Agriculture Reference
In-Depth Information
combination have contributed to an overreliance on glyphosate and a
concomitant reduction in the diversity of weed management practices by U.S.
crop producers. This, in turn, has contributed to the evolution of glyphosate
resistance in some weed species and a shift in weed composition in fields,
favoring weeds that are naturally resistant to glyphosate. This leads to higher
management costs, reduced yields and profits, and increased use of less
environmentally benign herbicides. Glyphosate resistance is currently
documented in 14 U.S. weed species (Heap, 2012), and the potential exists for
much more acreage to be affected (Frisvold et al., 2009; Shaw et al., 2011).
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Because no new major herbicide chemistry has been made commercially
available in the last 20 years, and because few new ones are expected to be
available soon (Harker et al., 2012), many plant scientists believe that slowing
the rate of glyphosate resistance and the spread of glyphosate-resistant (GR)
weeds are among the most important problems facing U.S. crop producers
(NRC, 2010, 2012). In addition, private and public programs seeking to
promote the adoption of BMPs are in their infancy and do not address the
reduced incentives to adopt BMPs caused by the ability of weed seeds to
disperse between farms—the programs do not discourage the use of weed
management practices that contribute to resistance.
Best management practices (BMP) may help sustain the efficacy of HT
crops.
Because weeds tend to inherit resistance to glyphosate as a dominant
trait, the mandatory refuge requirement, which has been successful in
sustaining the efficacy of Bt crops, might not be a viable option for HT crops
(NRC, 2010). Depending on the weed, several BMPs, which are relatively
difficult to monitor and enforce, might be required. These include using at
least one other herbicide (particularly a residual herbicide that takes longer to
decompose and thus stays in the soil longer), rotating crops, increasing the
intensity of tillage, cleaning equipment between use in different fields to
prevent the spread of weed seeds and pollen, and optimizing application by
using the application rate recommended on the herbicide label and applying
herbicides at the appropriate time and uniformly throughout the field. Some of
these practices have been associated with increased weed management costs
(Hurley et al., 2009), and many farmers, perhaps due partly to the incentive
problems described above, are only adopting BMPs in the presence of
glyphosate-resistant weeds, as opposed to adopting preventative approaches.
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Another approach currently being promoted by technology providers is
the use of HT crops that are tolerant to two herbicides. However, the
commercial availability of these types of crops does not address the incentive
