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How herbicides affect nodulation and N fixation

Growers generally use inoculants to improve nitrogen (N) fixation and nodulation of high protein pulse crops...

November 28, 2007  By Donna Fleury

Growers generally use inoculants to improve nitrogen (N) fixation and nodulation
of high protein pulse crops, such as field peas, to increase nitrogen supply
and thereby increase yields, improve protein levels and decrease input costs
of nitrogen fertilizer. The high nitrogen requirements of these crops are typically
met through inoculation with effective nitrogen fixing rhizobia. Therefore,
any factors influencing either the rhizobia or the ability of the plant to send
energy to the nodules may have a negative impact on nitrogen fixation and ultimately,
crop yield.

Researchers at the University of Saskatchewan have been investigating whether
herbicides are one of the factors that can affect nitrogen fixation and yields
of field pea and chickpea crops. The project was initiated in 2004 by co-investigators
Dr. Fran Walley, professor and head of the Department of Soil Sciences, and
professor Rick Holm of the Crop Development Centre. They are also working with
Dr. Newton Lupwayi of Agriculture and Agri-Food Canada (AAFC) at the Beaverlodge
Research Farm in Alberta. Field pea plots were established at both locations,
while chickpeas were only grown at the Saskatchewan site.

Recommended products at recommended rates on peas and chickpeas
are not likely to see any impact on N fixation.

"We are looking at the impact of both in-crop use of herbicides, as well
as herbicides applied in the wheat year just prior to the pea crop that could
have residual effects," explains Walley. The research compared the impact
of herbicides on yield and N fixation. "We assessed N fixation in the field
over the growing season and also determined total N fixation at final harvest."
The in-crop herbicides included a number of commonly used products for field
peas. For chickpeas, one registered product and others being evaluated for potential
minor use registration were used. Herbicides were applied at recommended rates.


Although the final research results are not available until after the 2006
yield data are analyzed, there are some preliminary observations that can be
made. "In some instances, where herbicide application resulted in observable
crop damage, there were indications that N fixation was reduced. However, temporary
reductions in N fixation did not always translate into reduced crop yields,
presumably because the crop was able to recover relatively quickly and may have
benefitted from improved weed control," explains Walley. In those cases
where herbicides caused significant crop damage, there was definitely a subsequent
impact on N fixation.

"We're not surprised by that finding because N fixation depends not only
on the ability of the rhizobia to fix N, but also on the ability of the plant
to supply energy in the form of photosynthates to rhizobia in the nodules,"
says Walley. "If the plant is damaged, then it also impairs the plant's
ability to support N fixation. In some cases, it can be a temporary impact and
the plant can recover, but not always." The plant responds to damage by
trying to repair itself and in doing so, will preferentially allocate its resources
to the repair process, limiting the resources sent down to the nodules. If there
is in-crop or residual herbicide damage that takes a toll on the crop, it is
almost a double impact because that means it is also limiting how much N can
be fixed.

"We found that where the herbicide damage was either minimal or the plant
recovered quickly, there was no detectable impact on either yield or total N
fixation at the end of the season," notes Walley. "In many cases,
the crop had the ability to fully recover." However, with chickpeas for
instance, where there was significant leaf loss from some unregistered herbicide
treatments, those situations showed a negative impact on N fixation. "It's
really the degree of damage as much as anything, and anything that will limit
crop damage will limit the impact on rhizobia."

At one site, although there was some herbicide damage, the final yield results
were still better where herbicides had been used because of better weed control.
All of the plots were hand weeded as well. In the end, the benefits of good
weed control may outweigh any impact on N fixation from herbicide use. "In
terms of the residual herbicides, we haven't detected any significant effect
of those products when we've used them," says Walley. "However, it's
important to note that we've also had a couple of years where the environmental
conditions were very favourable for maximum breakdown of the products."

Overall, it is really a good news story as much as anything, says Walley. Growers
who use recommended products at recommended rates on peas and chickpeas are
not likely to see any impact on N fixation at the end of the season, assuming
there is no herbicide damage. However, it is important to consider the differences
between locations and years. "Our results have really differed between
sites and between years," notes Walley. "It's very clear that weather
conditions and factors unique to different locations are going to have an impact
on the outcome of what happens when you use herbicides on pulse crops. It could
be very site specific and reflect general environmental conditions." This
is true of herbicide damage in general, which can be very difficult to predict.

The 2006 crop year was the final year of the project and final results will
be available in the spring of 2007. The project was sponsored primarily by the
Alberta Pulse Growers Association, with support from the Saskatchewan Pulse
Growers Association. "We also have another related research project underway,
where a graduate student is looking at the survival of rhizobium in the lab,
and to measure the actual impact of herbicides on the survival of rhizobia,"
says Walley. That project is still ongoing and the results are not yet available.


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