Plasma inorganic iodine and thyroxine concentrations for beef cows in latepregnancy and early lactation associated with different levels of dietaryiodine supplementation

摘要

Plasma thyroxine concentrations (T4) have been used routinely to assess the iodine status of cattle. Whittaker (1999) has suggested that concentrations < 20 nmol/L would be a better index than < 50 nmol/L to indicate long-term dietary iodine inadequacy. Plasma T4 values tend to be relatively stable and hardly respond to short-term dietary iodine supplementation. Plasma inorganic iodine concentration (PlI) may be a better diagnostic aid as it is associated with current dietary iodine intake (Rogers & Mee, 1996). PlI values, however, may change markedly almost on a day-to-day basis (Whittaker, 1999) and there may be considerabe individual variation within a herd depending on the particular daily intake of each cow. PlI concentrations between 105-287 ng/mL are frequently taken as the normal reference range 1or adequacy (Rogers & Mee, 1996) although these amounts are not based on production data. The purpose of the present study was to determine both mean PlI and T_4 concentrations in groups of beef cows in late pregnancy and early lactation when given iodine either in a general mineral supplement or as a constituent of a sustained-release rumen bolus system.