The critical weed-free period (CWFP) in corn takes place from emergence to V4 stage (six-leaf stage). Although corn is typically seen as a longer-season crop, maintaining a clean, weed-free field during this period is critical.
A weed-free field is important during all stages of the CWFP. Corn isn’t a very competitive crop, so during the early stages of development the roots are small and the plant has to compete for resources. When corn is under pressure for resources, it will focus on outgrowing weeds by increasing leaf growth rather than root growth to outcompete weeds, resulting in a compromised root growth. During the V6/V7 stage, the number of rows around the ear is set, which is correlated to yield. Larger populations of weeds result in increased stress in the plant, which can decrease the number of rows and yield.
The environment and weed species present affect the critical weed-free period. Depending on the early-season environment, different weed species could be seen that flush at different times. Weeds such as wild buckwheat and volunteer Roundup Ready canola continue to flush during the CWFP. Currently, corn hybrid selection does not play a large role in the CWFP, though some hybrids will have stronger early season vigor.
The right herbicide chemistry applied at the right time will help control targeted weed populations. Pre-emergent herbicide products are becoming more popular for controlling flushing weeds and helping with resistance management; many products that can be applied pre-emergent provide flushing control for a few weeks. Good fertility or using a starter fertilizer can help corn get off to a stronger start, allowing for better competition. Effective weed management in crop rotations will also keep weed populations in check for subsequent crops.
Growers have a number of management tools and techniques available to them to keep field weed-free during the CWFP, but it’s crucial that they know what weed spectrum they are dealing with in order to keep fields weed-free for optimum corn yields and an increased bottom line.