe to make sure that you add the proper adjuvant and proper surfactant,” Blackshaw says.

“Soil fertility is just another one of those factors. If you have a very low soil nitrogen level, then the weeds would probably not be growing as well. So perhaps you might choose a herbicide that relies on contact action, where translocation doesn’t make a difference to its effectiveness,” Blackshaw offers.

He adds, “We don’t know the full story yet; we don’t have a list of herbicides that would be affected and ones that wouldn’t be. But in generalities, we can say that the herbicides that require systemic activity to do their job are the ones that could be affected the most by low soil fertility or other less than ideal growing conditions.” n