INFLUENCE OF HERBICIDES ON GROWTH AND NODULATION OF WHITE CLOVER, TRIFOLIUM-REPENS

Five herbicides commonly used for suppression of weed growth in white clover seed crops were tested for toxicity to white clover (Trifolium repens), the nodule-forming bacteria Rhizobium trifolii, and the nitrogen-fixing symbiosis of these organisms. Trials were carried out under bacteriologically-controlled and glasshouse conditions. Overall, paraquat was more toxic to R. trifolii growth than MCPB, followed by bentazone, fusilade and kerb as the least toxic. Bentazone showed the least toxicity to plant growth, nodulation and N2 fixation under bacteriologically-controlled conditions. All the other herbicides inhibited C2H2 reduction, while nodule number and plant fresh weight were lowered by paraquat, MCPB and kerb. However, when plants grown in soil were treated, kerb showed the least toxicity with fusilade, bentazone and MCPB being intermediate in toxicity. This study also showed that changes in growth medium, nitrogen status and soil moisture content can alter the activity of particular herbicides. Possible toxicity of herbicides to plant growth and N2 fixation should be considered when using herbicides to control weeds in leguminous crops.