Green manures and crop rotations: South America's no till success by Rolf Derpsch, Paraguay*
GroundCover™ Issue: 36
Crop rotation and green manure cover crops (GMCCs) are proving to be the cornerstone of no-tillage sustainable agriculture in Latin America, particularly in the high-rainfall Mediterranean climate of Brazil and eastern Paraguay
Despite what many Latin American farmers believe, green manure and cover crops are not costly. When used in conjunction with crop rotations, they also pay off in terms of yields, economic returns, weed reduction, less chemicals, and a host of other benefits for soils.
One of the biggest challenges facing farmers is to fit green manure cover crops into their current crop rotation, or to develop new rotations that take full advantage of these benefits. Each farmer has to find the ‘window’ where a specific cover crop will fit to accomplish specific purposes.
Farmers need to keep in mind that cover crops function in the ‘off season’ of crops and that they should not compete with cash crops, in terms of labour, time and space.
To find those ‘windows’, research in Latin America has focused on:
- screening adequate GMCCs for different agroecologic regions
- studying the residual fertiliser effect of GMCCs on following cash crops
- showing the economics of cover crops.
In drier areas, such as the Cerrados of North-Central Brazil, farmers and researchers have developed production systems where cover crops are established immediately after harvesting of the main crop.
If cover crops die in the dry season, it is not a problem as long as they have produced enough biomass before then.
In southern Brazil and Paraguay conditions are such that some cash crops or GMCCs can be seeded at any time of the year if soil moisture is available.
Follow not fallow
One of the most fruitful lessons learned in no-tillage farming in Latin America is that farmers should aim to never leave the land in fallow. Even a few weeks’ fallow can result in weed proliferation, seeding of weeds, reduction of soil cover, soil erosion as well as leaching of nutrients.
‘Knife roller' — instrumental to success
Closely linked to the widespread use of cover crops in South America is the use of a ‘knife roller’ to put the cover crops down to the ground. The knife roller is reasonably inexpensive and in many cases can be made in the local area or by the farmer. The implement can be pulled by medium-sized tractors or by animals and has greatly helped reduce herbicide rates in the no-tillage system.
The knife roller has become an essential tool for managing GMCCs. An alternative approach is to weld steel bars on top of the discs of disc harrows and use this set-up to achieve a similar result.
Popular cover crops
Following systematic research with GMCCs in the late 1970s, and testing of more than 100 species and some varieties of these same species, a range of crops has been identified and they are now available for use by farmers, especially in Brazil and Paraguay.
The main cover crops used in South America are:
- winter legumes: white and blue lupins, common and hairy vetch, forage peas, serradella (cadiz), chickpea
- winter grasses: black oats, common oats, rye, triticale, ryegrass
- other winter species: oilseed radish, sunflower
- summer legumes: sun hemp, pigeon pea, dolichos lab-lab
- summer grasses: sorghum, millet, foxtail millet or German millet.
Herbicide costs reduced
Research in Paraguay has shown that crop rotation and short-term GMCCs can reduce the cost of herbicides drastically to US$39/ha in the case of sunflower (57 days GMCC). This compares with costs of US$105/ha for sunflowers when only herbicides and monoculture were used.
By avoiding a fallow altogether it has been possible in some areas to use no herbicides in no-tillage for as much as three years in a row.
In some cases when farmers are using crop rotations, weed elimination with a total herbicide is necessary only before planting and no herbicide application is required during the growing season. If some weeds escape, these can be efficiently and economically controlled in South America by hand hoeing.
Integrate GMCCs for best results
Cover crops must be integrated into individual farming systems and trialed for their beneficial effects. Tillage, for example, counteracts the benefits of cover crops, as it removes the mulch needed to sustain a no-tillage system.
Less weeds with crop rotation
Research conducted in southern Brazil shows consistent reductions in weed infestation with crop rotations in both no-tillage and conventional tillage.
|Occurrence of weeds||With rotation||Without rotation|
|Broad-leafed weeds in wheat||36||24||102||167|
|Narrow-leafed weeds in wheat||17||30||41||44|
|Broad-leafed weeds in soybeans||4||20||15||71|
NT = No-tillage, CT = Conventional tillage
Keep in mind...
Green manure cover crops (GMCCs) should:
- be of low cost (seeds)
- be easy to seed and manage
- provide good weed control and shading
- produce a positive residual fertiliser effect on following cash crops
- not compete in area, labour, time and space with cash crops.
This is an edited version of a paper presented at the 2001 Western Australian Crop Updates, supported by growers and the Federal Government through the GRDC.
Region North, South, West