A new herbicide-carrying hydrogel’s droplets can land on leaves and stay there without bouncing off or breaking up (ACS Sustainable Chem. Eng. 2020, 10.1021/acssuschemeng.0c03396). Such hydrogels could help reduce agrochemical losses when they are sprayed, potentially cutting the amount needed to achieve the same level of crop protection.
Although herbicides and pesticides are usually sprayed onto crops, not all the droplets stay on the foliage where they need to be to kill weeds and insects. By one estimate, 50% of agrochemical droplets bounce or roll off leaves (Sci. Adv. 2017, DOI: 10.1126/sciadv.1602188). These losses are not only costly but also waste water and could pollute the environment, says Scott McCue, a mathematician at Queensland University of Technology who was not involved in the study.
Typically, agrochemical companies address this problem by adding surfactants to their pesticide formulations to reduce the surface tension of each droplet, says McCue, who helps companies model how droplets behave when they hit leaves. Although adding surfactants helps each droplet adhere to a leaf’s surface, these mixtures could produce smaller droplets that are more easily carried by the wind. Surfactants also add to the cost of a formulation, and some of them may not be environmentally friendly, he notes.
Hoping to come up with herbicide formulations with sustainable components that benefit plants, Chong Cao and Qiliang Huang of the Chinese Academy of Agricultural Sciences and their colleagues turned to hydrogels containing folic acid and zinc—essential plant nutrients—instead.
Cao mixed together aqueous solutions of folate ions, zinc ions, and the herbicide dicamba, and after 20 min, a viscous, orange-yellow liquid formed. The folate and zinc cross-linked into a loose 3-D network with large gaps housing the dicamba molecules. This network is flexible enough to allow the hydrogel to be sprayed like a liquid, but strong enough to hold individual droplets together and help them recover when they hit leaves, Cao explains.
The researchers sprayed the hydrogel on the waxy, hydrophobic leaves of Chenopodium album, a weed that interferes with corn and soy crops in North America and Europe, in a wind tunnel to simulate how pesticides are applied in the field. Using a high-speed camera, the researchers observed that droplets of the hydrogel fell flat like pancakes on contact and then recovered as spherical droplets that stuck to the leaves. In contrast, droplets of aqueous solutions of dicamba, folate ions, or zinc ions shattered on impact, forming smaller droplets that then bounced off.
“There is evidence in this study that splashing and bouncing of droplets is reduced through this hydrogel,” McCue says. But he notes that the study is preliminary, and there is a long way to go before commercial adoption. He suggests that the researchers repeat their experiments on different leaf surfaces and use a nozzle that produces smaller and faster droplets to better mimic commercial spraying. “There’s so much time and energy being put into this problem in industry, any new idea for designing agrochemical spray practices that reduce environmental impact is definitely worth following up on,” McCue says.
The researchers have successfully incorporated a fungicide, thifluzamide, into a similar hydrogel system. They are also investigating the mechanism through which dicamba is released from the gel to kill weeds.