Composites prepared with whole sorghum stem fragments reinforcing a polyethylene matrix were studied using ten different sorghum genotypes. We first built of a robust protocol ensuring an accurate assessment of composite physical and mechanical properties. It shows that for a given sorghum genotype, the composition of the stem fragments is depending on the size of the fragments. This justifies the need to use a similar stem fragment size for comparing genotypes. During compounding and injection, fragments are broken, to an extend which depends on genotypes. This robust processing protocol shows that there are large differences in the final mechanical properties of the composites depending on the genotype. This is correlated with the histological structure and the biochemical composition of each genotype, showing correlations between the amount of cellulose, hemicellulose and lignin present in the stem fragments with the mechanical properties of the composites, as well as the importance of the location of cellulose (sclerenchym fibers versus parenchym tissues and vessels). We will show the difficulties of working with small, well identified amounts of biological materials when dealing with composite-filler biochemical and structural characteristics which are heavily varying between on position to another along the plant stem. This work is the first step of a plant breeding process for identifying the best sorghum varieties to be used for preparing polymer composites.