LUBBOCK — A fast-spreading weed is causing problems in Texas cotton fields and could cost growers much of their harvest if left unchecked.Pigweed, which can grow as tall as 10 feet and overtake crops, is spreading aggressively in the High Plains this year after becoming resistant to the herbicide that once easily eradicated it. Above-average rainfall has also helped the growth of pigweed in the world’s largest contiguous growing patch, where cotton producers are checking their fields and seeking other herbicides to ensure that their crops aren’t affected.“It just happens so fast,” said Walt Hagood, who farms about 1,600 cotton acres southwest of Lubbock. “If we don’t get on top of them early, it’s just like a baseball game. You get down a few runs and it’s hard to come back.” Put simply, pigweed is a cotton-picking headache and could cut producers’ yields by 40 percent. Some fields near Lubbock recently looked clear of the weed, but pigweed plants dotted cotton rows in others. For those areas, eradication gets more difficult if pigweed goes to seed.One female plant can have as many as 600,000 seeds that are carried by water but not wind. Rainfall in the High Plains since late May helped flush the seeds to other areas of fields, though they’re light enough to be blown around sometimes . To overcome the proliferation of pigweed, cotton producers must use different herbicides throughout the growing season. The plants can also be manually hoed from fields, though the weed’s thick stems and deep roots make that hard work.“Its reproductive capacity is tremendous,” said Wayne Keeling, an agronomist with the Texas AgriLife Extension Service in Lubbock. “The problem is here now and it needs to be dealt with before it gets worse.” Pigweed, also known as Palmer amaranth, is a summer annual that’s native to the Sonoran Desert and the Lower Rio Grande Valley, and it affects all row crops, including vegetables and peanuts. By the late 1990s, pigweed had become a major agricultural weed in the southern Great Plains, and it remains an issue in the Carolinas and Midwest states such as Iowa and Illinois. Cotton growers in the South already spend about $100 million a year to try to keep it out of their fields.Growers found that herbicides containing glysophate were highly successful in killing the weed, though over the years several states had seen it become resistant to the chemical. This year that resistance reached the Texas fields, forcing producers to find a mix of other herbicides to combat the problem. “There’s nothing really new on the horizon,” said Larry Steckel, a weed specialist at the University of Tennessee at Knoxville. But he said using different herbicides, including one containing the chemical glufosinate, has proved effective. Growers will not have one herbicide in their toolbox after the U.S. Environmental Protection Agency recently denied an emergency exemption applied for by Texas agriculture officials for the chemical propazine. The EPA cited drinking water and environmental hazard concerns. Pete Dotray, a professor of weed science at Texas Tech University, said cotton growers need to rotate either crops or the herbicides they use against pigweed. He said agricultural users in the Southeast have had problems with resistance for years because they relied solely on glysophate products. “It seemed like the technology lasted longer here because we were more diversified” in the herbicides, he said.