Heat-treated rapeseed expeller press cake with extremely low glucosinolate content reduce transfer of iodine to cow milk

The main objective of this study was to investigate the effect of increasing dietary levels of heat treated low glucosinolate rapeseed expeller press cake (RSC) on the transfer of iodine from feed to cow milk. Eight cows of the Norwegian red cattle breed were split in two 4 × 4 Latin squares, using 4 treatments and 4 periods of 14 days each. The 4 different treatments were 1) Control, 0.0 kg RSC/day, 2) RSC-Low, 0.6 kg RSC/day, 3) RSC-Medium, 1.4 kg RSC/day and 4) RSC-High, 2.0 kg RSC/day. Irrespective of a planned constant dietary iodine content, the analysed concentration of iodine ranged from 1.4 mg/kg DM in the RSC-High diet to 1.9 mg/kg DM in the Control diet. From day 11-14 in each period, samples were collected and the total iodine concentrations in feed, milk and plasma were determined by inductively coupled plasma mass spectrometry. The iodohormones, triiodothyronine (T3) and thyroxin (T4) in plasma were determined by fluoroimmunoassay. No differences (P > 0.05) in total iodine as well as the T3 and T4 plasma concentrations were observed between the four treatments, even though the plasma iodine reflected the somewhat varying dietary iodine. Feed intake, milk production and milk composition was not affected by the different treatments (P > 0.05). Although the levels of glucosinolates were low and no glucosinolate metabolites (e.g., goitrin and indole acetonitrile) were found in the RSC, an increasing offer decreased the milk iodine concentration from 0.35 mg/kg in the Control to 0.25 mg/kg with RSC-Low, to 0.15 mg/kg with RSC-Medium and to 0.10 mg/kg with RSC-High treatments. The iodine transfer, i.e. the output of iodine via milk related to the iodine intake, amounted to 25, 19, 13 and 10% in the Control and the 3 groups with increasing dietary RSC level. This study indicates that milk iodine transfer is severely inhibited at considerably lower levels of glucosinolates in RSC than previously anticipated.