Research Highlights of the CIMMYT Wheat Program 1999-2000

Advances in Mulch-based
Conservation Agriculture
P.C. Wall
Over the past two decades there has been a
remarkable spread of a new type of agriculture
based on residue retention (mulch) and direct
seeding into this residue with little soil movement.
There are many names for this type of agriculture
(no-tillage, zero tillage, direct seeding, and,
occasionally, conservation tillage), all of which try
to convey the two basic requirements of the system:
residue cover and seeding into residue without soil
inversion and as little soil movement as possible.
Today there are over 50 million hectares under zerotillage
in the world, up from only a few hundred
hectares at the beginning of the 1980s. This zerotillage
revolution has required scientists and
farmers to rethink and redefine many conventional
theories and wisdom on crop and soil management.
The major benefits of residue retention and direct
seeding are increased water infiltration (usually
leading to higher water use efficiency), a reduction
in soil erosion, build-up of soil organic matter,
carbon sequestration, and improved soil chemical,
physical, and biological fertility. The major benefits
to the farmer are the reduction in soil erosion,
increased yields (especially in dry years), more
timely sowing, less use of manual labor, and large
reductions in machinery and fuel costs.
The major characteristic of successful no-tillage
systems is the maintenance of sufficient mulch on
the soil surface, for mulch is the motor behind the
chief benefits of the system. As some pathogens that
cause crop diseases are able to survive on the
residues, crop rotation becomes even more
important in no-tillage systems than it is in
conventionally tilled systems. In designing crop
rotations, balancing residue production with
economic production is important to maintain
sufficient soil cover for the system to work.
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One of the major reasons for tilling the soil is for
weed control; in systems where the soil is not tilled,
controlling weeds becomes critical. Initially weeds
are controlled by desiccating herbicides prior to
seeding, and then with normal post-emergence
herbicides. With time, however, in well-managed
systems where weeds are not allowed to set seed,
weed populations are reduced as seed is no longer
incorporated into the soil, the soil seed bank
produced by previous conventional tillage is
reduced, and the mulch soil cover helps control the
weeds. However, good integrated weed control
remains a prerequisite for successful systems.
Dissemination of Zero-Tillage Agriculture
Mulch-based conservation agriculture has been
adapted to a wide variety of soil, topographical,
and climatic conditions. There are areas of no-tillage
agriculture from the equator to 50º north latitude
and nearly to 40 o south. No-tillage is practiced on
soils ranging from heavy clays (80% clay fraction in
Brazil) to light sands, and on plains and hillsides.
The key to this wide adaptation has been the
understanding by farmers and scientists of the
principles of conservation agriculture and the
creative adaptation of equipment and farming
systems to overcome practical problems. However,
as yet, the system has not worked well in soils with
a severe drainage impediment, or where rainfall is
too low for annual cropping and residue production
is very low.
To date most of the expansion in zero-tillage area
has been on large mechanized farms in the USA
(19.75 million ha), Brazil (12 million ha), Argentina
(8 million ha), Canada (4.08 million ha), Australia
(8.64 million ha), Paraguay (0.8 million ha), Mexico