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 Craig

Conyza canadensis Resistant to glyphosate in TN Wind-blown seed Need more herbicides Need more tillage Photo courtesy of Chism Craig

Amaranthus palmerii Dioecious summer annual Prolific seed producer Can grow 2.5 cm/day in summer Greatly reduces crop yield

Cotton field kg ae/ha glyphosate 14 DAA

West Tennessee kg/ha glyphosate in 1 st spray 3 kg/ha glyphosate applied in 2 nd 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…..

Canada Questions??