Cotton gained the most in more than three weeks on signs that hot, dry weather eroded crop quality in the U.S., the worldΆs top exporter.

For the season started Aug. 1, 46 percent of harvested cotton has met the standards needed to be delivered against futures traded in New York, a unit of the U.S. Department of Agriculture said in a report today. ThatΆs below a historical average of about 70 percent, according to Peter Egli, a Chicago- based director of risk-management for Plexus Cotton Ltd. Plants are producing coarse fiber in growing areas including Texas, the biggest producer, he said.

“The extreme heat and drought this summer have taken a toll on fiber characteristics,” Egli said in an e-mailed report. “We believe that this low percentage of tenderable grades has traders nervous” that there wonΆt be enough supplies to rebuild inventories, he said.

Cotton for December delivery rose 0.9 percent to settle at 71.36 cents a pound at 2:31 p.m. on ICE Futures U.S. in New York, the biggest advance since Sept. 18.