ZINC EXCRETION AND RETENTION IN GROWING PIGS FED INCREASING LEVELS OF ZINC, OXIDE

A balance study comprising six litters of five female pigs was conducted to investigate the retention and excretion of zinc when a barley, wheat, and soybean meal diet was supplemented with increasing levels of zinc oxide. Stainless steel metabolism cages were used and precautions against contaminations were undertaken. The natural zinc content of the basic diet was 42 mg per kg DM, and to this diet was added 0, 30, 60, 120, or 200 mg zinc per kg. Before the first balance period the pigs were allowed 5 days of adaptation. The pigs were subjected to two balance periods (at 35 and 45 kg body weight) during which urine catheters were inserted into the bladder to separate urine and faeces. The pigs were fed 1.2 and 1.4 kg feed per day during balance period 1 and 2, respectively. Zinc content in diets, faeces and urine was determined, and daily zinc excretion in urine and faeces and zinc retention was determined for each treatment group within the balance period. Furthermore, zinc concentration and alkaline phosphatase activity in plasma were determined. When dietary zinc supplementation increased from 0 to 200 mg per kg, the retention of zinc increased from 8 to 37 mg per day and from 14 to 43 mg per day in balance periods 1 and 2, respectively. Zinc retention did not increase when the zinc supplement exceeded 120 mg per kg, indicating a strong homoeostatic regulation of this mineral. The excretion of zinc in urine was between 0.6 and 1.0 mg per day in all treatments. This shows that the homoeostatic regulation takes place in the gastrointestinal tract and not via the kidneys. Based on zinc retention, the apparent digestibility (bioavailability) of zinc in zinc oxide was about 20%. The results on zinc retention, plasma zinc concentration and plasma alkaline phosphatase activity indicated that the intrinsic zinc content in the diet was insufficient to ensure the pigs' requirement.