Herbicide Resistant Weeds & Crops: A North American Perspective.

Tom Mueller

University of Tennessee

Knoxville, TN, USA

Overview of presentation

• Herbicide resistant crops– How widespread?– Effect on weed control– Effect on development of herbicide

resistance

• Herbicide resistant weeds– Occurrence– Effect on farmers

• My Perspective….

My perspective

• Herbicide tolerant crops can be an emotional issue

• If one wants to see a problem, one will see one

• If one does not want to see a problem, one will not see one

• As is often the case, we only see what we want to see…..

Defining a few terms

• Herbicide Resistant Crop– Genetically Modified Organisms (GMOs)– Transgenic

• Herbicide Resistance = – Inherited ability of a weed population to

survive and reproduce after exposure to a herbicide dose (rate) that would control an unselected (sensitive) population

Glyphosate Tolerant Crops

• RoundupReady (RR) varieties now common in USA– Soybean (>90%)– Cotton (70%)– Corn (~50%, still increasing)– Canola (not a US crop, but in Canada)

• Mainly Monsanto Ag Products

Why do US farmers like RR?

• The System works!!– Kills many weeds, both small and large– Crop safety, can have overlaps– Safe to people spraying (glyphosate is not toxic)

• Monsanto owns seed companies….

• System is simple– One chemical… very easy

Other types of HT crops

• LibertyLink Crops (Bayer)– Tolerant to glufosinate– Canola– Corn, soybeans

• Some very promising lines

Some resistance to GMOs

• In practice, the United States has conducted a large-scale feeding trial

• ~300 million US citizens have consumed GMO crops for ~ 12 years.

• No negative dietary effects

• None…

• There is no toxicological reason not to allow GMO crops to be used.

Herbicide Resistant Weeds

• Weeds have been adapting to herbicide use for many years

• Selection pressure allows for survivors to make seed and fill that open niche

(Conyza canadensis)

(Amaranthus palmerii)

Distribution of HR weeds

• More prevalent where glyphosate continually used

• Cotton/soybean farms

• Southeastern United States

Two main weedy species

• Conyza canadensis– Winter annual

• Amaranthus palmerii– Summer annual

Conyza canadensis

• Very widespread– Several million hectares infested

• Only a problem in no-tillage systems

• Farmers using additional chemicals– 2,4-D/dicamba + flumioxizan prior to planting

(Conyza canadensis)

Photo credit: Chism CraigPhoto credit: Chism Craig

Conyza canadensisResistant to glyphosate in TN

• Wind-blown seed

• Need more herbicides

• Need more tillagePhoto courtesy ofChism Craig

Amaranthus palmerii

• Dioecious summer annual

• Prolific seed producer

• Can grow 2.5 cm/day in summer

• Greatly reduces crop yield

Cotton field 2004

0.8 kg ae/ha glyphosate 14 DAA

West Tennessee 2008 4 kg/ha glyphosate in 1st spray

3 kg/ha glyphosate applied in 2nd application

HR Weeds effect on Farmers

• Use more herbicides

• In extreme cases, hire people to hoe the fields

• Still using RR crop varieties

Other areas

• Canada, upper midwest US

• Minimal glyphosate Resistance problems

• No GMO wheat

• GMO canola has several modes of action– 50% RR, 30% LL, 15% Imi, 5% non HT– Can rotate between alternate modes of action

• Other weeds have developed resistance– ACCase or ALS resistance

Over the larger area

• “Most” farmers have no “major” weed resistance problems

• More farmers will have HR weeds in the next few years

• As RR corn use increases, will see more HR problems

The most common problem

• RR corn “volunteers” in soybeans

• A RR crop (corn) becomes a weed in a different RR crop (soybeans)

Do you see RR corn plants?

Final thoughts

• HT crops widely used

• Few HR weeds, but more each year

• Once a farmer gets them, is a major problem

One sees what one wants to see…..

• CanadaQuestions??