with the extendable rod of a hydraulic tensioning cylinder that applies biasing closing force to an upper conditioning roll or to other structure that changes position as the opening distance is increased or decreased other suitable devices such as for example light sensors and acoustic sensors may also be utilized in determining the amount of crop materials present or the conditioning roll opening distance without departing from the principles of the present invention another means for sensing either independently of or in combination with opening distance sensor 22 the amount of cut crop materials being cut from the ground by the windrower 10 may take advantage of the fact that conditioning rolls 18 are typically yieldably biased toward one another to maintain the opening distance at a minimum such yieldable biasing means typically comprise pneumatic hydraulic or mechanical spring u201c tensioning u201d systems the greater the volume of cut crop materials passing between conditioning rolls 18 the greater the exertion of such materials against the rolls when separating them accordingly an exertion sensor 24 may be operably associated with one pair of the rolls 18 for providing an electrical output in accordance with the force pressure exerted by the moving stream of cut.