Refers to the pathway by which carbon dioxide is fixed. Cool season grasses typically fix carbon dioxide by the C3 cycle. In the first step of carbon fixation, carbon dioxide is fixed to a 5 carbon molecule called ribulose bisphosphate. The initial 6-carbon compound that is formed from the combination of ribulose bisphospate and carbon dioxide quickly splits into two 3-carbon compounds called phosphoglyceric acid (PGA). Thus the term C3 comes from the initial formation of the 3-carbon compounds.