Transcript Weed control in smallholder conservation agriculture

Weed control in
smallholder
Conservation Agriculture
Pat Wall and Christian Thierfelder
CIMMYT
MR
International Maize and Wheat Improvement Center
Why Soil Tillage?
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Weed Control
Prepare a seed-bed
Mineralise nitrogen
Eliminate compacted zones
Incorporate fertilizers and ammendments
Control diseases and pests
Control water run-off
Accumulate water
Weed control by tillage is easy
and can be complete
Weed control without tillage
is more complicated and
requires much more
knowledge.
Conservation Agriculture requires
an integrated approach to weed
control
 Use of chemical herbicides and/or manual
weed control
 Use of crop rotation
 Use of green manure cover crops
 Use of crop residues
 Restriction of seed set by weeds
Some characteristics of weed control
techniques
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Manual weeding
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moves a lot of soil and residues – basically shallow tillage
time consuming, very labour intensive
not knowledge intensive
not very risky
Chemical weed control
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effective when done properly
time saving
knowledge intensive
risky depending on herbicides used and management skills
requires purchased inputs - capital
Hand weeding is labour intensive
Excellent weed control by Crotalaria juncea
Green Weight of Weeds
(t/ha)
Effect of ground cover on weed biomass
(85 days after rolling)
16
12
8
Y = 19.1 - 2.6X
4
r = 0.95
0
2
3
4
5
Residues t/ha
Source: Almeida y Rodriguez, 1985
6
Decline in weed populations with
conservation agriculture
600
Weeds/m2
500
400
300
200
100
0
88/89
Skora Neto et al, 1996
89/90
90/91
91/92
92/93
93/94
Reasons for decline in weed
populations:
 Seed not incorporated into the soil profile
 “Seed bank” depleted by efficient control
 Restriction of weed seed-set
 Weed seed on the surface is destroyed by
insects and fungi.
 Surface seed germination restricted by lack
of moisture and light.
It is important to restrict seed set of
the weeds. Therefore a late weeding
before harvest is very important.
Old saying: “One year’s seeding,
seven years weeding”.
Chemical herbicides
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Contact vs. Systemic
Specific (selective) vs. Complete (dessicant)
Pre-sowing vs. Pre-emergence vs. Post-emergence
Residual effects vs. No residual effects
Very toxic to Relatively benign
Different formulations
Different requirements for application:
timing/conditions/water volume/surfactants etc.
Glyphosate is the main herbicide
used in CA
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Originally from the Monsanto Company
and called RoundUp
Monsanto’s patent has now expired
Many companies now produce
glyphosate-based herbicides
This has resulted in a very marked
reduction in price
Some Characteristics of
Glyphosate
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Is a desiccant – total weed control (if managed correctly).
Rapidly fixed by clay particles
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Must use with clean water
No residual effects in the soil – immediately immobilized
Very low toxicity – Green label
Weeds must be actively growing
Needs + 6 hours without rain after application
Works better with more concentrated solutions (less water)
Requires 7-21 days for symptoms of control to be apparent –
longer for weed control
Options for the application of
Glyphosate
 Pre-sowing – complete cover
 Pre-emergence – complete cover
 Post-emergence – directed application
Weed Control Options
Activities depending on intensity of
labour and chemical input
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Manual weeding as a sole measure
Initial glyphosate application followed by hand
weeding
Initial glyphosate application followed by weed
wipes
Glyphosate plus other herbicide(s)
Weeds sprayed with glyphosate prior to
cowpea seeding. Monze, Zambia
CIMMYT
Uniform application is important
MR
International Maize and Wheat Improvement Center