Executive Summary
Introduction
This report describes an
RIRDC-funded study of the causes and management of slimes affecting rice
production in the irrigated areas of southern New South Wales. At the start
of the project it was believed that two types, green slime and brown slime,
were problematic and that both were caused by algae – hence the title of
our project. However, green slime is already very well managed and is,
therefore, a minor problem. Bacteria proved to be the major cause of significant
brown slime formation, and this became the major focus of our research.
The occurrence of slime in
rice bays is important because it can reduce the yield of rice by killing
seedlings. Organisms in the slime also compete for nitrogen with the rice
plants. Potential benefits of the research include better yields of rice,
lower fertiliser costs for farmers and possibly further economies in the
use of water.
Prior to this project, there
had been very little research on the effects, causes and management of
slime in Australian rice production. The main work was that of Noble and
Happey-Wood (1987).
Intended Audience
This report provides information
on green and brown slime for rice farmers, agricultural extension workers,
educators and future researchers. It includes descriptions of the components
and causes of slime and recommendations for control, supported by field
observations (by farmers themselves and by researchers) and experimental
evidence gained in our laboratory.
Background
Coherent, floating slime
is common in rice fields. When rice seedlings are small, particularly before
the 3-leaf stage, slime may prevent them from growing through the water/slime
surface layer, reducing rice establishment. Even at later stages, slime
may compete with the rice for nutrients.
Farming practices have changed
in the last 20 years or so. Currently rice production is intensive, with
a crop each year and shorter periods of crop rotation; these two changes
have led to more intensive use of fertilisers, particularly urea. It is
now more common to flood fields before sowing rice, rather than after sowing
into dry soil. Our research suggests that these changed practices have
caused the increase in nuisance slime reported by farmers.
Green slime is caused by
algae – several types of filamentous green and blue-green algae form mats
that float on the water in rice bays. However, existing management practices
deal with this effectively, and farmers were not concerned about green
slime affecting yields. Brown slime is stickier and more tenacious, and
may form mats up to 1 cm thick on the water surface; they may cover large
areas (at least 2m x 50m). These mats may reduce rice establishment, and
are seen as a problem by some farmers.
Previous work by Noble and
Happey-Wood (1987) suggested that brown slime was caused by diatoms – golden-brown
algae with silica cell walls. We found that bacteria cause brown slime,
and that relatively few diatoms are found in it. Farmers variously attributed
growth of slime to the presence of decomposing organic matter, poor water
circulation, poor land preparation and arrival of algae in irrigation water.
Evidence from their study led Noble & Happey-Wood (1987) to conclude
that input of algae from rivers was small and that conditions within rice
bays increased algal populations. In spite of this, farmers we spoke to
were still concerned that algae from rivers or irrigation channels might
be causing slime. We were able to show that the algal populations of this
incoming irrigation water are, indeed, very low.
Aims and Objectives
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Identification and control of
green slime
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Determining the nature of brown
slime, and why and when it is a problem.
-
Determining the conditions that
increase brown slime
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Suggesting strategies that farmers
can use to reduce the brown slime problem
All objectives were achieved.
Methods Used
During the rice growing
seasons of 2002 to 2004:
-
Information was gathered from
rice growers in the western Murrumbidgee, Coleambally and Murray Valley
regions about the occurrence of nuisance slime and about which water and
fertiliser practices had changed over recent years. We collected information
about irrigation supply water from scientists in various relevant organizations.
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Collection of soil and water
samples and analysis of green slime: Samples of water and green algal slime
were collected from farms, from several sites on the Murrumbidgee River
and from irrigation supply channels (2002-3). Algae were identified and
the chlorophyll content of the water measured as a way to assess algal
population size.
-
Collection of soil and brown
slime samples: We visited farmers who had reported brown slime to photograph
the affected rice bays and to collect slime samples for microscopic examination.
We also collected soil samples from bays on a number of farms for laboratory
experiments.
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Microscopic examination of organisms
in brown slime: We used these slime samples to identify the main components
of brown slime.
-
Laboratory experiments on the
effect of flooding and urea on rice growth: Laboratory work allowed us
to use replicates and to test a number of different parameters rapidly
and within our budget.
We did laboratory experiments to determine the effects of sowing into flooded
soil vs. dry sowing and of urea on rice growth.
Having found that bacteria, particularly iron-oxidizing bacteria, were
the main cause of brown slime, we also determined the effects of nutrients
(particularly urea) and iron and the role of bacterial urease (the enzyme
that allows organisms to use urea) on slime formation in culture.
Farmers were concerned about the role of pH in slime formation, so we measured
soil and water pH.
-
Consequences of brown slime
for rice yield: Just before harvest, we revisited farms which had experienced
slime problems during seedling establishment in 2004 to find out if yield
had been reduced.
Results
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Information gathering: Farmers
and fellow scientists were generous in providing information about the
occurrence of slime, the conditions believed to cause them and their effects
on yields. These data have been incorporated throughout the Results.
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Collection of soil and water
samples and analysis of green slime:
The green alga Spirogyra predominated in green slimes. The bluegreen alga
Anabaena, and the green algae Oedogonium and Hydrodictyon were sometimes
present. Information from 11 farmers showed that green slime was common
but could be controlled well by lowering the water level or adding a copper-based
algicide (note that copper harms many organisms); one farmer used Round-Up.
Water samples from the river and the irrigation supply channels showed
very low levels of chlorophyll – that is, numbers of all algae (green and
blue-green algae and diatoms) are very low in the incoming water. The highest
concentration measured was in a rice bay. We conclude (as did Noble &
Happey- Wood, 1987) that algal growth is encouraged, sometimes strongly,
by conditions in the rice bays themselves. No further study of green slime
was warranted.
-
Collection of soil and brown
slime samples: Samples of brown slime from 6 farms contained large populations
of bacteria. They also contained diatoms, but too few to cause the slime
or its colour.
We saw and photographed rice seedlings that were held back by brown slime.
Control measures used by farmers include dropping the water level, minimizing
use of urea and recirculating or flushing the water. One farmer noted that
brown slime is not a problem if the rice is sod-sown. If water is kept
moving, either no film forms or it sticks to the soil, and does not affect
the rice plants.
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Microscopic examination of organisms
in brown slime: Examination of further samples confirmed the role of bacteria
and showed that brown slime contains many types of organisms and also organic
debris, all stuck together by the “biofilm” (biologically produced film)
secreted by iron-oxidizing bacteria. Without the bacteria to produce this
sticky, coherent film, the other components of the slime would remain dispersed
in the water.
Diatoms were present but rare - they were rarer than protozoa and invertebrates.
Green algae were insignificant. Other than the large numbers of bacteria,
the most interesting finding was the presence of deposits of iron oxide
in all but one sample of brown slime. They give the slime its colour and
their presence indicates that iron-oxidizing bacteria are the main slime
forming organisms.
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Laboratory experiments on the
effect of flooding and urea on rice growth: Rice sown into damp soil (dry
sown) grew faster than rice sown into flooded soil for the first 18 days.
Addition of urea before sowing into flooded soil inhibits the growth of
the first 3 leaves of rice seedlings, for the first 8 days after sowing.
This is significant, since rice is vulnerable to being held under by brown
slime until the stronger third leaf is established.
Experiments in which bacteria were cultured in flooded soil collected from
rice bays showed that (a) no biofilm formed in controls (only water added)
and oxidation of iron was slight; (b) addition of phosphate alone did not
cause the formation of a biofilm or oxidation of iron; (c) however, addition
of urea did cause the development of a biofilm and led to the oxidation
of the soil iron, staining the overlying water orange. Increasing the concentration
of urea increased the rate at which iron was oxidized.
Thus bacteria, iron and urea interact to form brown slime. Urease, an enzyme
that breaks urea down, is needed by bacteria to utilize urea. It is not
produced by all bacteria. We showed that adding urea to soil samples increased
the amount of urease present, by selecting for soil bacteria that produce
it. No biofilm is formed unless urea is added to the samples. If iron is
added without urea, iron oxidation occurs but no film is formed. When both
urea and iron are added a strong biofilm is formed and iron is oxidized.
Thus we conclude that for brown slime formation, iron is required to allow
the growth of the iron-oxidizing bacteria that cause the film. However,
without urea, their growth is too slow to form a coherent film.
There was no correlation between pH and the presence of brown slime.
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Consequences of brown slime
for rice yield: We saw one site at which a patch of rice had been lost
due to persistent brown slime. However, the overall yield for the site
was still considered good.
Implications for Stakeholders
Although some farmers were
happy with the control measures they use for brown slime, others had been
less successful. We have shown that bacterial growth is the cause of brown
slime and that urea increases the growth of the relevant iron-oxidizing
bacteria very strongly.
Evidence from our discussions
with farmers and our observations show other factors that are involved
as well (see Recommendations, below), including the presence of still water
during rice establishment.
Thus we have now provided
a rationale for control measures for brown slime. This will benefit rice
growers and hence the whole industry.
Recommendations to rice
growers
Suggested management practices
to reduce brown slime.
-
avoid using urea at rates above
57.5 kg N/ha in slime-prone soils prior to flooding where possible, sow
clover in rotation to fix nitrogen dry sow rice or sow as soon as possible
after flooding
If brown slime does form
-
keep water moving, especially
in corners of bays, to prevent the formation of large slime mats reduce
water levels to keep rice leaves above slime BUT only after rice seedlings
have reached the 3 leaf stage if the slime mats are limited, try raking
the mats and throwing them onto nearby banks
How can farmers tell
whether there will be a likelihood of brown slime?
-
past experience of particular
bays/paddocks soil type; e.g. brown slime is more likely if the surface
of grey soils always turns orange after flooding, as iron is probably abundant.
Many grey soils contain reduced iron.
-
about 3 weeks before sowing,
test soil from bays thought to be likely candidates for brown slime. Test
by adding urea to soil plus water in a glass jar - watch for the appearance
of brown-orange coloured iron oxides in the next two weeks.
Last updated: June 2006
Copyright RIRDC
http://www.rirdc.gov.au/reports/RIC/06-010sum.html
