Higher yields with conservation agriculture are only possible in dry regions. This is the conclusion of a metastudy on the conservation agriculture technique. The study was carried out by an international team of researchers under the leadership of the University of California in Davis, and the results were published in October 2014.
The scientists analysed the data from several hundred field trials reported in the literature. They concluded that conservation agriculture leads to lower yields in wet regions with sufficient precipitation. Compared to conventional agriculture, yields are on average six to nine per cent lower. This changes in dry regions, where if all the measures of conservation agriculture are consistently implemented, yields can actually be expected to increase by seven per cent on average.
Besides leaving out ploughing, these measures include leaving the stubble from the previous crop and rotating crops in a field instead of monoculture.
A major advantage of conservation agriculture is that it makes a field less vulnerable to erosion, which is particularly a problem in regions with heavy rainfall. Heavy rains have a far greater impact on a ploughed field without plant cover than on one where the stubble has been left from the previous crop. Soil in which plant residues are left can also store more moisture. With direct seed, farmers need to water less during dry periods. However, there are also disadvantages. Tilling ploughs weeds under, and omitting this means farmers must use more weedkiller. Pests such as snails are also more frequent with direct seed, and may under some circumstances have to be treated with chemical agents.
All three components must be taken into consideration
As the research has shown, the three components – no tilling, leaving stubble from the previous crop, and crop rotation – are central to the productivity of conservation agriculture.
In many places in Africa, farmers leave their draught animals in the fields so that they can eat the residues from the harvest. In dry regions, this reduces the yield of conservation agriculture to the level of conventional cultivation techniques. The residual vegetation is particularly important, so that the moisture remains in the soil rather than evaporating. The scientists explain the higher yield of conservation agriculture in dry regions by this protection against evaporation. Where residual vegetation is not left to stand in dry regions, an important advantage of conservation agriculture is lost.
If farmers also omit leaving residuals as well as crop rotation, the yield with direct seed falls ten per cent or more compared to conventional techniques in both dry and wet regions. The researchers report that this is largely due to the fact that the pressure from plant diseases and pests is much greater in monoculture than in crop rotation.
Pittelkow CM, Liang X, Linquist BA, van Groenigen KJ, Lee J, Lundy ME,
van Gestel N, Six J, Venterea RT, van Kessel C: Productivity limits and
potential of the principles of conservation agriculture. Nature, Online-
Published 22 October 2014, doi: 10.1038/nature13809