Application Time and Cropping Management
Optimum glyphosate application time is a complex subject, primarily due to large variations in weather, weed populations, and cropping management practices under which corn and soybean are produced. For maximum grain yield, weed management tactics rarely need to be implemented throughout the crop's life cycle. In fact, weed management tactics need to be concerned with preventing weed interference during the first 4 to 6 weeks after planting (Wood et al. 1996). This critical weed-free period or critical period is defined as a period of time in crop development that weeds must be controlled to prevent yield loss. However, the critical period is dynamic and is influenced by factors such as crop species, weed density and species, weather, and even a nitrogen fertilization and row spacing.
Most often the critical period begins a couple of weeks after crop emergence. Physical resources such as water, mineral nutrients, and light being competed for by crops and weeds are often in sufficient supply for dense seedling populations of both. Initial size difference between crops and most Corn Belt weeds due to seed size also tends to delay the onset of the critical period. For corn production the critical period can begin as early as VE to as late as V7, while the end of the period ranges from V5 to VT. These wide ranges have been observed to be caused by weed density, nitrogen fertilization, and drought stress (Evans et al. 2003; Knezevic et al. 2003; Norsworthy and Oliveira 2004; Dalley et al. 2004). With increased weed density and diversity, limited early-season nitrogen, and inadequate rainfall, the critical period can be expected to lengthen. Conversely, low weed density and diversity, well-fertilized corn, and ample rainfall will shorten the critical period.
Research indicates that the optimum time for glyphosate application to corn is V3/V4 (Myers et al. 2005; Gower et al. 2003; Cox et al. 2006). Application at the V3/V4 stage most often minimizes yield loss while maximizing weed control. Producers may have to choose between yield losses due to early-season weed competition and herbicide efficacy when basing weed control on a single glyphosate application. Eliminating early-season yield loss by targeting two-inch weeds invites reinfestation by later germinating weeds, some yield loss, and unacceptable weed control. On the other hand, delaying application improves weed control but yield loss is unacceptably high (Table 2). A comprise may be to apply glyphosate when weeds are six inches tall, although weed removal at that point may only be marginally acceptable for control and yield loss. Attempting to target a very narrow window for weed removal also poses difficult logistical challenges, especially for large operations and those with additional in-crop field work.
Table 2. Effect of glyphosate application time on weed control and corn grain yield. Adapted from Gower et al. 2003.
1 Weeds emerging after herbicide application controlled with hand weeding.
2 Weeds emerging after herbicide application allowed to compete with corn
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.
Figure 5. Influence of weed removal timing and row spacing on soybean yield loss. Knezevic et al. 2003.
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.