Pyroxsulam Persistence, Adsorption, and Movement in Palouse Soils
Raeder, Alan James
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Instances of herbicide persistence contributing to injury of sensitive rotational crops in the Palouse are common. As a consequence, the Palouse has a reputation for unexpectedly long herbicide persistence among industry, extension researchers, and agricultural professionals. Pyroxsulam was first registered in 2008 (EPA 2008) and was a newly registered ALS inhibitor labeled under the trade name PowerFlexTM. In the spring of 2010 and 2011, symptoms of pyroxsulam injury in lentil were observed due to residual pyroxsulam in the soil, leftover from the previous year’s application in winter wheat. To better define pyroxsulam soil persistence, studies were established to evaluate the impact of pyroxsulam soil residual to rotational crops, the adsorption and desorption in Palouse soil, as well as the vertical movement of pyroxsulam in soil columns collected from field studies where liming applications had modified the soil pH. Pyroxsulam can persist in Palouse soils at levels resulting in sensitive crop injury beyond the 9-month rotation interval that was initially listed on the PowerFlex™ herbicide label. When considering the physical and chemical characteristics of pyroxsulam, it is possible that the potential for sensitive crop injury due to soil persistence in the Palouse is increased in fields with decreased soil pH. Based on pyroxsulam adsorption to and desorption from 17 Palouse soils, it is likely that pyroxsulam adsorption is primarily dependent on soil pH when evaluated over a wide range of soil pH, such as the range of soil pH (4.3 to 8.4) observed in 17 Palouse soils. In situations where the soil pH is greater than the pKa of pyroxsulam (4.67), but below a pH of 6, it is likely that organic matter content more heavily impacts how pyroxsulam interacts within the soil water fraction. Pyroxsulam is relatively immobile in the Palouse soil profile and the observed immobility in the soil profile may be related to the unique persistence of pyroxsulam in Palouse soils.