C I M M Y T MR International 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, Residues t/ha Green Weight of Weeds (t/ha) Y = X r = 0.95

Decline in weed populations with conservation agriculture Skora Neto et al, 1996

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 T MR International Maize and Wheat Improvement Center Weeds sprayed with glyphosate prior to cowpea seeding. Monze, Zambia Uniform application is important