John Deere has spent $305m on Blue River Technologies, an agricultural startup that has developed a way to accurately destroy weeds. Blue River’s main product is a machine-vision system that can identify weeds for precision spot-spraying, that lets farmers optimize their herbicide and pesticide spraying – cutting costs, and also helping to lower their use of these chemicals. Part of a trend called Precision Agriculture, the approach is all about only using exact amounts, rather than the blanket-coverage approach of old – which was time efficient, but often wasteful. It also enables farmers to use far stronger chemicals in precision applications, which would kill off an entire crop if sprayed in a conventional manner. With a precise burst, only the…