Five potential negative effects of zinc oxide on piglets

　　Zinc oxide, in high doses, can promote piglet growth and reduce diarrhea. However, excessive use of high doses of zinc oxide to prevent diarrhea in piglets often leads to hidden negative effects. Several hypotheses exist regarding the mechanism of these negative effects, but the exact mechanism is not yet fully understood. The optimal level of zinc oxide for controlling diarrhea is approximately 3,000 ppm; however, using zinc oxide at this dose may have some negative impacts on piglet health.

　　1. Heavy Metal Contamination

　　Commercial zinc oxide has impurity issues, and the quality of zinc is not strictly controlled. A study by the French Institute of the International Federation for Information Processing (IFIP) showed that after pigs consumed contaminated feed (0.5 mg cadmium/kg feed) for 42-160 days, the cadmium concentration in the kidneys of humans who consumed the pork exceeded the legal limit (1 mg/kg). The zinc in this study was added at the normal standard level (nutritional). High doses of zinc oxide should be fed for a short period, but the cadmium level in the diet will increase. Some analyses show that cadmium can reach up to 2.5 mg per kilogram of feed. Cadmium is known to be an organ toxin with a long elimination half-life. Therefore, if piglets' tissues are contaminated with cadmium after weaning, the cadmium level in the tissues may increase at slaughter. In Thailand, high doses of zinc oxide are permitted, while the EU prohibits their addition. A recent study of 214 samples of pork muscle and kidney from these regions showed that cadmium concentrations in over 25% of muscle and kidney samples exceeded regulatory limits. Contaminated zinc oxide may be the main source of cadmium in the diet.

　　2. Nutritional Interactions

　　High zinc concentrations cause the body to produce more metallothionein. In this case, intestinal transport preferentially binds copper, which may lead to a sub-deficiency of this trace mineral. Due to the high safety margin, the bioavailability of iron does not appear to be significantly affected by high levels of zinc. High doses of zinc oxide have a negative impact on phytase activity. Studies on the interaction between zinc and phytase show that high doses of zinc oxide negatively affect phytase activity, thus affecting the release of phytate phosphorus. This means that phytase function is reduced, which may lead to phosphorus deficiency in animals. Zinc oxide has the strongest acid-binding capacity in feed: at pH=4, the acid-binding capacity of zinc oxide is approximately 16,000 meq, lime powder is 13,000 meq, and sodium bicarbonate is 12,000 meq. Therefore, high doses of zinc oxide may antagonize feed acidifiers such as organic acids.

　　3. Zinc Toxicity

　　High doses of zinc oxide may be beneficial for weaned piglets, but the National Research Council (NRC) believes that high doses of zinc oxide may affect piglet health if used for a long time. The industry knows that long-term addition of high doses of zinc oxide has negative effects, but the mechanism is unclear.

　　4. Environmental Issues

　　When the zinc concentration in feed does not exceed 150 ppm, zinc enrichment in soil does not exceed 3000 μg/kg DM/year. In Europe, adding 3 kg of zinc oxide per ton of feed for 2 weeks after weaning results in up to nearly 30% zinc excretion during the pig's growth cycle. The processing of pig manure also highlights this problem, as zinc is concentrated in the solid fraction, and the zinc content in by-products will exceed the EU's maximum limit for zinc in organic fertilizers.

　　5. Zinc and Microbial Resistance

　　Excessive use of zinc in animal diets may enhance bacterial resistance. Bacteria regulate intracellular zinc concentration through efflux pump systems. These pumps are non-specific to zinc and can also excrete other molecules, such as antibiotics. High zinc concentrations tend to increase the synthesis of efflux pumps; therefore, the use of high doses of zinc oxide may reduce bacterial sensitivity to antibiotics. A genetic relationship can also be inferred: heavy metal resistance and antibiotic resistance are sometimes correlated. Therefore, high concentrations of zinc may lead to bacterial resistance to some antibiotics.