Factors affecting iodine concentration of milk of individual cows

Variations were measured of iodine concentrations of milk during complete lactations of 36 Holstein cows from the University of California herd in Davis and 24 Holstein and 12 Guernsey cows from the California State University herd in Fresno. At Davis no iodine was added to the concentrate, whereas at Fresno iodine as ethylene diamine dihydriodide was added to the concentrate at 4 ppm. At Davis, the mean milk iodine concentration was 166 micrograms/kg; at Fresno, the mean milk iodine concentration was 745 micrograms/kg. Holstein milk had higher iodine concentrations than Guernsey milk, 839 versus 554 micrograms/kg. Iodine concentrations of milk increased during lactation for all cows. At Davis, samples taken in the 1st mo of lactation had 105 micrograms/kg compared with 218 micrograms/kg in the 9th mo. At Fresno, samples taken in the 2nd wk of lactation had 183 micrograms/kg, compared with 1017 micrograms/kg in the 40th wk. Addition of as little as 4 ppm ethylene diamine dihydriodide to the concentrate throughout lactation will lead to greatly increased iodine concentrations in the milk, particularly in late lactation.