By Rich Zollinger, Extension Weed Specialist
Question: I sprayed generic Liberty “Interline” at 1 p.m. in the afternoon on a field of soybeans. It controlled all weeds except lambsquarters. There is no control of this weed and my neighbors also had no control. I used 32 fluid ounces per acre of Interline, 20 gallons per acre of water, 3 pounds AMS per acre, and droplet size was at 286 microns. The conditions were perfect for good control last week. Why didn’t the generic Liberty kill the lambsquarters? I will need to respray all my fields again. How do we achieve control? Has anyone looked at different additives with Liberty?
The following are a few comments from my research and observations about glufosinate:
1. Pre-emergent soybean herbicides are much more effective and consistent than post-emergent soybean herbicides for lambsquarters control.
2. Liberty/glufosinate requires two sequential applications for adequate broad-spectrum weed control.
3. Field research has shown that most generic formulations are similar in efficacy to Liberty.
4. You did everything right with the rate at 32 fluid ounces per acre, 20 gallons per acre of water, droplet size and 3 pounds AMS per acre.
5. You ask if some adjuvants can improve control of glufosinate. Glufosinate chemically is similar to glyphosate but very different in certain aspects including adjuvant enhancement. Unfortunately, I have found no adjuvant class that increases efficacy other than AMS. See pages 1-3 (especially page 3) in file: https://www.ag.ndsu.edu/weeds/ndwcr-files/13-tan
In fact, you will see that most adjuvants antagonize control. The best and only adjuvant we have found to enhance glufosinate is AMS. Do not reduce the AMS rate!
6. It is not surprising that some lambsquarters escaped the glufosinate. On page 97 in the North Dakota weed guide, we state that lambsquarters is less susceptible to glufosinate (Liberty) than other weeds. On page 114, we give Liberty a rating of F-G on lambsquarters, which is better than your results/observations.
7. One piece of information you did not disclose was the size of lambsquarters at application. Any lambsquarters over 2-3 inches would be less likely to be controlled.
8. One last comment: Lambsquarters is the HARDEST plant to wet. It retains fewer spray droplets than other weeds. This is where adjuvants facilitate the wetting process. This may be why Liberty and glyphosate do not control this weed in some situations. NIS increases droplet retention with glyphosate, but unfortunately, NIS does almost nothing to increase efficacy from glufosinate (page 3 in file). Continue to use high spray volume and keep the droplet size at the same 250-350 micron size range.
9. All post soybean herbicides are weak on lambsquarters, so you may try spraying again with the same recipe with full sunlight, high temperatures and high humidity.
10. I strongly suggest you read paragraph B9 on page 77 in the weed guide for additional information on environmental conditions that affect Liberty. Humidity is one of the most important factors that affect efficacy. The humidity was probably very low on your June 12 application, which was several days prior to the wet weather that brought higher humidity that moved across the region. Low humidity can significantly reduce glufosinate activity.