By Mark Loux
We are starting to see the availability of soybean varieties with more than two herbicide resistance traits, which can expand the herbicide options, improve control, and allow multiple site of action tank mixes that reduce the rate of selection for resistance. One of these is the Enlist soybean, with resistance to glyphosate, glufosinate, and 2,4-D. As of this writing, full approval for the Enlist soybean is still being held up by the Philippines (because they can apparently). The other is the LL-GT27 soybean, which has resistance to glyphosate, glufosinate, and isoxaflutole (Balance). There is no label for use of isoxaflutole on this soybean yet, but it is legal to apply both glyphosate and glufosinate. In Ohio, as long as neither label prohibits applying a mixture of two herbicides labeled for a specific use, it’s legal to apply the mixture. So, it’s also legal to apply a mixture of glyphosate and glufosinate to the LL-GT27 soybean. There is no label that actually mentions or provides guidance for this mixture, which does not affect legality, but could affect who assumes liability for the recommendation to apply a mixture if that matters to you. Some seed companies are making the recommendation for POST application of the mix of glyphosate and glufosinate to the LL-GT27 soybean in printed materials. Our interpretation after discussion with ODA, is that these materials are essentially supplements to labels, and so the seed company would assume some liability for the recommendation.
Our assumption is that for the POST application of this mixture, the glufosinate is carrying most of the load for the control of ragweeds, marestail, and waterhemp, which generally have resistance to glyphosate. Glufosinate could use some help on larger giant foxtail, and definitely needs help on several grasses that it’s weak on – barnyardgrass, yellow foxtail, etc. Many users of LibertyLink soybeans have made it a practice to consistently add a POST grass herbicide such as clethodim. For the LL-GT27 soybean, one would have the choice of going this route or replacing the clethodim with glyphosate to control grasses or perennial weeds. We have had a lot of insightful questions from growers about the wisdom of mixing a systemic and contact herbicide together. While there’s not much research-based information yet on how well glyphosate and glufosinate work together, there’s probably not much issue with adding a low rate of glyphosate to glufosinate to control grasses. And we have other examples where contact and systemic herbicides are successfully used together – e.g. Gramoxone + 2,4-D + metribuzin; glyphosate + fomesafen. A colleague at Purdue related that the two herbicides require different types of surfactants which creates an interesting dilemma. Both products contain a full surfactant load though, so a mixture should have plenty of whatever surfactants are being used for sure. A couple of other things to keep in mind:
- In mixtures of systemic and contact herbicides, it’s important to optimize the application parameters for the contact herbicide. In this case, it’s especially important to optimize the glufosinate since it’s controlling the glyphosate-resistant broadleaf weeds. Optimizing glufosinate means higher spray volumes and smaller spray droplets, compared to what is needed for glyphosate alone (which works with almost any spray parameters). Both herbicides require the addition of AMS. Keep in mind also that glufosinate requires warm, sunny conditions for maximum activity. And the activity of both herbicides is reduced in late evening through early morning.
- Sources tell us that, depending upon glyphosate rate, the cost of adding clethodim versus glyphosate to glufosinate is a wash to a slight advantage for glyphosate. Higher rates of glyphosate will be needed where perennial weeds are the target. The long-term potential for herbicide resistance could also be considered as part of the cost-benefit analysis here. Populations of glyphosate-resistant annual grasses have become more prevalent in soybean-growing areas. Continuous use of glyphosate for control of annual grasses in soybeans can increase the rate of resistance development, especially where glyphosate is used as a primary POST herbicide in corn also. Paying somewhat more for clethodim in certain years of the rotation may therefore provide long-term weed management benefits compared with use of cheaper glyphosate.