www.dal.cadal.ca

Weed Management under Organic

Production

Andrew Hammermeister

Organic Agriculture Centre of Canada

Dalhousie University, Faculty of Agriculture

Ecology – The Science of

Connections

• “In living nature, nothing is

unconnected from the

whole”

What is a weed?• Liebman et al. (2001) define agricultural

weeds as “plants that are especially

successful at colonizing disturbed, but

potentially productive sites, and at

maintaining their abundance under

conditions of repeated disturbance.”

• Schonbeck (2013): any plant not

intentionally sown or propagated by the

grower that requires management to

prevent it from interfering with crop or

livestock production.

• http://www2.ca.uky.edu/agc/pubs/pat/pat1-6/pat1-6.htm

• http://www2.ca.uky.edu/agc/pubs/pat/pat1-6/pat1-6.htm

Plant community succession

• A disturbance creates a vacuum, unused nutrients, light, water

• Natural ecosystems undergo ‘succession’ after disturbance.

Empty space is quickly filled

Annuals herbaceous perennials shrubs forest

• Cultivated agricultural systems artificially maintain the

ecosystem at an immature state.

Disturbance

Ecological function of weedshttp://www.extension.org/pages/18529/an-ecological-understanding-of-weeds#.UlXFI8qDmSo

• Protect the soil from erosion

• Replenish organic matter, feed and restore

soil life

• Absorb, conserve, and recycle soluble

nutrients that would otherwise leach away

• Absorb carbon dioxide from the

• Restore biodiversity

• Provide habitat for insects and animals

Weeds have a variety of adaptations to

disturbance (Mohler, 2001a):

• Take up and utilize large amounts of soluble nutrients

• High tolerance to stresses, excess nutrients, drought,

waterlogging; temperature extremes; or repeated grazing,

mowing, or tillage.

• Rapid growth and formation of mature seeds, vegetative

propagules, or both

• Rapid seed germination in response to light, nutrients,

scarification (scratched seed coat), etc.

Weeds are adapted to disturbance (Mohler,

2001a):(Mohler, 2001a):

• Prolific seed production (up to hundreds of thousands

per plant)

• Seed characteristics that promote wide dispersal:

small size, pass through digestive tracts, burrs attach to

fur, and easy wind dispersal

• Seed dormancy mechanisms and seed longevity in

the soil that allow seeds to “wait” for favorable growth

conditions before germinating

• Ability to regrow or reproduce from small fragments

of root, rhizome (underground stem), tuber, or other

underground structures

(Mohler, C. L. 2001a. Weed life history: Identifying vulnerabilities. p. 40–98. In M. Liebman

et al. Ecological management of agricultural weeds. Cambridge University Press, New

York.)

Weed seeds• Some weeds produce dozens of seeds per

plant, others may produce 10s of thousands

• Weed seeds can survive in the soil for less than

1 year to several decades depending on the

species

• Some weeds germinate immediately, others may

require several years of burial before

germinatingSeed Longevity of 41 Weed Species Buried 17 Years in Eastern and Western Nebraska.

Burnside et al. 1996. Weed Science , Vol. 44, No.1, pp. 74-86

http://www.jstor.org/stable/4045786?seq=1

Diagram of the maximum emergence depth of a range of weed species according to seed weight (size). (Based on Roberts,

H. A. (Ed.). (1982). Weed Control Handbook (7th ed.). Oxford: Blackwell Scientific Publications.)http://physicalweeding.com/information/

Weed problems occur when a sufficient weed seed

population encounters a favorable environment for weed

growth in the presence of a crop that is susceptible to the

effects of weed competition. Figure credit: Ed Zaborski, University of Illinois (adapted from Schonbeck and McCann, 2007)

Production costs of weedshttp://www.extension.org/pages/18529/an-ecological-understanding-of-weeds#.UlXFI8qDmSo

• Competing directly for light, nutrients, moisture, and space

• Releasing natural substances that inhibit crop growth (allelopathy)

• Physically hindering crop growth and development, especially climbing vines like morning glories, Ipomoea spp.; and hedge bindweed, Calystegia sepium

• Hosting pests or pathogens that may attack crops

• Promoting disease by restricting air circulation around the crop

• Interfering with or contaminating crop harvest

• Reproducing prolifically, resulting in a greater weed problem in the future

• Parasitizing crops directly (e.g., dodders, Cuscuta spp.; and witchweed, Striga asiatica)

• Creativity

– Look at the problem from another angle

– Why is the weed there?

– Why is it more competitive than the crop?

• Pesticide applications are a reaction to the

problem

– Weed outbreaks are not due to a herbicide

deficiency!!!

How do we make money and

reduce pesticide use?

Rotations

Seeding rate

Crop selection

Cultivar choice

Delayed seeding

Integrated Weed Management

• Few biopesticides for weeds

• Roguing

• Mechanical weeding

• Flaming

• Herbicides

Chemical Physical

BiologicalCultural

Weed management options

Weed Management Strategies

•Enhance crop competitiveness

•Remove or curtail weed growth in

critical early crop development

•Reduce weed seed bank in soil

CULTURAL WEED CONTROL

Prevention of Weeds • Diversify your cropping system

• Control weeds in hedge rows, ditches,

manure/compost piles

• Use weed-free seed

• Clean equipment to prevent spread

• Work patches of rhizomatous weeds separately,

and clean equipment

• Clean equipment between fields

• Compost manure properly (60oC for 3 d)

• Catch chaff from combine

• Prepare the planting site well

– Get rid of perennial weeds before planting

– Optimize fertility for the crop (incl. pH)

– Consider summer fallow if coming out of

grass

– Have a good seed bed

– Have proper drainage

Preparation

Improve crop competitiveness:

seedbed preparation• Level fields

• Reduce planting speeds to ensure uniform depth

• Plant into a firm seedbed and/or pack behind the seeder

• Good seed soil contact essential for rapid and even establishment

• Packing before planting:– Stimulates a flush of weeds

– Creates a firm seedbed

– Creates uniform and firm soil for fingerweeding

• Packing after planting:– Improves seed-soil contact

– Can stimulate competitive weed growth

(use a seeder that packs only in row)

Improve Crop Competitiveness:

Seed and Seeding• Seed

– Competitive cultivar

– Seed purity: damaged seed, weeds, other materials

– Viability, germination, vigour

• Rate– Higher rates are more competitive

• Depth– Deeper planting delays emergence, can be used strategically,

but not for small-seeded crops or less competitive crops

– Deeper seeding allows pre- and post-emergent tillage

• Timing – Crop and weed problem dependent– Early planting – competes with late emerging weeds

– Delayed planting allows control of early weeds, better vigour for some crops

• Spacing– Optimize competitiveness, want even seed distribution

Soil Nitrogen Status and

Seeding RateHigh N, low seeding rate Low N, high seeding rate

Features of Good Crop Rotations

For Weed ManagementFor weed suppression

• Crops that develop slowly or are less competitive should follow weed suppressing crops

• Alternate leaf crops with straw crops

• Alternate between spring sown and winter sown crops

• Select crops that are competitive with weed problem

Row crops can be used to control some weed problems

Intercropping to increase crop competitiveness, compensate for variability in soil

Rotation Examples:

Prairie Organic Farm (Ian Cushon)

• Alfalfa x2: seed + bees– Seed 50-100 lbs/ac @ $2-3/lb

– Bees $100/10,000

– Gross $250-$500/ac

• Hemp– 500-1200 lbs/ac @$1/lb for oil

• Lentil– 600 lbs/ac (10bu/ac) $0.6/lb

• Flax (underseeded to alfalfa)

– 10 bu/ac @$30/bu

• Total: $1660-$2860/ac/5yrs

– $332-$572/yr (3,500 acres)

Rotation example:

Quebec Dairy Farm - 30 cows– Forage

– Forage

– Forage

– Forage

– Forage• Break after 1st cut

• 2 months of weed (couch grass) control

• Apply manure & lime

– Spelt or corn for feed

– Mixed crop for feed underseeded to forage

• Critical success factors:– Optimize soil pH, drainage,

land levelling

– Forages for weed control and soil building

– Roguing weeds in corn

– Would like to add soybean

• Grows short season corn

Canada thistle

• Alfalfa for 3 years

• Mow when in

flower

• Frequent shallow

spot tillage

• Till patch and

clean off

equipment

Green Manure Crops

• Pest Management– Smother weeds

– Break pest cycles

• Weed control:– Ryegrass

– buckwheat

– fall rye

– winter wheat

– oilradish

– mustard

– legumes (alfalfa, sweet clover, white clover once established)

MECHANICAL/PHYSICAL

WEED CONTROL

Critical weed free period

Critical weed free period

http://www.extension.org/pages/18529/an-ecological-understanding-of-weeds#.Uooq2sqDmSo

Critical weed free period

http://www.plant.uoguelph.ca/research/weedsci/extension/ext_iwm.html

Critical weed free period

http://www.plant.uoguelph.ca/research/weedsci/extension/ext_iwm.html

Earthworm: numbers & biomass

Nelson and Lynch, 2008

Phase of the Rotation (years)

Pot

ato

1 yr

sin

ce2

yrs

sinc

e3y

rs s

ince

4/5

yrs

sinc

eRef

eren

ce

Num

bers

(in

div

iduals

m-2

)

0

200

400

600 2006 data

2007 data

Biomass

Pot

ato

1 yr

sin

ce2

yrs

sinc

e3y

rs s

ince

4/5

yrs

sinc

eRef

eren

ce

Bio

mass (

fresh m

ass g

m-2

)

0

100

200

300

2006 data

2007 data

a a

ab

b

cc

aa a b

c

c

aa

b

bc

c

c

a a a

b

cc

Numbers

• False seed bed: A seedbed is prepared, weed seeds in the top 5 cm / 2” of soil germinate and then emerge, the soil is then re-tilled (cultivated) with the minimum disturbance necessary to kill weed seedlings, the crop is then sown, germinates and emerges from mostly weed free soil. http://physicalweeding.com/information/

Stale seed bed: Final seedbed is prepared, weed seeds in the top 5 cm / 2” of soil germinate, crop is sown, weed seedlings emerge, immediately prior to crop emergence weed seedlings are killed by a thermal weeder, crop emerges from weed free soil.

http://physicalweeding.com/information/

• Control weeds before the crop emerges or

just prior to transplanting.

• Burn down herbicides

• Flame weeding?

– Kills small weeds

– Doesn’t disturb soil

Stale seed-bed

Hit weeds when small; light cultivation

Finger/tine weeder

http://www.youtube.com/watch?v=AqwfX3zF-t8

Camera guided wide blade

Wet conditions… major challenge

Roguing

Reigi Weeder– 6 acres a day, 2 people & small tractor

– Best on small weeds– http://www.youtube.com/watch?v=k_R-fTC7uYk

Till and mowDiscourage rodent damage by removing weeds

Mulching in apples

Black Currant Weed

Management Research

• The Questions

– What is the most

cost effective

means of

controlling weeds in

small bush fruits?

– How do these

treatments affect

soil biology?

• The Treatments• Mowing

• Cultivation

• Flaming

• Acetic Acid + tilling

• Mulches

• Black plastic

• Black woven cloth

• White woven poly

• Straw

• Bark

Weed management trial in Dalhousie orchard(Note: the crop in the following slides is black currants but the following discussion should apply to haskap

also.)

Cultivation - 6wks after planting

Acetic Acid herbicide - 6wks after planting

Mowed – 6 weeks after planting

Plastic - 6wks after planting

Plastic vs. mowed

14 months after planting

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7B

ush

siz

e (

m3 p

lan

t-1)

c

a

b

b b

ab abab

Figure 1. Bush size at the end of 2012. Error bars show standard error of the mean. Bars

with the same letter group are not significantly different (p<0.05).

Cultivation+Acid

Black Fabric

Cultivation

Mowing

Black Plastic

Straw

Hay+Plastic

White Fabric

0

50

100

150

200

250

300

350

400

0.140 0.150 0.160 0.170 0.180 0.190 0.200 0.210

So

il D

eg

ree

Da

ys

Average Soil Moisture (m3/m3)

Figure 2. Soil temperature (degree days) vs. average soil moisture from June-

September 2012 as measured by soil probes buried 10 cm deep in each plot

(n=32).

.

-1.0 1.0

-0.6

1.0

Black plastic + hay

Cultivatated soil

Straw

Black plastic

Slender-

B. rupestris

E. anceps

C. obscura

M. subterranea

Small-bl

T. corticinus

S. notangulus

MediBlac

D. globulosusL. intermedius

MediTan-P. carbonarius

Speci2

A. apicalis

H. capillaricornus

A. familiaris

D. filiformis

SmalAtht

Speci3

Fig. 5. Relationship between extracted beetle taxa and plot

types (cultivated soil, black plastic, black plastic + hay, or straw)

as shown by a principal component analysis biplot.

cd

a

d

b

cc

cd cd

0

50

100

150

200

250

300

350

400

450C

os

t E

ffe

cti

ve

ne

ss

(c

m3

/$)

Figure 3. Cost effectiveness (bush volume per dollar spent)

at the end of two years of growth. Columns with the same

letter are not significantly different (p<0.05).

Summary• Organic weed management requires an

integrated approach

– Prevention

– Cultural control

– Physical control

– Chemical or flaming control

• More approaches used at the same time

dramatically increase success of weed

management

• Keep weeds in perspective

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