When using postemergence herbicides on weeds like marestail and lambsquarters, timing is critical, Hager emphasizes. “Herbicide performance is best if weeds are still small, when using either with a burndown or a timely post application,” he says. “However, sometimes common lambsquarters can be a challenge to control with a postemergence application, depending on environmental conditions.”

Gunsolus agrees. “Common lambsquarters is very sensitive to small weed size for successful postemergence control,” he says. “You need to spray lambsquarters while it is still 2 to 3 in. tall. There can also be a lot of environmental barriers for good, postemergence herbicide uptake, such as conditions that are too dry or too cool; it’s a very difficult weed to control solely with postemergence applications.”

Gunsolus recommends using a preemergence herbicide with soil residual chemistry for lambsquarters, waterhemp and ragweed control. “Another reason common lambsquarters is so difficult to manage is the fact that it has a really long life in the seed bank,” he says. “So to reduce the weed seed bank in your fields later in the season, you need to resolve your weed problems early.”

Giant ragweed is also competetive with soybeans, Hager says. “It’s the worst weed for soybean production from an interference standpoint [taking into consideration all negative aspects of weeds growing with crops],” he says. “Switching from conventional tillage to a no-till or to a limited tillage system can help. Other strategies for control include employing an integrated herbicide program that includes a soil residual herbicide at around planting time and timely postemergence applications [at 3- to 4-in. weed heights].”