1 Development and Employment of Slow-Release Pendimethalin 2 Formulations for the Reduction of Root Penetration into Subsurface 3 Drippers 4 Yifat Zait, † Dekel Segev, § Avraham Schweitzer, § Yaakov Goldwasser, ‡ Baruch Rubin, ‡ 5 and Yael G. Mishael* ,† 6 † Department of Soil and Water Science and ‡ Institute of Plant Science and Genetics in Agriculture, The Robert H. Smith Faculty of 7 Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel 8 § Netafim Ltd., Derech Hashalom 10, Tel Aviv, Israel 9 ABSTRACT: Subsurface drip irrigation supplies water directly to the root zone and is an efficient irrigation technology. One of 10 the main challenges is preventing plant roots from clogging the drippers. With the aim of inhibiting root penetration, slow-release 11 pendimethalin formulations based on its solubilization in micelles adsorbed and unadsorbed to clay were developed. In the past 12 unadsorbed micelles were considered inadequate for slow release, because release was too fast. In contrast, the advantage of a 13 two-mode release formulation, composed of adsorbed and unadsorbed micelles, is demonstrated. A bioassay to study 14 pendimethalin leaching at a refined scale of 1-2 cm was developed and reduced leaching from the micelle-clay formulations in 15 comparison to the commercial formulation (Stomp) was exhibited. In a greenhouse study the application of the formulations by 16 injection into an irrigation system was extremely efficient with 0-10% root penetration in comparison to 100% penetration upon 17 Stomp injection. 18 KEYWORDS: subsurface irrigation, herbicide, drippers, pendimethalin, slow-release formulations, micelle-clay formulations, leaching 19 ■ INTRODUCTION 20 The shortage in water for irrigation along with the awareness of 21 water conservation enhanced the interest and application of 22 subsurface drip irrigation systems all around the world. 23 Subsurface drip irrigation supplies water directly to the root 24 zone through buried drippers and is one of the most efficient 25 water-saving irrigation technologies. The advantages of the 26 technology include improved water and nutrient management 27 and in many cases also improved yield and enhanced crop 28 quality. 1,2 29 One of the main challenges of subsurface drip irrigation for 30 long-term operation is preventing roots from penetrating and 31 clogging the drippers. 2 Due to high water and nutrient content 32 in the soil around the drippers, the concentration of crop roots 33 in the soil increases with proximity to the dripper 3,4 High root 34 concentration near the drippers increases the probability of 35 roots penetrating the dripper. 5 Dripper penetration and 36 clogging are enhanced when irrigation is lowered or terminated 37 (between crops) and the remaining roots seek water in the 38 emitters. 39 To prevent root penetration self-protection drippers with 40 impregnated trifluralin are manufactured. The incorporated 41 trifluralin is released slowly after the system is buried and 42 sufficient concentrations remain in the vicinity of the drippers 43 to protect against root penetration. Although the reduction in 44 root penetration employing the self-protected drippers was 45 efficient for turf grass irrigation, 6,7 it is not economically feasible 46 for lower value crop production. 5 A more economical approach 47 is injecting root growth inhibiters through the subsurface drip 48 irrigation system such as thiazopyr, 6 phosphoric acid, 1 or 49 trifluralin, 5 as demonstrated in a detailed field study with winter 50 wheat showing the high efficiency of trifluralin injection. These 51 authors point out that the main advantages of trifluralin are its 52 long half-life time (a few months) and its extremely low 53 migration in the soil, so it remains in close proximity to the 54 dripper as simulated by numerical calculations. 8 55 In 2007 trifluralin was excluded from Annex I due to its 56 toxicity to aquatic organisms including fish (Kyprianou). In 57 contrast, pendimethalin, which is also a dinitroaniline herbicide 58 with a bioactivity similar to that of trifluralin, was included. 59 Pendimethalin is commonly used for pre-emergence control of 60 grass and broad-leaved weed seedlings in several broad-leaved 61 crops and in corn, sugar cane, and spring wheat. It is usually 62 applied before sowing and incorporated into the soil to prevent 63 its volatilization and photodegradation. Although pendimetha- 64 lin migration is considered low, 9-12 it is higher than the 65 migration of trifluralin. 13 Minimizing pendimethalin leaching 66 would concentrate it in close proximity to the dripper, root 67 penetration would be eliminated, and adequate dripper 68 protection would be achieved. 69 The main approaches to reduce herbicide leaching are 70 herbicide dose optimization, site-specific weed management, 71 and nonchemical weed control such as mechanical and thermal 72 methods. These strategies are more relevant for surface 73 application. Another approach widely pursued in the past few 74 decades is developing slow-release formulations, 14-22 many of 75 which are based on organically modified clay minerals 18-24 Received: October 12, 2014 Revised: January 27, 2015 Accepted: January 27, 2015 Article pubs.acs.org/JAFC © XXXX American Chemical Society A DOI: 10.1021/jf504839q J. Agric. Food Chem. XXXX, XXX, XXX-XXX emr00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.6.i7:4236 | 2.0 alpha 39) 2014/12/19 13:33:00 | PROD-JCAVA | rq_4368577 | 2/02/2015 16:20:01 | 7 | JCA-DEFAULT