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Fluid Journal : Fluid Journal 2005-2007
14 Fluid Journal EARLY SPRING 2007 DRS. K. NELSON, P. MOTAVALLI, G. STEVENS, A. KENDIG, M. NATHAN, D. DUNN SUMMARY Soybeans were produced on over five million acres in Missouri in 2003 and 83 percent of the soybean varieties were Roundup Ready® or contained another form of transgenic herbicide resistance. Roundup Ready varieties allow farmers to apply glyphosate- based products for broad-spectrum post-emergence weed control. The incidence of K deficiency has increased in recent years due to 1) reduced K availability under drought conditions, 2) soil compaction, 3) reduced applications of K for soybeans due to low commodity prices, 4) higher corn grain yields, and 5) increased soybean acreage in rotation with corn, increasing K fertilizer requirements. Soil test K data from the University of Missouri Foliar K Applications Safe With Glyphosate Expect minimal soybean injury and reduction in weed control, depending on product selection and application rate. Soybean injury resulting from foliar applications of up to 19.2 lbs/A of K2O from several potassium (K) fertilizer sources (i.e., potassium chloride, potassium thiosulfate, and 3-18-18) was generally less than 10 percent. K fertilizer sources tank-mixed with glyphosate, such as 3-18-18 at 2.4 and 9.6 lbs K2O/A, 5-0-20-13 (KTS + urea-triazone) at 2.4 lbs K2O/A, and 0-0-62 at 9.6 and 19.2 lbs K2O/A, controlled more than 90% of weeds and produced grain yields similar to herbicide applications with diammonium sulfate (DAS), while providing additional K fertilizer to the soybean plant in a single-pass weed management system in northern Missouri. However, two-pass weed management in southern Missouri provided excellent weed control for all additives, and grain yields were greater than or similar to glyphosate plus DAS. The results of the study indicate that foliar K applications can be mixed with glyphosate with minimal crop injury and reduction in weed control, depending on product selection and application rate. Soil and Plant Testing Lab indicate that over 50 percent of the soil samples tested in the low-to-medium range for K. This situation indicates that nearly 2.5 million soybean acres in Missouri and large acreages in adjacent states could be at risk or are currently experiencing yield loss due to inadequate soil K test levels. Several studies have evaluated response of soybeans to foliar fertilizer mixtures. However, no research has evaluated the interaction between macronutrient foliar fertilizer and weed control with post-emergent herbicides. Potassium is an essential nutrient that increases drought tolerance, stem strength, and improves plant growth. Movement of K to plant roots and subsequent uptake of K is primarily controlled by diffusion. Drought and other stress conditions such as soil compaction and low soil temperature may limit K uptake. Previous research on a farm field in northeast Missouri on crop response to a foliar application of K sulfate at the V4, R1-R2, or R3-R4 stages of development demonstrated that soybean grain yield increased over 10 bu/A when compared to a non-treated or MgSO4 control. The calculated increase in profit due to this yield increase from foliar K applications was approximately $50/A. However, possible limitations for the use of K sulfate combined with a post- emergent herbicide application are: 1) the large carrier volume required for an optimum foliar K application and 2) the possible incompatibility of the K fertilizer source with a glyphosate-based herbicide. In addition, the K source and herbicide mixture must result in minimal crop injury and not affect weed control.
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