The aim of this investigation was to study allelopathic and competitive relationships between weeds and small grains by grain harvest index (GHI) of small grain in order to provide more information about these complex relationships. In the study were included small grains: wheat ( Triticum aestivum), barley ( Hordeum sativum), oat ( Avena sativa L.), triticale (Triticosecale Witt.) and rye ( Secale cereale) and some weed species: Agropyron repens, Cirsium arvense, Sonchus arvensis, and Sorghum halepense. Applied extracts of weed species mainly expressed suppressed effect on GHI, but stimulating actions were noticed, too. Positive influence, on GHI was registered at Agropyron repens and Sonchus arvensis variant in wheat and at Agropyron repens segments variant in oat. The most expressive grain harvest index decreasing was noticed in Sinapis arvensis variant, while oat showed the least sensitiveness towards weed presence by GHI aspect. The intensity of competitive relationships between weed species and wheat, considering grain harvest index, depended on sprouting quickness and percentage of studied weed species.
