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High Zinc Levels May Affect Bone Development In Pigs

High levels of zinc in nursery pig diets can affect bone growth and development, a University of Wisconsin-Madison study has shown.

Nursery pigs fed 3,000 parts per million of zinc from zinc oxide for four weeks had two to three times the normal level of zinc in their bones, according to Tom Crenshaw, an animal scientist at the UW-Madison”s College of Agricultural and Life Sciences.

“The data suggest a slight but immediate effect on bone mineral deposition and bone strength,” Crenshaw says. “We don”t know what the long-term consequences will be.” An experiment now underway at the CALS Swine Research Center should reveal whether high zinc levels in bone are linked to increased lameness and bone breakage in market hogs, he says.

Over the last three years, a number of pork producers and processors have called Crenshaw, reporting spontaneous fractures in hogs and bones breaking after slaughter. All nutrients seemed to be in the normal range in the affected animals” diets, Crenshaw found.

Zinc came to mind as a culprit because of the way trace minerals interact during bone development, he says. While zinc is now a common supplement in nursery pig diets, there”s some evidence that zinc at 3,000 parts per million approaches toxicity. The skeleton may act as a reservoir, absorbing excess zinc to protect the animal from toxic zinc compounds that might be absorbed into the body. Once zinc is absorbed into bone it stays there; the animal”s body has a hard time mobilizing it out.

In the current trial at the Swine Research Center, Crenshaw hopes to find out whether high levels of zinc in young pigs” bones affect bone growth and development as the animal grows. If so, the negative consequences – broken bones and lameness – might show up in market-weight animals and breeding stock.

Zinc is often supplemented in swine nursery diets because it helps to maintain growth rates and performance. Low levels of antibiotics in animal feeds also improve animal performance and growth rates. However, this “subtherapeutic” antibiotic feeding raises concerns about the development of antibiotic-resistant bacteria that pose human health problems. As routine antibiotic feeding comes under increasing scrutiny, the industry has begun looking for non-antibiotic feed additives that will maintain growth rate and performance. Additions of 3,000 parts per million of zinc is one approach being used.

Crenshaw notes that the livestock industry is also under pressure to reduce phosphorus and calcium in animal manure, which means reducing levels of these elements in livestock diets. Lower phosphorus and calcium levels in diets, plus extra zinc, may be combining to produce weak-boned animals, he says. Low levels of phosphorus and calcium may increase the toxicity of zinc in the animal.

Crenshaw is also part of a North-Central Region study looking at another form of zinc that might maintain animal performance without affecting bone development. Other feeding and management strategies are also being looked at to maintain performance without using trace elements or antibiotics, he says.

This research was supported by PIC, USA of Franklin, Kentucky and state funding to the UW-Madison College of Agricultural and Life Sciences. Crenshaw discussed these findings at the 1999 national meeting of the American Society of Animal Science in Indianapolis.