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