Nitrogen (28% UAN) as a spray carrier can affect winter wheat sensitivity to herbicides

Here’s a summary of our research with new recommendations for Michigan wheat producers.

Herbicide applications that cause injury to wheat can potentially reduce yield. A practice that may increase the risk for crop injury to wheat is applying herbicides with 28% liquid urea ammonium nitrate (UAN) fertilizer as the spray carrier. Since this is a common practice for many Michigan wheat growers, over the last three years with funding provided by Michigan Crop Improvement Association we conducted research to address the following questions:

  • What effect will applying my herbicide with 28% UAN (nitrogen) as a spray carrier have on weed control and wheat tolerance?
  • Can I reduce this risk by applying my herbicide with a 50:50 mixture of 28% UAN and water as the spray carrier?
  • Will the timing of these herbicide-nitrogen carrier applications influence crop tolerance?

This research was designed to help Michigan wheat producers make informed decisions about weed management options to maximize weed control and yield.

28% UAN (nitrogen) as a spray carrier

The herbicides:

  • 2,4-D amine (1 pt/A)
  • 2,4-D ester (1 pt/A)
  • Affinity BroadSpec (0.75 oz/A) plus surfactant (0.25% v/v)
  • Huskie (11 fl oz/A) plus surfactant (0.25% v/v) and 28% UAN (2 qt/A)

These are all common herbicide treatments used for broadleaf weed control in winter wheat. The herbicides were applied in the spring when winter wheat was at Feeke’s stages 4 to 5 with three different spray carriers. The spray carriers were water, a 50:50 ratio of water and 28% UAN (28 lb actual N), and 100% 28% UAN (56 lb actual N) all applied at 19 gallons per acre. All plots received 102 lb actual N per acre, 46 lb actual N was applied as urea in mid-March and the remainder was applied as 28% UAN, within three days of the herbicide applications. This was done so that the entire wheat crop had the same fertility program.

Applying any of the herbicides with 28% UAN at 100% or a 50:50 mixture with water did not affect winter annual weed control. Differences in weed control were dependent on the herbicide. Affinity BroadSpec and Huskie provided the greatest spectrum of weed control, providing good to excellent control of field pennycress, bushy wallflower, mayweed chamomile, and common chickweed. 2,4-D amine and 2,4-D ester also provided good to excellent control of all of these species, except common chickweed. 2,4-D is not very effective for controlling common chickweed. All herbicide treatments also help reduce common ragweed populations in-season and after wheat harvest. One exception is if the common ragweed population is resistant to ALS-inhibiting herbicides, then Affinity BroadSpec would not be effective.

Applying Affinity BroadSpec or Huskie with 28% UAN (100%) caused significant injury (~20%) to wheat. Remember both of these herbicides were applied with 0.25% non-ionic surfactant, which could have contributed to the increased leaf burn (Photos 1 and 2). Wheat did not show any significant signs of injury when 2,4-D amine or 2,4-D ester were applied with 28% UAN. Early-season wheat injury is not always the best predictor of wheat yield. At the 0.05 significance level (which means we are 95% confident), there were not any differences in wheat yield when any of the herbicide treatments were applied with 28% UAN (100%). However, at a 0.10 significance level (90% confident), wheat yields were lower than the highest yielding treatment when Affinity BroadSpec and Huskie were applied with 28% UAN (100%) as the carrier. This raises some concerns that these treatments may reduce yield under certain conditions.

Left, wheat injury when Affinity BroadSpec (0.75 oz/A) + NIS (0.25% v/v) was applied with 28% UAN (100%) as the spray carrier. Right, wheat injury when Huskie (11 fl oz/A) + NIS (0.25% v/v) was applied with 28% UAN (100%) as the spray carrier.
Photos 1 and 2. Left, wheat injury when Affinity BroadSpec (0.75 oz/A) + NIS (0.25% v/v) was applied with 28% UAN (100%) as the spray carrier. Right, wheat injury when Huskie (11 fl oz/A) + NIS (0.25% v/v) was applied with 28% UAN (100%) as the spray carrier.

50:50 ratio of 28% UAN:water as a spray carrier

Reducing the 28% UAN spray carrier amount to a 50:50 water:28% UAN mixture resulted in 10% to 15% less injury from Affinity BroadSpec and Huskie. This reduction in injury also helped protect yield from these applications, even at the 0.1 significance level. Again, there were no significant signs of injury from the 2,4-D amine or 2,4-D ester applications with the 50:50 28% UAN:water carrier. Wheat tolerance was good from all herbicides when water was the spray carrier.

Timing of application

The wheat stage at the time of herbicide application can also affect wheat tolerance and yield. In two years of this research we compared applications of 2,4-D ester, Affinity BroadSpec and Huskie sprayed with the different spray carriers applied to wheat prior to jointing (Feeke’s stages 4 to 5) and when wheat was just past the jointing stage (Feeke’s stages 6 to 7). Similar to results from the early-season applications, Affinity BroadSpec and Huskie applied with 28% UAN (100%) spray carrier at wheat after jointing resulted in the greatest injury to wheat (~30%). Applying these herbicides with 50:50 water:28% UAN spray resulted in reduced injury (15% to 18%), but it was still significantly higher than the non-treated controls.

At the later application timing, it also took longer for wheat to recover from this injury. Similar to the early-application timing results, spray carrier composition did not affect wheat yield at the 0.05 significance level. However, at the 0.1 significance level, wheat yields for the Affinity BroadSpec and Huskie treatments with 28% UAN (100% or 50%) were lower than the highest yielding treatment.

When we compared the two application timings early (Feeke’s stages 4 to 5) and late (Feeke’s stages 6 to 7), we observed a significant difference in wheat yields. The early application timing of Affinity BroadSpec and Huskie resulted in higher yields that the later application of these two herbicides. Even though we did not observe a differences in the 2,4-D ester application timings, it is not recommended to apply 2,4-D or any plant growth regulator herbicide after jointing (Feeke’s stage 6). Late herbicide applications of plant growth regulator herbicides can cause kernel abortion and blank wheat heads, ultimately reducing yield.

Summary

  • Spray carrier composition (water, 50:50 water:28% UAN, or 100% 28% UAN) did not affect winter annual weed control for any herbicide.
  • Affinity BroadSpec and Huskie provided the best control of common chickweed.
  • Control of field pennycress, bushy wallflower and common ragweed was similar for all herbicide treatments.
  • Spray carrier composition (water, 50:50 water:28% UAN, or 100% 28% UAN) did not affect wheat tolerance from 2,4-D formulations. A surfactant was not included with 2,4-D applications. Addition of a surfactant, fungicide or other herbicide could increase chances of wheat injury.
  • Wheat injury was greatest from Huskie and Affinity BroadSpec when they were applied with 28% UAN (100%) as the spray carrier. Under certain conditions, such as lower levels of significance, these treatments may result in lower wheat yields. Remember these treatments included 0.25% v/v of non-ionic surfactant that may have contributed to greater injury from the nitrogen. Caution should be used if applying these treatments.
  • Wheat injury from Huskie and Affinity BroadSpec was lower when applied with the 50:50 water:28% UAN spray carrier compared with 100% 28% UAN as the spray carrier.
  • Later application timings of Affinity BroadSpec and Huskie resulted in lower yields compared with plots that were treated earlier.

Recommendations

  • 2,4-D amine or 2,4-D ester at 1 pt /A can be applied with liquid nitrogen fertilizer solutions (28% UAN) as the spray carrier at 100% or a 50:50 28% UAN:water mixture. 2,4-D ester formulations generally mix easier with fertilizer solutions than 2,4-D amine formulations. When applying 2,4-D products with 28% UAN as the spray carrier, do not include surfactant. The addition of other herbicides or fungicides to these mixtures will likely increase the risk for crop injury. Applications of 2,4-D amine or 2,4-D ester should be made in the spring to actively growing wheat following tillering (Feeke’s stage 3) but prior to jointing (Feeke’s stage 6).
  • MSU does not recommend applying Affinity BroadSpec or Huskie with 100% (56 lb actual N) 28% UAN as the spray carrier. The risk of crop injury and potential yield reductions is higher with these combinations. The full load of surfactant at 0.25% v/v used in these combinations was likely the cause for increased injury.
  • Affinity BroadSpec and Huskie can be applied with 50:50 ratio of liquid nitrogen fertilizer solutions (28% UAN) and water (28 lb actual N). Reducing the surfactant rate in these mixtures to 0.125% v/v will also reduce the risk for crop injury. Wheat tolerance is also greater if applications of these combinations are made prior to wheat jointing (Feeke’s stage 6).

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