SAN RAFAEL, Calif. (AP) - Researchers here are looking to horse manure to study the spread of invasive weeds. Dominican University has received a $100,000 National Park Service grant to study how to slow the spread of nonnative plants and weeds in state parks, school officials said. Horse manure might be part of the problem, according to scientists. Researchers began collecting horse manure samples from trails and pastures this summer. They want to test an assumption that seeds can pass through horses, leading to sprouts of invasive weeds. "We need to know through scientific research if horses do or do...