Smooth barley is an annual weed species that is infesting crops and pastures in South Australia. Complicating control options is the presence of herbicide-resistant biotypes. A field trial was conducted to identify alternative herbicides for the management of acetyl coenzyme A carboxylase (ACCase)-inhibiting herbicide-resistant smooth barley in field pea. Preplant (PP) soil applications of pyroxasulfone; prosulfocarb plus S-metolachlor; dimethenamid-P; propyzamide; trifluralin alone or with triallate or with diuron; or imazamox applied POST were evaluated for their effectiveness and crop safety. Propyzamide, pyroxasulfone, or imazamox applied POST provided a high level of smooth barley control, did not cause any crop injury, and increased field pea grain or forage yield compared with the nontreated. Furthermore, propyzamide or pyroxasulfone reduced panicle density and seed production in smooth barley, whereas the effectiveness of POST imazamox varied over the two seasons. Dimethenamid-P reduced the impact of smooth barley on field pea yield, but cause stunting, and was less effective than propyzamide, pyroxasulfone, and imazamox in reducing smooth barley seed production. Negative relationship between field pea yield and smooth barley panicle density indicated that smooth barley is highly competitive in field pea crops and can cause large yield losses. The results of this investigation suggest that propyzamide or pyroxasulfone applied PP and imazamox applied POST could be used effectively in the field for the management of ACCase-inhibiting herbicide-resistant smooth barley in South Australia.

Weedy barley species have emerged as important weeds in southern Australia, where they can be particularly difficult to control in cereal crops. Knowledge of seed dormancy mechanisms, germination ecology, and recruitment behavior in the field would facilitate development of effective weed-control programs for these weed species. Based on somatic chromosome number, smooth barley was identified as the species infesting all the sites sampled in South Australia. Smooth barley populations from cropping fields and noncrop habitats showed large differences in their pattern of dormancy loss. Noncrop populations (EP2, EP3, and MN2) rapidly lost dormancy during dry after-ripening and showed 70 to 95% germination at 3 mo after maturity. Five populations collected from cropping fields (EP1, EP4, EP5, MN1, and MN3), on the other hand, showed < 30% germination, even at 8 mo after maturity, when germination was assessed at 20/12 C day/night temperatures. These dormant, smooth barley populations from cropping fields were found to be highly responsive to cold stratification, with germination increasing in response to the duration of the treatment. Germination of dormant, smooth barley populations increased with the addition of gibberellic acid (0.001 M GA3), but only when lemma and palea had been removed. Recruitment behavior of smooth barley in the field was influenced by the population and the tillage system. A nondormant population, collected from a long-term pasture (MN2), showed high seedling emergence (> 90%) during autumn, which was well before planting of the winter crop (lentil). In contrast, the other three populations sampled from cropping fields showed very little seedling establishment (< 10%) before crop planting, which would make them difficult to control in cereals because there are no selective herbicides available for the control of weedy barley species. There was a significant seeding system by emergence time interaction (P < 0.001), which was reflected in greater in-crop, smooth barley plant densities under zero-till than under conventional tillage and no-till systems.