Herbicide resistance, documented in many economically damaging weed species, is a major threat to global crop production. The injudicious use of herbicides, often in the absence of diverse weed control strategies, poses an immense selection pressure on weed communities for resistance evolution and weed adaptive traits such as high seed dormancy. This study evaluates the interaction among developing herbicide resistance, seed size, and seed dormancy of ripgut brome (Bromus diandrus Roth), wild oat (Avena fatua L.), and hare barley [Hordeum leporinum Link; syn. Hordeum murinum L. ssp. leporinum (Link) Arcang.] collected from within intensively managed fields (in-crop) in comparison with populations in surrounding ruderal (non-crop disturbed) areas with no history of exposure to herbicides within the Western Australian grainbelt. Seed size of the three species varied by farming system (continuous cereal-intensive annual crops, diverse annual crops, pasture based) and habitat (in-crop, ruderal). Field populations of H. leporinum and B. diandrus tended to have greater seed size compared with ruderal populations. Larger seeds had significantly more dormancy in all three weed species. Field-collected populations that were exposed to herbicide applications for at least the past 5 yr exhibited significantly greater seed dormancy compared with their counterparts present in ruderal areas within the same geographic area. The association between increased seed dormancy and developing multiple herbicide resistance further complicates effective weed management.

A biotype of the annual grass weed hare barley infesting an alfalfa field with a 24-yr history of the use of the bipyridylium herbicides paraquat and diquat, was investigated for resistance to these herbicides. Rates of up to 800 g ai ha–1 of each herbicide caused no mortality in the hare barley plants from this field. The same species, collected from an adjacent pasture field with no history of bipyridylium herbicide application, exhibited LD50's of 57 and 160 g ai ha–1 for paraquat and diquat, respectively. Tiller numbers and dry matter production in the biotype from the alfalfa field were not affected by the normal rate recommended for both herbicides. These results clearly show that hare barley from the alfalfa field is resistant to paraquat and diquat. Both biotypes were equally sensitive to fluazifop, glyphosate, and sethoxydim.

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.