HORTSCIENCE VOL. 40(3) JUNE 2005 620 Effect of Foramsulfuron and Isoxaflutole Residues on Rotational Vegetable Crops Nader Soltani, 1 Peter H. Sikkema, and Darren E. Robinson Ridgetown College, University of Guelph, Ridgetown, Ontario, Canada N0P 2C0 Additional index words. carryover, foramsulfuron, isoxaflutole, preemergence, postemergence Abstract. There is little information published on the effect of residues from postemergence (POST) applications of foramsulfuron and preemergence (PRE) applications of isoxaflu- tole, and isoxaflutole plus atrazine in the year after application on vegetable crops. Three trials were established from 2000 to 2002 in Ontario to determine the effects of residues of foramsulfuron, isoxaflutole, and isoxaflutole plus atrazine on cabbage, processing pea, potato, sugar beet, and tomato 1 year after application. Aside from a reduction in sugar beet plant stand, there were no visual injury symptoms in any crop at 7, 14, and 28 days after emergence (DAE) in any of the herbicide carryover treatments. Isoxaflutole residues reduced shoot dry weight and yield as much as 27% and 28% in cabbage, and 57% and 60% in sugar beets, respectively. The addition of atrazine to isoxaflutole caused further reductions in shoot dry weight and yield of cabbage and sugar beet. Isoxaflutole plus at- razine residues reduced shoot dry weight and yield as much as 42% and 43% in cabbage, and 58% and 82% in sugar beets, respectively. There were no adverse effects on shoot dry weight and yield of processing pea, potato, and tomato from isoxaflutole or isoxaflutole plus atrazine residues in the year following application. Foramsulfuron residues at either rate did not reduce shoot dry weight or yield of any crops 1 year after application. Based on these results, it is recommended that cabbage and sugar beet not be grown in the year following the PRE application of isoxaflutole or isoxaflutole plus atrazine. fonylurea herbicides, which have resulted in some restrictions in the selection of rotational crops (Al-Khatib et al., 1992, 1993; Greenland, 2003). There is little published information on the potential for foramsulfuron applied to corn to injure vegetable crops in rotation, especially under northern temperate environ- mental conditions. Isoxaflutole plus atrazine (trade name Converge) is another recently registered soil- applied isoxazole plus s-triazine herbicide in corn that provides residual control of broadleaf and grass weeds such as velvetleaf (Abutilon theophrasti), redroot pigweed (Amaranthus retroflexus L.), smartweed (Polygonum spp.), common lambsquarters (Chenopodium al- bum), annual nightshades (Solanum spp.), fall panicum (Panicum dichotomiflorum), common ragweed (Ambrosia artemisiifolia L.), giant foxtail (Setaria faberii), green foxtail (Setaria viridis), yellow foxtail (Setaria glauca), and barnyard grass (Echinochloa crusgalli L. Beauv.) (OMAFRA, 2004; Vencill, 2002). Degradation occurs through chemical and biological processes. The rate of isoxaflutole breakdown is concurrent with increases in soil moisture and temperature (Beltran et al., 2003). The authors are unaware, however, of any published data that illustrate the ef- fect of isoxaflutole residues alone or in tank mix combination with other herbicides on vegetable crops grown in Ontario 1 year after application. The objectives of this research were to determine the potential for foramsulfuron, isoxaflutole alone, and isoxaflutole plus atrazine residues to cause injury to cabbage (Brassica oleracea L.), processing pea (Pisum sativum), potato ( Solanum tubersum L.), sugar beet (Beta vulgaris) and tomato (Lycopersicon esculentum Mill.), which are grown in rotation with field corn in Ontario. Materials and Methods Field studies were established in 2000, 2001, and 2002 at Ridgetown College, Ridgetown, Ontario. Glufonsinate-tolerant field corn was planted in a loam soil (pH 6.8, OM 4.6%, sand 51%, silt 31%, clay 18%) in 2000, a silt loam soil (pH 6.3, OM 6.7%, sand 47%, silt 36%, clay 17%) in 2001, and a very fine sandy loam soil (pH 6.4, OM 4.7%, sand 79%, silt 15%, clay 6%) in 2002. Seedbed preparation at all locations consisted of fall moldboard plowing followed by three passes with a field cultivator in the spring. In the year of herbicide application (i.e., 2000, 2001, 2002), the experimental design was a randomized complete block design with four replications. Each plot was 6 m wide and 22 m long. The following treatments were applied: isoxaflutole preemergence (PRE) at 105 and 210 g·ha –1 a.i., a tank mix of isoxaflutole plus atrazine (PRE) at 105+1063 and 210+2126 g·ha –1 a.i., and foramsulfuron postemergence (POST) at 70 and 140 g·ha –1 a.i., representing the label rate and twice the label rate of the above herbicides or herbicide tank mixes. PRE treatments were made 1 to 2 d after planting to the soil surface and POST treatments were applied to 6 to 7 leaf field corn. Foramsulfuron treatments included 1.75 and 3.50 L·ha –1 of methylated seed oil (MSO) and 2.50 and 5.00 L·ha –1 of 28% urea ammonium nitrate (UAN) at 70 and 140 g·ha –1 a.i., respectively. Treatments also included an untreated control as a check. Field corn was maintained weed free with two applications of glufosinate-ammonium (500 g·ha –1 a.i.), grown to maturity and harvested according to standard agronomic practices. Herbicide applications were made with a CO 2 -pressurized backpack sprayer calibrated to deliver 200 L·ha –1 of spray solution at a pressure of 200 kPa using Teejet 8002 flat-fan nozzles (Spraying Systems Co., Wheaton, Ill.). The boom was 2.5 m long with six nozzles spaced 0.5 m apart. In the year following herbicide application (i.e., 2001, 2002, and 2003), the trial areas were shallow disked (10 cm depth) followed by two passes with a field cultivator. The experimental design was a randomized complete block with a split-plot arrangement and four replications with herbicide treatment as main plot and rotational crops as subplots. All sub-plots con- sisted of 6-m-long rows of the five vegetable crops spaced 1.5 m apart planted across the main plot. Plots were planted with cabbage (‘Atlantic’), processing pea (‘Bolero’), potato (‘Superior’), sugar beet (‘E 17’) and tomato (‘Heinz 9478’) during mid to late May and fertilized according to recommended Ontario crop production practices. To avoid any weed competition with vegetable crops, the trials were maintained weed free by hand-weeding as required in the re-cropping year. Percent visual injury of each vegetable In southern Ontario, vegetable crops are commonly grown in rotation with agronomic crops such as field corn, as they offer growers greater value than traditional agronomic crops. Vegetable crops differ in their response to residues from herbicides applied in previous years (Greenland, 2003; O’Sullivan et al., 1998, 1999; Vencill et al., 1990). Foramsulfuron (trade name Option) was recently registered for postemergence (POST) control of grasses and small-seeded broadleaf weeds in field corn [Ontario Ministry of Agri- culture, Food and Rural Affairs (OMAFRA), 2002; Vencill, 2002]. Sulfonylurea herbicides are applied at very low use rates, have low mammalian toxicity, and have unprecedented herbicidal activity, and hence have become popular in many parts of the world (Sarmah and Sabadie, 2002). Some sulfonylurea herbicides have minimal carryover problems while others such as metsulfuron have a minimum rotation restriction of 34 months for some vegetable crops (Schroeder, 1998). Degradation occurs first through acid hydrolysis, which is pH dependent, and later through biological pro- cesses involving microorganisms (Sarmah and Sabadie, 2002). Vegetable crops have shown sensitivity to soil residues of a number of sul- HORTSCIENCE 40(3):620–622. 2005. Received for publication 28 Oct. 2004. Accepted for publication 5 Dec. 2004. The authors would like to acknowledge D. Bilyea for his expertise and technical assistance in these studies. Funding for this project was provided by the Ontario Fruit and Vegetable Growers Association and Ontario Processing Vegetable Growers. 1 To whom all correspondence should be addressed; e-mail nsoltani@ridgetownc.uoguelph.ca.