Effect of Shading on the Growth of Some Common Weeds of the Semi‐Arid Tropics1

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Abstract
Light is one of the major factors influencing crop‐weed balance. The influence of shading on the growth, leaf area, dry matter production, and seed production of some common weeds of the semiarid tropics was investigated in a field study at ICRISAT Research Center near Hyderabad, India, during the 1978 and 1979 rainy seasons. Different shade treatments were achieved by erecting rectangular bamboo frames over each plot. The weeds studied were Cyperus rotundus L., Digitaria ciliaris (Retz.) Koeler, Celosia argentea L., Acanthospermum hispidum DC., Tridax procumbens L., Dactyloctenium aegyptium (L.) P. Beauv., and Amaranthus viridis L. At 90% shading plant height was reduced to 30% of the control with Celosia and Tridax. Digitaria and Dactyloctenium showed 70 to 80% reductions in leaf area index (LAI) at 90% shading. Dry matter production in Digitaria, Dactyloctenium, and Acanthospermum was reduced up to 80% at higher shading levels. In Cyperus, the effect of decreasing light levels on plant height was not pronounced, but the LAI and dry matter production were severely affected. Photosynthetic photon flux density (PPFD) measurements showed a negative correlation between levels of shading and the seed production in these weeds.The study indicated that by manipulating crop canopies to create desired shading, substantial weed suppression could be achieved. The agronomic significance of these results is discussed in the light of the weed‐suppressing capacity of crop canopies in different cropping systems of the semiarid tropics that may complement other control measures to obtain better weed management.

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