Influence of floating plant density on potential herbicide spray loss

Mesocosm experiments were conducted to evaluate the effect of floating plant density on over-the-top spray solution loss to the column using a tracer dye. Experiments quantified in-water rhodamine water tracer (RWT) dye concentration after foliar treatment at 935 L ha^-1 to waterhyacinth [Eichhornia crassipes (Mart.) Solms], waterlettuce (Pistia stratiotes L.) and giant salvinia (Salvinia molesta D.S. Mitchell) at 0, 25, 50 and 100% area covered (PAC).

As expected, spray loss to the water surface decreased with increasing plant density in all species. However, each species exhibited an unique relationship between density and percentage spray loss. The plant material required to result in 50% spray loss (ED₅₀ ) was 32, 62 and 55 PAC for waterhyacinth, waterlettuce and giant salvinia, respectively. Greater ED₅₀ estimates in waterlettuce and giant salvinia were attributed to plant architecture and leaf orientation compared to waterhyacinth, which grows more vertically and has a greater overall surface area to intercept and retain spray solution. However, when treated at 100 PAC, waterhyacinth and waterlettuce resulted in 20-25% spray loss, whereas giant salvinia resulted in only 10% loss. Consequently, giant salvinia exhibited a near 1:1 relationship between spray loss and PAC (slope = -0.93).

These data suggest that potential herbicide spray loss, as affected by plant density, is largely species-specific and dependent on leaf morphology and plant architecture. Further research will confirm these findings under field conditions as well as to identify other parameters that might affect spray loss when treating floating and emergent plants. © 2021 Society of Chemical Industry. This article has been contributed to by US Government employees and their work is in the public domain in the USA.