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How Does My Fertilizer Application Impact Herbicide-Resistant Weeds?

By Clark Brenzil, PAg., Provincial Specialist Weed Control, Regina

May 2020

More weed biomass is a good indicator of a healthier weed and more weed seed production, the same as it is for a crop. For the most part, any seeds that are produced by resistant weeds will go back into the "soil seed bank." This will be the source of the next generation of weeds in the field. This means the more weed seeds produced in a field, the more likely it is for that rare herbicide-resistant mutant in a weed population to be present when its corresponding herbicide group is used. Increasing the likelihood of finding herbicide-resistant mutants in a field leads to the quicker evolution of herbicide resistance with greater weed biomass production than those with lower weed biomass.

Managing fertilizer placement is one of several integrated weed management practices that can be implemented to reduce your odds of developing herbicide resistance in key weeds on your farm.

Nitrification inhibitor fertilizer technology has made great advances in preventing urea nitrogen sources from volatilizing from the soil, before they can be incorporated by rainfall. This has created a substantial increase in the amount of nitrogen that is applied as broadcast granules at the expense of side-banded and fall-banded fertilizer applications. The benefits to broadcast applications include efficiencies at seeding that allow for larger seed loads, reduced filling stops and reduced soil disturbance with narrow single-shoot hoe or disc seeding units.

Evenly distributing the fertilizer over the whole field with a broadcast application allows all plants access to the urea. In some cases, broadcasting may preferentially feed weeds and place the crop at a disadvantage. For example, a one-inch knife on 12-inch rows is eight per cent seedbed utilization, so less than 10 per cent of the surface fertilizer is initially available to the crop. A three-inch spread on 10-inch rows, meanwhile, is 30 per cent utilization, and that amount of surface fertilizer is immediately available to the crop.

With a side-banding application, the fertilizer is concentrated close to the crop seed and the crop is the primary beneficiary of that fertilizer, since the crop roots will reach the fertilizer before any weeds do. With mid-row banding, there could be a race between the crop roots from below and the weed roots from above. The edge can be given to the crop if mid-row banding is integrated with narrow row spacing (10 inches or less).

Surface applications also initially benefit the weeds, as the nitrogen is more available to the shallow roots of the weeds emerging from near the surface as compared to the crop roots that are below the seeding depth. Eventually this will be corrected as the nitrogen moves down and deeper into the soil profile, or crown (grass) or lateral (broadleaf) crop roots begin to form near the soil surface and reach out to scavenge nutrients from inter-row spaces. However, keep in mind that the early growth of the crop is the most vulnerable to competition from weeds, as at the same time the weeds have the greater access to surface fertilizer.

Figure 1 shows the results of research conducted in spring wheat on the production of weed biomass under three different fertilizer application methods and expressed as a percentage of the control that had no nitrogen fertilizer applied. The three nitrogen application methods were broadcast, side-banded and fall-banded with anhydrous ammonia. The actual nitrogen rate was the same for each of the application methods.


Weed biomass chart
Figure 1: A chart showing the results of research conducted in spring wheat on the production of weed biomass under three different fertilizer application methods.

The first result that stands out is that weeds respond to fertilizer by producing more biomass, relative to the fertilizer amount they have access to. But when the fertilizer is broadcast on the surface, the biomass produced by the weeds is more than double (125 per cent) that of the unfertilized area, compared to a 50 per cent increase from unfertilized for the side-banded application. Fall banding, and quite likely mid-row banding, fall somewhere in the middle because of the more random or distant nature of the distribution in the field relative to the crop row.

How does this impact your risk of herbicide resistance? Herbicide resistance is a numbers game. In each weed population there will be a very small number of individuals carrying mutations that will result in herbicide resistance. The proportion of these mutations will vary with species and with the herbicide mode-of-action or Group. Scientists believe that weeds can have one mutant that has a gene for resistance to Group 2 herbicides in every 100,000 plants, whereas the frequency of resistant individuals for glyphosate is one in 100,000,000 plants.

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