I was recently asked about the mode of action of the feed additive, zinc oxide. This should be an easy one, because the animal feed industry has used zinc oxide for more than 20 years. But, after so much research, practical experiences and empirical evidence, I cannot say with any degree of certainty that I am fully convinced of any proposed theory.
Yes, zinc oxide will improve growth performance and reduce diarrhea in piglets (when fed at pharmacological levels). However, it remains unclear whether this is an effect of reduced total bacterial load, stabilized gut microflora, enhanced intestinal integrity and immunity, or improved enzymatic secretion and activity to name but a few of the proposed modes of action!
In my opinion, zinc oxide cannot be easily replaced by another source of zinc, be it organic or inorganic. Many will certainly disagree with this statement, but zinc oxide remains the most widely used additive against piglet diarrheas (at least where it is still allowed!) The fact that zinc oxide cannot be easily replaced by other forms of zinc, leads me to believe that zinc oxide has bacteriostatic or bacteriocidic activities within the intestinal tract.
At least, this activity must be what brings about the majority of the positive effect on piglet growth performance and reduced incidence of diarrhea. I must also note that zinc oxide products of hugely disparate zinc bioavailability also have been shown to perform equally well! This aspect alone puts into question theories that involve zinc internal metabolism (that is, everything that comes after zinc is absorbed).
In support of the above, I cannot but refer to human medicine where zinc oxide ointments are frequently used to “sterilize” external superficial wounds. Similarly, zinc oxide is used in several dental pharmaceutical products for the same purpose.
But, all these are just my own interpretation of available information. I would be certainly very interested to read about your opinions. So, please do leave a comment!