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Crop Updates - Crop rotations and green manure cover crops in Latin America

Crop rotations and green manure cover crops in Latin America
Rolf Derpsch, GTZ Paraguay

INTRODUCTION

Crop rotation and green manure cover crops (GMCC's) are an essential element in the success story of no-tillage expansion in Latin America. Only those farmers that have understood the importance of these practices are obtaining the highest economic benefits from this system. Cover crops do not cost but will pay. When practiced in monoculture or even in double cropping, i.e. when the same crop or crops are repeated on the same land each year, no-tillage is an imperfect and incomplete system, in which diseases, weeds and pests tend to increase and profits tend to decrease. Adaptive research in this area is the most important factor to make no-tillage work, that is take advantage of all the benefits of the system, reduce weed pressure and increase economic returns!

Research conducted in southern Brazil shows consistent reductions in weed infestation with crop rotations in no-tillage and conventional tillage (Table 2).

Table 1. Number of weeds per m³ with and without crop rotation in two tillage systems in Rio Grande do Sul, Brazil (Ruedell, 1990, adapted by Gazziero, 1998)

NT = No-tillage, CT = Conventional tillage.

Good no-till farmers in Latin America see it as good farming practice to use GMCC's and crop rotations independently of the price situation of crops. Once farmers have discovered the benefits of these practices they don't want to miss them. Sorrenson (1984), between others, has clearly shown the economic advantages of using crop rotation and the right cover crops. While many people still think that when using GMCC's you are adding costs without getting anything back, farmers especially in Brazil and Paraguay have learned that economics of no-tillage can be substantially increased with their use.

Research performed in Brazil has shown significant yield increases of some GMCC's on cash crops. In the average of two years, the highest yield of soybeans (2670 kg/ha) could be achieved after black oats (Avena strigosa) as a green manure crop. This yield was 770 kg/ha higher than the average of all other treatments. Phaseolus beans also produced the highest yield after black oats. Corn responded with the highest yields after white lupins (Lupinus albus) (6.410 kg/ha) and hairy vetch (Vicia villosa) (6320 kg/ha) as compared to yields of less than 4.100 kg/ha after wheat, oats and rye. Also after oilseed radish (Raphanus sativus) a high grain yield of corn (without N) was achieved (5.800 kg/ha). Higher crop gross margins could be achieved when using black oats as a cover crop before soybeans as compared to double cropping wheat and soybeans

In Paraguay, already in the first year, yield increases of almost 1 ton of cotton could be achieved when this crop followed Crotalaria juncea (3.140 kg/ha) or Mucuna pruriens (3.040 kg/ha), as compared to traditional fallow (2.210 kg/ha). Yield increases of 95% and 76% respectively could be achieved after Mucuna pruriens (black seed) and Mucuna pruriens (gray seed), as compared to winter fallow. Tobacco showed yield increases of 105% after Mucuna pruriens (gray seed) as compared to winter fallow.

Research conducted by Kliewer et al. (2000) in Paraguay has shown, that crop rotation and short term GMCC's can reduce the cost of herbicides drastically to US$ 43,05/ha in the case of Crotalaria juncea (52 days GMCC) and to US$39,27/ha in the case of sunflower (57 days GMCC), as against costs of US$105,10/ha when only herbicides and monoculture were used. Kliewer et al. (1998) also reported soybean yields after black oats of 2600 kg/ha without using any herbicides at all. Weed measurements 96 days after seeding soybeans showed 93 kg/ha of dry matter of weeds/ha after black oats, as against 7390 kg/ha after fallow. In the last case soybeans yielded not more than 780 kg/ha. Using a rotation where long and short term GMCC's or cash crops are seeded as soon as possible after harvesting the previous crop, or after rolling down GMCC's with a knife roller, it was possible not to use herbicides in no-tillage for as much as three years in a row. In some cases when farmers are using crop rotations, only eliminating weeds with a total herbicide before planting is necessary without any herbicide application during the growing season at all. If some weeds escape, the few weeds that develop can be efficiently and economically controlled by hand hoeing because labor is cheap.

Research conducted in Brazil has shown that black oats used as a green manure cover crop before soybeans can increase soybean yield by as much as 63% as compared to soybeans after wheat (Derpsch et al. 1991).

Good knowledge about green and dry matter production and profitability of green manure cover crops, how to fit them into different crop rotations and what residual fertiliser effect we can expect of each GMCC planted before the main cash crops is essential for dissemination of their use. Several publications have contributed in filling this knowledge gap mainly in Brazil (Sorrenson and Montoya, 1984; Monegat, 1991; Derpsch, 1991; Derpsch and Calegari, 1992; Calegari et al. 1992).

PRINCIPLES OF GREEN MANURE COVER CROPS (GMCC'S) IN CROP ROTATIONS

Green manure cover crops (GMCC's) 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;

they should not compete in area, labor, time and space with cash crops.

The continuous seeding of the same crop, in the same place during many years has only been possible in the case of rice. This crop has been cultivated as the only crop sometimes for centuries in highly populated areas in Asia. In general monoculture results in diminishing productivity per unit area, the maintenance of low productivity's, or in extreme cases the complete loss of production.

In general the following factors are responsible for this situation (Franke, 1976, 1980):

Increase of specific diseases and pests.

Increase of specific weeds.

Reduced availability of nutrients due to changes in biological activity and physical degradation of the soil.

Reduced root development.

Accumulation of specific toxic substances that inhibit growth.

The principles and fundamentals of crop rotation

The principles and fundamentals of crop rotation are:

To alternate plant species:

with different rooting depth;

with different ability to absorb nutrients;

that are susceptible to diseases with those that are resistant;

taking into account positive and negative effects of one crop on the next;

that tend to undermine with those that tend to increase soil fertility;

with different needs in terms o labor peaks, machines and implements, water, etc. (Anon., 1972).

Green manure cover crops (GMCC's) are the cornerstone of sustainable agriculture and should always be included in crop rotations. Green manure's and cover crops are used as synonyms in this paper. Experience has shown, that tillage negates cover crops. On the other hand cover crops are essential for producing the mulch needed in the no-tillage system. Cover crops have to be integrated in the agricultural system of each farm and show their beneficial effects. Cover crops, in combination with no-tillage and crop rotations ensure the sustainability of agricultural production. "But, rotation isn't just a helter-skelter array of crops" (Rick Bieber, 2000). Without the knowledge of positive or negative residual effects of one species on the succeeding crop, any attempt of organising a crop rotation is merely a theoretical model. Not only legumes are adequate green manure cover crops. Black oats for instance (Avena strigosa Schreb) are planted on 3,2 million ha only in the States of Paraná and Rio Grande do Sul in Southern Brazil. They are planted on more than 300.000 ha in Paraguay.

The main cover crops used in South America

The main cover crops used in South America, specially in Brazil and Paraguay, 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 legume: sun hemp, pigeon pea, dolichos lab-lab. Summer grasses:  sorghum, millet, foxtail millet or German millet.

Climatic conditions in Eastern Paraguay close to the Brazilian border

The climate of Colonia Yguazú, 45 km from the city of Iguazu Falls, Brazil, is classified as Cfa by Koeppen.

One of the biggest challenges is to fit green manure cover crops into your current crop rotation, or to develop new rotations that take full advantage of their benefits. Each farmer has to find the 'window' where a specific cover crop will fit to accomplish specific purposes. It has always to be kept in mind, that cover crops function in the 'off season' of crops and that they should not compete in terms of labor, time and space with cash crops.

In order to find those 'windows' or 'niches' research in Latin America has focused on:

screening adequate GMCC's for different agroecologic regions;

studying the residual fertiliser effect of GMCC's on following cash crops;

showing the economics of cover crops.

FUNCTIONS OF GREEN MANURE COVER CROPS

Provide soil cover for:

no-tillage;

increasing water infiltration into the soil;

reducing water evaporation;

reducing soil temperature;

protection against erosion;

reducing weed infestation;

accumulation of organic matter in the soil;

adding and recycling nutrients;

improve soil structure;

promotion of biological soil preparation.

BENEFITS OF GREEN MANURE COVER CROPS

Cover crops are a key element to make sustainable agriculture possible and have shown the following benefits in Latin America:

Higher economic returns when appropriately chosen.

Reduce the need for herbicides and pesticides.

Improve yields of following cash crops.

Conserve soil moisture.

Prevent soil erosion.

Enhance organic matter content of the soil.

Provide nitrogen.

Improve soil fertility.

Reduce fertiliser costs.

POSSIBILITIES OF REDUCING WEEDS AND HERBICIDE COSTS WITH COVER CROPS IN NO-TILLAGE

One of the most recent and fruitful lessons we have learned in the no-tillage system is that farmers should, if possible, never leave the land in fallow. In general fallow periods of only a few weeks will result in weed proliferation, seeding of weeds, reduction of soil cover, soil erosion as well as lixiviation of nutrients. If instead of leaving the land in fallow, farmers seed any crop immediately or as soon as possible after harvest of the previous crop, they will reduce weed proliferation, avoid weeds producing viable seeds, increase soil cover and the biomass returned to the soil, increase organic matter content of the soil, avoid soil erosion as well as the washing out of nutrients, and improve biological conditions of the soil. After initiating a more intense and systematic research with GMCC's in the late 1970s, testing more than a 100 species and some varieties of these same species, a range of crops have been identified and are now available for the use by farmers especially in Brazil and Paraguay. Some of the winter cover crops are black oats (Avena strigosa Schreb), rye (Secale cereale L.), triticale (Tritico-cereale), oilseed radish (Raphanus sativus var. Oleiferus Metzg), white bitter lupins (Lupinus albus L.), blue bitter lupins (Lupinus angustifolius L.) common vetch (Vicia sativa L.), hairy vetch (Vicia villosa Roth), chick peas (Lathyrus sativus L.), sunflower (Helinthus annuus L.), ryegrass (Lolium multiflorum L.), etc. The most commonly used summer cover crops are millets (Penisetum americanum L., Sorghum bicolor L, etc.), crotalaria (Crotalaria juncea L.), lab-lab (Dolichos lablab L.), pigeon pea (Cajanus cajan L.) and even plants that up to now have been considered to be noxious weeds like Brachiaria plantaginea are used in the Cerrados of North-Central Brazil as cover crops in no-tillage. The Cerrados have only one growing season. Here farmers and researchers have developed production systems where cover crops are established immediately after harvest 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. In Southern Brazil and Paraguay conditions are such, that some cash or GMCC's can be seeded at any time of the year if soil moisture is available.

GMCC's and crop rotation are the key factors for the unprecedented growth of no-tillage especially in Brazil and Paraguay. Linked to the spread of cover crops is the use of a 'knife roller' to put the cover crops down to the ground. This implement is not terribly expensive and in many cases can be made locally or by the farmer himself. The implement can be pulled by medium sized tractors or by animal traction and has contributed a lot in reducing herbicide rates in the no-tillage system. The knife roller has become an essential tool for managing GMCC's in many countries of South America. Alternatively steel bars can be welded on top of the discs of disc harrows and the implement used for the same purpose.

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