23 IV. Fresh ‘tlk from Chicarco Dairies Samples of 2 to 3 gallens of fresh milk were obtained monthly from several of the larger Chicago dairies for one vear beginning in March 1955, The results in Sunshine Units are oresented in Ficure 5 which shows the very interesting seascnal variation in the Sr90 level of forage ard dairy products. The January-February 1956 Sr°0 level apnears to be a good average of the levels observed over the 1955 growing season. That is to be exnected, since over the ~anter months the cows feed on hay gathered at various times during the previsus growing season, Similerly, the March-April 1955 level must repre- sent the average for 195), The sharp rise in the Spring of 1955 mav %e due in nart to leaf retention of fallout from the concurrent Nevada tests. able in terms of other factors. However, the rise may be explain- “Yhen first pastured in the Spring, the cows change from feed grovm an averare of eight months earlier to new provth, representing an incr-ase in Sr°0 level du2 to the total awditional Sr79 fallout curing the eight months period, Furthermore, the spring pasturage for the most pert may be unploved fields, The fall-off during the sumrer may be due to several factors: a larger mroportion of feed from recently plowed fields, greater root depth and thus lover Sr9O level for mre mature clants, and lower Sr? retention in milk as a result of richer calcium diet. To the extent that leaf retention is involved, it should have lower effect in summer consistert -ith the lower observed fall- out durin: the summer months (see 195% Chicago Rain Data, Table 2 above }» 00 The increase fron the March 1955 level to the February 1956 level is somewhat greater than the corresponding increase in the total Sr70 soil level from July 19S) to July 1955 (see Figure l}). The sreater increase in milk level may te the consequence of hicher fallout, and thus higher leaf retention, in 1955 SY