Field Assessment of Crop Residues for Allelopathic Effects on Both Crops and Weeds

The use of herbicides on crop fields has created shifts in weed populations and the emergence of herbicide resistance. Because of this phenomenon, there is interest in the exploitation of allelopathic activity of crop plants. This was assessed in field experiments at the Tygerhoek Research Farm (19 degrees 54 '' E, 34 degrees 08 '' S), South Africa, to determine whether significant weed control could be achieved via allelopathy. This locality contains weakly developed residual soils (pH 5.1) of Mispah (Entisol) type containing 22% clay and 1.6% carbon. Uniform scattering of a quantity of plant residues preceded drilling. The rotational plant species planted into the plant residues consisted of barley (Hordeum vulgare L. variety Clipper), canola (Brassica nap us L. variety ATR Hyden), wheat (Triticum aestivum variety SST 88), lupines (Lupinus albus L. variety Tanjil), alfalfa (Medicago sativa L. variety SA standard), medic (Medicago truncatula Gaertn. variety Parabinga), and ryegrass (Lolium multiflorum Lam. variety Energa). These crops are used in crop rotation in this grain production area. Six plant species were used in the second experiment, planted at 90 degrees angle across the first after the latter was harvested, namely barley, wheat (variety SST 027), lupines (variety Tanjil and variety Quilinock), ryegrass and ryegrass weed type (L. multiflorum x perenne). Broadleaf weeds comprised 88.5% of total weed spectrum. Medic suppressed ryegrass weed type while lupines suppressed grass weeds. An acceptable production practice using allelopathic crops for weed control will likely consist of combining continued limited amounts of herbicides with leguminous crop residues.