RESIDUAL EFFECTS OF ALFALFA AND BROMEGRASS CROPPING ON YIELDS OF WHEAT GROWN FOR 15 SUBSEQUENT YEARS

Twelve crops of wheat (Triticum aestivum L.) were grown over a 15-yr period after forage hay swards or a fallow-wheat rotation (control) on a fine-textured soil in northern Alberta. Nitrogen fertilizer was not applied to the forage crops nor to the wheat. Yields of wheat following alfalfa (Medicago sativa L.) ranged from 66 to 114% greater than those following control for the first eight crops. Yield differences started to decline thereafter, although the wheat yield after alfalfa was significantly higher than that after the control for the 10th crop in the 13th year after the sward breaking. The alfalfa contributed to the wheat yields through both the subsoil and topsoil as determined by interchanging topsoil between the former alfalfa and control plots. Yields of wheat after an alfalfa and bromegrass (Bromus inermis Leyss.) mixture were only slightly less than those after alfalfa alone, however, those after bromegrass alone were much less than those following alfalfa. Key words: Alfalfa, bromegrass, yie...