Questions about the safety of a popular herbicide made by Monsanto Co have resurfaced in a warning from a U.S. scientist that claims top-selling Roundup may contribute to plant disease and health problems for farm animals. Plant pathologist and retired Purdue University professor Don Huber has written a letter to U.S. Agriculture Secretary Tom Vilsack warning that a newly discovered and widespread electron microscopic pathogen appears to significantly impact the health of plants, animals, and probably human beings." Huber coordinates a committee of the American Phytopathological Society as part of the USDA National Plant Disease Recovery System. Huber said the organism has been found in high concentrations of Roundup Ready soybean meal and corn, which are used in livestock feed. He said laboratory tests have confirmed the presence of the organism in pigs, cattle and other livestock that have experienced spontaneous abortions and infertility. The organism is also prolific in corn and soybean crops stricken by disease, according to Huber. "It should be treated as an emergency." He requested USDA participation in an investigation, and he urged a moratorium on approvals of Roundup Ready crops. USDA officials declined to comment about the letter's contents. Roundup has long been a draw for critics, who say the herbicide promotes widespread weed resistance, or "super weeds." "While the evidence is considered preliminary, the potential damage to humans and animals is severe," said Jeffrey Smith, executive director of the Institute for Responsible Technology. There have been other alarms raised about Roundup, including a report last year from Argentine scientists who claimed that Roundup can contribute to birth defects in frogs and chickens.