Although clear evidence for benefits in crop production is partly missing, several natural compounds and microorganisms have been introduced to the market as biostimulants. They are supposed to enhance nutrient efficiency and availability in the rhizosphere, reduce abiotic stress, and improve crop quality parameters. Biostimulants often derive from natural compounds, such as microorganisms, algae, and plant extracts. In this study, the commercial plant extract-based biostimulant ComCat^® was tested in two field experiments with maize in the communities of Banikoara and Matéri in Northern Benin and six pot experiments (four with maize and two with winter barley) at the University of Hohenheim in Germany. Maize was grown under nutrient deficiency, drought, and weed competition, and winter barley was stressed by the herbicide Luximo (cinmethylin). ComCat^® was applied at half, full, and double the recommended field rate (50, 100, and 200 g ha⁻¹) on the stressed and unstressed control plants as leaf or seed treatment. The experiments were conducted in randomized complete block designs with four replications. The above-ground biomass and yield data of one experiment in Benin were collected. The biostimulant did not promote maize and winter barley biomass production of the unstressed plants. When exposed to stress, ComCat^® resulted only in one out of eight experiments in higher barley biomass compared to the stressed treatment without ComCat^® application. There was a reduced phytotoxic effect of cinmethylin after seed treatment with ComCat^®. Crop response to ComCat^® was independent of the application rate. Basic and applied studies are needed to investigate the response of crops to biostimulants and their mechanisms of action in the plants before they should be used in practical farming.