The influence of herbicide placement in soil on toxicity to emerging seedlings Public Deposited

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  • Previous research has indicated several soil-applied herbicides are more effective by exposure to emerging shoots than from root uptake. Studies were conducted in the greenhouse and growth chamber to investigate effects of herbicide placement in soil on toxicity to emerging seedlings. In these studies, an inch of treated soil was placed above the seed, below the seed, or both above and below the seed. The seed was placed in the middle of a 1-inch layer of untreated soil. Each soil layer was brought to field capacity separately and the entire container was covered with a plastic bag. The use of this plastic bag avoided the need for irrigation and subsequent herbicide movement. By this method, numerous herbicides were active on oats (Avena sativa) primarily through shoot exposure, with root exposure having little or no effect on foliage growth. These herbicides included EPTC (ethyl N, N-dipropylthiocarbamate), dichlobenil (2, 6-dichlorobenzonitrile), trifluralin (α, α, α-trifluoro-2, 6-dinitro-N, N-dipropyl-p-toluidine), nitralin (2, 6-dinitro-N, N-dipropyl-4-[methylsulphonyl] aniline), propachlor (2-chloro-N-iso-propylacetanilide), DCPA (dimethyl 2, 3, 5, 6-tetrachlorotereph-thalate), and CDEC (2-chloroallyl diethyl dithiocarbamate). Sindone (isomers of 1, 1-dimethyl-4, 6-diisopropylindanyl ethyl ketone) was more effective on sugar beets (Beta vulgaris) and annual rye-grass (Lolium multiflorum) through shoot exposure than root exposure. Some herbicides are more effective on oats through root exposure. These included bromacil (5-bromo-3-sec-butyl-6-methyluracil), R 11913,¹ Nia 11092 (1, 1-dimethyl-3-(3-[N-tert-butylcarbamyloxy] phenyl) urea), pyrazon (5-amino-4-chloro-2-phenyl-3[ 2H]-pyridazinone), diuron (3-[3, 4-dichlorophenyl]-1, 1-dimethylurea), and atrazine (2-chloro-4-ethylamino-6-isopropyl-amino-s-triazine). Dicamba (2-methoxy-3, 6-dichlorobenzoic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic acid) were more effective on cucumber through root exposure than through shoot exposure. Some herbicides appear to be taken up differently by different plant species. R 11913 was more effective through shoot exposure on sugar beets and more effective through root exposure on oats. Atrazine proved more effective on oats through root exposure; however, with green foxtail (Setaria viridis) and annual ryegrass, shoot exposure was more important when an untreated buffer layer was present between the seed and the treated zone. The possibility existed that a reduction in transpiration, caused by the plastic bag could affect the mode of uptake. In studies without the use of the plastic bag, atrazine was still more effective on oats through root exposure and more effective on green foxtail through shoot exposure. Irrigation in this study was accomplished through straws extended into each layer to minimize herbicide movement. We noticed that green foxtail produced secondary roots as early as five days after planting or two days after emergence. This indicated that these newly-formed secondary roots, as well as the emerging shoot, could absorb atrazine placed in the shoot region. When the buffer layer was removed and atrazine-treated soil was placed adjacent to the seed, placement immediately below the seed showed the greatest toxicity to both oats and green foxtail. This indicates the variation in sensitivity of plants by exposure to different parts of the seedling. Studies were conducted on varying placement positions of the herbicide as well as concentrations. From these studies, proximity to the seed was a major factor in increasing toxicity to oat seedlings by atrazine and EPTC, and to annual ryegrass seedlings by diuron. Atrazine was also more effective to green foxtail and annual ryegrass seedlings with increased proximity to the seed. A narrow concentrated layer was more effective than a wider dilute layer with diuron, atrazine, and EPTC when placed either above or below the seed. With atrazine and diuron, placement of the dilute wider layer completely around the seed was just as effective as the concentrated layer adjacent to the seed. With EPTC, a concentrated layer adjacent to the seed was more effective than a dilute layer completely around the seed. The effects of a constant temperature regime (24 C) was compared with a temperature regime with a low night temperature (27 C/ 13 C) in relation to uptake of EPTC, expressed by toxic action on oats. The temperature regime with a low night temperature caused a slight increase in toxicity to oats through shoot exposure, as well as through root exposure. However, the increase in uptake of EPTC through root exposure was considerably greater than through shoot exposure. ¹No chemical name available, an experimental herbicide of Stauffer Chemical Company.
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