Even as crops resistant to 2,4-D herbicide are being developed, populations of weeds are also developing a resistance. A 2,4-D–resistant variety of the waterhemp weed has been found, and its spread could lessen the impact of an herbicide widely used in grassland and crop production. Despite worldwide use of 2,4-D since the 1940s, only 17 weeds were previously known to be resistant to it.