Introduction The acetolactate synthase (ALS) herbicides are a group of pre- and post-emergent herbicides for use in cropping, and are important because they are inexpensive, easy to use and have low mammalian toxicity. They include the sulfonylureas, the sulfonamides and the imidazolinones. Though widely used in cereals, they are reputed to persist in soil and can damage sensitive crops such as legumes and oilseeds sown in subsequent years. Residues of chlorsulfuron have persisted for several years to damage sensitive crops at concentrations as low as 0.1 ng/g in alkaline soils in Canada (Moyer et al. 1990). The sulfonylureas persist longer under alkaline conditions (Beyer et al. 1988). In alkaline soils of south- eastern Australia they have persisted for at least 18 months (Ferris et al. 1995; Stork 1995). Another study found that sulfonylureas reduced dry matter (DM) of medic, lentil and canola crops as long as 2 years after application (Black et al. 1999). Our own recent study showed that persistence of chlorsulfuron and triasulfuron can exceed 2 years (Hollaway et al. 2006a), causing concern regarding the potential for these residues to damage sensitive crops. As no south-eastern Australian studies continued long enough to fully understand persistence in alkaline soils, we sought information from other locations. The Canadian experience is of considerable concern, as chlorsulfuron residues damaged lentil crops up to 7 years after application of 40 g a.i./ha in alkaline soils (Moyer et al. 1990). In contrast, sensitive species, including chickpeas, were able to be safely planted less than 12 months after treatment with in alkaline soils in Queensland (Osten and Walker 1998). Clearly, comparing data from other locations with similar soil types is not sufficient. To address the concerns of the local farming community local information was required. We needed to measure residues for longer than in previous studies and to assess the impact of known soil residues of chlorsulfuron, triasulfuron, metsulfuron-methyl, imazethapyr and flumetsulam applied 1, 2 or 3 years before sowing sensitive crops such as canola, pea, lentil and medic. This aim was justified by the very long persistence of chlorsulfuron and imazethapyr compared with the other herbicides, and the ability of some sensitive crops to recover from residue damage when others failed to yield. Australian Journal of Experimental Agriculture, 2006, 46, 1323–1331 0816-1089/06/101323 10.1071/EA05053 © CSIRO 2006 K. L. Hollaway A,D , R. S. Kookana B , D. M. Noy A , J. G. Smith A and N. Wilhelm C A Department of Primary Industries, Private Bag 260, Horsham, Vic. 3401, Australia. B CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia. C South Australian Research and Development Institute, Plant Research Centre, GPO Box 397, Adelaide, SA 5001, Australia. D Corresponding author. Email: katherine.hollaway@dpi.vic.gov.au Abstract. Grain growers in south-eastern Australia have reported unexpected crop failures with theoretically safe recropping periods for acetolactate synthase herbicides in alkaline soils. This experience has led to the concern that these herbicides may degrade very slowly in alkaline soils, and herbicide residues have at times been blamed for unexplained crop losses. To address this issue, we established 5 recropping trials across Victoria and South Australia with 5 acetolactate synthase herbicides (chlorsulfuron, triasulfuron, metsulfuron-methyl, imazethapyr, and flumetsulam). The herbicides were applied to separate plots in years 1, 2 or 3, and sensitive crop species were sown in year 4 to measure the impact of herbicide residues. We observed that the persistence of the sulfonylureas (chlorsulfuron, triasulfuron, metsulfuron-methyl) varied between herbicides, but all persisted longer in alkaline soils than in acid soils, and were, therefore, more likely to damage crops in alkaline soil. Imazethapyr persisted longer in clay soils than in sandy soils and was, therefore, more likely to damage crops in clay soils. All herbicides persisted longer when rainfall was below average. Canola was more sensitive to imazethapyr than either pea, lentil or medic, but was less sensitive to the sulfonylureas. In contrast, lentil and medic were the most sensitive to sulfonylureas. Despite some damage, we found that safe recropping periods could be predicted from the product labels in all but one situation. The sole exception was that metsulfuron-methyl reduced dry matter and yield of lentil and medic sown 10 months after application in a soil with pH 8.5. We hypothesise that the real cause of crop failure in many situations is not unusual herbicide persistence, but failure to take full account of soil type (pH and clay content including variation in the paddock) and rainfall when deciding to recrop after using acetolactate synthase herbicides. Crop damage caused by residual acetolactate synthase herbicides in the soils of south-eastern Australia www.publish.csiro.au/journals/ajea CSIRO PUBLISHING