I’m assuming we can say March came in like a lion, so hopefully, it goes out like a lamb! My thoughts have also been with our livestock producers, especially everyone calving with this extra difficult winter. It’s also been an interesting winter programming season for me-probably the worst travel wise ever with some scary trips. Grateful winter programming is concluding and extra grateful for safety on all the bad roads. My out of state travels often were to speak on palmer amaranth management. I don’t claim to be an expert on this, just seek to read, observe, and learn for helping our farmers. Well, palmer had another ‘win’ with the announcement this week of a population in Kansas being confirmed to be 2,4-D and dicamba resistant. Populations in Kansas had already been confirmed to be resistant to ALS, atrazine, glyphosate, and HPPD chemistries. The 2,4-D and dicamba resistant population was found at K-State Agronomy’s long-term (45 year) conservation tillage study in southern Riley County. This study compares long-term monocrops to various crop rotations. The seeds from plants that survived in the field were collected, grown, and exposed to dose rate studies at the K-State Agronomy Department greenhouse. Twenty-one days after treatment, the resistant progeny survived up to a 16X rate of 2,4-D (8 lb acid equivalent/acre (ae/a)) while susceptible progeny were killed with 1 lb ae/acre or less. The seed from plants that survived in the field were also treated with 0.5 lb ae/acre rate of dicamba with 81% of the plants surviving. Studies are ongoing to determine the level of resistance and additional cross-resistance to other growth regulator (Group 4) herbicides.
That’s why when I talk about palmer, waterhemp or frankly any of our weeds, to me, it’s about a system’s approach. We can’t rely on herbicides alone. I think of weed control beginning at harvest by not combining patches of weeds or extra weedy endrows. There’s research documenting 99% of palmer seed survives the combine. There’s also research proving seed dispersal from the combine throughout the field the following growing season by counting plants that resulted from the first several combine passes. Instead, I recommend to consider disking once or shredding those areas at harvest. Then get a small grain seeded to reduce light interception onto the soil surface. Why? Natural and red light has been proven by the research to stimulate germination of palmer seed more than soil temperature. Light interception onto bare soil can allow for a flush of palmer to germinate. So in managing palmer, I’m thinking of anything we can do that can delay or reduce germination. Palmer seed in general is short lived…7-10 years. But plants are prolific seed producers. A plant inside the field can produce up to ½ million seeds. The large plant at the field edge can produce up to 1.8 million seeds. Adding a small grain such as wheat or rye for grain back into the rotation can delay palmer germination for a few months as the crop canopy delays germination until after harvest. Research and observation has proven this as well. The exception to this has been when tram lines were in the field as the bare soil in the tramlines has allowed for palmer germination. After using a burndown to kill the germinating palmer flush after small grain harvest, a cover crop can keep the ground covered for the rest of the season and allow for managed livestock grazing if desired. Even if the small grain crop isn’t taken for grain, the cover alone helps reduce light interception onto the soil surface and palmer germination.
Going back to the tillage, the southern U.S. has gone back to the plow. We can’t afford that. There’s also many no-till guys where disking is a hard option to consider. Several research studies showed that a 1 time tillage to bury the seed at least 3-4” and keep it buried for at least 3 years reduced palmer seed viability by 80-100%. That’s why I’ve mentioned the tillage. I did ask Dr. Jason Norsworthy from the University of Arkansas about the possibility of just shredding weed patches at harvest instead. He doesn’t have research on that and I don’t have observation but it could be another option to consider instead of running the combine through weed patches at harvest. Regarding herbicides, I’m so proud of an increasing number of farmers last year using pre’s with residual followed by posts with residual. Herbicides are part of the strategy, but we’ve got to look at the whole system. And, we’ve got to rotate our use of dicamba! We rely on dicamba a lot for our corn apps. But if we use it in corn and soybeans, we have the potential in 3-4 years to have resistance develop here. Take Home Considerations: palmer/waterhemp/weed management begins at harvest by not combining major weed patches; Consider one-time tillage (or shredding) of endrows on fields with heavy palmer pressure. Then plant a small grain to remove light interception; Plan herbicide program for burndown, pre with residual, post with residual, and potentially a second post if in beans; Narrow row beans may help with canopy closure; Consider adding a small grain in the crop rotation; Use at least two effective modes of action; Rotate use of dicamba to maintain as a tool. What is perhaps positive is we have an opportunity to learn from the southern U.S. and manage palmer better here! If you missed the palmer amaranth webinar by Dr. Jason Norsworthy, you can view it here: https://unl.box.com/s/al5zrhxjwml7s31liv1bryne320bf6r6.