Paraquat use in a wiper does come with severe drawbacks compared to the use of glyphosate. First is safety. All applicators and mixers of paraquat need to complete a paraquat-specific EPA training program due to the high acute toxicity of paraquat. Second is performance. Paraquat does not translocate. It will only kill the plant parts with which it comes in contact. The University of Georgia’s Pest Management Handbook for peanut lists up to 85% control as possible with a weed wiper, when 60-70% of the of the weed surface is wicked. Getting that 60-70% coverage is only possible if there is a large difference in plant height between the crop and weed, and by the time this height difference is reached, yield loss has likely already occurred.
In 2020, I conducted a trial using a rolling carpet-style wiper, manufactured by GrassWorks, with a 50/50 mixture of Gramoxone® 2.0 and water. There were three study factors, application timing (late July or first week of August), the addition of crop oil, and one vs. two passes. In 2020, none of the treatments differed significantly. When wiping occurred, complete control of Palmer amaranth, kochia and common lambsquarters was achieved (Figure 1); along with a complete prevention of seed production (Figure 2); and while crop injury was observed, the beets largely recovered (Figure 3).

Figure 1. A Palmer, left, and kochia, right, plant a week after paraquat wiping.

Figure 2. Left, seed from a Palmer plant not wicked with Paraquat; right, seed from a head wicked with paraquat; center, germination test of seed from the head pictured to the right. Palmer amaranth seed is identified as black in the photo on the left.

Figure 3. Sugarbeet injury and weed control a week after paraquat wiping.
In 2021 and 2022, weed control was far worse and crop injury far higher. I believe the reason for this lack of success is related to weed density at the time of wiping. For moderate infestations, the wiping resulted in good coverage with limited off-weed dripping. In 2021 and 2022, weed density in the test plots was so severe that paraquat coverage was poor and increased contact between treated Palmer amaranth and sugarbeet caused crop injury.
And this speaks to a bigger issue with researching this topic. A lot of the efficacy of wiping on paraquat is function of experience rather than science. You need to have the right speed, wiper height and wiper saturation. Wiper saturation is an especially delicate adjustment to make. Control is best if the paraquat is running down the stem of the weed to the base of the plant. But over-saturating the wick leads to dripping and increased crop injury. Finding that right balance takes experience, and with several different manufactures of many different styles of wipers, the equipment setup in a research plot may not carryover to what a grower may choose to do.
So, is wiping of paraquat a solution to herbicide-resistant weeds in sugarbeet? No, but under the right conditions it can limit seed production, preserve yield and make harvest easier late in the season when other options fail. Under less-than-ideal conditions, the best it might offer is cathartic weed control.
Source : unl.edu