Field studies were conducted in 2007 and 2008 at the University of Arkansas at Pine Bluff farm near Lonoke to evaluate and compare the effects of low rates of glufosinate and glyphosate on rice. Two rice cultivars were seeded, and glyphosate and glufosinate were applied at 1/2, 1/4, and 1/8 of the labeled use rate of 870 g ae ha⁻¹ and 616 g ai ha⁻¹, respectively, at the three- to four-leaf, panicle initiation (PI), and boot stages. Rice canopy height reductions, reduction in flag leaf length, prolonged maturity, and yield losses were caused by both herbicides at all evaluated application timings. Although both herbicides caused significant injury, symptoms varied greatly between the two herbicides. Glufosinate injury to rice was more rapid and visually intense than with glyphosate. Glufosinate symptoms, which consisted of rapid necrosis, were visible in 1 to 2 d, whereas glyphosate symptoms, stunting and chlorosis, became visible after 7 to 10 d or not at all depending on time of application. Glyphosate applied at the 1/2× rate to rice in the boot growth stage caused less than 10% injury at 3 wk after treatment but resulted in 80% yield loss. Glufosinate at boot caused 80% injury and 80% yield loss. Glyphosate symptoms from PI and boot timings were typically only visible at heading and included malformed panicles and shortened flag leaves. Harvested grain seed weights were reduced as much as 14% by either herbicide applied at PI and boot. Germination of harvested grain was not affected by any treatment. At the rates evaluated in this research, glufosinate-induced injury to rice can be just as detrimental as glyphosate in reducing yield.

Nomenclature: Glufosinate; glyphosate; rice, Oryza sativa L. ‘XL723’, ‘Wells’