In one aspect the invention is directed to a mower configured to cut plant material as it moves over a field and deposit the cut plant material onto the field, the mower having a header assembly having a cutter bed, conditioning rollers, and a swathboard, the swathboard being positionable to shape the cut plant material into windrows. The cutter bed has a plurality of rotating rotary cutters that includes at least one pair of oppositely rotating cutters that sends a rearwardly-directed stream of cut plant material between the oppositely rotating pair of rotary cutters. The header assembly includes a sensor system mounted on the swathboard. The sensor system has a sensor configured to collect nutrient information of the plant material being cut by the mower. The sensor is positioned on the swathboard to coincide with the location where the stream of cut plant material interacts with the swathboard.