EFFECT OF CROP ROTATIONS ON WEED-CONTROL, BIDENS CERNUA AND ERIGERON-CANADENSIS POPULATIONS, AND CARROT YIELDS IN ORGANIC SOILS

Organic coils in Quebec are prone to infestation by nodding beggartick s (Bidens cernua L.) and Canada fleabane (Erigeron canadensis L.), and by the root-knot nematode (Meloidogyne hapla Chitwood). Yield losses can be very severe in fields where carrots have been grown continuousl y for several years. Six 3-year crop rotations, including onion (Alliu m cepa L.), barley (Hordeum vulgare L.) or a weed fallow, were compare d to continuous carrot production to establish their effect on weed co ntrol and carrot yield. Weed populations and above ground dry biomass were monitored four times each season. Total and marketable yields of carrots also were measured each year. Results show that weed populatio ns increase when onions are part of the production cycle, and decrease with barley. Beggarticks and fleabane populations were static when cr opping sequences included 2 years of carrots with or without onions. T otal carrot yield increased by 35-50% and marketable yield increased 1 7-25-fold due to a drastic reduction in root-knot nematode populations when barley was included in the rotation. The onion-barley-carrot rot ation provided good weed control, including nodding beggarticks and Ca nada fleabane, and yielded high quality carrots.