The Effects of Post-emergence Herbicide Treatments on Sugarcane in South Africa

Results of nine post-emergence herbicide phytotoxicity experiments are reported. The experiments were conducted on medium to heavy soils at Pongola in the northern irrigated part of the industry or on light soils at Shakaskraal on the Natal north coast. Tests were conducted using the recommended rates of herbicide combinations and double these rates, applied directly over the sugarqane foliage at a susceptible stage of cane growth (about 400mm leaf height). The standard treatment, diuron + 2,4-D + S, was included as a basis for comparison, and alternative standards such as paraquat, or Lasso + atrazine + paraquat, were included when new com, binations used for a similar purpose were tested. Of the newly registered herbicides, Bladex Plus + S was safer to use than the standard diuron + 2,4-D + S, while the effects of Bimate + S appeared to be similar to those of the standard treatment. MSMA was more damaging than the standard treatment but not as phytotoxic to cane as was paraquat, either alone or in combinations with diuron. The new combinations of registered herbicides were in general no more phytotoxic than the equivalent standard treatments. Symptoms of phytotoxicity included leaf chlorosis and necrosis, stunting of cane, and the effects on stalk population. Where these effects persisted until harvest yield reductions were also measured. In general, most combinations caused mild to severe early stunting of the cane but by the time of -. harvest, the crops had recovered. Yield reductions were common when the cane was sprayed at a late stage of growth ( > 500 mm cane leaf height). Reduction of yield would not be expected from any of the treatments tested provided that they were applied at the optimum cane growth stage or that the spray was directed away from the sugarcane foliage.