Changes in weed populations in the conversion of two arable farms to organic farming

There is increasing interest in policies encouraging organic or biological farming. Farmers in the UK are not converting to organic farming in large numbers, with one principal reason being the problem of weed control. There is some evidence that weed problems reduce over time, but it is uncertain to what degree weed populations increase during conversion at the field rather than plot scale and how management strategies affect that increase. The results of surveying two farms in conversion over 4 seasons suggest weed populations increase rapidly, but there is evidence on one farm with fields in longer conversion periods that the weed population growth stabilises. However, periods of grass ley of greater than two seasons duration greatly reduce weed population growth during the conversion period. There is no evidence of an increase in weed species number during the conversion period.