FACTORS AFFECTING HERBICIDE EFFICACY

Lynn M. Sosnoskie, PhD

Department of Plant Sciences

University of California, Davis

Photo from: A.S. Culpepper

Herbicides are, for many, a critical component of weed control programs

(21 selected crops 1960-2008)

Herbicides may not be 100% effective…

Photo from: A.S. Culpepper

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Almond image: Mullookkaaran, Sugar beet image: Jacopo Werther. Grapes: Ian L. Use does not signify endorsement.

ACCase ALS inhibitors

glyphosate

ACCase ALS inhibitors

ACCase glyphosate

Herbicide Resistance in Washington Year Species Site of Action Actives 1970 Common Groundsel Photosystem II inhibitors simazine

1987 Russian-thistle ALS inhibitors chlorsulfuron

1988 Yellow Starthistle Synthetic Auxins picloram

1989 Kochia ALS inhibitors chlorsulfuron

1991 Wild Oat ACCase inhibitors diclofop-methyl

1992 Powell Amaranth Photosystem II inhibitors terbacil

1993 Prickly Lettuce ALS inhibitors chlorsulfuron

2000 Spiny Sowthistle ALS inhibitors imazamox, others

2007 Prickly Lettuce Synthetic Auxins 2,4-D, dicamba, MCPA

2010 Redroot Pigweed Photosystem II inhibitors metribuzin, terbacil

2010 Common Lambsquarters Photosystem II inhibitors metribuzin, terbacil

2010 Mayweed Chamomile ALS inhibitors cloransulam-methyl, others

Trade Name Chemical WSSA Code Resistance PNW? Fusilade fluazifop 1 Yes Poast sethoxydim 1 Yes Select (and others) clethodim 1 Yes Matrix (and others) rimsulfuron 2 Yes Kerb pronamide 3 Yes Prowl H2O (and others) pendimethalin 3 Yes Surflan (and others) oryzalin 3 Yes Treflan (and others) trifluralin 3 Yes Princep (and others) simazine 5 Yes Karmex (and others) diuron 7 Yes Roundup (and others) glyphosate 9 No Rely (and others) glufosinate 10 Yes Solicam norflurazon 12 No Aim carfentrazone 14 No Chateau flumioxazin 14 No Goal (and others) oxyfluorfen 14 No Venue pyraflufen 14 No Devrinol napropamide 15 Yes

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Herbicide resistance is an evolutionary process

Herbicide applications don’t change the weeds genes to make them resistant The gene changes that confer resistance are already present in the species Repeated use of a herbicide removes susceptible biotypes…leaving resistant plants that then reproduce After time, resistant offspring from the resistant source begin to dominate the population

Evolution of herbicide resistance

A A A 2.0 A B B

Time

Poor Weed Control Is More Than Just Resistance

Boom height

Weed height

Factors affecting herbicide efficacy

Weed

Herbicide Environment

Herbicide MOA, Application, Rate, Spray Solution

Image: Tamina Miller. Use does not signify endorsement.

MOA affects Selectivity (Choosing the right herbicide for the weed)

AUXINIC HERBICIDES

Selective for broadleaf spp.

Phenoxycarboxylic acids

Picolinic acids

Pyrachlor

Benzoic acid

Quinoline carboxylic acid

ACC-ASE INHIBITORS

Selective for grass spp.

Cyclohexadienes (-dims)

Aryloxyphenoxypropionates (-fops)

Differences in Efficacy (CSU Extension Turf Weed Management CMG Garden Notes #552)

Herbicide class a.i. Dandelion Bindweed Crabgrasses Foxtails

Phenoxycarboxylic acid 2,4-D Good

Picolinic acid triclopyr Good Good

Quinoline carboxylic acid quinclorac Good Good Good

Method of Application (How the herbicide is applied in the environment to control weeds)

Pre-emergence Post-emergence

Credit: A.S. Culpepper

Differences in Weed Control Spectrum PRE vs. POST (CA Tomatoes)

RIMSULFURON (MATRIX SG) PRE

Barnyardgrass

Giant, green, yellow foxtail

Henbit

Kochia

Redroot and smooth pigweed

Common purslane

RIMSULFURON (MATRIX SG) POST Barnyardgrass Annual bluegrass Barley, giant, green, yellow foxtail Fall panicum Henbit Kochia Redroot and smooth pigweed Common purslane Shepherd’s purse Wild radish Chickweed Mustard spp.

Source: Matrix SG label, Dupont

Differences in Weed Control Spectrum PRE vs. POST (CA Tomatoes)

RIMSULFURON (MATRIX SG) PRE RIMSULFURON (MATRIX SG) POST

Source: Matrix SG label, Dupont

Bindweed: YES Bindweed: NO (Vine suppression)

Herbicide Rate (Control of grass spp. in citrus with Roundup Powermax)

Weed 0.7 QT/A 1.3 QT/A 2 QT/A 3.3 QT/A

Bermudagrass Burndown - Partial control Control

Paragrass Burndown Control Control Control

Torpedograss Suppression - Partial control Control

Source: Roundup Powermax label, Monsanto

Spray Solution Water quality (Spray solutions are >95% water)

What goes in can affect what comes out

Glyphosate is one of the best examples

pH – high pH causes glyphosate to dissociate

Cations – Mg, Ca, Na can bind to glyphosate

Turbidity – glyphosate tightly bound to soil and OM

Tank Mixing (via Andrew Kniss U. Wyo.)

Mixtures of two herbicides are far more effective at slowing the evolution of herbicide resistant weeds compared to an annual rotation of herbicides (You still need to rotate, tho!) BUT… BOTH HERBICIDES must be effective on the target weed If one of the herbicides is not effective, then there is still heavy selection pressure for weeds resistant to the other herbicide AND… The tank-mixture strategy also only works with the most common resistance mechanisms; other types of herbicide resistance (like metabolic resistance) are potentially more likely to occur when multiple herbicides are used

2010 1980 1940

1950-1954 diuron

1955-1959 simazine

1960-1964 dichlobenil trifluralin

Timeline of Herbicide Introductions for PRE Products Registered in WA Vine Crops*

* Timeline is based on 2015 registrations in CA; not all herbicides are registered in all crops. Years represent data reported in Timmons (1970) Weed Sci. 18:294-307, Appleby (2005) Weed Sci. 53:762-768, and the EPA. Dates are of the first

recorded registration/use in any crop, not necessarily tree nuts, fruit trees, and/or vines.

1970-1974 napropamide norflurazon oryzalin pendimethalin pronamide

1975-1979 oxyfluorfen

1980-1984 isoxaben

1985-1989 flazasulfuron

1990-1994 flumioxazin sulfentrazone

2010-present indaziflam

2010 1980 1940

Timeline of Herbicide Introductions for POST Products Registered in WA Vine Crops*

* Timeline is based on 2015 registrations in CA; not all herbicides are registered in all crops. Years represent data reported in Timmons (1970) Weed Sci. 18:294-307, Appleby (2005) Weed Sci. 53:762-768, and the EPA. Dates are of the first

recorded registration/use in any crop, not necessarily tree nuts, fruit trees, and/or vines.

1970-1974 glyphosate

1980-1984 fluazifop glufosinate sethoxydim

1965-1969 paraquat

1990-1994 carfentrazone pyraflufen

1985-1989 rimsulfuron

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Cumulative a.i. Introductions versus Issued Herbicide Patents per Year

Weed Biology/Ecology Identity, Life Cycle, Morphology, Size

Herbicides and Nutsedge Control

Metolachlor = <20% Glyphosate = 70%

Metolachlor = 55-75% Glyphosate = 55%

Life Cycle (Annual vs Perennial)

An extensive root system makes field bindweed very tolerant of control measures

Root system of field bindweed, Convolvulus arvensis. Redrawn from B. F. Kiltz. 1930 J. Amer. Soc. Agron. 22:216-234

Plant morphology can affect herbicide capture and absorption

Leaf orientation Location of meristems Leaf area Hairiness

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Plant Density and Spray Interference Greater numbers of weeds will require greater weed

control efforts to prevent escapes

Credit: T.M. Webster

Probability of finding a mutation that confers resistance increases as plant density increase

Jasieniuk et al. (1996)

Est. Mutation Rate Weeds/m2 Probability of occurrence of at least one mutant plant in 30 ha fields

1 x 10-6 1 0.45 5 0.95

50 1.00 500 1.00

1 x 10-8 1 0.006 5 0.03

50 0.26 500 0.95

Est. “typical” spontaneous genetic

mutation rate (gametes/locus/gen.)

Est. spontaneous genetic mutation rate

for ALS-R (gametes/locus/gen.)

Results are estimates for a single dominant nuclear gene mutation in a random-mating system.

Environment Soil, Water, Temperature, Wind

Soil ◦ Clay, OM can make herbicides

unavailable; affects rate required for control

◦ In less adsorptive soils, herbicide leaching can occur; too much herbicide can lead to crop injury

◦ Slope can lead to erosion or drainage; physical loss of treated soil

Water

◦ Soil moisture: ◦ Needed for activation of

residual herbicides

◦ Precipitation: ◦ Activation of herbicides ◦ Erosion, leaching ◦ Rain-fastness

Wind

◦ Spray drift (droplets)

◦ Dust drift (particles)

◦ Dust (barrier)

2,4-D damage on grapes. UC IPM

Temperature ◦ Herbicide effects ◦ Degradation

◦ Volatilization

◦ Plant stress ◦ Plant growth rate

◦ Cuticle development/herbicide absorption

◦ Herbicide translocation

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Time, in days, to reach 3” height following Palmer amaranth emergence

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Herbicides Read the label, calibrate the sprayer, use adjuvants wisely Use multiple herbicide MOAs Apply herbicides at label rates and at recommended weed sizes Use mechanical, cultural, and biological practices when appropriate

Weeds Understand the biology of the weeds present Plant into weed free fields and keep fields as weed free as possible Prevent the movement of weed seed within and between fields Scout fields, borders routinely

Environment Apply sprays as are appropriate for soil, moisture, temperature, wind

BMPS FOR WEED RESISTANCE MANAGEMENT

Thank you for your time! I am a new scientist and I need you to help me to help you! I need you to tell me: • What YOUR biggest weed concerns

are.

• What management strategies (organic, conventional, mixed) YOU want investigated.

• What application tools (i.e. nozzles, spray rates) need more study.

• How the environment affects your weed control.

lynn.weed.science@gmail.com

@LynnSosnoskie on Twitter