Changes in the transfer of fallout radiocaesium from pasture harvested in ibaraki prefecture, Japan, to cow milk two months after the fukushima daiichi nuclear power plant accident

Abstract

Changes in the radioactivity of 131I, 134Cs, and 137Cs in milk produced by cows given pasture that was contaminated with these radioactive nuclides caused by the Fukushima Daiichi nuclear power plant accident on 11 March 2011, were examined between 16 May and 26 June 2011. Pasture (Italian ryegrass) was seeded on September 2010, and cultivated in the Animal Resource Science Center of the University of Tokyo (about 140 km south-west of the power plant). Pasture was harvested 2 months after the accident and prepared for fermented grass forage (haylage). The cows examined were born and kept in the Animal Resource Science Center and were given commercial mixed feed (total mixed ration forage: TMR) that contained no radioactive 131I, 134Cs, or 137Cs, for 2 weeks before being examined. They were given haylage and TMR (10 and 25 kg/600 kg of body weight/day, respectively) for 2 weeks, and then were given only TMR (35 kg/600 kg of body weight/day) for 2 weeks. During the examination, milk was collected twice a day and mixed in each cow. The weight and radioactivity of 131I, 134Cs, and 137Cs of the mixed milk in each cow were measured daily. No radioactive 131I was detected in either the milk or haylage. The radioactivity of 134Cs and 137Cs contained in the mixed feed of haylage and TMR was 380 Bq/kg (radiocesium radioactivity was represented as total concentrations of 134Cs and 137Cs). Radiocesium radioactivity concentrations in the milk rapidly increased to 30 Bq/kg after 4 days from the start of feeding and equilibrated to 36 Bq/kg after 12 days. After that, cows were given TMR containing no radiocesium, and radioactivity concentrations of radiocesium in the milk rapidly decreased. Two weeks after stopping radiocesium feeding, radioactivity concentrations were less than 5 Bq/kg (background level). In summary, when the cow (approximately 600 kg of body weight) was given feed with radioactive 134Cs and 137Cs (12,600 Bq/600 kg body weight/day), 5.71% of 134Cs and 137Cs was secreted into the milk (720 Bq/20 kg milk/day). Radioactivity concentrations of radiocesium in the milk were lower than new standard (50 Bq/kg) of Japan.