- Chemicals are being sought that can control weeds in lettuce
under different environmental conditions, soil types and weed populations.
The extensive or repeated use of a herbicide may lead to important
changes in the weed population which call for new materials
that can take care of the modified situation.
In the search for new chemicals, RH 315 has appeared promising
in some respects.
Greenhouse and field experiments were conducted to determine
the tolerance of lettuce (crisphead type) to RH 315 in three types of
soil. Its performance on weeds under different environmental conditions, its residual action on other vegetables, as well as pre and
postemergence activity on weeds were also tested. Other herbicides
were used in some experiments.
Lettuce of the crisphead type proved to possess high tolerance
to RH 315. The tolerance was greater in a silty clay loam and a
muck soil than in a sandy soil. In all types of soil, the safety factor
was greater than 2X when susceptible weeds are considered. The
degree of tolerance varied with the season in field experiments.
RH 315 at 1.5 to 2 lb/A in a silty clay loam and at 1 to 1.5
lb/A in a sandy loam provided excellent control of annual bluegrass,
Italian ryegrass, barnyardgrass, green foxtail, common chickweed,
purslane, mouseear chickweed, lambsquarters, henbit, red dead-nettle,
shepherspurse and bittercress. The control of pigweed and
wild mustard required higher rates. No control of composite species
was achieved with this compound.
Delaying sprinkler irrigation after application caused reduction
in RH 315 activity. It was more effective in controlling weeds in
early fall than during the summer.
Two months after application, RH 315 residues from rates
which were effective in controlling susceptible weeds did not cause
injury to bush beans, sweet corn, cucumbers, red beets, common
chickweed and annual bluegrass, under summer conditions.
When applied at the stage of two true leaves or before, under
conditions of early fall, RH 315 proved to possess postemergence
activity on weed species that are controlled by preemergence applications
of this compound. From 17 to 2O days were required for RH
315 to exert its effect in postemergence application.
Trifluralin at 0.75 lb/A provided excellent control of Italian
ryegrass, barnyardgrass, pigweed and purslane both in a silty clay
loam and a sandy loam. Benefin and EL 179 both at 1 lb/A gave good
control of the same weeds in the silty clay loam, but their performance
was inferior to that of trifluralin. Benefin at 0.75 lb/A did not
perform very well in the sandy loam.
Delaying incorporation 68 hours after the herbicide application
caused 70% loss of trifluralin activity and 35% loss of benefin action
in a sandy loam soil. The residual action of trifluralin was greater.
than that of EL 179 and benefin when tested by vegetable plant growth
82 days after application. The least residual action was obtained
with RH 315 and CDEC when tested 60 days after application.
Bensulide at 6 lb/A performed poorly in a silty clay loam,
whereas CDEC at 5 lb/A provided good control of pigweed, Italian
ryegrass and barnyardgrass in the same type of soil.
The results obtained in this research showed that RH 315 is
effective for selective control of some weeds in lettuce that are not
controlled by standard herbicides. Furthermore, its biological
residual life is sufficiently long to provide weed control for an entire
crop cycle but not long enough to cause injury to succeeding, susceptible
vegetable crops. The disadvantages of RH 315 are its poor
activity on pigweed and its dependence on the supply of moisture immediately
after application under summer conditions.