Plants act to conserve available water during droughty times by thickening epicuticular wax and closing stomata. Both responses reduce glyphosate absorption. Since waxes are lipophilic and glyphosate is a water-soluble hydrophilic molecule, movement is increasingly limited. Although little glyphosate is absorbed through stomata, reduced CO2 absorption lowers photosynthetic rates, translocation, and thus absorption. Certain broadleaf weed species such as velvetleaf are known to orient their leaves more vertically when stresses are applied, decreasing droplet retention time on the leaf and decreasing herbicide efficacy (Zhou et al. 2007). Velvetleaf stressed by drought or flooding had much greater glyphosate tolerance than non-stressed plants. Cold stress also increases tolerance, but the magnitude is much less than drought stress.
One of the most common of these species is annual morningglory species. Morningglory are some of the most glyphosate-tolerant weeds common to the U.S. Corn Belt. This tolerant species also has an extended emergence window, allowing some individuals to completely avoid exposure. Improved control of many tolerant weed species can be obtained by increasing the glyphosate rate and applying to smaller weeds.
Wild buckwheat is another highly tolerant weed. At the standard use rate of 0.75 lb a.e. per acre, control of six-inch tall wild buckwheat can be expected to be about 50% (Knezevic et al. 2006). By tank-mixing a PPO-inhibiting diphenylether or increasing the glyphosate rate, control can be greatly improved.
Glyphosate products also vary in their parent acid concentrations. For example; Roundup WeatherMax® contains 4.5 lb a.e. per gallon, while Gly-4® contains 3 lb. The standard glyphosate application rate is 0.75 lb a.e. per acre; determine the product rate by using 0.75 as the numerator and the parent acid concentration as the denominator to determine gallons of product per acre (0.75/parent acid concentration = gallons/acre * 128 = oz/acre). Rates should be adjusted for weed height by using the standard rate for weeds < 6 inches tall, and increasing the rate by 50 and 100% for weeds between 6 and 12 inches and > 12 inches in height.
Recommended spray volumes differ by glyphosate product label; minimum spray volumes range from 3 to 5 gallons per acre and maxima from 20 to 40. Research indicates that glyphosate performance improves with decreasing spray volume to rates as low as 2.5 gallons per acre (Ramsdale et al. 2003). Reduced spray volumes decrease the likelihood of antagonism with hard water and increase glyphosate concentration per droplet. Since foliar-applied herbicides move by simple diffusion, maintaining a high concentration gradient improves absorption. Ultra low carrier volumes may provide insufficient spray coverage in dense weed/crop canopies, however, and the orifice size of spray tips necessary for such volumes are easily plugged. Carrier volumes of 10 to 15 gallons per acre are probably a good range for sufficient performance under a diversity of field conditions.
For some situations a better approach to a single glyphosate application in corn and soybean is to utilize herbicides with soil residual activity. Herbicides with soil activity tank-mixed with postemerge glyphosate have increased soybean yield and weed control (Grey, 2007). Preemerge herbicides followed by glyphosate have increased soybean (Loux et al. 2007) and corn (Tharp et al. 2004) grain yields. Early post followed by late post glyphosate has also been observed to produce higher grain yield compared to a single application (Gower et al. 2003). Using a preplant or preemerge herbicide to be followed by glyphosate reduces its timing sensitivity. Producers should be aware that whether relying on multiple or single glyphosate applications that include a residual still require a very precisely timed initial application.
Soybean yield is less affected by weed interference when compared to corn (Dalley et al. 2004). This is probably in part due to nitrogen fertilization of corn increasing weed competitiveness (Clay et al. 2005). Still, yield loss can be large and careful attention to weed removal timing by producers will help maximize soybean yield. As with corn, the onset of the critical period will vary with weed communities, weather, and cropping management. Ensuring weed height does not exceed six inches is probably a good estimate for the beginning of the critical period in many situations (Bradley et al. 2007). Reducing soybean row spacing can delay the beginning of the critical period (Knezevic et al. 2003) by reducing the competitiveness of weeds (Hock et al. 2006). Figure 5 depicts soybean yield loss associated with three row widths. Note that at any weed removal time yield loss increases with increasing row width.
Narrow-row soybean also reduces weed resurgence, while corn does not share the same benefit (Bradley, 2006). Unlike soybean, reducing corn row spacing does not improve its competitiveness (Norsworthy and Oliveira, 2004) and may even reduce it (Dalley et al. 2004). Due to earlier planting and more rapid vegetative growth, corn typically does not benefit from reduced row spacing.