Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
Evaluating herbicides for activity under drought stress is something we have limited data on. We know that all types of plant stress can reduce herbicide activity and drought stress is no different. But we don’t have a lot of data or recommendations for spraying under drought stress like we are experiencing in the area.
Some weed species are well adapted to continue to grow under these types of conditions, albeit slower than under well-watered conditions. So, while the crops may be suffering and slowing their growth to conserve water, weeds like Palmer amaranth are still growing. Palmer amaranth evolved in the deserts of the southwest US and have mechanisms to cope with drought stress. They have extensive, shallow roots that are very efficient in capturing limited moisture. They have the ability to continue photosynthesis during the heat of the day through “C4-photosynthesis”. So, while weed growth is slowed, it is not slowed as much as soybeans. Waiting for rains to reduce moisture-stress means some weeds maybe too large to control once rain occurs.
Leaf cuticles is the waxy layer on leaves to help prevent moisture loss from the plant and thus more challenging for the herbicide to penetrate the leaf. Read the label and use the product that allows better absorption through the cuticle. Methylated seed oil (MSO) is generally the most effective at penetrating leaf cuticle, followed by crop oil concentrated (COC), and then non-ionic surfactant (NIS). Include nitrogen if it is allowed on the label.
Spray early morning or evening when leaves have unrolled from the heat of the day. Unfortunately, spraying at this time of the day will reduce the effectiveness of Liberty (glufosinate) so keep this in mind.
One of the causes of spray drift is the evaporation of spray droplets from the time they leave the spray nozzle until they land on the target. With high temperatures, spray drift is more likely to occur, so use nozzles that produce larger droplets. This is particularly important with Enlist or any dicamba products.
Leaves will curl to reduce surface area and minimize evaporation, but that also reduces the ability to get complete spray coverage, so use spray volumes of at least 20 gallons per acre to improve spray coverage.
While spraying under drought conditions may limit herbicide effectiveness, the application can slow weed growth and limit its impact on yield and prevent the weeds from being too large to control once the rains do return. Spraying these fields may not eliminate the weeds, but it will improve the level of weed control with later applicat