Field studies were conducted in Mississippi, USA, from 1998 to 2000, to compare weed population shifts in soyabean and cotton using a total glyphosate system, preemergence (PRE) herbicides followed by glyphosate, and a conventional herbicide programme. In the first year of the soyabean study, populations of hemp sesbania ( Sesbania exaltata) were highest for treatments of PRE herbicides followed by either glyphosate or the conventional herbicide programme because of better control from the total glyphosate system. Barnyardgrass ( Echinochloa crus-galli) populations in the first year of the study for the untreated plots were 0 plants/m 2 but increased in the third year to 61 plants/m 2. Flumetsulam + metolachlor followed by glyphosate at the lower rates and the untreated control were the only treatments in which there was an increase in barnyardgrass over the 3-year study. Broadleaf signalgrass ( Urochloa platyphylla) populations increased in the third year with 0.1 kg ai flumetsulam + 2.1 kg ai metolachlor/ha followed by 0.84 kg ae glyphosate/ha, primarily because of reduced competition from lower populations of other weeds such as hemp sesbania. Pitted morningglory ( Ipomoea lacunosa) populations for all treatments decreased in the third year because of good control of this species and the high level of interference from other weed species in the first 2 years. Johnsongrass ( Sorghum halepense) populations decreased in the third year with 0.4 kg ai flumetsulam + 1.1 kg ai metolachlor/ha followed by 0.84 kg glyphosate/ha. Johnsongrass populations decreased with timely glyphosate sequential applications, with 5 plants/m 2 in 1998 and 0 plants/m 2 in 2000. Yields increased from the first year to the second year, corresponding to reduced weed pressure, and yields varied from 710 to 1420 kg/ha. Because of weed pressure, soyabean yields were not different in any of the treatments, including the untreated, although treatments changed the species present. In the cotton study, weed populations over 3 years decreased, with the most significant reductions from the treatments of fluometuron + prometryn + metolachlor followed by either pyrithiobac or glyphosate. Weeds that showed the most significant decline were barnyardgrass and hemp sesbania, whereas johnsongrass increased, with 27 plants/m 2 in treatments of 0.6 kg ai fluometuron + 0.3 kg ai prometryn + 0.7 kg ai metolachlor/ha followed by 0.84 kg glyphosate/ha. Lint cotton yields varied from 0 to 128 kg/ha. Because of the weed pressure, cotton yields were not different in any of the treatments, although treatments changed the species present. This research has shown that weed species can decrease over time with the continued use of any of these herbicide programmes.
