LONG-TERM EFFECT OF A NONIRRIGATED CROPPI NG SYSTEM ON GRAIN-YIELD OFMAIZE

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 ano ther field located on the loamy sandy soils of southwest France. The e xperimental treatments studied were: i) continuous maize with aerial p arts residues returned (TR); ii) continuous maize with aerial parts re sidues removed (TE); iii) continuous maize and a winter Italian rye-gr ass cover crop, sown in August and ploughed back in spring at the same time as the maize residue (RG); iv) rotation involving a continuous m aize 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 mai ze 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 r espectively) was noted during the course of the experiment, probably d ue 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 a veraging 8.1 and 9.2% respectively was observed after the 3-yr fallow (grassland) period.