So where did the cow get the dose of radioactive cesium? Air? Water? No one knows, because everyone in the government and the producers have been looking only at radioactive rice hay.
From NHK News Japanese (2:17PM JST 8/19/2011):
厚生労働省によりますと、福島県産の牛の肉から国の暫定基準値を超える放射性セシウムが検出されたことが分かりました。この牛肉は、食肉処理場に保管されていたということで、これまでの調査では、高濃度の放射性セシウムを含む稲わらは餌として与えられていなかったとみられるということです。政府は福島県の肉牛について、１９日、出荷停止を解除する予定でしたが、出荷停止を当面継続するとともに、さらに詳しい調査を行うよう福島県に指示しました。
According to the Ministry of Health, Labor and Welfare, radioactive cesium in the amount exceeding the national provisional limit has been detected from the beef from a cow raised in Fukushima Prefecture. This meat has been stored at a meat processing facility, and according to the investigation so far the cow was never fed the radioactive rice hay. The national government was going to lift the shipping ban on meat cows in Fukushima Prefecture today (August 19) but instead has instructed the Fukushima prefectural government to continue to halt shipping for the time being and conduct further investigation.
On a separate piece of news (link in Japanese), the manure made from chicken poop mixed with dead leaves and sawdust in Tama district of Tokyo (west) has been found to contain 890 becquerels/kg of radioactive cesium. I hope it is from dead leaves or sawdust, and not from chickens. Chickens are fed with the chicken feed from the US, and they are raised indoors.