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Iodine Supplementation of Cattle.
Rogers, Philip
Rogers, Philip
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1999-12-01
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Philip, A.M., Rogers, M.V.B., Iodine Supplementation of Cattle, End of Project Reports, Teagasc, 1999.
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.