We are well into the corn planting season, and are switching focus toward soybean planting for 2017. As we approach planting soybeans, spring burndown remains a huge topic to consider. A lot of weeds, especially kochia, got a great head start this season, and the solid mat of weeds out there can be a real challenge for burndown herbicides, especially if programs are based primarily on Roundup. The addition of a phenoxy to burndowns is a good addition, but caution must be taken to allow enough time before planting. A rule to remember is, roughly, we need to allow 1 day/oz of a 2,4-D in a burndown and 2 days/oz of dicamba.
One alternative product that deserves attention this year is Gramoxone Herbicide from Syngenta. Gramoxone (paraquat) is a non-selective herbicide with zero residual, and zero pre-plant interval. When applied correctly, it can be effective in a burndown program. Gramoxone pricing has been reduced significantly, making it an economical choice. Gramoxone does have some handling considerations, but are not hard to overcome. Here are some tips on managing Gramoxone application ahead of soybeans effectively.
- Gramoxone is rainfast in 30 minutes.
- Works in cool weather- much better than Roundup under these conditions.
- Is a good resistance management partner, as the mode of action is different than most burndowns. In addition, Gramoxone is compatible with most classes of herbicide, including the PPO herbicides.
- Best performance will be obtained when a PSII herbicide (metribuzin) is in the mix (a triazine in the mix improves paraquat performance SIGNIFICANTLY. There is a synergy in combination.)
- Products like Dimetric EXT or other dry 75% metribuzin, liquid metribuzins, or products like Boundary herbicide, with metribuzin in them are a great partner choice.
- Mix with 2,4-D ester when possible. However, a preplant interval as listed with the 2,4-D must be followed.
- Apply in a minimum of 15 gallons per acre, as coverage is critical.
- Apply at <10 mph.
- Addition of a nitrogen source can improve control, as it does with Roundup.
Why a Triazine (PSII inhibitor) with Gramoxone?
Triazine herbicides (Group 5) inhibit photosystem II, and Paraquat depends upon products from photosystem II to work. Slowing down photosynthesis slows the activity of paraquat, providing a more thorough kill. Reduced photosynthesis allows paraquat to move locally in tissue to kill more cells. Without the Group 5 herbicide, in bright sun, paraquat kill can be too quickly, as cells desiccate quickly and do not allow movement of the paraquat in plant tissues.
Field Observations of Efficacy of a paraquat plus PSII inhibitor:
The Bristol location has had some excellent success on a heavy kochia population this year when using the following herbicide mix ahead of soybean planting: 30 oz of BroadAxe, 4 oz Dimetric (metribuzin), 32 oz Gramoxone, 16 oz Exuro (MSO designed for burndown) and 20 gallons of water.
Kochia control with a Gramoxone plus a PSII herbicide (metribuzin) in a burndown/residual program. Photos taken before and three days after application.
As with any burndown herbicide program, weeds covered by residue are not controlled. A planned post-emergence herbicide program is still needed.