Progressive Crop Consultant July/August 2019

SELECTIVE
FORCES THAT
ACT ON WEEDS
and How They Can Alter Plant Populations and Communities
Figure 1. All photos courtesy of Lynn Sosnoskie.
BY LYNN SOSNOSKIE | UCCE Agronomy and Weed Science Advisor, Merced and Madera Counties
Weeds are problematic in
crops, primarily because
they compete with commodities
for water, light, and nutrients,
which can result in yield loss.
Weeds can also impact crops, indirectly,
by serving as alternate hosts for insects
and pathogens (Del Pozo-Valdivia 2019;
Petit et al. 2011), providing habitat for
vertebrate pests (White et al. 1998),
or by impeding harvest operations
(Morgan et al. 2001; Smith et al. 2000),
among many other effects.
Resistant Weeds
Consequently, growers employ a
variety of control strategies, including
the application of herbicides, to
manage unwanted vegetation in
their production systems. Although
herbicides can be extremely effective,
weeds may escape chemical control
for a variety of reasons, including
the evolution of herbicide resistance.
Currently, there are 500 confirmed cases
(species x site of action) of herbicide
resistance, worldwide (Heap 2019).
With respect to the United States, 164
unique instances of resistance have
been documented. Most resistances (52
cases) are to the acetolactate synthase
(ALS) inhibitors followed by the
photosystem II (PS II) inhibitors (26
cases), 5-enol-pyruvyl-shikimate-3-
phosphate synthase (EPSPS) inhibitors
(17 cases), and the acetyl-CoA
carboxylase (ACCase) inhibitors (15
cases) (Heap 2019). Examples of active
ingredients for these sites of action
would be rimsulfuron (ALS-inhibitor),
atrazine (PS II-inhibitor, glyphosate
12 Progressive Crop Consultant July /August 2019
(EPSPS-inhibitor), and sethoxydim
(ACCase-inhibitor), respectively.
Herbicide Resistance
Herbicide resistance is an evolutionary
process. Herbicides do not directly
cause the mutations that lead to
herbicide resistance, rather their
repeated use over space and time
‘selects’ for the genetic mutations that
result in reduced herbicide efficacy. In
short, the genetic mutations that confer
herbicide resistance are already present
before the herbicide is applied. The
herbicide treatment eliminates all the
weeds that do not contain the mutated
gene (i.e. the susceptible plants); if no
further intervention is undertaken,
the resistant survivors will continue to
grow, flower, and set seed, which will be
added to the soil seedbank. Over time,
the resistant trait becomes dominant
in the population as susceptible
individuals die out without successfully
reproducing (Figure 1) (Hanson et al.
2013).
Adaptations to Weed Control
Herbicides are not, however, the only
selective forces that can alter the
structure of weed populations and
communities. Any weed management
or crop production practice can select
for weed species that are adapted
to the resulting environment. For
example, repeated and consistent
mowing (Pirchio et al. 2018) can favor
the development of species that are
naturally prostrate or spreading in habit,
like clovers (Trifolium spp.) (Figure 2,
see page 14). The use of drip-irrigation
in processing tomatoes can lower the
numbers of annual weeds that emerge
and compete with the crop (likely due to
reduced surface wetting that stimulates
germination) while facilitating the
establishment of field bindweed
(Convolvulus arvensis), a deep-rooted
and drought-tolerant perennial weed
(Shrestha et al. 2007; Sosnoskie and
Hanson 2015; Sutton et al. 2006).
The adoption of reduced tillage in
processing tomatoes favors the spread of
field bindweed which can be suppressed
by frequent soil disturbance. Even
hand-weeding can serve as a selective
pressure; Echinochloa crus-galli subsp.
Oryzicola, a form of barnyardgrass
that mimics cultivated rice both in
physical form and phenology, is difficult
to visually identify and may escape
removal in labor-intensive production
systems (McElroy 2014).
Managing Herbicide Resistance
When it comes to managing herbicide
resistance, the Weed Science Society of
America (WSSA) has a list of strategies
to employ in order to increase the
diversity of tools in a production
system. However, these tools have value
beyond the prevention and mitigation
of resistance; varying the types and
timing of disturbances should help to
combat difficult to control species that
arose in response to the repeated use
of a weed control strategy. Some of the
best management practices endorsed by
the WSSA include:
Continued on Page 14