The cutter implement may include a housing, a cutter, and a curtain. The housing has a forward end for engaging the crop material when moving in a direction of operation. The housing at least partially defines a cutting region disposed at the forward end. The cutter is coupled to the housing and can cut the crop material in the cutting region. The curtain is coupled to the forward end of the housing and is positioned forward of the cutter relative to the direction of operation for engaging and leaning the crop material upstream of the cutter. The curtain includes a stiffness control feature operable to control a vertical stiffness of the curtain to provide a variable vertical stiffness that increases with an increase in a bend angle of the curtain rearward relative to the direction of operation.