C.S. Lamb

Novel approaches to the estimation of intake and bioavailability of radiocaesium in ruminants grazing forested areas

It is difficult to measure transfer of radiocaesium to the tissues of forest ruminants because they can potentially ingest a wide range of plant types. Measurements on undomesticated forest ruminants incur further difficulties. Existing techniques of estimating radiocaesium intake are imprecise when applied to forest systems. New approaches to measure this parameter are discussed. Two methods of intake estimation are described and evaluated. In the first method, radiocaesium intake is estimated from the radiocaesium activity concentrations of plants, combined with estimates of dry-matter (DM) intake and plant species composition of the diet, using plant and orally-dosed hydrocarbons (n-alkanes) as markers. The second approach estimates the total radiocaesium intake of an animal from the rate of excretion of radiocaesium in the faeces and an assumed value for the apparent absorption coefficient. Estimates of radiocaesium intake, using these approaches, in lactating goats and adult sheep were used to calculate transfer coefficients for milk and muscle; these compared favourably with transfer coefficients previously obtained under controlled experimental conditions. Potential variations in bioavailability of dietary radiocaesium sources to forest ruminants have rarely been considered. Approaches that can be used to describe bioavailability, including the true absorption coefficient and in vitro extractability, are outlined.

The effect of treating pastures with bentonite on the transfer of 137Cs from grazed herbage to sheep

Abstract The persistence of high levels of 134 Cs + 137 Cs in vegetation of certain upland areas of the United Kingdom has prompted investigations into possible methods of reducing levels in sheep tissues. The introduction of the clay mineral, bentonite, into the diet of ruminants has been shown to reduce the uptake of caesium from the gut. The effects of treating pasture with bentonite at two rates (single or repeated application of 80 g m −2 ) on the intake and transfer of 137 Cs to sheep tissues was investigated. Whilst there were reductions in the radiocaesium activity concentrations of tissues of sheep grazing pasture treated at both rates, transfer coefficients were only affected for sheep on the repeated treatment. However, ewes grazing the repeatedly treated pasture experienced a loss in body weight (18%) associated with a decrease in herbage intake (39%). Considerable variation in the herbage intake by individual ewes and lambs, both between and within treatments, showed the need for direct intake measurements in order to determine relevant transfer coefficients in field experiments. Transfer coefficients for 137 Cs of 0.33 d kg −1 and 1·61 d kg −1 were obtained for muscle of control ewes and lambs respectively.

Transfer of radiocesium from different environmental sources to ewes and suckling lambs

Indoor experiments are described that compare the transfer of radiocesium to ewe and lamb tissues from different sources. Lactating ewes were fed either perennial ryegrass contaminated by Chernobyl fallout, or saltmarsh vegetation contaminated by marine discharges from the Sellafield reprocessing plant. The transfer to ewe tissues and milk was greater from the Chernobyl contaminated herbage than from saltmarsh vegetation. Lambs receiving a mixture of vegetation and milk were given radiocesium from one of the two vegetation sources or from milk obtained from the experimental ewes. Transfer to lamb tissues declined in the order milk greater than Chernobyl fallout greater than Sellafield discharge. The radiocesium transfer to lamb tissues exceeded that to ewe tissues. Transfer coefficients for 137Cs in the Chernobyl fallout were higher than most previously published figures at 0.12 d kg-1 for ewe muscle and 0.50 d kg-1 for lamb muscle. The transfer coefficient for 137Cs from ewe milk to lamb muscle was 1.20 d kg-1.

Dynamic distribution of radioisotopes of cerium, ruthenium and silver in sheep tissues

A number of radioisotopes of cerium, ruthenium and silver may be released into the environment and potentially contaminate foodstuffs. For ruminants, data on the absorption, transfer and excretion rates of these radionuclides are sparse and were previously insufficient to allow the development of reliable dynamic models to enable the prediction of contamination levels in food products. In this study, groups of sheep were given single oral administrations of radiocerium, radioruthenium, and radiosilver and slaughtered at various periods up to 1 year. The resulting data were used to develop compartment models to describe the transfer of the three radionuclides to sheep tissues. Values for gut absorption and transfer to muscle (the most important tissue in terms of contamination of the food chain) derived from the models were considerably lower than previously advised values for the radionuclides studied. For all three radionuclides there were certain organs which contained significantly higher activity concentrations than muscle. The models developed allow prediction of the likely levels of these radionuclides in the tissues of sheep as a result of single or continuous contamination events.

A comparative assessment of the potential use of alginates and dietary calcium manipulation as countermeasures to reduce the transfer of radiostrontium to the milk of dairy animals

The potential of using different alginates or supplementary calcium as feed-additives to reduce the transfer of ingested radiostrontium to milk was assessed in dairy cattle fed a haylage/concentrate diet. The feed-additives compared were: calcium alginate (4% by dry matter), sodium alginate (4% by dry matter) and four levels of supplementation with CaCO3. Both alginates reduced the transfer of radiostrontium to milk by 30-40% without effecting diet palatability. However, the high present cost of alginates precludes their use as countermeasures. Dietary calcium supplementation reduced the transfer of 85Sr to milk broadly in agreement with previous predictions. From data relevant to dairy cattle in the United Kingdom it is suggested that dietary calcium intake could be doubled without exceeding recommended maximum intakes, thus decreasing the transfer of radiostrontium to milk by approximately 50%

A model of radioiodine transfer to goat milk incorporating the influence of stable iodine.

Abstract Previously reported models for radioiodine in ruminants cannot account for the effect of variations in stable iodine intake including large countermeasure doses of stable iodine on the transfer of radioiodine to goat milk. A metabolically based model of radioiodine transfer in goats has been parameterised using new experimental data on the effect of countermeasure doses of stable iodine on radioiodine transfer to milk. To account for the effect of dietary stable iodine levels, the model represents the transfer of iodine from the extracellular fluid to milk with Michaelis-Menten kinetics. The model shows good agreement with the experimental data, and the estimated parameters compare favourably with values which can be estimated from the literature. The parameterised model accounts for 95% of the variation in the observed data for milk, faeces, urine and thyroid (n=199). The model has been used to predict the effects of variation in stable iodine intake and the extent of consequent chemical contamination of milk by stable iodine. The time taken for radio-iodine to reach peak concentrations in milk following a deposition event is predicted to vary significantly (ca. 2 days) over a range of expected stable iodine intakes. Doses of stable iodine sufficient to reduce the radioiodine transfer to milk will result in stable iodine concentrations in milk greatly in excess of internationally advised limits. Therefore, we recommend that stable iodine supplementation not be used as a countermeasure to reduce radioiodine transfer to milk. Indeed, model predictions suggest that reductions in stable iodine intake would be a more effective countermeasure. However, this is unlikely to be feasible since the short physical half-life of 131I may not allow adequate time to implement changes in feed manufacture. The model described in this paper is freely available in ModelMaker 3.0 format (http://www.notingham.ac.uk/environmental-modelling/).

Dynamic radiocaesium distribution in sheep: Measurement and modelling

An experiment was carried out to provide a data set specifically designed to allow the development of a model to describe the dynamic behaviour of radiocaesium in sheep tissues. A single intraruminal dose of 134CsCl was given to each of 15 Scottish Blackface ewes, aged 1–2 years. Radiocaesium activity concentrations were measured at various time intervals after dosing in blood, faeces and urine and, after slaughter, in a range of tissues and digestive tract contents. A first order compartment model was developed comprising nine compartments representing various organs and tissues in the body. Rate constants for the model were obtained by optimisation to the experimental results. A limited validation of the model was carried out by using it to calculate tissue transfer coefficients (Ff) and biological half-lives (0.5) of radiocaesium in sheep. The values generated compare favourably with published figures. Applications for the model and constraints on its usage are examined along with possibilities for further development.

Caesium dosing reduces uptake of radiocaesium by sheep

In sheep grazing pasture contaminated by radiocaesium, administration of stable caesium was found to inhibit the uptake of radiocaesium. Eight lambs feeding on a contaminated pasture received an oral dose of 500 mg CsCl and after 6 days had a radiocaesium load 17% lower than that of a control group on the same pasture. The rate of radiocaesium excretion from 4 lambs, in metabolism cages and fed uncontaminated grass, was unaffected by CsCl dosing in comparison with controls. Thus. caesium dosing appeared to inhibit uptake but did not alter rates of caesium loss by normal processes of metabolism. Measurements of stable and radioactive caesium excretion rates were interpreted using a compartment model, based on the hypothesis of two major caesium reservoirs within the sheep, with derived biological half-lives approximately 4 and 25 days, and capacities 20 and 80% of total, respectively. Caesium dosing appeared to saturate the latter reservoir and consequently inhibit further uptake. If a practicable method of application could be achieved, caesium dosing could be useful in the management of sheep and other grazing animals on radiocaesium-contaminated pasture and could find application to radiological protection in humans.

The effectiveness of alginates to reduce the transfer of radiostrontium to the milk of dairy goats

In the event of a nuclear accident the radiation dose to human populations arising from radiostrontium ingested as contaminated milk is a major cause of concern. We report a study to determine if calcium alginate incorporated into the diet can be used as an effective countermeasure to reduce radiostrontium transfer to the milk of dairy goats. When Ca-alginate was included into a pelleted ration at 5% dry weight the transfer of radiostrontium to the milk of the goats was reduced by approximately 50%. No effects on diet palatability or the absorption of iron or calcium were observed. Ca-alginate was readily fermentable and hence its potential binding capacity is likely to be reduced in ruminants compared to monogastrics. The Ca-alginate also supplied additional calcium to the diet in an amount which may explain the observed reduction in radiostrontium transfer to milk. Therefore, currently, we cannot be certain if the effect we observed was due to alginate or calcium.

Transfer of 131I to sheep milk from vegetation contaminated by chernobyl fallout

The transfer of 131I to sheep milk was measured in a controlled feeding experiment using herbage recently contaminated by fallout from the Chernobyl accident. The transfer coefficient (fm) of131I from the Chernobyl-contaminated herbage was 0·29±0·017 day litre−1. The daily proportion of 131I intake which was secreted in milk was 56±0·035%. This is an order of magnitude higher than for cattle and agrees with the higher transfer of stable iodine from plasma to milk which occurs in sheep and goats. At the same time the biological half-life of 131I was measured in ewes which had been grazing outside during deposition of the Chernobyl fallout and were then housed and fed an 131I-free diet. The loss of 131I in sheep milk was described using a double exponential relationship. The calculated biological half-life for the first component in the milk was 1 day, accounting for 97.4% of the reduction in the concentration of 131I activity in the milk. The 131I-free diet had a comparatively high stable iodine content since it was saltmarsh vegetation. however, the calculated half-life was similar to previously estimated values for goats. The transfer parameters presented here, which have been estimated using measured dietary intakes and milk outputs, appear to be the first such dates reported for sheep for an environmentally contaminated source.