|
Background and Objective:
In order to benefit
from hog manure applications, producers should have access to
information on the effects of manure on weed pressures and herbicide
performance. Therefore, the objective of this study was to quantify
the effect of liquid hog manure applications to agricultural land on
weed emergence and densities and on herbicide efficacy and carry
over risks.
This objective was met.
Procedure and Project Activities:
This study focused on
the herbicide trifluralin and had both a field and laboratory
component. The field component was designed to quantify the effect
of manure applications and synthetic fertilizers on weed emergence
and densities; and on herbicide efficacy and carry over risks. The
field trials were conducted on three different soil types: a fine
sandy loam, a loam, and a clay soil. For each soil type treatment,
the field experiment was a randomized block design with five
nitrogen treatments and four replicates per treatment. The manure
and urea fertilizer treatments were applied in 2001. Following
these applications Treflan EC (trifluralin) treatment was applied at
a recommended field rate of 0.93 L/acre. Canola was grown on all
plots. In
2002, all plots received an equal amount of synthetic fertilizers
and were seeded to oats, a crop that is sensitive to trifluralin
residues in soil. As such, the oat crop served as a bioassay to
compare the level of crop injury among fertilizer treatments and
soil type. The amount of trifluralin in soil in 2001 and 2002 was
also quantified by chemical analysis. Weed emergence and growth was
measured in 2001, 2002 and 2003.
This study also included a laboratory component
that further examined the impact of manure management on trifluralin
persistence in soil. Ms. M. Reimer, a graduate student whose stipend
was supported, in part by ARDI, conducted these laboratory studies.
Ms. Reimer made greater progress than initially projected. This
allowed us to conduct additional laboratory studies with available
resources. It was decided to select two additional herbicides:
glyphosate and 2,4-D. Since there was some impact of manure
applications on herbicide persistence in soil, Ms. M. Reimer also
conducted studies that helped to explain the reasons for these
observed differences in herbicide persistence.
Both herbicide mineralization and sorption were
measured. Herbicide mineralization refers to the transformation of a
herbicide into carbon dioxide. This transformation process is the
result of microorganisms in the soil. When herbicide mineralization
is high, the herbicide product is less persistent in soil. Herbicide
sorption refers to the retention of herbicides in soil. When the
sorption is high, herbicides are strongly retained by soil and are
less available for plant uptake. Herbicides that are bound by soil
are also less likely to be degraded by microorganisms in soil.
Results and Discussion:
Results indicated that there were a total of
11 different weed species present across the fertilizer and soil
type treatments in the three years. The most prominent weed
species were volunteer canola, volunteer wheat, dandelions and
wild oats. Both soil type and fertilizer treatment were
significant factors in influencing total weed densities. The fine
sandy loam had much lesser weed infestations than the clay and
loam soils. Also, plots that had received twice the recommended
rate of manure, based on soil N, demonstrated greater weed
infestations relative to other treatments. The application of
high rates of urea also resulted in larger weed infestations. The
effect of soil type on weed densities was most pronounced for
dandelions. The effect of fertilizer treatment on weed densities
was particularly significant for wild oats in 2003.
The amount of trifluralin in soil decreased
during the growing season, but there were no significant trifluralin
losses from soil during the winter months. As such, the potential for
trifluralin carry-over and crop injury risks can be assessed at the
end of the growing season in the year that herbicides are applied. A
similar trend in herbicide persistence over time has been observed for
the products Odyssey (imazethapyr and imazamox) and Pursuit
(imazethapyr) in Manitoba soils. The amount of trifluralin present in
soil in 2002 was sufficiently high to cause crop injury to oats at the
3 to 6 leaf stage. Despite similar levels of trifluralin in soil
(based on chemical extractions), the loam soil demonstrated lesser
visual signs of oat injury at the 3 to 6 leaf stage than the clay and
fine sandy loam soils. This suggested that soil type has an influence
on the bioavailability of trifluralin to plants. There was no effect
of fertilizer treatment on the amount of oat injury caused by
trifluralin residues in soil.
Oat yields in 2002 were greater for the fine
sandy loam soil than for the clay and loam soils because there was
less weed competition in the fine sandy loam soil. Thus, crop injury
induced by trifluralin early in the growing season had a lesser effect
on oat yield than weed density. Oat yields were also better in the
soils that received either manure or urea in 2001, relative to soils
than received no fertilizers in the first year of study.
Results of the laboratory experiments indicated
that the effects of fresh manure application and long-term manure
applications on trifluralin and glyphosate persistence in soil, varied
by soil type. This suggests that trifluralin and glyphosate
persistence in soil is influenced by factors other than manure
applications. Long-term (10 to 40 years) manure applications to
agricultural land decreased the persistence of trifluralin and
glyphosate in some soils, perhaps due to better soil tilth induced by
manure amendments. The effect of fresh manure applications on
trifluralin persistence in soil was influenced by the timing between
manure and herbicide application. When trifluralin was applied soon
after manure applications, trifluralin persistence in soil was
decreased. When the time between manure and herbicide application was
delayed, trifluralin persistence in soil was increased.
The effect of fresh manure applications on
herbicide persistence was more pronounced for 2,4-D than for
glyphosate and trifluralin. The persistence of 2,4-D increased with
increasing applications of fresh manure to soil. The greatest effects
of fresh manure applications on 2,4-D persistence in soil were
observed when the herbicide was applied at 7 to 28 days after manure
applications, as compared to herbicide and manure applications at the
same time.
There was no impact of manure on soil microbial
activity in soil, but there was some indication that manure influenced
the amount of herbicides sorbed by soil. Fresh manure application to
soil increased trifluralin sorption in only one soil type which, when
amended with fresh manure, also showed greater trifluralin persistence
in soil. The sorption of 2,4-D was also enhanced in soils that
received high rates of fresh manure, thereby explaining the observed
increases of 2,4-D persistence in soils amended with fresh manure. A
history of manure applications onto agricultural land decreased
trifluralin sorption in soil, contributed to the lesser persistence of
trifluralin in some fields with a history of manure applications. It
is possible that long-term manure applications alter soil organic
matter characteristics, therefore altering the degree of trifluralin
sorption by soil. Either fresh or long-term manure applications had
no effect on glyphosate sorption by soil. Glyphosate is strongly
sorbed onto mineral soil particles such as clay so that soil organic
matter characteristics are generally less important to glyphosate
retention in soils.
This study suggests that
high nutrient applications to agricultural land could
encourage an increase in weed densities for some species.
Specifically, the application of twice the recommended rate of
manure based on soil N increased wild oat densities in agricultural
land. It was also clear that trifluralin is persistent in Manitoba
soils and can cause injury to sensitive crops such as oats. The
level of visual injury to oats at the 3 to 6 leaves stage was less
pronounced for loam soils than fine sandy loam and clay soils.
However, oat yields were more influenced by density of weeds in
plots that the level of trifluralin residues in soil.
It is clear that manure applications had no
influence on trifluralin persistence and carry-over in the field
experiment. However, the results of the laboratory experiments
provided some indication that long-term (10 to 40 years) manure
applications to agricultural land decreased trifluralin and glyphosate
persistence in some soils, perhaps due to better soil tilth induced by
manure amendments. The effect of manure on herbicide persistence in
soil varied with soil characteristics and is expected to be also
dependent on land management practices other than manure. Fresh
manure applications to soil had a greater and more consistent effect
on 2,4-D persistence than on trifluralin and glyphosate persistence.
The persistence of 2,4-D in soil decreased with increasing manure
application rates. There was evidence that the effect of manure on
trifluralin and 2,4-D persistence in soil was due to the effect of
manure on herbicide sorption in soil and, hence, the availability of
the herbicide to microorganisms that degrade herbicide in soil. Manure
did not influence the activity of microorganisms in soil.
Acknowledgements:
We gratefully
acknowledge the funding that was provided for this research by the
Canola Council of Canada and the Agri-Food Research and Development
Initiative. This
work was also supported by other agencies, including the Natural
Sciences and Engineering Research Council support to students, and
in-kind contributions from technical support at the Carman Research
Station, University of Manitoba.
|
HomeContact
UsAbout
ManitobaDepartmentsLinksPrivacy 
