Abstract
Alfalfa (Medicago sativa L.) fed as hay to livestock, was the first forage crop in the Middle East and the Arabian Gulf countries, which contributed to the success of dairy industry. The invasion of alfalfa fields by weeds was found a serious problem for farmers since it tends to reduce the quality and the quantity of the crop. In this study, 60 weed plant species were found in 13 private fields in Hail region (Saudi Arabia). Invasive weeds reduce alfalfa yield from 22.66 to 11.94 t/ha after 5 years of culture. In the same period, weed density increased from 21.3 to 69 plants/m(2). The family dominance index (FDI) shows that Poaceae, Asteraceae and Chenopodiaceae were the most abundant families in alfalfa fields. Cynodon dactylon (L.) Pers., Portulaca oleracea L., Conyzabonariensis (L.) Cronquist, Malva parviflora L., Chenopodium murale L., Setaria verticillata (L.), P. Beauv., Eleusine indica (L.) Gaertn, Digitaria sanguinalis (L.) Scop, Tribulus terrestris L., Convolvulus arvensis L. and Echinochloa colona (L.) Link. were the most abundant, frequent and noxious weeds in alfalfa fields according to their Total Relative Dominance (TRD). The harmful effect of the most noxious weeds was reported.