ISU Researchers Say Bt Corn Not to Blame for Sow Problems
October 14th, 2002
AMES, Iowa — Researchers at Iowa State University have concluded that genetically altered corn isn't to blame for higher-than-normal pseudopregnancy rates in sows on five Iowa farms.
An Iowa farmer first raised concerns that the Bacillus thuringiensis (Bt) corn he had been feeding his sows might have had an impact. Bt corn contains a gene from a soil bacterium that helps the plant combat a yield-reducing pest, the European corn borer. Following publicity about the situation, a few other farmers said they had fed Bt corn and also had reproductive problems in their swine operations.
Some of the producers met with veterinary and plant pathology experts at Iowa State in July. As a result of the meeting, John Carr, assistant professor of diagnostic and production animal medicine, visited five farms where reproductive problems occurred. In addition, two farms where Bt corn was fed but no fertility problems existed were evaluated.
The visits revealed that all the farms fed the animals a combination of Bt and non-Bt corn varieties. Several different Bt corn hybrids were used in the feeds.
Carr, Thomas Carson, professor of veterinary medicine, and Gary Munkvold, associate professor of plant pathology, concluded there was no link between the pseudopregnancy reproductive problem and the feeding of Bt corn.
Pseudopregnancy is a condition in which a sow or gilt has a delay in its normal estrus recycle period. During this delay, many behavioral or physical signs typical of pregnant animals may be exhibited, but no pigs are ever delivered.
The occurrence of a pseudopregnancy most often is related to management or proper pregnancy testing. Another reason may be the presence of estrogen-like compounds produced by Fusarium, a fungus found in corn. Some kinds of Fusarium infect corn kernels and can, under certain conditions, produce mycotoxins.
The ISU researchers said zearalenone, a mycotoxin that acts like estrogen, is the one most often associated with pseudopregnancy. Corn from the five farms studied was tested for the presence of mold spores. On all the farms, pigs were fed a combination of feeds containing Bt and non-Bt corn varieties. Fusarium molds were found, but none of the corn samples contained any detectable zearalenone.
"It's been known for many decades that Fusarium infection is very common in corn kernels," said Munkvold. "However, the presence of Fusarium does not necessarily mean that mycotoxins are present at harmful concentrations."
Bt hybrids typically have lower levels of Fusarium, so the producers involved were surprised by the mold spores that were found.
"When insects feed on corn kernels, the kernels are more susceptible to infection," Munkvold said. "Although Bt corn is not immune to Fusarium, it rarely has insect injury to the kernels, so the risk for infection is reduced."
Carr said the cause of pseudopregnancy can be difficult to determine. "Pseudopregnancy has been recognized for many years and is present on pig farms throughout the world, irrespective of the type or consistency of the feed being used," he said.
Carr said a detailed examination of breeding management programs helped resolve the problem on three of the farms involved. Problems on another farm were resolved prior to the start of his investigation. The fifth operation closed.
Two 750-sow units, which were not experiencing pseudopregnancy problems, also were examined over a six-month period while the animals were fed a variety of Bt corn. A normal farrowing rate of 82 to 84 percent was recorded with no significant pseudopregnancy problems.
Munkvold said there is no evidence that feeding grain from Bt hybrids poses any danger to livestock. "Numerous feeding studies with a variety of animals have been conducted and no detrimental effects have been observed," he said.
John Carr, Veterinary Medicine, (515) 294-8455
Thomas Carson, Veterinary Medicine, (515) 294-1950
Gary Munkvold, Plant Pathology, (515) 294-6708
Susan Thompson, Agriculture Communications, (515) 294-5616
Teddi Barron, News Service, (515) 294-4778