The aim of this research was to examine the effect of fertilizer treatment on the content of polyphenols in velvetleaf and maize, grown without use of herbicides under controlled conditions (12h/12h, T 25º/22ºC day/night). Plants were treated with organic - F1 and synthetic fertilizers - F2 and F3. The content of total phenols, individual polyphenolic acids and antioxidant activity of plant extracts were examined, as important indicators of the plant condition and its resistance to oxidative stress. The content of total phenols was determined spectrophotometrically (modified Folin-Ciocalteu method). The content of individual polyphenolic acids was determined by the high-performance liquid chromatography method, while measurement of polyphenols antioxidant activity was determined by the DPPH method. It was established that in maize, the application of fertilizers, neither in monoculture nor in competition with velvetleaf plants, caused statistically significant changes in the content of polyphenolic acids. However, in velvetleaf, the use of fertilizers led to an increase in the content of polyphenolic acids in conditions of competition vs monoculture. The content of chlorogenic, p-coumaric and trans-ferulic acids was statistically, significantly higher in plants of velvetleaf competition vs monoculture, and the content of chlorogenic, p-coumaric and cinnamic acids was lower in plants of maize competition vs monoculture. A statistically significant increase in the content of total polyphenols and their antioxidant activity in velvetleaf plants indicates the difficulty of removing weeds from crops, especially resistant populations. The research conducted and the results obtained confirm that weeds in competitive conditions manifest their natural competitive advantage, which is further enhanced by crop feeding.