Long before pork producers stopped feeding “slop” to pigs, they knew there was something in that galvanized bucket that kept the pigs healthy. It turned out that zinc prevented poor growth and rough, cracked skin with open sores, characterized by the disorder called parakeratosis. Pork producers seldom, if ever, see this condition today because we routinely add zinc to swine diets.
Recently, however, when zinc was intentionally not supplemented in traditional nursery diets (composed of corn, soybean meal, soy protein concentrate, dried whey, plasma protein, etc.), weaned pigs developed skin lesions that resembled parakeratosis three weeks after weaning. (See photo.)
The bottom line is that the large amount of zinc transferred to the pig in utero was not enough to support growth in this young, rapidly growing animal. It’s no surprise that these zinc-deficient pigs grew at half the rate of pigs that received supplemented minerals.
The most recent zinc recommendations for swine by the National Research Council are based on research completed in the last century. But dietary ingredients, management and genetics have changed immensely, so one might guess that nutrient needs also have changed. In recent work at The Ohio State University it was determined that average daily gain was maximized when 75 ppm of an organic zinc (0.88 pound per day) or 100 ppm inorganic zinc (0.82 pound per day) was added to nursery diets in a 28-day study. It’s worth noting that the feed intake was not altered by dietary treatment. (See chart.)
Metallothionein, the protein involved in “handling” many minerals, including zinc, was found in greater concentration in the duodenum (where zinc is primarily absorbed) when pigs were fed an organic zinc source versus when they were fed inorganic zinc (zinc sulfate).
There is much work that needs to be done at the cellular level before the mechanism for this positive influence of organic minerals can be elucidated.
Adequate zinc also is known to be important in establishing good trabecular bone formation. However, as yet there have been few studies done involving livestock that would validate this role. If we look at research using the rat as a model, it has shown that more dietary zinc was needed for modulation of osteoclast maturation of bone (30 ppm) than for growth of the animal (7.5 ppm).
When zinc is limited, the bone plate growth is limited and trabecular volume is decreased, resulting in increased osteoclast resorption and bone remodeling. However, too much zinc can result in poor joint structure and, ultimately, cartilage erosion and joint inflammation with excessive synovial fluid. (See accompanying photo.) This will ultimately lead to swollen joints, pain and lameness.
The joint lesions result from the biological interaction of zinc and copper. Excessive zinc in the diet decreases the amount of copper available to the animal to meet its metabolic needs. Hence, very high or pharmacological zinc can induce a copper deficiency if fed for an extended period of time such as throughout the grow/finish period. Also, excessive zinc can reduce the calcium available for the animal to build healthy bones.
In recent years, the pork industry has benefitted from increased and inexpensive gains associated with feeding pharmacological concentrations of zinc (2,000 to 3,000 ppm) from zinc oxide to nursery pigs. While there are many hypotheses, the biological mechanism of action has not been elucidated.
Our laboratory was the first to report an increase in the zinc handling protein, metallothionein, and improvement in gut morphology when feeding pharmacological zinc levels. This work also showed that pharmacological zinc only needs to be fed the first 14 days after weaning for the pig to attain the growth and health advantages.
Newly weaned pigs fed pharmacological zinc load their bodies with zinc for about 10 days before beginning to excrete more in their feces than pigs fed zinc at the NRC-recommended levels. Thus, feeding pharmacological zinc is not an environmental concern if we closely manage the time period when it’s incorporated into the nursery diets.
Recently, some researchers have reported increased feed intake when organic zinc was added to a diet already supplemented with ractopamine and 110 ppm zinc from zinc sulfate. However, several producers report inconsistent results when additional zinc is supplemented, and this research has not been peer reviewed.
Because zinc has numerous interactions with other minerals such as iron, copper and calcium, as well as with protein sources, it is possible that this benefit is seen when nutritional interactions have occurred due to imbalances or variations in dietary ingredients.
It is clear that zinc continues to be needed in pork production for many aspects involving the animal’s health and well-being, but equally important is feeding the correct dietary concentration. Grains and plant proteins that contain high amounts of phytate will increase the zinc requirement as will high concentrations of calcium. If pharmacological zinc is fed in the nursery, producers may be able to reduce the amount in the grow/finish phase to as low as 25 ppm added zinc.
As research continues to reveal more about the biological zinc needs of today’s fast-growing pigs and the prolific dam, we should be able to more closely match dietary concentration with their metabolic needs. In the end, it will save money for the producer, protect the environment and better accommodate the pig.