Summary
Residues from mature, harvested crops of sorghum (Sorghum bicolor Moench.), sunflower (Helianthus annuus L.), oilseed rape (Brassica napus L.), wheat (Triticum aestivum L.) and field pea (Pisum sativum L.), exhibited selective effects on weed germination and growth under field conditions. Weed species in the study included Avena fatua L., Avena sterilis ssp. ludoviciana (Durieu) Nyman, Echinochloa crus‐galli (L.) Beauv., Phalaris aquatica L., Phalaris paradoxa L., Lolium perenne L., Vulpia myuros (L.) Gmel., Hibiscus trionum L., Polygonum aviculare L., Bilderdykia convolvulus (L.) Dumort. and Lamium amplexicaule L. All crop residues tested, and in particular wheat and pea residues, promoted the germination and growth of A. fatua and A sterilis ssp. ludoviciana. Other grass weeds, however, were inhibited by the presence of crop residue, the extent of inhibition being dependent on residue type. Germination response of dicotyledonous weed species was also a function of residue type, H. trionum numbers were significantly higher in plots where oilseed rape, sorghum or sunflower residue had been incorporated, while L. amplexicaule was inhibited by these residue types and stimulated by pea and wheat residues.
Field studies undertaken at Armidale in 1982-84 demonstrated that the principal factors determining the effect of retained stubble on the growth and yield of wheat were the species of the crop from which the stubble had been derived, the amount and distribution of rainfall and the degree of decomposition that the stubble had undergone prior to the next crop. Stubbles were phytotoxic only prior to leaching by rain or decomposition, with unweathered sorghum stubble proving more inhibitory to wheat than unweathered sunflower, field pea, wheat or oilseed rape stubble. The availability of soil nitrate had no influence on the degree of phytotoxicity of any stubble type. Decomposed crop stubbles stimulated wheat growth, with the greatest stimulation recorded in the presence of oilseed rape stubble.
The allelopathic potential of the unweathered stubble of field-grown sorghum and sunflower was examined under controlled conditions in a series of glasshouse experiments. Wheat emergence was significantly inhibited in the presence of stubble from the eleven sorghum, and ten of the twelve sunflower cultivars tested. Inhibition of wheat emergence in the presence of sorghum stubbles ranged from 10 to 31%, and in the presence of sunflower stubbles from 4 to 33%, demonstrating a high degree of intraspecific variation in allelopathic potential. Many of the wheat seedlings emerging in stubble treatments exhibited an abnormal geotropic response. A threefold increase in stubble quantity, from 0.5 to 1.5% w/w soil, resulted in an approximate doubling of phytotoxicity. Wheat emergence was significantly reduced when stubble was incorporated into all three soils tested, with greater inhibition recorded in chocolate soil and a 50: 50 chocolate soi1:sand mixture than in sand. The choice of crop cultivar and the management of stubble quantity, especially in harvester trails, have important implications for the minimization of negative allelopathic effects in stubble retention systems.
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