Biodegradation Characteristics of Imazaquin and Imazethapyr

Abstract

The extent of14C-imazaquin and14C-imazethapyr abiotic vs. biotic degradation in soil was investigated. Degradation was measured in an in vitro system which allowed 90% recovery of applied herbicide. Triallate biodegradation is well documented and therefore used as a standard. Herbicide degradation was compared in two soils, a Cisne silt loam and a Drummer silty clay loam. Herbicide degradation in gamma-irradiated soil was compared to fresh soil. Biomass quantities were measured for the duration of the experiments.14CO2evolution, extractable parent, metabolites, and unextractable residue were measured. After 12 weeks of incubation, 95% of the radioactivity could be extracted as parent from sterilized soil. In unsterilized soil, imazaquin and imazethapyr degraded at a similar rate which was dependent upon soil type. All herbicides degraded slower in the Drummer soil and triallate degraded two to three times faster than the imidazolinones in either soil.14C-imazaquin degradation products included14CO2and unextractable residues. The major product from14C-imazethapyr degradation was14CO2. Evolution of14CO2from an imazethapyr-treated Cisne soil, containing a serial dilution of activated charcoal, demonstrated that adsorption of herbicide was negatively correlated with degradation. Therefore imidazolinone microbial degradation is regulated by the amount of herbicide in soil solution as determined by soil characteristics.

Response of four fall-seeded cover crops to residues of selected herbicides

“…The effect of soil pH on imazethapyr persistence has been reported (Cantwell et al, 1989;O'Sullivan et al, 1998;Stougaard et al, 1990) and may have played a role in imazethapyr persistence in this study. As soil pH increases there is reduced adsorption, greater microbial degradation and therefore greater dissipation of imazethapyr resulting in reduced concentrations in the soil that may injure sensitive plants (Cantwell et al, 1989;O'Sullivan et al, 1998;Stougaard et al, 1990).…”

VA-mycorrhizae and mycorrhiza stimulating isoflavonoid compounds reduce plant herbicide injury

“…Crop phytotoxic symptoms include stunting, shortened internodes, deformed leaves, interveinal chlorosis, loss of apical dominance and premature senescence (Hess, 1989;Shaner, 1989b) and have been found to occur in double cropped soybeans and other crops (Barnes et al, 1989;Basham et al, 1987;Mills and Witt, 1989a, b;Renner et al, 1988). Under normal soil conditions these herbicides are degraded rapidly and do not reach the threshold concentrations for non-target phytotoxicity (Cantwell et al, 1989;Shaner, 1989a). However, under unfavorable conditions for degradation, such as severe drought, high soil pH, low soil temperatures, and high clay content, residual herbicide levels may build up and adversely affect crop plants (Cantwell et al, 1989;Marking, 1988;Stougaard et al, 1990).…”

“…Under normal soil conditions these herbicides are degraded rapidly and do not reach the threshold concentrations for non-target phytotoxicity (Cantwell et al, 1989;Shaner, 1989a). However, under unfavorable conditions for degradation, such as severe drought, high soil pH, low soil temperatures, and high clay content, residual herbicide levels may build up and adversely affect crop plants (Cantwell et al, 1989;Marking, 1988;Stougaard et al, 1990). Imazaquin persistence has been related to carryover injury in succeeding crops such as rice, wheat, corn, sorghum and cotton (Barnes et al, 1989;Barrett, 1989b;Basham et al, 1987;Basham and Lavy, 1987).…”

Differential sensitivity of plant-associated bacteria to sulfonylurea and imidazolinone herbicides

“…In addition, the target e n z y m e is present only in plants and microorganisms, so that m a m m a l i a n toxicity is very low (Beyer et al, 1988;Nakata, 1991). Moreover, these compounds are generally subjected to a relatively fast mineralization by either chemical hydrolysis or microbial degradation (Beyer et al, 1988;Cantwell et al, 1989). In some cases, however, especially under alkaline conditions, residues may persist in soils for several months (Cantwell et al, 1989;Peterson and Arnold, 1985).…”

“…Moreover, these compounds are generally subjected to a relatively fast mineralization by either chemical hydrolysis or microbial degradation (Beyer et al, 1988;Cantwell et al, 1989). In some cases, however, especially under alkaline conditions, residues may persist in soils for several months (Cantwell et al, 1989;Peterson and Arnold, 1985). In the last few years much work has been done to ascertain both the sensitivity of rotational crops and the occurrence of natural variability of response among inbred lines of sensitive crops to soil residues of these herbicides (e.g.…”