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www.dal.cadal.ca
Weed Management under Organic
Production
Andrew Hammermeister
Organic Agriculture Centre of Canada
Dalhousie University, Faculty of Agriculture
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.
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
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
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)
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
Reigi Weeder– 6 acres a day, 2 people & small tractor
– Best on small weeds– http://www.youtube.com/watch?v=k_R-fTC7uYk
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.)
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