The effects of population density and intercropping with cowpea on the water use and growth of sorghum in semi-arid conditions in Botswana

Abstract

Soil moisture was measured below sorghum crops of differing density in rainfed trials in increasingly droughty conditions from 1981/2 to 1983/4. Additional measurements were made in sorghum/cowpea intercrops and cowpea monocrops during the drought seasons of 1982/3 and 1983/4. Increasing crop density, by addition of either sorghum or cowpea, resulted in increased effective root activity, which was accompanied by small increases in actual/potential evapotranspiration ( E a/ E p) early in each season. These small increases were followed by small decreases later in the season, but these effects were rarely statistically significant. Both sorghum density and intercropping had little effect on total seasonal water use. Dry matter production increased linearly with increasing seasonal water use. Water use efficiencies of dry weight production were increased by increasing sorghum density. Intercropping in dry conditions did not have significant effects on water use efficiencies. Estimates of soil evaporation suggested that the effects of density on water use efficiency could be attributed to reductions in soil evaporation. The effects of density on reproductive yields were governed by two opposing processes: increasing crop density increased water use efficiencies of dry weight production, but decreased the proportions of dry weight allocated to grain.