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Wild oats steal away nutrients

With little information available on nutrient uptake by wild oats and subsequent release from residue, soil scientist Dr. Jeff Schoenau at the University of Saskatchewan, set out to find out just how much nitrogen (N), phosphorus (P) and potassium (K) wild oats use in the spring. “Anecdotally, it is recommended that weeds be controlled early to reduce competitive effects. However, the effect of the age of the weed at the time of control on competition specifically for soil nutrients has received little attention,” says Schoenau.


March 18, 2010
By Bruce Barker

Topics

With little information available on nutrient uptake by wild oats and
subsequent release from residue, soil scientist Dr. Jeff Schoenau at
the University of Saskatchewan, set out to find out just how much
nitrogen (N), phosphorus (P) and potassium (K) wild oats use in the
spring. “Anecdotally, it is recommended that weeds be controlled early
to reduce competitive effects. However, the effect of the age of the
weed at the time of control on competition specifically for soil
nutrients has received little attention,” says Schoenau. 

Visiting scientist Amarsanaa Bayar from the Mongolian State University
of Agriculture carried out the research while she was attending the
University of Saskatchewan, along with Schoenau and his colleagues Rick
Holm and Ken Sapsford.

Wild oat nutrient uptake is large
The first part of the study looked at wild oat nutrient uptake. In the
spring of 2008, a heavily infested wild oat plot was selected and soil
sampled to estimate the basic soil properties and nutrient content of
the soil.

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Beginning one week after emergence (one to two leaf stage), and every
subsequent week for a total of 10 weeks, a square metre sample of wild
oats was obtained from the plot, dried and its biomass weight recorded.
Nutrient uptake of N, P and K was estimated from these samples. “There
was the typical S-shaped uptake, with about one-half of total N taken
up by four weeks, with maximum uptake by seven to eight weeks, about
the milk stage for wild oats. At that stage, we estimated 107 kg per
hectare (95.5 lbs per acre) of N, 10 kg (22 lbs) of P and 177 kg (158
lbs) of K were taken up by the wild oats,” explains Schoenau.

Schoenau says that the nutrient uptake by wild oat follows a pattern
similar to that observed for cereal crops by Dr. S.S. Malhi on a Black
Chernozem in east-central Saskatchewan. There, it was observed that
maximum rate of uptake occurred at tillering to stem elongation, while
maximum total uptake occurred at flowering to medium milk stages,
similar to that observed above for wild oat. Similarly, Malhi observed
a peak of N uptake for several wheat, barley and oat varieties after
about seven to eight weeks of growth with stabilization thereafter. A
decline in uptake after about eight weeks was noted in one year of
their study.  Schoenau says that it seems likely that with wild oat,
the reduction in nutrient contained in the above ground biomass after
Week 8 is due to seed shed.  

Uptake of nutrients by wild oats (kg/ha)
picture-13

Dead wild oats slow to release nutrients
The second part of Schoenau’s research looked at how timing of
herbicide application might influence nutrient recycling, and whether
the nutrients that are taken up by the wild oats before they are
controlled with a herbicide would be released back to the soil in time
for the crop to use them during the growing season. A growth chamber
experiment was conducted in the fall of 2008 to determine the extent to
which the nutrients in dead wild oat residue is conserved and recycled
for crop uptake.

Soil was collected, analyzed and placed into pots. The soil was
considered to have moderate N and P fertility and sufficient K
fertility. Above-ground wild oat samples harvested from the field study
were cut into pieces of approximately three centimetres in length to
allow placement on to the surface of soil in the pots. A rate of wild
oat residue equivalent to that produced and measured in the 2008 field
trials at each of 10 sampling periods was then added to the surface of
the soil in the pots.

Subsequently, spring wheat was planted in the pots. At the end of a
six-week period, the above ground portions of the wheat were harvested,
and a percentage recovery of nutrient added in the wild oat residue by
the wheat plants was calculated.

Wheat above-ground biomass yield after six weeks’ growth in a
controlled environment chamber with treatments of wild oat residue
added that was harvested at different growth stages (weeks) in the
field.
(
Source:  Schoenau et al. 2009)
picture-14

Schoenau notes that the unamended control had the highest wheat biomass
yield, with all treatments that received wild oat residue amendment
having lower wheat yield, especially those that received wild oat
residue harvested at an advanced growth stage. Lesser amounts of
younger wild oat residue, as represented by one and two week old wild
oat residue additions, had little effect on yield. Lower yields in the
seven to 10 week-old wild oat residue treatments may be explained
partly by reduced nutrient availability compared to the young wild oat
residue due to immobilization of nutrients when the wild oat residue
decomposes.

A significant enhancement in N uptake by wheat with added wild oat
residue treatments up to seven weeks of age indicates that the residue
is having a positive effect on N availability, especially the young
wild oat residue. A similar trend was observed with P. Still, even at
the highest recovery rates, only about six percent of residue N was
recovered from the residue by the wheat crop. Recoveries of residue
nutrient decreased to less than one percent with old wild oat residue.

Schoenau says these findings support the need for early wild oat
control. “Early wild oat control should significantly reduce
competitions for nutrients, and should promote more rapid recycling,
but don’t expect a lot of the nutrients to come back in nutrient
recycling in the year of control.”