Feeding weanling pigs a diet containing an organic form of zinc rather than a traditional inorganic zinc supplement helped maintain their growth performance while levels of the mineral in their manure decreased 76 percent, according to a team of University of Missouri swine nutrition researchers.

"Zinc is a key micronutrient required for normal growth, proper reproductive development and immune system function," says Trygve Veum, MU professor of animal sciences. "In the swine industry, inorganic zinc is commonly fed in high concentrations to nursery pigs to enhance their growth performance and to prevent diarrhea."

Feeding high concentrations results in large amounts of the nutrient in the pigs' manure. "The manure gets spread on fields as fertilizer, but when the zinc builds up faster than crops can use it, nutrient pollution and water quality become a real concern," he adds.

Previous research shows pigs digest organic forms of zinc more easily than inorganic forms. Carolyn Huntington, Mt. Vernon, Ill., Angela Bradley, Palmyra, Mo., and Veum designed an experiment to determine if lower levels of a commercially available organic zinc-polysaccharide supplement would provide growth performance similar to the inorganic zinc ration typically fed to pigs.

The researchers assigned 50, 21-day-old barrows and gilts to one of five dietary treatments that contained different concentrations of organic or inorganic zinc. During the five-week experiment, they collected blood plasma, fecal, feed and urine samples, which they used to evaluate zinc absorption and retention.

"Our results indicate organic zinc supplements can be fed at rates almost seven times less than inorganic zinc and still maintain pig growth performance," says Huntington.

The environmental benefit is not without cost, says Bradley. "The reason producers traditionally supplement with inorganic zinc is because it's relatively inexpensive."

The organic zinc-polysaccharide supplement the researchers tested in the study cost almost four times as much per pound as the inorganic zinc. Because it was rationed at much lower levels, the additional cost averaged about 9 cents per pig.

   "For small to mid-sized producers with ample crop land on which to spread manure, it may not make economic sense to feed organic zinc," says Veum. "But for large producers already struggling with nutrient issues, feeding pigs organic zinc could be part of an overall nutrient management plan.”

The next step is to identify the molecular pathways in the pig intestines that are responsible for the differences in organic and inorganic zinc uptake. Learning more about these pathways could further improve utilization of zinc, copper and iron, and lessen potential environmental impacts, he adds.

University of Missouri