Citation: Kurniadie, D.; Umiyati, U.; Widianto, R.; Kato-Noguchi, H. Effect of Chitosan Molecules on Paraquat Herbicidal Efficacy under Simulated Rainfall Conditions. Agronomy 2022, 12, 1666. https://doi.org/10.3390/ agronomy12071666 Academic Editor: Gabriella Kazinczi Received: 14 June 2022 Accepted: 11 July 2022 Published: 13 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). agronomy Article Effect of Chitosan Molecules on Paraquat Herbicidal Efficacy under Simulated Rainfall Conditions Denny Kurniadie 1, *, Uum Umiyati 1 , Ryan Widianto 1 and Hisashi Kato-Noguchi 2, * 1 Department of Agronomy, Faculty of Agriculture, Universitas Padjadjaran Jl. Ir Soekarno Km 21, Jatinangor, Sumedang 45360, Jawa Barat, Indonesia; umiyati_uum@yahoo.com (U.U.); ryanwidianto21@hotmail.com (R.W.) 2 Department of Applied Biological Science, Faculty of Agriculture, Kagawa University, Miki, Kagawa 761-0795, Japan * Correspondence: denny.kurniadie@unpad.ac.id (D.K.); kato.hisashi@kagawa-u.ac.jp (H.K.-N.) Abstract: Unexpected rainfall before herbicide absorption by plants can wash away herbicides from plant tissue surfaces, which may reduce the herbicidal efficacy and increase the adverse effects on nontarget organisms and the environment, including water networks. The objective of this study was the evaluation of the effect of chitosan on paraquat efficacy under simulated rainfall conditions. Simulated rainfall within 3 h after paraquat application decreased its herbicidal efficacy. A mixture of paraquat (280 g a.i./ha) and chitosan (0.05% w/v) significantly increased the herbicidal efficacy against Ageratum conyzoides L. (21% increase), Borreria alata Aubl. (15%) and Paspalum conjugatum Bergius (8%) under the rainfall conditions. The chemical structure of chitosan may contribute to the penetration of paraquat into plant tissues. However, a mixture of paraquat and chitosan did not affect the herbicidal efficacy against Imperata cylindrica (L.) Beauv. The morphological characteristics of I. cylindrica may interfere with the enhancement effect of chitosan. Chitosan is a degradable, nontoxic and easily available and low-cost material made from crustacean shells. These results suggest that chitosan may increase paraquat efficacy against some noxious weed species under rainfall conditions, which may reduce the risk of paraquat contamination into the environment. Therefore, the application of herbicides with chitosan may provide the economic and environmental benefits. Chitosan may enhance the efficacy of other herbicides under unexpected rainfall conditions; however, this possibility requires further investigation. Keywords: chitosan; herbicide efficacy; nanoparticle; paraquat; rainfall 1. Introduction The herbicide paraquat (1,1 ′ -dimethyl [4,4 ′ -bipyridine]-1,1 ′ -diium dichloride) is clas- sified as a methyl viologen, and is normally synthesized in the form of a dichloride salt from pyridine [1,2]. It is a quick-acting, nonselective and post-planting contact herbicide with plant surfaces [3,4]. It works as a photosystem I electron diverter and leads to rapid leaf wilting and desiccation [5,6]. Paraquat is one of the most common herbicides and has been used worldwide for post-emergence weed management in crop fields such as rice, tobacco and cotton [5,7]. However, the herbicide can contaminate the environment, including water networks, because of its high aqueous solubility, and threatens aquatic animals and algae [8]. Paraquat also causes lung and renal problems in humans [9,10]. The herbicide binds to soil easily, and its half-life varies from 16 months to 13 years [11,12]. Thus, paraquat has been banned in some countries due to its potential for environmental contamination and toxicity to humans [9,10,13,14]. The application of nanoparticles has been investigated in the production of effective herbicide formulations with less potential for toxicity and environmental contamination [15–17]. It is considered that nanoscale molecules penetrate plant surfaces easily and into their cells through cell membranes [18,19]. Chitosan is one of the potential nanoparticles for the Agronomy 2022, 12, 1666. https://doi.org/10.3390/agronomy12071666 https://www.mdpi.com/journal/agronomy