C I M M Y TMRInternational Maize and Wheat Improvement Center
Weed control in smallholder
Conservation Agriculture
Pat Wall and Christian Thierfelder
Why Soil Tillage?
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
Manual weeding moves a lot of soil and residues – basically shallow tillage time consuming, very labour intensive not knowledge intensive not very risky
Chemical weed control effective when done properly time saving knowledge intensive risky depending on herbicides used and management skills requires purchased inputs - capital
Manual weeding moves a lot of soil and residues – basically shallow tillage time consuming, very labour intensive not knowledge intensive not very risky
Chemical weed control 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
Effect of ground cover on weed biomass (85 days after rolling)
Source: Almeida y Rodriguez, 1985
0
4
8
12
16
2 3 4 5 6
Residues t/ha
Gre
en
Weig
ht
of
Weed
sG
reen
Weig
ht
of
Weed
s
(t/h
a)
Y = 19.1 - 2.6X
r = 0.95
Decline in weed populations with conservation agriculture
Skora Neto et al, 1996
0
100
200
300
400
500
600
88/89 89/90 90/91 91/92 92/93 93/94
Wee
ds/
m2
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
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
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
Is a desiccant – total weed control (if managed correctly). Rapidly fixed by clay particles
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
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)
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)
Activities depending on intensity of labour and chemical input
C I M M Y TMRInternational Maize and Wheat Improvement Center
Weeds sprayed with glyphosate prior to cowpea seeding. Monze, Zambia
Uniform application is important
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