LONG-TERM EFFECT OF A NONIRRIGATED CROPPING SYSTEM ON GRAIN-YIELD OF MAIZE

Yields of non-irrigated maize crops were measured over a 21-yr period (1968-1988) in 1 experiment and over a 24-yr period (1968-1991) in another field located on the loamy sandy soils of southwest France. The experimental treatments studied were: i) continuous maize with aerial parts residues returned (TR); ii) continuous maize with aerial parts residues removed (TE); iii) continuous maize and a winter Italian rye-grass cover crop, sown in August and ploughed back in spring at the same time as the maize residue (RG); iv) rotation involving a continuous maize for 4 yr, followed by 3 yr temporary grassland (PT). In spite of the wide interannual variations due to fluctuations in seasonal wafer deficit, maize grain yields were not affected either by continuous maize aerial parts residues returned or removed. On the contrary, a trend toward a progressive increase in grain yield (0.76 and 1.19 q/ha/yr respectively) was noted during the course of the experiment, probably due to genetic progress in plant breeding. The use of Italian rye-grass as a winter crop significantly enhanced grain production in the maize crops by 5.4 and 7.8% respectively Likewise, a grain yield increase averaging 8.1 and 9.2% respectively was observed after the 3-yr fallow (grassland) period.