- 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.