Soybean (Glycine max) influences metolachlor mobility in soil

This study was conducted to evaluate the mobility of14C-metolachlor over 1 yr for three seasons when applied preemergent to undisturbed field lysimeters with and without soybean representing cropped and noncropped zones, respectively. Leachate was collected weekly and analyzed for total14C, metolachlor, and metabolites. Lysimeters were removed, sectioned, and analyzed for14C. Sixty and 90 days after treatment (DAT), there was less soil water in lysimeters with soybean. Recovery of14C in lysimeters decreased with time and ranged from 54 to 74% 30 DAT followed by a slower rate of loss with 35 to 49% remaining 365 DAT. Comparable amounts of total14C were observed in soybean lysimeters as in fallow lysimeters 30, 60, and 90 DAT.14C distribution in the lysimeters, however, was quite different. Sixty and 90 DAT,14C mobility in soybean lysimeters was less than in fallow lysimeters. Also, less leachate was collected from soybean lysimeters, which resulted in later appearances and lesser amounts of14C in the leachate. Cumulative leachate from lysimeters with and without soybean 365 DAT contained 2% and 10 to 18% of the applied14C, respectively. Peak concentrations of14C in leachate from fallow columns occurred about 90 DAT and were two to 19 times higher than14C concentrations in leachate from soybean lysimeters. Metolachlor concentrations in leachate were well below the National Health Advisory level for drinking water in all cases. Apparent volatilization losses of14C amounted to 26 to 46% of the applied14C-metolachlor 30 DAT. These results suggest that herbicide mobility is different in cropped vs. fallow sites and possibly in intra- and interrow crop positions.

Fate and Transport of Alachlor, Metolachlor and Atrazine in Large Columns

14C ring-labelled atrazine, alachlor, and metolachlor were surface-applied at 3.14 kg a.i./ha in greenhouse lysimeters containing two soils in an ongoing experiment. Bromide (Br) – a conservative tracer – at 6.93 kg/ha as KBr and nitrate-nitrogen (NO3-N) at 112 kg/ha as KNO3 were mixed with each herbicide and surface-applied. Growth of Red top (Agrostis alba) was established in each column (105 cm long and 29.4 cm i.d.). The experiment consisted of 12 columns (2 soils × 3 herbicides × 2 replicates) each fitted with four sampling ports for leachates, a volatilization chamber, and an aeration and irrigation system. Volatile materials are being trapped directly in solvents. One column replicate was dismantled for soil and plant analyses. Columns of Plainfield sand and Piano silt loam treated with alachlor and metolachlor were sampled after 23 and 28 weeks, respectively; the atrazine columns after 35 weeks. Herbicide residues are determined by liquid scintillation counting, extracted and separated by thin-layer chromatography using autoradiographic detection. Volatilization was ≤ 0.01% of the amount of herbicide applied. The order of herbicide mobility was alachlor > metolachlor >> atrazine. As many as 8 to 12 alachlor metabolites and 2 to 6 metolachlor metabolites were separated in leachates.

Self-Limitation of Herbicide Mobility by Phytotoxic Action

Translocation of phloem-mobile herbicides was inhibited by their phytotoxic action on processes that maintain assimilate translocation. Glyphosate lowered import into developing sink leaves soon after it was applied to exporting sugarbeet leaves. Later, photosynthesis slowed down and starch accumulation stopped, but export of both assimilate and glyphosate continued until it was limited by starch availability at night Experiments with field pennycress and Tartary buckwheat indicated that self-limitation of chlorsulfuron translocation probably occurred and that it resulted from lowered assimilate entry into phloem rather than from inhibition of photosynthesis or carbon allocation. Leakage of chlorsulfuron from the phloem when export was slowed down also may have contributed to its reduced translocation.

Comparison of Different Soil Leaching Techniques with Four Herbicides

Herbicide mobility in soils was compared by three laboratory methods. The Rf values calculated from soil thin-layer chromatography correlated closely with those obtained from soil thick-layer chromatography (r = 0.96). Herbicides leached slightly further in slotted column chromatography as compared with the other methods. The working hours required to conduct a study with each method were in the increasing order of thin-layer, thick-layer, and column chromatography. However, the thin-layer method required the longest waiting times, followed by the column and thick-layer chromatography. If radioactive herbicides are not available or obtainable, the thick-layer chromatography is simplest and quickest. The relative mobility of herbicides studied was fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea] > napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide] > terbutryn [2-(tert-butyl-amino)-4-(ethylamino)-6-(methylthio)-s-triazine] > trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine). Less herbicide mobility was observed in heavier soil than in sandy soil.

Diffusion of Three Chloros-Triazines in Soil

Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triaazine] diffused at a rate of 15.2 × 10−8sq cm/sec at 25 C which was faster than the diffusion rate of 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) and 2-chloro-4,6-bis(isopropylamino)-s-triazine (propazine) in the same soils. Diffusion rates were highly correlated with total surface area of the soil. Increasing soil moisture content, soil pH, and soil temperature resulted in greater diffusion rates. A relative indication of herbicide mobility may be obtained from some of the physical and chemical properties of the soils and herbicides involved; however, rate of diffusion appears to be governed by a combination of soil and herbicide properties.