Iodine Supplementation of Cattle.

Abstract

Plasma total iodine (I) and thyroxine are of no practical value to assess I status of cattle. When interpreted carefully, plasma inorganic iodine (PII), is a sensitive index of current dietary I status. PII can confirm suspicions of deficiency or excess of dietary I. Between 1988 and 1999, 32- 62% of commercial herds had low or very low I status and the I status of the national herd has fallen since 1995. 2. I deficiency is the most important trace-element deficiency in Irish cattle and sheep. Our ruminants, especially those at pasture, and at critical times of their annual physiological cycle, need routine I supplementation. The most critical times for cows are from 1 month prepartum to 4 months postpartum, except in herds with unexplained abortions, when supplementation throughout pregnancy may be needed. Also, calves, lambs and growing stock may need regular I supplements if unknown factors compromise their performance or immune status. 3 Even in the absence of goitrogens, Irish forages supply <33% of the minimum I needed by cattle and 97% of our forages are deficient in I. 4. Irish mineral mixes supplied circa 32-44 mg I/cow/d in 1989. In the past few years they supply close to 60 mg I/cow/d. Increased use of iodised minerals would greatly improve the I status of the national herd. 5. Oral supplements of 30-60 mg I/cow/d via feed or drinking water maintained normal PII levels. Weekly skin application of 9 ml of 5% tincture of I to the flank-fold pocket was also effective, as was Ionox, a new slow-release bolus. A mean oral supplement (mg I/100 kg LW) of 11.8 is suggested for dairy cows, 4.7 for beef calves, heifers and cows and 6.4 for steers. These are higher inputs than are recommended or used in some countries. However, they are <45% of the inputs defined as safe by EU Feed Legislation, which allows a total intake of 11 mg I/kg feed DM, or 165 mg I/cow/d at an assumed DMI of 15 kg/cow/d. 6. Though Lipiodol injection increased PII for 42-90d, it is not registered on the therapeutic veterinary product list and oral I supplements can maintain normal PII levels for a fraction of its price. Because other methods are effective, faster acting and cheaper, and there is little evidence to support its use as an effective preventative of stillbirth in I deficient herds, Lipiodol is not recommended as an I-supplement for cows. 7. Milk I levels give no cause for concern as regards the risk of human thyrotoxicosis. However, there is a case for monitoring larger numbers of bulk-tank samples and, especially, samples of milk at retail outlets, on grounds of herd health and human health.