A recent research article by Purdue weed scientists highlights the damage that giant ragweed can cause in corn. Obviously, these tall weeds are going to be highly competitive for light and they are also going to compete for water. However, this new study also documents their competition for nitrogen. 

In the study, 180 lb/a of nitrogen was applied before corn planting. Giant ragweed emerged at 1 to 3 days after corn planting and were thinned to a low density that was equal to 2,020 plants/a (0.5 plants/m²). Giant ragweed emerging along with the corn would be highly competitive. On the date when giant ragweed reached 16 inches tall and the corn was at the V8 growth stage, samples of both plants were harvested and nitrogen accumulation was determined. Plants were also sampled at the end of the season at a couple weeks before corn harvest.

Giant ragweed was effective at taking up a significant amount of nitrogen despite its low density (Figure 1). Giant ragweed had removed 10 lb/a N by the time it was 16 inches tall at a time when corn had taken up 24 lb/a N. By the end of the season without any control, giant ragweed had removed 58 lb/a N while corn had accumulated 97 lb/a N. Perhaps this is not a surprise that this low density of giant ragweed could accumulate this much nitrogen when the size of individual giant ragweed plants is considered. Still, when nitrogen prices are considered, it doesn’t seem to be wise to be “fertilizing” giant ragweed.

Figure 1. Nitrogen uptake by a low density of giant ragweed and corn plants when sampled early and at the end of the season.

The effect of giant ragweed competition, including competition for nitrogen, on corn yield was also predictable. When this low density of giant ragweed was removed early enough in the season, the corn yield was not statistically reduced compared with the weedfree corn (Figure 2). However, season-long giant ragweed competition reduced corn yield by 19%. If the giant ragweed density was greater, yield loss would likely occur before giant ragweed reached 16-inches tall and yield loss would be greater than 19% if not controlled at all.

Figure 2. Corn yield when a low density of giant ragweed was controlled early in the season or allowed to compete all season compared to weedfree corn.  Bars with the same letter are not statistically different.

 

Giant ragweed is a highly competitive plant and is generally difficult to control with a single herbicide application. Consequently, we encourage two-pass herbicide programs to control giant ragweed in corn. The first pass should be a preemergence residual herbicide to control the early emerging giant ragweed. Preemergence herbicide options include atrazine or premixtures with atrazine, Hornet, Lumax, and Princep. The residual control from such herbicides often will not provide adequate full season control and a second postemergence herbicide application is usually required. Hence, this becomes a two-pass program. The advantage with the postemergence application after the preemergence herbicide is that the “window” to make the application is longer and the ragweed is smaller and easier to control.  The temptation to control giant ragweed with a single postemergence herbicide application is risky because 1) the giant ragweed may be too large to get complete control; 2) the ragweed may have already competed too long (i.e. removed nitrogen, etc.) and reduced corn yield; and 3) more ragweed may still emerge after this postemergence application, which would still require a second application.  

 

I will be discussing the research that Carrie Laboski and I have conducted on nitrogen accumulation by weeds and the effect on corn yield at the Fertilizer, Aglime, and Pest Management Conference in January. The results are more dramatic than the effects of this ragweed study because of the higher density and multiple weeds involved in our study. If you are interested, we hope you attend the session. 

 

Source: Johnson and others. 2007. Influence of nitrogen application timing on low density giant ragweed (Ambrosia trifida) interference in corn. Weed Technology 21:763-767.