Influence of environmental parameters on penoxsulam control of alligatorweed (alternanthera philoxeroides) in rice (oryza sativa)

Alligatorweed is a perennial plant which reproduces vegetatively and has spread from waterways into canals and ultimately into rice fields of Louisiana and Texas. Penoxsulam is a new acetolactate synthase (ALS) inhibiting broad-spectrum herbicide that was registered for rice in 2005. Previous research on alligatorweed control has focused mainly in aquatic areas and in the rice producing regions of Louisiana with little success. Research is limited using penoxsulam for alligatorweed control in rice production and results vary between year and location. Variability could be due to growth habit and resource allocation of this perennial species. Therefore, field and laboratory experiments were conducted from 2004 to 2007 to: 1) evaluate the effects of select rice herbicides on alligatorweed control, 2) determine the absorption and translocation efficiency and the effect of propanil on penoxsulam in alligatorweed 3) access the environmental effects of temperature on penoxsulam efficacy and determine application timing to avoid antagonism with propanil and, 4) evaluate the effects of flood timing and rice cultivars on rice root stunting and plant foliar injury from penoxsulam applications. Alligatorweed control was obtained from penoxsulam or bispyribac-sodium applied alone; however, mixtures with propanil were antagonistic. Day temperatures at 21 C increased efficacy of penoxsulam compared to 27 and 30 C day temperatures. Delaying propanil applications 3 days following penoxsulam applications were required at 21 and 27 C and 10 days at 30 C in order to avoid antagonism. Alligatorweed absorbed up to 33% of penoxsulam when applied alone, but most was retained in treated leaves (29%). Propanil reduced penoxsulam absorption into alligatorweed with only 22% of total penoxsulam recovered being absorbed by alligatorweed. More than 50% remained on the leaf surface of the treated leaf. Previous research has indicated root stunting of rice plants from ALS inhibiting herbicides. When various rice varieties were permanently flooded one week after herbicide application of penoxsulam, root stunting was greater compared to delaying flood establishment 7 or 14 days after treatment. Significant root stunting, however, did not affect rice yield.