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Factors affecting radiocaesium transfer to ruminants


A method has been developed to measure the true absorption of radionuclides through the gut of animals.
The methodology is now successfully used by a number of laboratories in different European countries. It has been used to answer localised problems (eg the bioavailablity of radionuclides close to Chernobyl NPP and the bioavailablity of marine discharges from the Sellafield reprocessing plant.

The introduction of At provides a method of estimating the amount of radioisotope absorbed. This should be used to provide imput data to models estimating dose in the event of a nuclear incident. Currently the parameter termed absorption within such models does not always represent the same biological process due to inconsistancy between experimental methodologies.

The extent of soil adhesion is under study in different pasture systems and on various fodders, including monthly sampling of grass and sheep faeces at 2 different locations in Belgium with different pedological characteristics. Winter fodder has also been sampled and soil samples have been taken from all sites when possible.

A study has been started on the effect of feed characteristics on radiocaesium transfer to sheep. A practical method for producing contaminated plant material was tested successfully. Rye grass was sown in polyvinyl chloride (PVC) trays filled with a synthetic, water retaining polymer. The vegetation was contaminated by adding caesium-134 as caesium chloride to the nutrient solution. 3 successive cuts were possible, the first one containing more than 16% of the radioactivity initially introduced into the system.

An in vitro digestion method was used to investigate whether such a technique could be used to obtain a rapid estimation of the bioavailability of radiocaesium associated with plant material. Italian rye grass, red clover and maize were internally contaminated by adding caesium-134 chloride to the nutrient solution. Extractions were performed on fresh, dried and frozen material. The digestibility of grass and clover was found to be similar and there was no difference between the 3 states of material apart from maize where the digestibility of frozen material was higher than that of dried and the fresh was lowest. The extractability was high for all species and all treatments. This extraction procedure was also carried out on maize which had been fed to sheep in an earlier in vivo experiment. In this experiment the availability of radiocaesium incorporated into maize was 25% lower than that of ionic radiocaesium. The caesium extractability measured in vitro, however, was comparable to that of ionic radiocaesium added to uncontaminated dry maize.

A study has been done to estimate the availability to adult sheep, in terms of the true absorption coefficient, of radiocaesium present in 2 vegetation types, heather and hill grass. Both contained mainly caesium-137 which had originated from the reactor accident at Chernobyl. Caesium-134, administered as a continuous intravenous infusion, was chosed as the means ofobtaining an estimate of the transfer of dietary radiocaesium across the gut wall. 8 mature Scottish Blackface ewes were used and the caesium-134 infusion, feeding of contaminated vegetation and excreta collections continued for 7 days. Samples of vegetation, faeces, urine and caesium-134 infusion solution were subjected to gamma analysis using a germanium detector.
2 methods of estimating the true absorption coefficient will be compared. In the first method, the coefficient is estimated as the guotient of the plasma caesium-137 turnover rate and the dietary caesium-137 intake rate. In the second, it is estimated from measurements of the intake, total faecal excretion rate and the faecal excretion of endogenous caesium-137. Results have been obtained from the first method and suggest that heather is less available. The true absorption coefficient for the heather was 0.67 +/- 0.034 and for the hill grass 0.88 +/- 0.093.

Work has been started to examine the behaviour of radiocaesium in the gut of sheep using 2 dietary sources of caesium-134, ionic and that present in an organic soil 2 years after injection. A grass diet and a cereal based concentrate diet will be used. The first experimental period has been completed with 8 mature Scottish Blackface ewes fitted with a ruminal fistula and a T shaped cannula at each end of the small intestine. Results are not yet available.

A study is being undertaken of the availability of radiocaesium associated with 2 different soil types for absorption in the sheep gut and its transfer to sheep milk and of the extent of soil adhesion on fodders and grazed pastures. Sampling sites were selected of: lowland alFalFa grassland; lowland alFalFa Fescue grassland; lowland maize; upland polyplyte meadow; upland Dactylis glomerata grassland; and upland grazed pature. Vegetation, superficial soil layer and fodder from the relevant farms have been sampled. The vegetation and soil characteristics have been determined and the meteorological conditions (rainfall and temperature) continuously monitored. Radiocaesium content of all samples will be carried out.

The availability of radiocaesium associated with ingested soil for transfer into body tissues of grazing animals was assessed by indoor feeding trials using 2 types of contaminated soil (clay and loam) and sheep housd in metabolism cages. 6 Bergamasca ewes were used and, after an initial adaptation period, the contaminated soil was administered orally each day for 33 days. Caesium-137 activity concentrations of milk and blood samples were determined by gamma spectrometry. For each soil treatment, caesium-137 activity concentrations in milk reached an equilibrium after only a few days. Both of the soil types had a day content exceeding 10%. Initial transfer coefficient values have been obtained of 3.5E-4 d/kg for the day and 6.2E-4 d/kg for the loam. These comparatively low values suggest that radiocaesium was not removed from the binding sites on the clay minerals in the sheep gut.

A soil adhesion sampling programme has been carried out in Ireland. Samples of permanent pasture and 3 winter fodders (hay silage and fodder beet) were taken from both mineral and organic soils. Permanent pasture mineral soil sites were compared in an upland and a lowland situation. Pasture vegetation, soil, sheep faeces and winter fodders, at harvesting and in storage, were all sampled. Rainfall was recorded for each site prior to each sampling event. Physical and chemical characteristics were determined for soil samples taken from a profile down to 30 cm deep at each site. All samples have been prepared and analyzed for the radionuclides caesium-137, caesium-134 and potassium-40.

The levels of radionuclides in the vegetation and faecal samples have reflected the levels in the associated soil. They have shown seasonal trends which follow the patterns of growth and senescence of the vegetation. Uptake of caesium-137 and caesium-134 by vegetation was lower in the well managed, mineral soils, which are treated with potassium fertilizer, than on the low quality, upland mineral sites and the organic soils. Initial comparisons of radionuclide levels in corresponding soil and pasture samples suggests that significant soil adhesion is occurring. A comparison of radionuclide levels in pasture vegetation and faeces indicates that soil contamination of fodder also seems to be occurring, probably by direct consumption of soil during grazing. Specific farming practices are suggested as having a role in the extent of soil contamination of winter fodder crops.

The effect of the stage of lactation in ewes on radiocaesium uptake is under study. The 3 week experimental protocol includes one week of acclimitization, one of contamination and one of decontamination. 200 ewes were selected for the experiment and about 75% of milk samples obtained have been analyzed for radiocaesium concentration. Data has also been collected which will enable investigation of the influence of several other factors affecting the transfer coefficient.

2 pastures grazed by sheep, one of predominantly clay soil and the other sandy, were selected for a soil adhesion study. A detailed soil analysis was conducted and daily precipitation data recorded. Samples of soil, grass and faeces were collected monthly from both pastures. Appreciable quantities of titanium were detected in the soil samples, 20 ug/g in the grass samples and 70 ug/g in the faeces samples. Approximately half the samples collected have been processed.

20 kg of heavily contaminated surface soil were collected from a site near the Chernobyl reactor and dried and seived. Gelatin capsules of the processed soil have been fed to lactating ewes in a soil ingestion study. Samples of milk, faeces and urine were collected and will be measured for radiocaesium activity concentrations.

A general multiple compartment model for the transport of trace elements through animals has been devised and implemented in a computer programme. It is particularly suited for the description of milkaccumulation in the mammary gland.

The RUINS package is intended to simulate transfer of radiocaesium in the soil, pasture and grazing animal agroecosystems. This package has been developed to demonstrate its sensitivity to factors related to the adhesion of contaminated soil to vegetation surfaces. The factors studied are: availability of radiocaesium in soil relative to plant incorporated material; fraction of vegetation radiocaesium which is attributed to soil contamination; and importance of soil type and the differing availability from various soil components. Simulations have been performed to test the importance of the assumption that radiocaesium fixed in the soil will behave differently in the gut of an animal than the labile or exchangeable radiocaesium.

A multicompartment model of lamb radiocaesium metabolism continues to be developed and validated. Improvements are being made to the dynamics of absorption and secretion within the gut. Work is under way to extend the animal model so that its parameters are based upon the underlying animal physiology rather than arbitrary fits. A model based on well founded physiological principles can be applied to animals other than sheep and will provide a sound theoretical framework within which counter measures could be developed and understood.

In parallel with the modelling programme, a limited experimental programme is under way to characterize the cellular uptake and efflux using standard in vivo culture techniques.

The uptake and transfer of radiocaesium from plants to sheep grazing freely in a natural ecosystem was studied. A flock of free ranging sheep have been followed during the grazing period in a mountainous area of Sweden. The botanical composition of ruminal samples taken at slaughter of the lambs has been investigated. A comparison with the previous year showed that the intake of ferns and mushrooms was greater in 1990 than in 1989. This difference in feed intake may explain the higher radiocaesium levels found in mutton in 1990 compared with 1989.
Recent reviews of the transfer of radiocaesium to ruminants after the Chernobyl accident have shown that there are a number of animal and dietary factors which may effect the transfer of radiocaesium. However, there is insufficient information available on the importance of these factors to enable predictive models to take them into account. Similarly, few attempts have been made to understand the relationship between radiocaesium levels in animal feed and tissues in terms of metabolic processes and mechanisms. Such an understanding may help to improve the effectiveness of countermeasures applied after environmental releases of nuclides. This 2 year research programme investigates some of these factors. The participants work in 10 laboratories in 7 different countries spread throughout western Europe and Scandinavia.

Although a limited number of experiments using cattle will be conducted, the sheep is being used as the model ruminant since it is universally important throughout EEC and Scandinavian countries, easy to handle and relatively inexpensive to use in large numbers. However, in subsequent years we hope to extend studies of factors which we have found to be comparatively important for sheep to other ruminants.


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Natural Environment Research Council
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Windermere Road
LA11 6JU Grange-Over-Sands - Cumbria
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