Performance of overwinter cover crops in coastal Lebanon

The effect of the cover crops, narbon vetch (Vicia narbonensis), oat (Avena sativa), forage radish (Raphanus sativus), on weed suppressionandsoil nitrogenwas studied in field conditions overwinter. The impact of their residues on the productivity of a subsequent corn crop was followed in the summerof the same year. Between mid-November andMarch oat produced, on a dry weight basis,915 g m-2against 402 g m-2for narbon vetch and 292 g m-2for radish. Consequently, weeds were suppressed best in the presence of oat. Narbon vetchwas less effective than radishin controlling weeds, as it was slowerto get established. In terms of mineral nitrogen, soils occupied by radish (13.1 mg kg-1soil) presented smaller concentrations of ammonium-Nthanthose by vetch(18.7 mg kg-1soil)and the control(19.7 mg kg-1soil). Following the overwinter covercrops, plots were split intoone half whereroots only were incorporated,and another half receivingwhole plants. Residues of whole plants promoted higher production of corn in the booting and flowering stages, as compared to roots only. At the 50% doughstage, the highest dry matter yield was 7.6 Mgha-1afternarbon vetch. Cornyield was the smallest after oat, due to N immobilization. This was demonstrated, using the litter bag technique, when oat residues (15.6g N kg-1) lost 31% of their N against 66% for radishresidues(21.1g N kg-1).For an effective control of weeds and nutrient recycling a cover crop, such as oat, is most efficient. In this case, the succeeding summer crop needs to receive some extra nitrogen.