Transfer of Fallout Strontium-90 to Cows’ Milk

Transfer of Fallout Strontium-90 to Cows' Milk E. P. HARDY, Jr. and J. RIVERA Health and Safety Laboratory, U. S. Atomic Energy Commission, New York, New York Abstract

From 1962 through 1966, the monthly ~°Sr fallout rate, the accunmlated deposit, and the concomitant ~Sr levels in animal forage and milk were measured in samples collected at the Colorado State University dairy farm in Fort Collins. During the growing period, 42% of the total ~Sr deposit was retained on alfalfa hay at the first cutting, 29% at the second cutting, and 23% at the third cutting. The difference between the first cutting and the two following was significant. These deposition coefficients combined with values for the area of forage utilized by the animaI can be used to predict the contamination level of alfalfa hay from the measured total deposit. The total intake of ~°Sr and the amount secreted in milk were measured in several groups of dairy cows. The transfer coefficients of ~°Sr from diet to milk ranged from 0.05-0.09% per liter of milk per day and were in agreement with tracer and field experiments reported by other investigators. By using the appropriate deposition coefficient, forage yield, forage intake, and diet to milk transfer coefficient, the amount of ~°Sr in milk was calculated from the measured total 9°Sr deposit over the growing period. The calculated ~°Sr in milk levels agreed reasonably well with measured values. For example, during the 1963 growing season--a period of relatively high fallout rate--the calculated amount of ~°Sr in milk was 1S pCi per liter based on a measured deposit of 2.66 nCi ~°Sr per m -~. Milk samples collected during the same period and analyzed for ~°Sr showed 20 pCi per liter. Since ]962, studies have been carried out by the Department of Animal Science at Colorado State University to delineate the transport steps by which fallout ~'~Cs passes from fallout to milk. The data generated by these studies have been used to empirically derive transfer coefficients for predicting contamination levels of various segments of the food chain (17, 18, 20). During this time, precipitation, soil, herbReceived for publication February 19, 1968.

age, and milk samples were nmde available to the Health and Safety Laboratory for 9°Sr analyses to provide similar information on this fission product. The pertinent data summarized in this report are directed toward the following objectives: 1) to determine the fraction of fallout "°Sr retained on alfalfa hay (the principal diet of these cows) during the growing season, 2) to determine the fraction of the total ~°Sr intake that appears in milk, and 3) to apply these so-called transfer coefficients and known agricultural parameters to predict intake and milk ~°Sr levels. Experimental Methods

Two groups of cows were used in the experiments at Fort Collins. One group of :11 high milk producing Holsteins was allowed to pasture within restricted areas. These areas consisted of ten acres of irrigated brome grass and alfalfa pasture divided into three sections for rotational grazing. The pasture area was floodirrigated four times during the summer months. The amount of ~°Sr added to the soil by irrigation water was less than 5% of the total accunmlated deposit. The second group of cows, consisting of 80 animals of the Holstein, Jersey, Guernsey, and Brown Swiss breeds, was maintained in a dry lot. Monthly fallout was collected using t I A S L designed funnel-ion exchange column devices. Soil cores were taken from the surface to 8 in. at four undisturbed soil sites on the University campus during April and October of each year. Weekly composites of alfalfa hay and green chop were made available as well as weekly composites of milk from both the pasture and dry lot animals. Supplemental feed such as grain also was analyzed. Parameters related to animal management practices at Fort Collins such as dry matter intake, area of foliage utilized by the cow, forage yield, and milk production are shown in Table 1. These values were used to convert from measured fallout per unit area to intake and milk secretion. Results and Discussion

Fraction of fallout retained on alfalfa hay. Average soil ~°Sr values for four undisturbed areas on the University dairy farm are shown in the last column of Table 2. The error term 1210

9°STI~ON T I U I ~ F A L L O U T

TABLE 1. Dairy herd management at Colorado State University. Total dry nlatter intake Type feed

Dry lot b

Pasture ~

--(kg/cow/day)-2-8 2 2-8 8-10 7-9 4-5 14-20 19-22

Pasture Green chop IIay Grain Total

Area of forage utilized 20 30 nl2 per cow per day Forage yield Cutting 1 2 3

Growing period from to May 1 June 16 July 21

No. of (lays

June 15 July 20 Aug. 31

45 35 40

kg--Dry per m~ 0.340 0.225 0.210

Milk production 17-28 liters per cow per day ~ ] 0-I 2 grazing a~imals. b 80 animals. is the coefficient of variation about the mean of the values at the four sampling sites. Using the first soil value obtained in April, 1962, as a base, the monthly deposition values were summed (column headed Cmnulative Total) for comparison with the other measured ~°Sr levels in soil. The cumulative ~"Sr deposition at F o r t Collins based on soil measurements and monthly precipitation collections are graphically illustrated in Figure 1. This figure also shows the deposition rates during the pasture season (April-October) and during the times the animals were on dry lot feeding (NovemberMarch). Deposition increments over seasonal TABLE 2. Cumulative 9°Sr fallout at Colorado State University Year

Month

Cumulative total ~

--(nCi 1962 1963 1964 1965 1966

April October April October April October April October April October

26 31 32 45 46 54 55 57 58 59

Soil b

9°Sr/m-~)- 26± 6% 34--+11% 38±10% 40-+ 3% 46-+10% 52± 9% 54± 4% 56-+ 5%

~ Summed monthly fallout measurements starting with April, 1962, soil value. b Samples taken fronl undisturbed soil areas. Error term is the coefficient of variation about the mean of four sampling site values.

121i

or growing periods were determined from the monthly fallout measurements. During the experimental period, the fallout rate was highest in 1963 as a result of the intensive weapons testing carried out by the United States and the U S S R from the fall of 1961 through the end of 1962. The preferential incursion of stratospheric debris into the troposphere during the early spring of each year is reflected in the markedly higher fallout rate during the pasture season as compared to the time the entire herd is on dry lot. Strontium-90 levels in composite samples of alfalfa hay harvested three times during the pasture season from 1962 through 1966 are given in Table 3. The corresponding total ~°Sr deposition and the calculated deposition coefficient (the fraction of the total ~°Sr deposited during the growing period which remained on the hay at the time of cutting) are also shown in this table. A significant difference between the average deposition coefficient for first cutting hay and the second and third cuttings was demonstrated at the 95% confidence level by a nonparametric rank test (19). On the average, a larger proportion of fallout ~°Sr was retained on first cutting hay than on subsequent cuttings. The deposition coefficients (Table 3) derived from the fallout and forage contamination measurements in this work are somewhat higher than those reported by other investigators (8, 15, 16). This may be owing to lower rainfall during the growing sea~on at F o r t Collins than at areas cited in these investigations, o1" to differences in exposure periods. These higher deposition coefficients could not he attributed to uptake from the soil, since the maximum soil contribution to the contamination of alfalfa hay in these experiments was only 3% for the first cutting. It should be emphasized that the deposition coefficient as used in this report expresses only the net effect over a growing period and does not separate the effects of dry and rainy periods. Several investigators have demonstrated that precipitation removes recently deposited radioactive strontium (13, 16) and ~S~Cs (7, 18, 20) from pasture grasses. Furthermore, it was recently shown that ~ C s levels in alfalfa hay actually increased during rainless periods and it was concluded that dry fallout was the primary means of contamination in the F o r t Collins area (20). I t is highly probable that '~°Sr deposited during the growing season at F o r t Collins is affected in the same way (16). I t is proposed that the average 9°Sr deposiJ. DAIRY SCI~SCE ¥0~. 51, NO. 8

1212

CUMULATIVE

60e

el

JR. AND

IIARDY

RIVERA

:

M O N T H L Y VALUES SOIL

i

@

G

5O

E 0

40

o. ~=

U'J

50

0

0,,

f

20

u

SEASONAL RATE FROM MONTHLY VALUES

l= I0 ¸

PAST0 E I

IPASTO ! DRY LOT I

1962

ASTO E

DRY LOT I

196:5

PAST0 E DRY LOT

DRY LOT

I

1964 YEAR

I

1965

1966

FIO. 1. Strontium-90 deposition at Fort Collins, Colorado, based on soil measurements ((2)) and monthly precipitation collections ( ). tion coefficients r e p o r t e d here, covering the growth periods of three hay cuttings, be used to predict the contamination of forage if the fallout during the growing period is known. Fraction of total '°Sr intake in milk. D u r i n g 1962 and 1963, amounts of pasture, hay, grain, and green chop consumed by groups of animals on pasture or in dry lot as well as the milk output were measured. Each feed component in the diet and the milk was analyzed for ~Sr. The daily calcium intake ranged f r o m 140 to 200 g. Strontium-90 transfer coefficients f r o m diet to milk are expressed in three ways in Table 4. I n the first column of data the observed ratio

or OR (2) is defined as the ratio of pCi ~°Sr p e r gram Ca in milk to pCi 9°Sr p e r g r a m Ca in diet. The percentage in milk of the 9°Sr ingested is given in the second data column. The last column is the percentage of the intake secreted per liter of milk p e r day. This latter value is subject to less fluctuation than the percentage secreted (17) and therefore is the p r e f e r r e d value to use for prediction purposes. The coefficients for t r a n s f e r of 9°Sr f r o m the cow's diet to milk (Table 4) are based on measurements of the animal's total diet. These coefficients, expressed as the OR, per cent 99Sr intake in milk, or the per cent of intake per

TABLE 3. ~°Sr deposition during growing periods.

Alfalfa hay

Total

Dep. coeff.

Year

nCi ~°Sr/kg

nCi 9°Sr/m~

nCi 9°Sr/m:

1962 1963 1964 1965 1966

2.24 5.33 3.30 1.59 0.46

0.76 1.81 1.12 0.54 0.16

2.47 3.95 2.35 1.05 0.50

0.31 0.46 0.48 0.51 0.32 Avg 0.42 -+ 21%

Second cutting hay 1962 1963 1964 1965 1966

1.20 4.71 0.54 0.57 0.24

0.27 1.06 0.12 0.13 0.05

1.12 2.66 0.55 0.35 0.25

0.24 0.40 0.22 0.37 0.20 Avg 0.29 -+ 31%

0.52 1.53 0.50 1.07 0.48

0.11 0.32 0.11 0.22 0.10

0.84 1.48 0.70 0.55 0.40

0.13 0.22 0.16 0.40 0.25 Avg 0.23 -+- 43%

First cutting hay

Third cutting hay 1962 1963 1964 1965 1966 J. DAIRY SGIEiWCE ~V-OL. 51, NO. 8

g°STRONTIU~[ FALLOUT

TABLE 4. Coefficients for transfer of ingested °°Sr to milk.

Period 1962 pasture animals 1963 pasture animals 1963 dry-lot animals

OR No. of milk/ animals iatake

% '°St intake per % ~°Sr liter intake milk in milk per day

12

0.10

1.2

0.051

12

0.10

1.7

0.074

5

0.14

1.6

0.087

liter of milk per day, agree reasonably well with reported tracer studies (4, 6, 9) and with values based on field experiments (1, 8, 11). Other investigators have pointed out that part of the 9gSr secreted by the cow may come from the body as well as from the immediate diet (2, 11, 12) and that the magnitude of the contribution from body stores depends on the calcium intake (2). The daily calcium intake of the animals at F o r t Collins was relatively high (140-200 g per day), which means that most of the Ca and 9°Sr in the milk came from tlle ingested feed (4). Prediction of i*~take and milk levels. The diet of dairy cattle is composed to a large extent of forage consumed either in pasture or dry lot. I f the area of forage utilized by a cow can be estimated, then the °°Sr intake of an animal can be calculated from total deposition measurements and deposition coefficients. F o r example, in 1963 the ~°Sr deposit from June 16 to July 20 was 2.66 n C i / m ~. The average deposition coefficient for second cutting hay is 0.29 (Table 3) and a good estimate of the area of forage utilized by each animal per day is 25 In~ (20). Multiplying these values together yields 19 nCi g°Sr ingested daily by each animal over the period in question. The measured amount of ~°Sr consumed was 20 nCi per day. The contanfination of cow's milk by fallout is affected by the retention of deposited nuclear debris on edible herbage, dilution of initially contaminated herbage with new growth, washoff of fission products from edible tissues of the plant, and methods of animal management (1, 5, 10, 11, 14). The ~°Sr level in milk during the pasture season can be empirically predicted if the fallout during the growing period of the forage in nCi per m ~ (A) is measured. From the known deposition coefficient (B), the forage yield in kg per m "° (C), the forage intake in kilograms dry matter per day (D), and the milk transfer coefficient in per cent per liter per day ( E ) , the average amount of ~°Sr in

!213

milk in pCi per liter is equal to A × B / C X D × E × 10. The 10 in the equation is a scale factor to convert from nCi to pCi and to eliminate the per cent term. F o r example, the measured 9°Sr deposit during the second growing period in 1963 was 2.66 nCi per m ~, B ---- 0.29, C ----0.225, D = 7, and E = 0.07. The calculated amount of g°Sr in milk is 18 pCi per liter, whereas the measured amount was 20 pCi per liter. The relationships reported here between fallout and forage contamination and diet to milk secretion pertain primarily to conditions of irrigated farms in arid regions, and may not be applicable t,o some other areas for prediction purposes. Wide variations exist in animal management practices and climatological conditions in the dairy farming regions of the United States; g°Sr fallout levels at F o r t Collins are about one-half of those observed in the northeastern United States and about twice those in the southwestern United States. Nevm~heless, each experimental situation contributes to a general description of relationships between deposition and contamination of the food chain. As knowledge of effects of a wide range of climatological and agricultural parameters is accumulated, it may be applied to larger areas to estimate the radiation doses to man from exposure to specific radionuclidcs such as °°Sr.

Acknowledgments The authors are grateful to Dr. GeraJd Ward of the Department of Animal Science at Colorado State University, who provided Che samples and contributed many helpful suggestions. Dr. Daniel Wilson and Dr. James Johnson, who worked with Dr. War/[ on the experiments at Fort Collins, also gave us their fullest cooperation.

References (1) Burmann, F. J., G. F. Fries, M. J. Anderson, and G. E. Stoddard. 1966. Influence of grazing intensity on radiostrontium concentrations in milk. J. Dairy Sci., 49: 1219. (2) Comar, C. L. 1963. Factors influencing the biological availability of fallout radionuelides for animals and man. Federation Proe. (Amer. Soc. Exptl. Biol.), 22: 1390. (3) Comar, C. L,, 1%. I-I. Wasserman, and M. M. Nold. 1956. Strontium--calcium discrimination factors in the rat. Froc. Soe. Exptl. Biol. Med., 92: 859. (4) Comar, C. L., R. N. Wasserman, m~d A. R. Twardoek. 1961. Secretion of calcium and strontium into milk. Health Phys., 7: 69. (5) Cox, G. W., A. Morgan, and 1~. S. Taylor. 1960. Strontium-90 from fallout in the J . DAII~Y SCIENCE VOL. 51, NO. 8

1214

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

HARDY JR. AND RIVERA diet and milk of a dairy herd. J. Dairy Res., 27: 47. Cragle, R. G., and B. J. Demott. 1959. Strontium and calcium uptake and excretion in lactating dairy cows. J. Dairy Sci., 42 : 1367. Csupka, S., M. Petrasova, and J. Carach. 1967. Seasonal variation in the concentration of caesiunl-137 in grass and alfalfa. Nature, 213 : 1204. DeBortoli, M., P. Gaglione, A. Malvillcini, and E. Van der Stricht. 1966. Five years' experience of Strontium-90 and Cesium-137 herbage to milk t r a n s f e r under field conditions. Intern. Radiation Protection Ass. Meet., Rome, September, 1966. Garner, R. J., and B. F. Sansom. 1959. T r a n s f e r of 131I and SgSr from diet to milk in cattle. Vet. Record, 71:670. Hansen, W. G., J. E. Campbell, J. H. Fooks, H. C. Mitchell, and C. H. Eller. 1964. F a r m i n g practices and concentrations of fission products in milk. Public Health Service Pub]. no. 999-R-6. Kahn, B., I. R. Jones, C. R. Porter, and C. P. Straub. 1965. Transfer of radiostrontium from cows' feed to milk. J. Dairy Sci., 48 : 1023. Kirchmann, R. 1965. Relationship observed under natural conditions between the concentrations of fallout-induced ~Sr in grass and milk. Atompraxis, 11: 382. Kircbmann, R., E. F a g n i a r t , and S. V a n Puymbroeck. ]967. Studies on foliar con-

J. DAI]~YSCIENe~ VOL. 51, NO 8

(14)

(15)

(16)

(17)

(18)

(19)

(20)

tamination by radiocaseium and radiostrontium. Radioecological concentration processes. Aberg, B. and Hungate, F. P., eds. 475-483. P e r g a m o n Press, Great Britain. Koranda, J. J. 1965. Agricultural factors affecting the daily intake of fresh fallout by dairy cows. UCRL Publ. 12479. Kreiger, H. L., ]3. Kahn, and S. Cummings. 1967. Deposition and uptake of °°Sr and 137Cs in an established pasture. Radioecological concentration processes. Aberg, ]3., and Hungate, F. P., eds. Pergamon Press, Great Britain. Milbourn, G. M., and R. Taylor. ]965. The contamination of grassland with radioactive strontium. Rad. Bot., 5 : 3 3 7 . Ward, G. M., J. E. Johnson, and N. F. Stewart. 1965. Cesium-137 passage from precipitation to milk. Radioactive fallout from nuclear weapons tests. AEC Syrup. Series 5. CONF-765. Ward, G. M., J. E. Johnson, and D. W. Wilson. 1966. Deposition of fallout Cesium137 on forage and t r a n s f e r to milk. Public Health Repts., 8: 639. Wilcoxon, F., and R. A. Wilcox. 1964. Some rapid approximate statistical procedures. Lederle Laboratories, Pearl River, New York. Wilson, D. W., G. M. Ward, and J. E. Johnson. 1967. Fallout 1~7Cs: Direct aerial transfer as an important source of fo]iar deposition. Rad. ]3ot., vol. 7.