Benefits and risks of using herbicides at reduced rates
Fabian Menalled. Cropland Weed Specialist
In recent years, reducing herbicides rates has received increased interest as an approach
to decrease production costs, minimize the risk of crop and non-target injuries, lessen
carryover concerns, and diminish surface and groundwater contamination. But, can
bellow-labeled herbicide doses provide sufficient weed control? Is this a viable and legal
approach to increase profits? What are the benefits and risks of using herbicide at reduced
rates?
Prior to release, herbicide companies conduct much research to identify the product dose
that will assure efficient and satisfactory weed control over a wide range of
environmental situations, weed species and growth conditions. However, the amount of
herbicide required to provide effective weed control at a given location and for a certain
weed species may be lower. Research conducted at many Universities has repeatedly
proven that, under certain circumstances, bellow-labeled herbicide rates can often be
sufficient to control weeds at or bellow the threshold level and maintain satisfactory crop
yield. In this article, I summarize results from several studies assessing the potential
benefits and risks of controlling weeds with reduced herbicide doses. Additional
information can be found in Zhang et al. (2000) and Blackshaw et al (2006).
Importance of a competitive cropping system. Many studies have shown the
overwhelming importance of competitive cropping systems as a tool to develop longterm integrated weed management programs. Combining several crop production
practices such as the use of competitive cultivars, increased seeding rates, reduced row
spacing and specific fertilizer and herbicide placement has been used to successfully
reduce herbicide applications. For example, Dr. Robert Blackshaw, a Canadian weed
scientist, observed that in a multi-site and multi-year study, higher seeding rates
combined with diversified rotations improved the performance of bellow-labeled
herbicide application in wheat, barley, canola and field pea.
Weed size and weed growth. A very well know fact on weed management is that small
and actively growing weeds are more susceptible to herbicide applications than large and
mature weeds. Thus, reduced herbicide applications may provide efficient control if they
are made to small and actively growing weeds. In fact, it has been suggested that farmers
could measure the leaf extension rate of wild oat as a decision-making approach to time
the use of reduced herbicide doses as this variable provides an easy to track estimate of
plant growth. However, farmers should be caution when spraying small weeds as the crop
may also be small and crop injury could arise even at reduced herbicide rates.
Application method. In a summary of 76 studies, Dr. Zhang and collaborators evaluated
the efficacy and risks of controlling weeds with reduced herbicide rates under various
management and environmental conditions. They observed that weed control efficacy
depends on the method of herbicide application with greater opportunities for herbicide
reduction using preemergence than preplant incorporated or postemergence herbicides.
Interestingly, these results did not vary with the use of interrow cultivation.
Crop characteristics. The success of reducing herbicide application increases with crop
competitiveness. For example, Dr. Zhang and collaborators observed than weed control
efficacy with reduced herbicide rates was higher in corn, intermediate in soybean, and
lower in cereals. Within winter small grains, the potential to reduce herbicide application
rates is higher in winter rye, intermediate in winter barley, and lower in winter wheat.
For spring varieties, reduced herbicide doses were found to be more efficacious in spring
barley than in spring wheat.
Environmental conditions. Labels describe the range of environmental conditions in
which herbicides can be used as soil characteristics, temperature, moisture and relative
humidity have been reported to affect product efficacy. For example, reduced herbicide
rates have been found to be more suitable to be used in coarse-textured than in finetextured soils. Also, most postemergence herbicides are more effective when
temperatures are below 75o F and when adequate soil moisture is available.
Consequently, caution should be used when using reduced herbicide rates in drought or
high temperature conditions.
Weed density. Herbicide efficacy depends on weed abundance with better control
observed in sites with lower weed density. For example, it has been observed that
reduced doses of Achieve (tralkoxydim) or Assert (imazamethabenz) were more
efficacious at low wild oat densities than at high wild oat densities. In accordance,
research conducted in Minnesota has shown that rates as low as one-half of the normal
use rate of Puma (fenoxaprop) or Discover (clodinafop) can be use to manage wild oats at
a density bellow 40 plants per square foot, provided that these products are applied at the
correct timing.
Risks. Using herbicides at bellow-labeled rates has the potential risk of weed control
failures and yield reductions.
Herbicide resistance…. In the end, it all depends on
weighting the potential benefits of reduced rates and degree to which you are willing to
accept the risks associated with this weed management approach.
References
Zhang, J., S. E. Weaver, and A. S. Hamill. 2000. Risks and reliability of using herbicides
at below-labeled rates. Weed Technology 14: 106-115.
Blackshaw, R., J.T. O’Donovan, K.N. Harker, G.W. Clayton, and R. Stougaard. 2006.
Reduced herbicide doses in field crops: a review. Weed Biology and Management
6: 10-17.
Disclosure. Common chemical and trade names are used in this publication for clarity by
the reader. Inclusion of a common chemical or trade name does not imply endorsement
of that particular product or brand of herbicide and exclusion does not imply nonapproval.
Categories: Integrated Weed Management, Herbicides
Date: 06/29/06