HERBICIDE RATE RECOMMENDATIONS - SOIL PARAMETER EQUATIONS VS. REGISTERED RATE RECOMMENDATIONS

Chlorimuron, clomazone, imazaquin, imazethapyr, and pendimethalin were each applied at five rates to soils at 10 different sites each year f or three years to determine which soil properties influenced their bio activity. Six soils in tbe U.S. (NC) and four in Zimbabwe were charact erized for their plow-layer contents of organic matter (OM), humic mat ter (HM), clay mineral (CM), and silt and for pH and cation exchange c apacity (CEC). At each site, the rate of each herbicide yielding 80% w eed control (I-80) was determined by visually evaluating the treated p lots. The I-80 values were then regressed with the various soil proper ties to determine which of them contributed to this rate. In the U.S. soils, the I,, was highly correlated with % OM (r(2) = 0.64-0.72) and % HM (r(2) = 0.55-0.69) for chlorimuron, imazaquin, and pendimethalin but was less correlated with % OM (r(2) = 0.30) and pH (r(2) = 0.31) f or imazethapyr. Equations relating the herbicide rate for 100% weed co ntrol to soil properties were derived for chlorimuron, imazaquin, imaz ethapyr, and pendimethalin then compared with registered recommended r ates for each chemical from the label. Herbicide rate equations based on similar soil parameters were taken from the Literature to compare w ith our rate equations and with registered rate recommendations for co mparison purposes and to add validity to the use of soil parameters fo r making herbicide rate recommendations. The rate equations, based on selected sell parameters, produced rates comparable with registered ra tes for chlorimuron, pendimethalin, and metribuzin and lower rates of application than registered rates for imazaquin, imazethapyr, alachlor , and metolachlor for soils with OM levels below 3.5%.