Can directed-spray strategies control Guinea grass stools?

Guinea grass is a troublesome perennial grass in our sugarcane farming system. Despite a range of effective pre-emergent and early post-emergent herbicides, optimum spray windows are often missed and established Guinea grass stools are commonly found. To control established Guinea grass stools, growers often rely on spot spraying, which is time consuming and often requires multiple passes. Four replicated field trials were established to assess if directed-spray strategies could replace spot-spraying strategies. Field trials showed that a late application of isoxaflutole (75 g/ha) + MSMA (2.16 kg/ha) applied at the base of the row generated the strongest phytotoxicity symptoms on Guinea grass stools and reduced by up to 67% the number of Guinea grass stools in the following ratoon. However, this herbicide mixture also reduced sugarcane yield by 34-41% depending on the application technique. The safest treatment for sugarcane was an early banded spray with asulam (3.4 kg/ha), followed by a late interrow application of glyphosate (2.7 kg/ha) using a shielded sprayer, but the reduction in the number of Guinea grass in the following year was highly variable from trial to trial (0-67% reduction). No directed-spray strategies were identified to control established Guinea grass in sugarcane rows without a significant sugarcane-yield penalty. Key words Guinea grass, Megathyrsus maximus, directed spray, isoxaflutole, MSMA, glyphosate