A system and method for preparing a sample area of a bale in order to more accurately evaluate the plant material incorporated into the bale. A baler receives, compresses, shapes, and secures material into the bale. A cutter mechanism cuts a portion of the material in the bale into similarly-sized particles. A mixer mechanism mixes the particles into a homogenous aggregate. A compression mechanism compresses the homogenous aggregate into the bale. An NIR testing system receives and analyzes near-infrared radiation reflected by the homogenous aggregate, and generates evaluation data reflecting properties of the material. The cutter may include knives mounted in a compression chamber of the baler. The mixer may be a relief feature on a center rail, the compressor may be a projecting feature on the center rail, and an NIR sensor may be mounted to the center rail so as to press against the surface of the bale.