Top Crop Manager

Features Agronomy Fertility and Nutrients
Are you paying to feed weeds?

Weeds respond to fertilizer, too.

November 20, 2007  By Top Crop Manager

It only makes sense. Weeds use soil and fertilizer nutrients but until recently, scientists did not know how much they actually used. Now, research scientist Bob Blackshaw at Agriculture and Agri-Food Canada at Lethbridge, Alta., can put some numbers to nutrient use by weeds. In some cases, he found that some species of weeds actually use more nitrogen and phosphate than wheat or canola.

“Our studies showed that many weed species are more responsive to soil nitrogen and phosphorus than our crops,” Blackshaw says. “That tells me that if we don’t manage our soil fertility properly, we may actually be feeding the weeds and encouraging their growth at the expense of the crop.”

In two different greenhouse studies, Blackshaw looked at the root and shoot growth of wheat, canola and 23 common weeds in the nitrogen study; and wheat, canola and 22 weeds in the phosphorus study. He used five different rates of fertilizer in each study to assess species responsiveness to fertilizer.


Some weeds more responsive to nitrogen than crops

In the nitrogen study, Blackshaw found that shoot and root growth for all weeds increased with added nitrogen, but response varied greatly. Many of the weeds responded similar to, or greater than, wheat and canola. Fifteen of the weeds had greater shoot growth than wheat and eight had greater root biomass. The response was measured six weeks after emergence, or around flowering or heading. This is a common time to measure nutrient uptake, as the majority of the crop nutrients will have been taken up by this plant stage.

Table 1. Groupings of species response to increasing nitrogen fertilizer; shoot biomass production: Group A < Group B < Group C < Group D
Group A (least responsive) Group B Group C Group D (most responsive)
Common groundsel Hairy nightshade Wild buckwheat Wild mustard
Persian darnel Green smartweed Green foxtail Redroot pigweed
Wheat Stinkweed Canola
Henbit Scentless chamomile Lamb’s quarters
Barnyard grass Flixweed Cleavers
Russian thistle Wild oats
Foxtail barley Stork’s bill
Round-leaved mallow
Common hempnettle
Downy brome

The research showed that all of the crop and weed species extracted greater than 80 percent of available nitrogen in low nitrogen soils. As nitrogen levels were increased, some weed species showed more shoot response than others. Compared to wheat, all of the weeds in Group C and D were more responsive to nitrogen than wheat. These included wild oats and common broadleaf weeds like wild buckwheat, wild mustard, redroot pigweed, kochia, cleavers, stinkweed and hempnettle.

Also of interest was that the shoot biomass of some weeds and crops tended to plateau at the mid-rates of the added nitrogen, while some species, primarily in Group D, progressively produced more biomass up to the highest fertilizer nitrogen applications. Wild mustard, redroot pigweed and common lamb’s quarters showed continued responses, even at the highest fertilizer nitrogen rates.

While canola was among the highest nitrogen users; 17 of the 23 weed species took up more nitrogen than wheat and would likely be serious competitors for nitrogen. Looking at grassy weeds, wild oats took up more nitrogen than wheat, while green foxtail, Persian darnel and foxtail barley had lower nitrogen uptake.

Phosphate use shows similar trends

A similar greenhouse study was conducted by Blackshaw to assess phosphorus response and uptake by 22 weeds, wheat and canola. The results show similar trends in responsiveness. Shoot and root growth of all weeds increased with added phosphate, but the response varied greatly.

Table 2. Grouping of species based on percent of available nitrogen taken up by each species, at highest nitrogen level:
Group F < Group E < Group D < Group C < Group B < Group A.
Group F (lowest percentage uptake) Group E Group D Group C Group B Group A (highest percentage uptake)
Foxtail barley Flixweed Common groundsel Barnyard grass Green smartweed Canola
Persian darnel Green foxtail Cleavers Kochia Lamb’s quarters
Russian thistle Common hempnettle Wild oats Hairy nightshade
Downy brome Redroot pigweed
Stinkweed Stork’s bill
Henbit Round-leaved mallow
Scentless chamomile Wild mustard
Wild buckwheat


Many weeds exhibited similar or greater responses in shoot and root growth compared to wheat. Canola was among several species taking up the greatest amount of available phosphate at all fertilizer rates; redroot pigweed and wild mustard were also heavy users of phosphate. Percentage uptake by wheat varied according to fertilizer rate. Only four weed species extracted more phosphate than wheat at low phosphate levels, but 17 weed species extracted more phosphate at higher levels. Wild oats and downy brome were heavy users of phosphate.

Wild oats increased phosphorus uptake by 50 percent as the phosphate fertilizer rates were increased and downy brome by 80 percent. On the other end of the spectrum, phosphate uptake by barnyard grass, foxtail barley and green foxtail was relatively unaffected by phosphate fertilizer rate.

Blackshaw says the results of this study show that several weed species are equally or more responsive than crops to higher soil phosphate levels, and many weed species are equally or more effective than crops in extracting soil phosphate.

Plotting a strategy early

With fertilizer prices moving higher, paying to feed weeds is only getting more expensive. Nitrogen prices are expected to move towards $0.50 per pound, so every pound put in the ground needs to pay back in yield – not in healthier weeds.

In other research studies, Blackshaw found that spring application of fertilizer in a banded or point-injected (spoke wheel) application provided advantages to the crop, while reducing fertilizer access to many of the weeds.

“When we look at crop yield, banding provided higher yields than expected,” Blackshaw says. “I think a big component of the yield response was that we were favouring the crop, which provided better nitrogen uptake by the crop and increased crop competition with weeds.”

Along with spring-banded fertilizer, the other obvious strategy is to control weeds with herbicides as early as possible. While Blackshaw’s research did not look at the time of nutrient uptake by weeds, he says it would likely resemble nutrient uptake by canola and wheat. Typically, that means plants exist off the nutrient reserves in the seed for a few days and then as they send out roots, start to take up nutrients from the soil. After a few weeks, nitrogen uptake rapidly accelerates.

Generally, a wheat plant will have taken up about 30 per cent of its nitrogen requirements in the first three weeks of growth (around the five leaf/tillering stage) and about 90 per cent of its nitrogen requirements by head emergence.

Tony Zatylny, herbicides product manager for Arysta LifeScience in Calgary, Alta., notes that the company’s technical staff are seeing similar results in their early weed removal trials in wheat. “In our trials with Everest, we’re seeing a marked yield increase in wheat when the weeds are removed early. Although not all of our data is in yet, we are taking a closer look at the impact of secondary flushes and weed densities during the critical periods of growth for both the wheat crop and weeds. These factors also have an impact as many weed species don’t tend to germinate at the same time.”

How early to control weeds to prevent them from feeding off your nitrogen reserves depends on several factors, such as time of emergence and the competitiveness of the crop. “We intuitively know that nutrient uptake happens early in both crop and weeds, and that you get to a certain stage after emergence when yield losses take off, so early weed removal is critical,” Blackshaw explains.




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