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Don’t let the (herbicide resistance)
cat out of the bag: US lessons
OO By Jeff Werth1, David Thornby2 and Graham Charles3
“We let the cat out of the bag and it won’t go back in.”
“We have been put back 20 years in terms of weed control
programs that rely heavily on residuals.”
“We are not farming cotton anymore, we are farming
against Palmer amaranth.”
“Glyphosate has now become a useful adjuvant.”
R
ECENTLY a group of Australian cotton growers, consultants
and researchers visited parts of the cotton growing region
in the US to see first-hand the problems growers are facing
with herbicide resistant Palmer amaranth. The group visited
several farms, researchers and trial sites to investigate how the
US cotton industry is managing the issue. Below are some of
the comments from US growers/consultants about their current
situation:
The problem
In short, the problem is Palmer amaranth and glyphosate.
Palmer amaranth is a weed with a tremendous ability to
reproduce and share genetic material. The plant is dioecious,
which means it has separate male and female plants. It therefore
generally has to share genes between plants, though both male
and female parts can sometimes be found on the same plant. It
can produce over one million seeds per plant and large amounts
of pollen. These traits have given Palmer amaranth the ability to
adapt to a wide range of climates, and also to evolve resistance
to a number of herbicides.
Growers in the US in the late 80s and early 90s were
Palmer amaranth can produce over one million seeds per
plant.
16 — The Australian Cottongrower
experiencing issues managing Palmer amaranth as populations
were starting to develop resistance to trifluralin, atrazine,
pyrithiobac-sodium and a number of ‘imi’ herbicides in cotton,
corn and soybean. Then along came glyphosate-tolerant
(Roundup Ready) crop technology.
The ability to use glyphosate over the top of crops solved
a number of issues such as soil and moisture depletion under
frequent tillage, and provided excellent control of Palmer
amaranth. Notable Arkansas consultant and ex-university
professor Ford Baldwin said that “Roundup Ready took the
thinking out of weed control” and fields in cotton and soybean
crops were clean again. But after about six years growers started
noticing some escapes, but for a number of reasons continued to
rely solely on glyphosate. Two to three years later these patches
of escapes spread across whole fields and growers were in serious
trouble right across the cotton and soybean growing regions.
What are they doing now?
Glyphosate is no longer working on Palmer amaranth across
the region, so growers are finding its management far more
complex. As Palmer amaranth germinates throughout the
season, growers have had to implement heavy residual herbicide
programs, with residuals being applied approximately every two
weeks from planting until canopy closure.
The use of glufosinate-tolerant (Liberty) varieties has also
increased to approximately half of crops planted. But as
glufosinate is less forgiving of late applications, and Palmer
amaranth grows rapidly, growers are facing challenges covering
their fields in a timely manner for effective control. Between
frequent residual applications and Liberty’s requirement for
Spray booms with Wilmar Fabrication hooded sprayers, now
a common sight on US farms in cotton growing regions.
October–November 2014
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residual programs used for Palmer amaranth, but there is
potential for future resistance development, and the US industry
will need to consider these weeds more carefully if they are to be
ready when that happens.
Glytol (glyphosate+glufosinate tolerance) is in
its first season, and Roundup Ready ExtendflexTM
(glyphosate+glufosinate+dicamba tolerance) will be introduced
next year. A number of growers are waiting eagerly for its
introduction and if they return to relying on the over-the-top
herbicides they will face additional issues in the near future.
Fortunately, companies are promoting the introduction of
Extendflex and Glytol with accompanying residual herbicide
programs.
Weed control in non-crop areas is a further apparent difficulty
for American growers. Even on farms with very clean fields,
adjacent areas often had thick stands of Palmer amaranth,
and the affected growers will never get control of the problem
effectively until they adopt a zero-tolerance mindset in non-crop
areas as well.
What is our current situation?
Ford Baldwin discussing Palmer amaranth management at
Dow Brantley’s (blue polo shirt) farm in Arkansas.
greater timeliness, growers are having to buy a substantial
amount of additional machinery in order to increase their capacity
to get across their fields.
The number of growers using chipping to control escapes
and late season germinations has also dramatically increased. It
was noted that growers who maintained a diverse weed control
system before glyphosate resistant Palmer amaranth appeared
on their farms now have a lower weed burden, and subsequently
lower chipping costs, than growers who used glyphosate alone
until it became impossible to do so.
The cost of weed management in cotton crops has
subsequently risen from $150 to $400 per acre. This, combined
with lower cotton prices, has resulted in a dramatic reduction in
areas of cotton being planted. Growers are finding that cotton
is the hardest crop in which to manage Palmer amaranth due to
its low competitive ability earlier in the season. Research at the
University of Tennessee has shown that corn planted in narrower
rows can grow quickly and shade out Palmer amaranth, thus
reducing the reliance on herbicides.
Corn and other crops that produce robust stubble also appear
to provide extra Palmer amaranth control through mulching. As
a result the majority of growers who have continued to grow
cotton have been those who own gins and/or are leasing country.
The growers we visited made comments on what they would
change if they could start again:
OO “Move away from a single herbicide.”
OO “Implement a residual program.”
OO “Use crop rotations to rotate chemistry options available.”
OO “Have a zero-tolerance to weeds in both crop and non-crop
areas.”
OO “Develop the capacity for timely application of herbicides.”
OO “Increase water rates to ensure effective control.”
Underlying concerns for the US
Growers are currently so focused on Palmer amaranth, that
glyphosate-resistant populations of goosegrass (crowsfoot grass),
Johnson grass and barnyard grass are not being considered in the
majority of fields where they appear.
These weeds are likely to be managed for now by the heavy
18 — The Australian Cottongrower
There are now populations of glyphosate-resistant awnless
barnyard grass, fleabane, sowthistle, windmill grass and ryegrass
present in cotton rotations. Fleabane and awnless barnyard grass
are the most widespread in the north, and ryegrass is creating
problems in the south, particularly in areas that have recently
started growing cotton. Though none of these weeds have
the ‘super-weed’ characteristics of Palmer amaranth, growers
who are dealing with them are finding their costs of weed
management rising.
What can we do now?
If we take into consideration the comments of US growers,
there are a number of things that can be done to either prevent
resistance occurring on Australian cotton farms, or manage
resistance that may already be present. Research has shown that
controlling survivors of herbicide application and preventing new
weed seeds from entering the seed bank is the most effective
option for resistance prevention and management. Using residual
herbicides and strategic tillage where applicable in crop and
fallow is an effective measure to minimise the numbers of plants
being exposed to post-emergent herbicides. Moving away from
glyphosate and adopting two non-glyphosate tactics in crop and
fallow (‘2+2’) will be a successful long-term strategy.
The focus of resistance management, and weed management
in general, should be aimed at the seed bank. Vigilance in
ensuring large cohorts that emerge are effectively controlled,
preventing new seed entering the soil by controlling survivors
and using residual herbicides to reduce emergences will result in
driving down the seed bank for long-term benefits.
The importance of farm hygiene to prevent incursions from
channels, roadsides and fences into the field underpin weed
management at a farm level. Keeping the thinking in weed
control, and adopting a diverse weed control program instead
of automatically reaching for glyphosate will result in long-term
benefits. These are lessons that growers in the US have learnt the
hard way.
Queensland Department of Agriculture, Fisheries and Forestry, Toowoomba
Innokas Intellectual Services, Upper Coomera
New South Wales Department of Primary Industries, Narrabri
1
2
3
More information on weed management is available through:
WEEDpak – http://www.cottoncrc.org.au/industry/Publications/Weeds
Weed Smart – www.weedsmart.org.au/
October–November 2014
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Herbicide Resistance Management
Strategy for Australian cotton
OO By Annabelle Guest, AGDel and Susan Maas, CRDC
A
COTTON grower’s 10 year son was recently heard
asking his father “What’s a cultivator?” Historically the
Australian cotton industry has had a strong integrated
weed management system, but the move to no-till farming and
extensive use of herbicide tolerant (HT) cotton varieties has seen
a reduction in the diversity of weed control tactics used, with
glyphosate now accounting for more than 80 per cent of all
herbicide used in cotton. Unfortunately this has resulted in an
increase in incidence of herbicide resistance.
The Australian cotton industry
has taken the first step in
responding to the risk of herbicide
resistance development, with the
launch of the Herbicide Resistance
Management Strategy (HRMS) at
the Australian Cotton Conference.
This strategy provides growers
and consultants with a tool to be
able to assess and manage their
risk of resistance. Experience with
conventional insecticide resistance
has encouraged a proactive
Susan Maas.
culture to resistance issues in the
Australian cotton industry. Responding to the threat now, is far
easier than reacting to a crisis in the future.
CRDC commissioned a project conducted by Annabelle Guest,
AGDel, to facilitate the development of an industry agreed strategy
to manage herbicide resistance in cotton. The strategy was initially
drafted by the TIMS herbicide technical panel with the aim of
raising awareness of herbicide resistance in the Australian cotton
industry and providing a tool to help implement management
systems that delay the timeframe to herbicide resistance
development. The draft strategy was circulated widely to industry
stakeholders and industry feedback addressed and incorporated
prior to its ratification by the TIMS committee. It is planned that
the strategy will be further developed to include a wider range of
herbicide groups and incorporate other HT cotton varieties as they
become available.
The strategy demonstrates how various combinations of
chemical and mechanical control tactics lengthen the timeframe
to expected resistance development and their impact on weed
seed bank levels. The strategy is based on data generated from
modeling barnyard grass resistance development in glyphosate
tolerant cotton where three over-the-top (OTT) glyphosate
applications are made per season. Scenarios for resistance
development in both back-to-back irrigated cotton and dryland
cotton every second year are presented. The keys to extending
glyphosate life in cotton systems are:
FIGURE 1: Herbicide use in cotton
Survivor control prior to seed set is a key factor in delaying
resistance development. Glyphosate resistance in common
sowthistle has been confirmed in northern NSW.
20 — The Australian Cottongrower
CCA Market audit data shows the increasing reliance on group M. Source CRDC/CCA.
October–November 2014
the way forward
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Resistance Management Strategy (IRMS). The IRMS started from
OO Use a range of weed control tactics in the summer fallow
Cnr Buckland & Yaldwyn Sts
a basic position and has matured over time with the benefit
phase;
Toowoomba
Qld 4350
of stakeholder feedback. It is anticipated that HRMS will be
over-the-top glyphosate
OO Control any survivors
of in-crop
expanded to include more scenarios, tactics, and combinations of
applications andPhone:
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07 them
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tactics as well as mature to be relevant in a multi-herbicide-trait
OO Control weeds in
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Fax:
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channels.
www.excelagr.com.au
Brian Moran
The dependenceBrian
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The Australian Cottongrower — 21
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FIGURE 2: Percentage of area consulted on
believed to contain resistant weeds
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Reducing
fertiliser needs
by accounting
for soil microbes
OO By Dennis O’Brien, USDA–ARS Information Staff
CCA Survey results indicate increasing incidence of herbicide resistance in cotton farming
systems. Source:CCA/CRDC.
tables enable you to determine which other weed control tactics
can be incorporated into your management system by providing
guidance as to how much extra time they will give you until
resistance develops, and demonstrating the effect they will have
on the weed seed bank, which is critical to effectively managing
resistance. The supporting information provides a concise
summary of other key issues for weed management, such as
monitoring, record keeping, timely implementation of tactics,
management of non-crop areas and Come Clean, Go Clean.
The strategy to manage glyphosate resistant weeds is similar
to the strategy to prevent glyphosate resistance – integrate a
range of different tactics throughout the weed lifecycle to rapidly
deplete the soil weed seed bank, and prevent further seed set/
recruitment. This means that the HRMS is just as relevant to
managing resistance weeds as it is preventing them, and can help
to inform a weed management plan.
The HRMS is not intended to be prescriptive, and is aimed
to be an industry mechanism for communicating the herbicide
resistance risks from different tactics. It has been designed to
present the risk related to a range of tactic combinations, to allow
growers and consultants to make their own, informed decisions.
The strategy is supported by a number of existing industry resources such as
Weedpak, cotton production manual, and the cotton pest management guide
that are available from CottonInfo or from www.mybmp.com.au, with other
resources under development. Additional information is also available at
http://www.glyphosateresistance.org.au, and www.weedsmart.org.au
22 — The Australian Cottongrower
F
ARMERS face a balancing act when deciding how much
fertiliser to apply. Applying too much wastes money and
adds to nutrient runoff problems. Applying too little
reduces yields.
Agricultural Research Service scientists in Temple, Texas,
have found a way to help get it just right, maximising profits,
minimising costs, and saving water bodies from unwanted
nutrient runoff. They have developed a test that accurately
portrays soil health by determining the levels of naturally
occurring nitrogen and other nutrients.
Traditional methods for determining fertiliser needs are
based on soil tests developed in the 1960s, which measure the
amount of nitrate in the soil. But these tests don’t account for
the contributions of soil microbes. The microbes play a key role
because they mineralise organic nitrogen and phosphate and
make them more available to the crop. As a result, farmers often
apply more fertiliser than the plants actually need, adding to their
costs and causing unnecessary nutrient runoff.
“The problem is that conventional tools are not measuring
the right soil characteristics. They test for inorganic nitrogen in
the form of nitrate, but that’s just one form of nitrogen available
to the plant,” says Richard Haney, a soil scientist with the ARS
Grassland, Soil, and Water Research Laboratory in Temple.
Richard has developed a more integrated approach. Known as
the ‘Soil Health Tool’ or ‘Richard test’ in commercial laboratories,
it involves drying and rewetting soil and analysing it in ways
that account for microbial activity and measure both nitrate and
ammonium, plus an organic form of nitrogen. It also measures
organic carbon and other nutrients,
in part by replicating some of the
natural processes that occur in a
field.
The drying and rewetting
mimics what happens in the
field before and after a rain.
Nutrients and other compounds
are extracted from the soil samples
with both a water-based solution
and a solution known as ‘H3A,’
which has the organic acids that
plant roots use to acquire nutrients
from the soil. Growers who use the
process receive a spreadsheet that
shows the amounts of nitrogen,
phosphorus, and potassium
available to plants, based on results
extracted by both the water- and
H3A-based solutions. Results also
include measurements of waterOctober–November 2014
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soluble organic carbon, water-soluble organic nitrogen, and soil
microbial activity, and they provide a calculation of soil health
and the ratio of carbon to nitrogen (a key in how much organic
nitrogen is released). Organic carbon and organic nitrogen are
natural byproducts of microorganisms breaking down the soil.
Growers can use the results to determine fertiliser needs.
Savings for farmers
The Soil Health Tool works for any crop produced with
nitrogen or other nutrient fertilisers. Richard has made it available
to commercial and university soil-testing laboratories, worked
with farmers to promote it, and published several papers detailing
its mechanics. The research is funded in part by the Texas State
Soil and Water Conservation Board and the US Department
NEXT ►
of Agriculture’s Natural Resources Conservation Service. This
enhanced soil-testing process is now offered by laboratories in
Maine, Nebraska, and Ohio. It adds to the time and costs for a
soil test, but farmers have learned that in the long run it saves on
fertiliser costs.
David Brandt, who farms 1200 acres in Carroll, Ohio, started
using Richard’s system three years ago to estimate the amounts
of nitrogen he needed to apply to his corn, soybeans, and wheat
fields. He also used it to estimate his phosphorus and potash
fertiliser needs.
“I estimate that it’s saved us at least 25 per cent in nutrient
costs,” he says. “The readings were more accurate than other
soil tests we had run, and we either maintained or increased our
yields.”
On average, fertiliser costs are reduced by about $10 to $15
per acre by adopting the system, Richard says. With less fertiliser
being applied, there is also less nutrient runoff into rivers and
bays.
“This means that less of the nutrients are going into the Gulf
of Mexico, Chesapeake Bay, and other waterways, where they
have been contributing to algae blooms year after year,” Richard
says.
Works well with no-till, cover crops
An ARS technician applies an organic fertiliser source on
plots in a study to optimise application rates of organic and
inorganic fertilisers. The study is part of efforts to evaluate a
new ARS-developed tool for soil testing that can be used to
help growers reduce fertiliser use without decreasing yields.
(Photo: Daren Harmel)
Another problem with conventional soil tests is that they are
based on tilled systems used from the 1940s through the 1960s,
so they often fall short in providing estimates in cover-crop
and no-till systems, which create entirely different soil profiles.
Richard’s system is able to measure the effects of cover crops and
no-till practices. “We can develop a soil health calculation and
suggest a cover crop mix,” Richard says.
David found that the results helped him understand the
contributions made by his cover crops. “We knew they were
helping, but we never understood why. This new information
gave us a better understanding of what was going on in terms
of nutrients in the soil,” David says. He used the information to
adjust his mix of cover crops and get a better ratio of carbon and
nitrogen, a critical factor in soil health. “It’s helped us to pick the
right cover crops to utilise in the field,” he says.
In a four-year field study conducted with Daren Harmel,
research leader of the laboratory in Temple, Richard evaluated
the enhanced soil-testing method in fields of wheat, corn, oats,
and grain sorghum at nine sites in Texas. They applied fertiliser
at traditional rates or at the amounts dictated by the Richard
soil tests, and they left some plots unfertilised. They planted
and harvested on the same dates at each site and kept track of
fertiliser costs, crop prices, and overall profits.
They found that the enhanced method reduced fertiliser use
by 30 to 50 per cent and reduced fertiliser costs by up to 39 per
cent. The enhanced method had little effect on corn production
profits, but increased profits 7 to 18 per cent in wheat, oat, and
sorghum fields. The results were published in the Open Journal of
Soil Science in June 2013.
“We’re asking farmers to think about what they’re putting
on the soil and whether it is necessary. It involves a new way of
thinking, but fertiliser costs are rising, so the idea is attracting
more interest,” Richard says.
This research is part of Climate Change, Soils, and Emissions, an ARS national
program (#212) described at www.nps.ars.usda.gov.
ARS scientists have developed a testing process that
accurately measures naturally occurring nitrogen and other
nutrients in soil. (Photo: Peggy Greb)
24 — The Australian Cottongrower
Richard L. Haney is with the USDA-ARS Grassland, Soil, and Water Research
Laboratory, 808 East Blackland Rd., Temple, TX 76502; (254) 770-6503.
“Reducing Fertiliser Needs by Accounting for Soil Microbes” was published
in the July 2014 issue of Agricultural Research magazine.
October–November 2014