May 12, 2009

Facial Eczema (FE). Farm Information. Part 4.Zinc oxide. General dosing information.

Agriculture, farming, animal husbandry, animal health, disease, Facial Eczema, prevention, zinc oxide, general dosing information, zinc, toxicity, copper, selenium

 By Dr Clive Dalton

Original 1991 information written by Dr Barry Smith and Dr Neale Towers, Ruakura Agricultural Research Station, Hamilton, New Zealand.

4. FACIAL ECZEMA:  Zinc oxide. General dosing information.  

When to Start
  •  Have supplies of Zinc Oxide and the equipment needed ready well before the FE season starts. Supplies can be difficult to buy during an FE outbreak.
  •  Zinc dosing should begin as soon as the weather conditions (warm, humid, grass minimum temperature more than 13"C, heavy dew or 3-4 mm rain) favour spore growth and/or at the first signs that spore counts are beginning to rise. Don't wait until dangerous conditions arise, or until clinical cases are seen. 
  • The spore rises precede clinical symptoms by some 10-1 4 days, and to be effective zinc must be dosed before or at the time the animals graze toxic pasture. 
  • Farmers regularly monitoring spore counts early in the season could delay starting dosing until counts begin to rise (don't wait until they reach danger level). It usually takes more than a week for the earliest spore rise of a season to reach danger levels, so starting dosing immediately spore rises begin should provide adequate protection.
  • On problem farms in particular begin dosing in mid-January and continue throughout the autumn. 
  • Stock should not be exposed to zinc unnecessarily and excessively prolonged zinc dosing lowers the safety margin.

How to Start
  • Begin dosing at long-term dose rates unless dangerous conditions already exist.
  •  If conditions become dangerous during the first week of dosing increase the dose rate to "crisis" levels. Keep dose rates at this level for two weeks then reduce to long-term dose rates.
When to Stop
  • Continue dosing through the expected FE season.
  •  Towards the end of the FE season dosing can stop when spore counts fall to low levels and weather conditions are dry and cool. But watch the weather and start dosing if conditions favour spore growth again.
  • After long-term zinc dosing ceases, protection will carry over for several days.
  • After prolonged zinc dosing, protection will be quickly re-established once zinc administration begins again.
Zinc Toxicity
  • Overdosing with zinc is toxic. Take care calculating dose rates, weighing zinc and mixing drenches. Check drench guns for accuracy.
  •  There is a 3-fold safety margin for dosing zinc over 60 days, i.e. if three times the recommended rate is given it will cause damage to the pancreas.
  • The safety margin for dosing for longer periods is progressively reduced. Hence the need to use the correct dose rates, and avoiding unnecessary dosing for long periods.
  • Pancreatic injury must be severe before effects on animal health are noted. The pancreas will recover when zinc dosing ceases.
  • Administering zinc oxide drenches in large amounts as may occur with weekly or fortnightly dosing interferes with calcium metabolism (and may cause milk fever) and is not recommended for lactating stock.
  • Because elevations of zinc occur in liver and kidney (not meat) a withholding period of 1 week should be allowed before animals are slaughtered.

Copper and Selenium
  • Long-term zinc dosing may interfere with copper and selenium metabolism, although it has not yet been shown to induce copper or selenium deficiency.
  •  In areas where these minerals are deficient supplement the animals with copper and selenium immediately after zinc administration ceases.
  • Don't give copper supplements during the FE season unless clinical deficiencies exist. If copper supplements are required use an injectable preparation.

Purity of Zinc Compounds
  • Ask the supplier if the zinc oxide meets the Animal Remedies Board's specifications.

A volumetric measure of the zinc oxide is sufficient when mixing. Weigh out the zinc oxide needed to dose all animals, smooth the surface and mark the surface height on the container. An independent check of calculations should be made and professional advice sought if there is any doubt about the methods.

This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.

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