Soil Persistence of Imazethapyr and Detection Using a Sensitive Bioassay Technique

The effects of imazethapyr on root growth of vegetable crops were compared between a lab bioassay and vegetative crop growth in field plots. In the bioassay study, at 5 d after seeding, the lowest imazethapyr rates to cause significant root length reduction of table beets (Beta vulgaris L. ‘Early Wonder’), cabbage (Brassica oleracea L. ‘Bravo’), cucumber (Cucumis sativus L. ‘Marketmore 76’), and sorghum [Sorghum bicolor (L.) Moench ‘Corral’] were 0.5, 2.5, 10, and 25 ppb (w/w soil), respectively. In the field, where crops were planted 5 d after preplant incorporated (PPI) herbicide application, the lowest rates to cause dry shoot weight reduction of table beets, spinach (Spinacia oleracea L. ‘Seven‐R’), cabbage, cucumber, and sorghum were 0.005, 0.01, 0.025, 0.025, and 0.025 lb ai/acre, respectively. These rates are approximately equivalent to concentrations of 5 and 10 ppb respectively for table beets and spinach; and 25 ppb for cabbage, cucumber, or sorghum, based on a 4‐in. topsoil weighing 1 × 106 lb. However, cabbage planted into treated plots 4 and 12 mo after herbicide application was not injured. In field soil samples collected at monthly intervals starting 45 d after herbicide application (DAHA), imazethapyr at 0.0025 to 0.05 lb ai/acre was not detected beyond 75 DAHA using beet root growth as herbicide indicator. However, at 0.10 lb ai/acre the herbicide persisted until 105 DAHA but was not detected 135 DAHA. Research Question Imazethapyr is an imidazolinone herbicide used to control a wide spectrum of weeds in soybeans and edible legumes. It is used soil applied or postemergence at 0.024 to 0.063 lb ai/acre. Imazethapyr persistence in the soil can limit its use when sensitive species must be planted in rotation. Current analytical methods cannot detect imazethapyr residues below 5 ppb in soils, and this may not be adequate to predict safe levels for highly sensitive species in crop rotations. At present, label restrictions stipulate a waiting period of 18 to 26 mo before planting sensitive crops to ensure that the residues have degraded to safe levels. This is too long where available land resources are limited. Therefore, it is desirable to develop a simple, inexpensive and reliable method of evaluating imazethapyr persistence in the field and to determine whether or not this period can be shortened in certain soils to allow more flexible crop production timetable. Literature Summary The rate of imazethapyr dissipation in soil depends on the prevailing transfor mation and degradation processes. The more tightly the herbicide is adsorbed onto the soil, the more slowly residues in the soil decline. Imazethapyr behavior is influenced directly by pH, clay content and type, organic matter, and moisture. Imazethapyr adsorption increases as the cation exchange capacity of the soil increases and as soil pH decreases. Imazethapyr is microbially degraded and, in dry soil, this microbial activity is reduced. Also, less imazethapyr is available in the soil solution in dry soil for microbial degradation. Study Description A Petri dish bioassay was developed to evaluate the phytotoxicity of treated soil (coarse‐textured, mixed‐mesic Typic Hapludult, pH 6.4, CEC 3.5 meq/3.5 05 organic matter 1.0%) on the initial root growth of crops. One pound of ovendried soil sample was mixed with 1 oz standard solution of imazethapyr to give concentrations of 0, 0.5, 1.0, 2.5, 5.0, 10.0, and 25.0 ppb (w/w). Additional water was added to provide adequate moisture for seed germination and root growth. The soil was pressed firmly into square petri dishes and leveled. Ten seeds were positioned in a row across the soil surface. Transparent lids were taped in place and sealed. A slotted wooden rack was used to position the dishes at a 15 degree angle from the vertical so the emerging roots would grow down along the transparent lid. The racks were placed in growth chambers set at 0 h light and 80°F for 5 d. A field experiment using a site with soil characteristics similar to the one used in the bioassay study was conducted in 1990 and 1991 to evaluate the effectiveness of the bioassay. On 3 May 1990, imazethapyr was applied to the soil at 0.0005, 0.001, 0.0025, 0.005, 0.01, 0.025, 0.05, and 0.1 lb ai/acre and incorporated to 3 in. depth. Beets, spinach, cabbage, cucumbers, and sorghum were seeded across each plot on 8 May 1990 (5 d after herbicide application). Cabbage was planted 5 Sept. 1990 (4 mo after herbicide application) and 23 May 1991 (12 mo after herbicide application). Spring plantings were harvested 3 mo after planting, while the fall seeding was harvested 2 mo after planting. Soil samples were taken and tested monthly by the bioassay until no imazethapyr was detected. Applied Question How persistent is imazethapyr in a sandy loam soil and can a simple bioassay technique be used to determine this? Significant reduction in early root growth of crops occurred as the imazethapyr concentration increased from 0 to 25 ppb in the bioassay. Beets were most sensitive to imazethapyr, with root length reduced at 0.5 ppb. Cabbage was less sensitive than beets, and root length was reduced by 2.5 ppb imazethapyr. Cucumber and sorghum were the least sensitive. Root length was reduced at an imazethapyr concentrations of 10 and 25 ppb, respectively. A linear relationship between root length and the log of imazethapyr concentration was determined for beets in the bioassay. In the field, visible injury and reduced yield were observed in all crops as imazethapyr increased. Beets were most sensitive to imazethapyr injury in the field. Stunting was observed when 0.0025 lb ai/acre was incorporated prior to planting. Spinach and cabbage were slightly less sensitive, with injury detected at 0.005 lb ai/acre. Cucumber and sorghum were the least sensitive. Both were injured at an imazethapyr rate of 0.025 lb ai/acre. Imazethapyr rates of 0.001, 0.0025,0.005, 0.01, 0.025, 0.05, and 0.1 lb ai/acre incorporated 4 in. deep were approximately equivalent to soil concentrations of 1, 2.5, 5, 10, 25, 50, and 100 ppb (w/w), respectively, at the time of planting. Crops were less sensitive to imazethapyr in the field at these concentrations than in the bioassay. Soil samples collected monthly from the field plots were evaluated with the bioassay for injury to beet roots. Sampling continued until no reduced growth was observed. Dissipation of imazethapyr occurred throughout the 1990 growing season. Samples collected after September 1990 did not affect beet root growth in the bioassay at any of the rates of imazethapyr applied in the spring. Cabbage planted into the field plots was not injured after the bioassay indicated that no imazethapyr was present. The bioassay provided a simple, inexpensive and relatively quick method for detecting imazethapyr residue in the soil and predicting the potential for injury to vegetable crops from herbicide carryover. It was also shown that in a sandy loam soil with pH between 6.0 and 7.0, imazethapyr applied up to 0.10 lb ai/acre in the spring dissipated to safe levels for sensitive vegetable crops to be grown within 12 mo after application.