Some areas of the Ohio are wet and have yet to make much progress with burndown applications or planting, and others are rapidly approaching the finish line thanks to a stretch of nice weather. In addition to fluctuating precipitation, weather patterns lately have brought about both very warm and freezing temperatures. In some areas with conditions that have been adequate for planting, wind has also complicated the timing of herbicide applications. Questions have come up related to herbicide efficacy across this range of scenarios, and the following are some considerations for each:

Areas that are waiting to dry out to make burndown applications

• Cold weather (nighttime temps below 40 and daytime temps below 50) can slow the rate of kill for systemic products like glyphosate, dicamba and 2,4-D. Contact products like paraquat and glufosinate work best in sunny, warm conditions, and control can be reduced in cool cloudy weather.
• Actively growing weeds are the most susceptible to control by herbicides. A period of warm weather following freezing temperatures before burndown applications can allow weeds to resume growth. Waiting one or two days after a freeze event can improve control.
• Using full labeled rates, appropriate additives and tank-mixing effective products can help improve control in suboptimal conditions. Also keep in mind that Group 5 products including atrazine and metribuzin can lead to increased antagonism concerns, which can be elevated in these conditions.

Burndown and residual applications have been made, and the crop has been planted

• Adequate rainfall of at least 0.5” is necessary to incorporate residual herbicides for effective control of germinating weeds. Though drought conditions have been a concern in southern regions of the state, many areas have now received sufficient rainfall for activation. 
• Cool conditions slow the rate of herbicide metabolism by crop plants and can lead to increased crop injury concerns. In general, applications that include sulfentrazone, flumioxazin, and metribuzin before soybean have increased risk of injury.
• Unless the damage is substantial, soybean yield loss is unlikely to occur due to early season injury. Stand reductions and delayed crop canopy can occur, which will have implications for lower crop competitiveness with weeds, and may increase importance of timely POST applications.

Crops have been planted but applications have been delayed due to wind

• Off-target movement risks injuring sensitive nearby vegetation and compromises the long-term utility of chemical control strategies. Appropriate wind speeds for most herbicide applications are between 3 – 10 mph. Check herbicide labels for specific requirements related to wind speed.
• Applications that occur at or after planting can increase the risk of crop injury from soil-applied residual herbicides. In most situations, there is a short window after planting during which applications can still be made. For many herbicides, labels restrict applications within a certain time frame following crop planting.
• Metribuzin (Tricor) products must be applied before crop emergence. Products containing flumioxazin (Valor, Fierce products) and sulfentrazone (Spartan, Authority products) cannot be applied later than 3 days after planting. Applications of products or premixes that contain these ingredients are off-label after 3 days post planting.