The purpose of the research is to determine the degree of influence of morphological features: the length of plants, their stems, ears of winter wheat grown in the southern subzone of the Rostov Region, and the choice of operating modes for combing headers. The problems arise from the discrepancy between the parameters and operating modes of the combing devices for the characteristics of the harvested grain, and also because of the lack of knowledge of the physico-mechanical characteristics of the crop plants currently cultivated. When harvesting with streaks, it is necessary that only the ear of the plant gets into the working zone, the parameters of which are determined by the diameter of the combing drum, the gap between it and the fairing, their mutual arrangement, and the position of the header relative to the field surface. Compliance with this condition is necessary, but not sufficient for cleaning with a streak without loss of grain. When modeling on the mathematical model of the process of combing grain crops realized by a combing header in a specific field, knowledge of more than ten physico-mechanical characteristics of the harvested plants is required. Data on the characteristics of plants of winter wheat, cultivated in the conditions of the south of Russia, was accumulated by us for six years. The data on the non-uniformity of plant characteristics of five varieties of winter wheat are obtained. Measurements were carried out within the field to be cleaned, individual plots and grown in one bush. Geometric characteristics of plants, such as their height, length of stem and ear can be estimated with a certain degree of accuracy with a small number of measurements and visually, with the selection of the most different sites on the harvested massif. This article illustrates the influence of the geometric parameters of the plants of the crop being harvested on the quality of the process being performed and the possibility of controlling it by regulating the operating modes and controlling them.