A simple and cost-effective technique has been suggested for online monitoring of grist concentration in fodder. The technique is based on fluorescence imaging with grow light lamp excitation and a consumer CMOS camera (DSLR or smartphone) for photo capturing. A prototype instrument has been developed and tested in the laboratory for quantitative express determination of rapeseed grist concentration in fodder. In situ measurement of grist concentration during cattle food preparation has been demonstrated, and the perspectives were discussed. The developed instrument has the potential to ensure more accurate preparation of individual cattle diets compared to currently available methods, which will improve the efficiency of the cattle food production.