COWPEA TOLERANCE TO SANDEA HERBICIDE

Abstract

Southernpea is a major vegetable crop in Arkansas and Oklahoma for commercial production and home gardens. Complete weed control is necessary for this crop in commercial production to keep the peas free of contaminants and achieve high harvest efficiency. Several weeds like pigweed, cocklebur, velvetleaf, lambsquarters, hophornbeam copperleaf, nightshade, nutsedge, and morninglories are difficult to control in this crop because of limited herbicide options. Sandea (halosulfuron) is an excellent herbicide for nutsedge control and has activity on most of the weeds mentioned above. It has both soil and foliar activity. Sandea is labeled for several vegetable crops and southernpea may have enough tolerance to Sandea to warrant a label expansion. Experiments were conducted in Arkansas and Oklahoma between 2002 and 2005 to determine the tolerance of southernpea to Sandea and its efficacy on some weed species. In Oklahoma, trials were conducted in LeFlore County and at the Bixby Research Station in 2002 and 2003. Treatments consisted of various herbicides applied preemergence (PRE) or postemergence (POST), among which were some Sandea treatments. The doses of Sandea tested ranged from 0.024 to 0.048 lb a.i./A with some treatments applied with Basagran (bentazon), POST. Preemergence treatments were applied at 20 GPA and POST treatments at 30 GPA. Experimental units were arranged in randomized complete block design with four replications. The cultivar used was Early Scarlet. Plots were comprised of four rows, spaced either 30 or 36 inches, depending on location, 15 ft long. The crop at Bixby was irrigated, but not at LeFlore. In Arkansas, two experiments were conducted in 2005 at the Vegetable Station in Kibler. One experiment was setup in a split-plot design, with four replications, with cultivar as mainplot and Sandea treatments as subplot. Eleven advanced breeding lines and Early Scarlet were used. Four Sandea treatments, using doses of 0.048 and .096 lb ai/A applied either PRE, at 1 to 2-trifoliate (early POST), and at 3- to 4-trifoliate (late POST) were tested. The second experiment compared the responses of 16 advanced breeding lines and Early Scarlet to 0.096 lb a.i./A Sandea applied PRE. Plot size at Kibler consisted of 4 rows, spaced 36 inches, 20 ft long. Herbicide treatments were applied at 20 GPA spray volume and the crop was sprinkler irrigated as needed. In Oklahoma, the commercial rate of Sandea (0.032 to 0.048 lb a.i.) did not cause any injury to southernpea when applied PRE regardless of availability of irrigation. However, when applied POST, significant stunting (up to about 20%) of plants was observed in both locations. This level of injury did not cause significant yield loss. The trial at Bixby could not be harvested due to excessive pigweed biomass later in the season. Sandea controlled Palmer amaranth and carpetweed >90% when applied PRE, but had no activity on these species when applied POST. Conversely, Sandea had excellent activity (100%) on common cocklebur when applied POST, but ineffective when applied PRE. Trials in Arkansas were strictly for tolerance evaluation so no weed control data was collected. In Arkansas, the PRE timing was also safer than POST when 0.096 lb ai Sandea was used. The 11 advanced lines tested in trial 1 were among the top 15 lines selected for tolerance to Sandea from a preliminary screen. These selected lines still showed different levels of tolerance to high rates of Sandea, but may not show any difference among each other at the recommended rates. The best lines were 00-609 and 00-178, which showed no yield reduction when treated with 0.096 lb ai Sandea PRE. All advanced lines had higher yield than Early Scarlet without herbicide treatment. In trial 2, 01-103, 01-180, and 01-181 had 0% to 10% yield loss when treated with 0.096 lb ai Sandea, PRE. All three had similar or greater yield than Early Scarlet. The commercial standard incurred about 20% to 30% yield loss from the high dose of Sandea applied PRE in both trials in Arkansas. Sandea is safe for cowpea, PRE at recommended doses. However, some advanced lines can tolerate high rates of Sandea. Some weeds are controlled by Sandea PRE, but not POST and vice versa.