9 Keys for Effective Residual Use
The battle against weeds requires farmers to remain vigilant. Keeping fields clean early and holding those rows until canopy is the key to minimizing a new season worth of weed seeds from finding their way into the soil. Pre- and postemergence residual herbicides can help limit the work knockdown chemistry needs to handle.
“Everything we do has to revolve around economics, and residuals control a weed without it impacting the bottom line,” says TJ Binns, a service rep with Syngenta from Leoti, Kan. “A weed that never comes out of the ground has no impact on yield, and thus it has no impact on profitability.”
Here are 9 keys to effectively use these important tools.
1. Find a mix.
Multiple modes of action cover different weed species and extend effectiveness. Many commonly used residual products include a Group 15 herbicide (Chloroacetamide and Oxyacetamide) and are shoot growth inhibitors, which control grass and small-seeded broadleaf weeds. Group 15 products need to be applied and activated before weed emergence.
Other mode of action groups can be added for post-knockdown, large seeded broadleaf control and additional resistance mitigation.
“Mixing modes of action is always a good plan, and you’ve got more flexibility to do that, especially on your pre-emergence residuals, where your crops are not out of the ground yet,” explains Mike Probst, a technical services rep with BASF. “At that point, you still have the flexibility to use your Group 15 in conjunction with labeled residual PPOs (Group 14) or metribuzin (Group 5).”
2. Rain is required.
Residuals need differing amounts of moisture to activate in the soil.
“Generally, a less water-soluble product takes more water to get it in to the soil/water solution, and it doesn’t wash out as quickly, typically lasting longer throughout the season,” says Cody Evans, a crop protection tech development rep with Bayer Crop Science. “That being said, it doesn’t get activated as quickly as some of the other more water-soluble products that may wash out more readily of the seed germination zone. This is why we see a benefit to adding a highly soluble product like dicamba to a low soluble residual product to hold off weed pressure until an activating rainfall.”
The mentality should be to get that residual down before the first one runs out.
3. Consider timing.
Balance planting and weed growth when selecting a residual mix.
”If we’re trying to control waterhemp, it usually doesn‘t begin to germinate until early May,” Evans says. “Plan residual applications closer to expected planting and weed emergence dates to get the most bang for your buck.”
4. Overlap applications.
In many residual programs, a second pass should be applied four to six weeks after the first, depending on weather and efficacy.
“Don’t wait until you get a flush of weeds to trigger spraying a postemergence residual,” Probst says. “The mentality should be to get that residual down before the first one runs out.”
5. Coverage is key.
Use plenty of water volume to help drive residual through field residue.
“Those higher carrier volumes are paying vast dividends, and I’m going up to 15 gal. per acre,” Binns says. “Water is probably one of the cheapest things we can put in that tank, and if it’s going to help things work, we might as well do it.”
6. Add knockdown power.
Include chemistries that can kill weeds that have emerged.
“I’m including a knockdown in that second pass, whether it’s a Liberty, Enlist or a Dicamba option, to clean up any weeds that made it through that residual,” Probst says. “When we have control coming from multiple avenues, we’re protecting our tools and extending the longevity of everything.”
7. Consider weed species.
Targeting small-seeded broadleaf plants can leave room for large-seeded plants.
“We rely heavily on Group 15 products in corn and soybeans to help control small-seeded broadleaf and grass weed species,” Evans says. “This can open up a species shift to more larger-seeded broadleaf species break. Group 15 products will not do much to control species such as velvetleaf, morning glory and cocklebur so combinations with Group 2, 5 or 27 may be necessary.”
8. Have a plan but be flexible.
“Know what your weed pressure is and be realistic with yourself,” Binns says. “If hail goes through late season, we’ll open up the canopy, and we know we’re going to have a lot more weed seeds out there for next year. Even if you can get 99% control, you’ll have one weed that’s putting a half a million seeds out there, so we have to think about what our weed pressure is and be realistic about it.”
9. The cost of doing nothing.
Weed escapes and the future impact on yield are hard to quantify.
“Understand that a clean field year after year isn’t going to be a field that’s building a resistant population,” Evans says. “Anytime we’re putting a little bit extra money into these herbicide programs to start clean, spray clean and stay clean, we are reducing the pressure on our post programs and there’s less chance that field is going to give us trouble down the line.”
