Influence of Grazing Intensity on Radiostrontium Concentrations in Milk

Influence of Grazing Intensity on Radiostrontium Concentrations in Milk FRANZ J. B U R M A N N 1

U.S. Department of Health, Education, and Welfare, Cincinnati, Ohio GEORGE F. FRIES~

U.S. Department of Agriculture, Beltsville, Maryland and MELVIN J. ANDERSON and GEORGE E. STODDARD

Department of Dairy Industry Utah State University, Logan Abstract

The effect of grazing intensity on radiostrontium concentrations in milk was studied by comparing milk of two groups of Holstein cows grazed rotationally. One group was included in the main herd, on a normal rotational grazing system, and the other group followed the main herd one plot behind. Milk samples from the group following the main herd showed a significant increase at the 1% level in strontium-90 units and strontium-89 units. A significant, positive linear correlation for S.U. 90 and S.U. 89 in milk between the groups was obtained. During the experimental period, a herd of Holstein cattle without access to pasture, consuming a weighed amount of feed, secreted 1.5% of their total strontium-90 intake into the milk. The resumption of nuclear testing by the U S S R in September, 1961, made possible the investigation of the transfer of weapons-test debris into food. I n August, 1962, a cooperative study was initiated by the U. S. Public Health Service, the U. S. Agricultural Research Service, and the Agricultural E x p e r i m e n t Stations at the University of Minnesota, Iowa State University, and Utah State University. The first phase of the study was an investigation of radioiodine concentrations in milk for various feeding and management systems (6). I n the second phase, which began in 1963, attention was focused on radiostrontium. As p a r t of the over-all objective, a corollary study was designed to determine whether the grazing in-

tensity by itself, i.e., the length of time a given area is grazed, has an effect on radiostrontium concentration in milk. Radionuclide concentrations in milk depend on a multitude of factors, such as rate of contamination of feed, seasonal influences, changes in feeding practices, kinds of pasture, and pasture management (9, 13). By knowing the important factors involved, one hopes to predict future levels and to initiate p r o p e r procedures when necessary. To reduce radionuclide concentrations in milk, a number of courses are currently being suggested. One effective method is to feed uncontaminated forage. Optimum fertilization of pasture also has 'been proposed (9, 16). To determine the effectiveness of these procedures, an Mteration of the environment or an abrupt change in the feeding practice of the animals is required. This study was designed to deteIznine whether the intensity of grazing is one of the factors affecting radiostrontium concentration in milk. Experimental Procedure

Eight Holstein cows were divided into two comparable groups by age, weight, and stage of lactation. Because milk-calcium levels fall during the first 5 wk of lactation (11), only cows that had passed the second month of lactation by the beginning of the experbnent were selected. No significant change of weight occurred in the cows during the experiment. Mille production for both groups was approximately 25 kg (55 lb) p e r cow per day at the beginning of the experiment and approximately 18 kg (40 ]b) per cow p e r day at the end. Semimonthly milk fat tests for the various cows showed no difference of the average for both groups. One group, referred to as top grazers, was included in the main herd in the usual rotational grazing system practiced at Utah State University. Cows in the second group~ referred to as bottom grazers, followed the top grazers in the same rotation, but al-

l~eceived for publication April 25, 1966. Division of Radiological Health, Public Health Service, t~obert A. Taft Sanitary Engineering Center, Cincinnati, Ohio. 2Animal Husbandry l~esearch Division, Agricultural ttesearch Service, Beltsville, Maryland. 1219

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ways one plot behind (2). To evaluate the effect of grazing intensity on the radiostrontium concentration in milk, the two herds were managed in identical ways, with the following exception: The main herd was grazed as always, but moved to a new plot early enough so that the immediately following bottom grazers had adequate pasture available. The pasture was divided into 16 experimental plots. During the 12 wk of the study, 37 changes were made. I n addition to pasture, 36 kg (80 lb) of hay and 36 kg of concentrate were fed daily to the eight cows as a group. I n case the bottom grazers had been grazed on uusatisfactory pasture, they could have received their fill by consuming more hay and concentrate, thereby lowering their radionuclide intake. The location of the herd governed whetimr the cows had access to spring, irrigation, or city water; therefore, 3.8 liters (1 gal) of water was obtained weekly from each of the sources possibly used by the cows. Adjacent to the grazing area, as many precipitation samples as possible were collected. Pasture samples consisting of the cut of 0.8 sq m (1 sq yd) from three random areas were collected one day before the main herd entered a new pasture. The green material was immediately weighed, dried and again weighed. Three random samples of each lot of hay and concentrate were collected, using the standard sampling procedure of the Department of Dairy Industry. Two air samplers collected air-particulate samples continuously for each 24-hr period; the samples were measured five days after collection with an end-window proportional counter. The air samplers were located approximately 4.5 m above the ground on top of a storage shed adjacent to the pastures. A composite milk sample was obtained twice weekly. Samples of 470 ml (1 pt) taken from the total milking from each cow in the study, for both evening and the following morning milking, were eomposited into the 3.8-liter sample from each group. I n addition, three Holstein cows, on a constant diet for at least one month at the university, were utilized to determine the per cent of radiostrontium transferred into the milk and to make certain there were no effects on the radiostrontium concentration in milk during the study, due to any factor other than the diet of the experimental groups. These animals received hay ad libitum and were fed a 1-to-4 grain to 4% fat-corrected milk produced in excess of 9 kg (20 lb) per day. Samples were obtained as previously outlined. Sample collection and animal management

ET AL

were under the supervision of the staff of the Department of Dairy Industry, Utah S~ate University, Logan. All samples were sent to Radiological Health Research Activities, Robert A. T a f t Sanitary Engineering Center, for radionuclide analysis. Aliquots from the samples were sent to the Agricultural Research Center, Beltsville, Maryland, for calcium analysis. Results and Discussion

The strontium-90 concentration of the milk from the control herd was 10.7 ± 0.8' picocuries (pCi) per liter for 23 observations, and the strontium-89 concentration was below the minimum detectable level of 5 pCi/liter. Total milk production for the entire experimental period, from June 4 to August 23, 1963, was 5,825 liters (12,445 lb), with a milk-fat average of 3.5%. During the same period the group consumed approximately 4,000 kg (8,800 lb) of hay and approximately 800 kg (1,750 lb) of grain. The strontium-90 secreted into the milk was calculated to be 1.5%, agreeing well with previous, published data (3, 10). F r o m June 3 through J u l y 8, approximately 6,000 pCi of strontimn-90 per square meter was deposited by precipitation. The contribution of dry deposition is not known. The beta activity of air particulates collected till June 29 fluctuated between 8 and 29 p / C i / m ~ of air; however, the June 30 air filter contained 44 p C i / m '~, a rapid decrease from the 16 p C i / m a of the previous day. The following decrease was not quite so rapid. The sample collected on J u l y 1 contained 33 p C i / m ', and the one obtained on July 2 had 18 pCi/mL Thereafter, the beta activity remained below 20 pCi/mL All drinking-water samples analyzed contained less than 5 pCi strontium-90 and less than 5 pCi strontium-89 per liter. Availability of different water sources did not affect the study, because of the low radiostrontium concentration in all of the water. Assuming a 50-liter consumption of water and 5 pCi strontium-90 per liter, this source would have contributed only 0.5% of the total intake for the control group, and less for the experimental herds. Two batches of concentrate were fed. One contained an average of 46 S.U. 90 (pCi strontium-90 per gram calcium), and the other, 48 S.U. 90. The average S.U. 90 for hay was approximately 120. The S.U. 90 and S.U. 89 (pCi strontium-89 p e r gram calcium) in milk for both groups are shown in Figures 1 and 2, respectively. Beginning with Point A, the milk of the bottom ~2

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Fro. 1. S.U. 90 for top and bottom grazers. grazers contained higher S.U. 90 and S.U. 89 values for approximately one month. The only exception is Point C, where identical S.U. 89 values were obtained. The period between B and D was without rain except for a trace, less than 0.2 ram, occurring on the morning of August 4. Radiochemical analysis of the sample received indicated a deposition of less than 200 pCi strontium-90 p e r m ~. The last rain before this period, 0.76 ram, occurred J u l y 8, but no sample was collected. An analysis of variance was performed, to determine whether there was a statistically significant difference between the milk strontium units of the top and bottom grazers. A f t e r it was determined that a significant difference existed for both S.U. 90 ( F = 11.1 P < 0.005) and S.U. 89 (t~ = 8.5 P < 0.01) levels, the data were classified into two groups. The selection was based on whether precipitation occurred during the collection period. No significant difference was evident during' the rainy period, probably because of the frequent pasture J

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changes; however, the difference in milk levels for the period without precipitation was significant for S.U. 90 ( F = 57.6 P < 0.005) and S.U. 89 (1~ = 14.7 P < 0.01). The same periods were used to determine the correlation coefficient and its degree of significance. I n all cases a significant positive correlation at the 99.9% confidence level was obtained. The consistent fall of S.U. in milk from Point B to Point D is most likely the result of two effects: first, the removal of activity by the grazing cattle, and secondly, by the dilution effect of new growth. The reason for the consistently higher S.U. 89 and S.U. 90 in the milk of the bottom-grazing group could not be determined with certainty in a limited study such as this. One explanation, consistent with the results, is that cows forced to graze more closely to the ground and possibly graze larger areas may consume more mat material than the topgrazing cows. I t has been reported that the mat horizon, sampled to a depth of 2.5 to 4 cm below the soil surface and including the bases of stems, decaying organic matter, and any surface roots present, contained the major portion of the total deposition (1). Supporting evidence for this possibility has recently been reported in work with sheep (5), where it was found that considerably more soil was consumed when grazing sparse pastures than when grazing lush pastures. Another possible explanation for the difference is that the top grazers had the choice of more palatable portions of the plants, possibly containing less radiostrontium. However, green-chopping, which eliminates selection, has resulted in lower milk radiostrontium concentrations than grazing the equivalent material (7). Radionuclides deposited by precipitation, dry deposition, and airborne activity did not account for the difference in the radionuclide concentrations in milk of the two groups, nor did the milk yields (4). An increase in mat material consumption is, therefore, the most likely explanation for the observed effect. That the same effect was not observable at all times during the period when precipitation occurred, is probably because the top grazers had a greater opportunity to consume the activity deposited by rain. I t is concluded that the increase in grazing intensity did increase the strontium-89 and strontium-90 units in milk.

Acknowledgments

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We acknowledge the assistance of Mrs. B. Jacobs and Mrs. M. ttawkins, Radiological ~ealth Research Activities, Robert A. Taft Sanitary Engineering Center, who performed most of the

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radiochemical analyses, and the Radionuclide Analysis Group, Health Research Activities, for performing the radioactive measurements. References

(1) Agricultural Research Council Radioblologieal Laboratory Annual Report, 1961-62. 1962. Govt. Brit. Agricultural Research Council, Radiobiological Lab., Wantage, Berks, England. (2) Bryant, H. T., Blaser, R. E., Hammer, R. C., Jr., and Hardlson, W. A. 1961. Method for Increased Milk Production with Rotational Grazing. g. Dairy Sci., 44:1733. (3) Buldakov, L. A. G., and Buror, N. I. 1961. Behavior of Strontium and Calcium in the Cow. Radiobiologeya, 1 : 418. (4) Burmann, F. J. Unpublished data. (5) Field, A. C., and Purves, D. 1964. The Intake of Soil by the Grazing Sheep. Proe. Nutrition Soc., 23, XXIV. (6) Fries, G. F., Burmann, F. J., Cole, C. L., Sims, J. A., and Stoddard, G. E. 1966. Radioiodine in Milk of Cows Under Yarious Feeding and Management Systems. J. Dairy Sci., 49 : 24. (7) Fries, G. F., and Burmann, F. J. Unpublished data. (8) Hanford Biology Research Annual Report for 1956. 1957. U. S. AEC Report HW47500. (9) Hansen, W. G., Campbell, J. E., Fooks,

(10) (11) (12) (13) (14)

(15) (16)

J. It., Mitchell, H. C., and Eller, C. H. 1964. Farming Practices and Concelttrations of Fission Products in Milk. Public Health Service, Publ. 999-R-6. Harris, J. Y. 1962. Bovine Metabolism of Sr-85, Bu-133, Cs-134, and 1-131. J. Dairy Sci., 45:1573. Jenness, R., and Patton, S. 1959. Principles of Dairy Chemistry, John Wiley and Sons, New York. t(lechkovskii, V. M., ed. 1957. Academy of Science, USSR, 1956. U. S. AEC Report TR-2867. Menzel, R. G. 1954. Competitive Uptake by Plants of K, Ru, Cs, and Ca, Sr, Ba from Soils. Soil Sei., 77:419. Merten, D., and Suschny, O. 1961. Some Factors Influencing the Food-Chaln Transport of Radioactive Materials into Cow's Milk. Nature, 189:806. Radioactive Materials in Food and Agriculture. 1959. Report of an F A 0 Expert Committee. Rome. Straub, C. P., and Fooks, J. H. 1963. Effect of Farm Practices on Radionuclides in Milk. In Proceedings of the North Central Experiment Stations Workshop on Radionuclides in Foods and Agricultural Products, February 19-21, 1963, Cincinnati, Ohio. F. l~aghiri and R. R. Johnson, eds. Ohio Agricultural Experiment Station.

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