Anticoagulant residues in the ricefields


The possibilities for food chain concentration and secondary hazard
are generally considered low when anticoagulant rodenticides are used
according to manufacturers' recommendations. A study is in progress
to examine the potential for residue accumulation in paddy water,
soil, plants, and polished grain in two simulated "worst-case"
situations: (1) when exceptionally high levels of bait (5 kg/ha/week
at manufacturers' recommended concentration) are used throughout the
growing season, and all bait is "spilled" into the paddy; and, (2)
when even greater levels (50 kg/ha/week) are used, and all bait is
"spilled" into the paddy.

Sixty-six contiguous (7 m X 8 m) paddies were constructed in experi-
mental fields at the International Rice Research Institute (IRRI),
and planted with C-168, a tall, high-yielding variety. Paddies were
constructed and maintained by IRRI field crews. Fifteen paddies were
selected for treatment. Each treated paddy was surrounded on three
sides by untreated paddies and on the fourth by an affluent canal.
For treatment, a bait formulation (polished rice plus toxicant) was
broadcast evenly over the paddy each week from the second week after
transplant (WAT) until harvest. Paddies were assigned randomly, three
paddies per combination, to five combinations of bait formulation/
application rates: (1) warfarin (0.05 percent), 5 kg/ha/week; (2)
warfarin (0.05 percent), 50 kg/ha/week; (3) diphacinone (0.005 per-
cent), 5 kg/ha/week; (4) diphacinone (0.005 percent), 50 kg/ha/week;
and (5) no toxicant (reference), 50 kg/ha/week.

At 12 WAT, a water sample was taken from each treated plot immediately
after bait application. A second water sample was taken 24 h later.
Water samples were frozen until analyzed for residues. At harvest,
plant and soil samples were removed from each of the treated plots.
The soil samples were dried, homogenized, and packaged. Plant samples
from each plot were split: half were dried, homogenized, and pack-
aged; grains from the remaining half were harvested, winnowed, hulled,
polished, and packaged. All samples were then sent to the Denver
Wildlife Research Center for residue analyses. These analyses are
underway (see page 85) and final results will be reported upon comple-
tion of these tests and statistical analysis of the data.

Preliminary trial of an electric rat barrier in a farmer's ricefield

The concept of using a nonlethal electric barrier to repel rats from
ricefields has been under study by DWRC staff for several years (see
previous annual reports). Most of this work has been done in the
laboratory or semifield situations and results of these earlier inves-
tigations have been encouraging. The purpose of this study was to
evaluate the electric barrier under actual field conditions.