Differential responses of weeds and rice to shading stress from oilseed rape straw mulch

Abstract

Oilseed rape (Brassica napus L.) straw mulch suppresses weeds in rice (Oryza sativa L.) cropping systems. This study explores the role of shading in weed suppression by straw mulch through three experiments. The first experiment determined how oilseed rape straw length (powder or 1, 3, 5, 7, or 9 cm), application rate (100, 300, 500, 700, 900 or 1100 g m⁻²), and moisture (dry or wet) affect shading. Shading was greater with shorter straw lengths, higher application rates, and dry straw. When wet 1 cm straw was applied at the rates above, it blocked 52%, 87%, 96%, 99%, 99% and 99% of light, respectively. The second experiment evaluated germination of rice and seven weeds under an alternating light/dark regime compared with continuous darkness. Rice germination did not differ between the two treatments. Two weed species, barnyardgrass (Echinochloa crus-galli (L.) P. Beauv.) and Chinese sprangletop (Leptochloa chinensis (L.) Nees), exhibited some germination in continuous darkness but higher germination in the light/dark treatment. The remaining five weed species, small-flowered nutsedge (Cyperus difformis L.), eclipta (Eclipta prostrata (L.) L.), eared redstem (Ammannia auriculata Willd.), ricefield flatsedge (Cyperus iria L.), and grasslike fimbry (Fimbristylis littoralis Gaudich.) failed to germinate under the dark treatment. The third experiment measured effects of shading on seedling growth of rice and two weed species, barnyardgrass and eclipta. A few weeks of shading triggered shade avoidance responses and inhibited seedling growth in all three species. These findings demonstrate that oilseed rape straw mulch can effectively reduce weed germination and early growth through shading. This practice does not threaten rice establishment, and indeed may protect rice from later weed–crop competition for light.