Full-season johnsongrass competition reduced sugar yields by 53 and 44% in ‘CP 70–321’ and ‘CP 72–370’ sugarcane, respectively, primarily by reducing sugarcane stalk populations. Stalk population and height and sugar yields of both cultivars were optimized when asulam was applied at 3.7 kg ai/ha during April and early May. Asulam at 3.4 kg/ha, applied a second time 8 weeks later, generally increased johnsongrass control but did not increase sugar yields over one early application. By delaying the application, the potential for sugarcane injury from asulam applied once or twice increased in both cultivars. Injury appeared to be related to the stage of sugarcane growth and temperature and moisture stress at application time with asulam phytotoxicity being more pronounced in CP 72–370. Populations of johnsongrass stems in the year after treatment reflected the degree of johnsongrass control by asulam and the competitiveness of the crop in the previous year.
