| A
study in 1976 revealed that weed control in 1 hectare
of paddy field required about 500 man-hours of work
in the traditional way, about 90 hours if herbicides
were used, and only 20 hours to hand pull remaining
weeds after the action of the shrimps. |
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Rice at left. Barnyard grass
at right.
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Annual weeds have continued
to be a problem for small-scale growers of paddy rice in Japan,
and biological controls, such as tadpole shrimps, are just one
of the alternative weed weapons that natural growers may find
useful. The continued use of herbicides in conventional rice
has effectively controlled many weeds, including barnyard grass
(Echinochoa crus-galli), one of the most troublesome annual
weeds in Asian paddy fields. However, rice farmers practicing
natural agriculture must be creative in order to customize a
wide array of alternative weed control tactics to suit their
needs.
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A tadpole shrimp: better than hand weeding! |
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Alternative weed control tactics include traditional hand pulling,
cultivation, crop rotation, and manipulation of plant spacing.
Less traditional control methods of annual weed control in paddy
fields include "allelopathy" (the use of naturally
produced plant or animal chemicals that affect other plants),
and the use of biological controls including weed-eating ducks,
grass carp (Ctenophryngodon idella) and red tilapia (Tilapiamossambica
XT. Nilotica-anrea); various pathogens that cause weed diseases;
and–believe it or not–several insects. A planthopper
(Sagatodes pusanus), a stemboring moth (Enosima leucotaeniella),
and a distant relative, the tadpole shrimp, have the potential
to help farmers control barnyard grass and other annual weeds
in transplanted paddy rice.
Some farmers in Japan have used tadpole shrimps (several
species of tiny aquatic crustaceans sharing the name Triops)
since the 1920s to control weeds in transplanted rice. In
Japan and elsewhere, the tadpole shrimps occur naturally in
newly flooded paddy fields and are referred to as "weed
picking bugs in rice fields". Their story provides a
good example of the delicate balance that exists among organisms
in agricultural ecosystems, and the way that natural farmers
can manage this balance in order to produce a high quality
crop of natural rice.
Tadpole shrimps control weeds in paddy rice in three ways:
- By uprooting small weed seedlings;
- By agitating the soil surface so that the water becomes
muddy, thereby inhibiting photosynthesis of small weed seedlings;
- By eating young buds and roots of plant seedlings.
The tadpole shrimp is simply a critter that is in the right
place at the right time, when it comes to annual weed control.
Their eggs, deposited at a depth of about 1-3 cm, can lie
dormant in the soil for many years, hatching within a few
days after paddy fields are flooded. A small portion of the
eggs will remain resting and ready to hatch at a later date
should the current paddy dry up. The young shrimps develop
quickly and may begin laying eggs within ten days of hatching.
Early in the season, shrimps eat decaying plants and animals
in the water and soil, along with germinating weeds and other
small seedlings. At the same time, they eat fungus off of
rice plant stems. They are one of the millions of microorganisms
that periodically work to change inorganic matter into organic
matter to nourish plants. The shrimps may live for up to three
months, but water quality in paddy fields often shortens their
lives to only one-month. Natural enemies such as birds, fishes,
frogs and insects take their shrimp share, too
Tadpole shrimps may be paddy pests where rice is direct seeded,
since rice seedlings are just as tasty as paddy weeds. However,
in transplanted paddy fields, scientists have found that from
25 — 80 shrimps/square meter are able to effectively
control annual weeds such as barnyard grass. At greater densities,
their population soon crashes due to a shortage of food and
to their cannibalistic appetite. A study in 1976 revealed
that weed control in 1 hectare of paddy field required about
500 man-hours of work in the traditional way, about 90 hours
if herbicides were used, and only 20 hours to hand pull remaining
weeds after the action of the shrimps. Although the shrimps
die within one month, the weed control effect lasts until
rice is harvested. The shrimps have little effect on perennial
weeds.
Unfortunately, shrimp numbers are unpredictable with each
flooding. A large number of shrimps are necessary for the
early weed control that will last all season, but you would
need an expert to tell you just how many shrimps you have.
How can you encourage sufficient shrimps after flooding?
One answer is to rotate! To maintain stable shrimp numbers
in paddy fields, the environment must be repeatedly disturbed.
The shrimps do well where paddy fields have been converted
from rice to upland crops for a few years, and then returned
to shallow flooded rice cultivation. In this way, natural
farmers may be able to use crop rotation to encourage sufficient
numbers of shrimps for annual weed control.
Wait! There’s another twist to the story: Scientists
found that shrimp numbers initially increased during the period
when conventional fields were changed to natural farming.
Further investigation revealed that tadpole shrimps could
not live in the low soil pH (4.0 to 6.0) common to natural
farms. By using lime to raise the soil pH, natural farmers
can benefit from the weed control that tadpole shrimps provide.
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