Samuel G. L. Kleemann

Differences in the distribution and seed germination behaviour of populations of Bromus rigidus and Bromus diandrus in South Australia: adaptations to habitat and implications for weed management

A field survey was undertaken in South Australia to determine the relative distribution of the brome grass species B. diandrus and B. rigidus. Seeds of brome grass plants were collected from locations across the Yorke (n = 10) and Eyre Peninsulas (n = 25). B. rigidus was found more frequently and at higher densities in South Australian crops than B. diandrus, which showed a distinct preference for undisturbed fence-line margins. Species identity of brome plants in each sample was initially determined by assessing morphology of the callus-scar of the caryopsis as well as the structure of the panicle. Species identity was later confirmed by counting somatic chromosome number. There was consistent agreement between the 2 approaches to identification, indicating that these morphological features can be used with confidence when identifying B. diandrus and B. rigidus in the field. Although B. diandrus and B. rigidus are morphologically very similar, they showed large differences in germination behaviour. B. diandrus seeds collected from fence-line margins were more germinable than B. rigidus from neighbouring cropped areas. Populations of B. rigidus also showed strong inhibition of seed germination when exposed to light. This inhibitory effect of light on seed germination was not seen in the B. diandrus collections. Two populations of B. rigidus from Yorke Peninsula showed little germination (<15% germination in complete darkness) until well after the start of the next growing season. These 2 populations did, however, show a large response to treatment with gibberellic acid (1 mm), indicating high seed viability but presence of deep dormancy. From a practical point of view, the germination behaviour (longer dormancy and light inhibition) exhibited by B. rigidus would allow this species to proliferate under conservation tillage systems such as no-till, where seeds only experience complete darkness after burial following the sowing operation. Germination behaviour of B. rigidus observed in this study is expected to contribute to greater seed carry-over from one season to the next, and favour its colonisation in crops, as seen in the current field survey.

Factors Affecting Seed Germination of Perennial Wall Rocket (Diplotaxis Tenuifolia) in Southern Australia

Abstract Germination response of perennial wall rocket to temperature, light, osmotic potential, and depth of burial emergence was evaluated under controlled environmental conditions. The effect of seed burial depth on seedling recruitment in the field was also investigated at Roseworthy, South Australia. Under optimal conditions (30 C, light/dark) germination of perennial wall rocket was rapid, with 90% of seeds germinating within 48 h of imbibition. Germination was reduced (20%) at lower, suboptimal temperatures (10 to 20 C) when seeds of perennial wall rocket were exposed to light. Germination declined with increasing osmotic potential and was completely inhibited at osmotic potentials of −1.5 MPa. Perennial wall rocket emergence was greatest from seeds placed on the soil surface, but some seedlings (< 10%) emerged from a depth of 0.5 to 2 cm. Under both field and growth-cabinet conditions, the greatest seedling emergence of perennial wall rocket occurred from seed present on the soil surface; however, the level of absolute recruitment from the seed bank was much lower (< 5%). Information gained from this study will further improve our understanding of the germination behavior of perennial wall rocket and contribute to developing sustainable strategies for its control. Nomenclature: Perennial wall rocket, Diplotaxis tenuifolia (L.) DC. DIPTE

Population Ecology and Management of Rigid Brome (Bromus rigidus) in Australian Cropping Systems

Abstract Rigid brome is a problematic weed of southern Australian cropping systems. Increased knowledge about the ecology of rigid brome and the influence of management strategies on its seedbank dynamics could facilitate development of more effective weed control programs. A field study was undertaken to investigate seedbank persistence and the influence of different management strategies on rigid brome control at Lock in South Australia during 2003 to 2005. Seeds of rigid brome were found to persist in the soil for up to 3 yr, with > 20% of the seedbank persisting from one season to the next. Therefore, a single year management program against this weed species is likely to be ineffective and could result in rapid buildup in weed infestations. However, management strategies that combined effective herbicides (ClearfieldTM technology) and crop competition over consecutive years provided effective control of this troublesome weed. Such cropping systems reduced rigid brome density (1 to 10 plants m−2) and seed production (8 to 160 seeds m−2) in the final crop of the 3-yr cropping sequence as compared to common grower practice of trifluralin and triasulfuron mixtures (138 plants m−2; 1,866 seeds m−2). These treatment combinations were able to deplete the initial seedbank (1,748 seeds m−2) to manageable levels (< 5 seeds m−2) within 3 yr. The results of this study should provide growers with confidence that severe rigid brome infestations can be managed effectively without compromising crop productivity. Nomenclature: Triasulfuron; trifluralin; rigid brome, Bromus rigidus Roth BRORI

Seed Dormancy and Seedling Emergence in Ripgut Brome (Bromus diandrus) Populations in Southern Australia

Abstract Ripgut brome is a difficult weed to manage in cereal crops of southern Australia because only a few herbicides can provide effective control in cereals. Knowledge of seed-dormancy mechanisms, germination ecology, and emergence behavior in the field could facilitate development of effective weed control programs for this weed species. Ripgut brome populations from cropping fields were found to possess much longer seed dormancy than that reported previously in the literature. Furthermore, some ripgut brome populations from cropping fields showed longer seed dormancy than those collected from adjacent noncropped fence lines. For example, all seeds of one of the populations from the fence line (SA-1F) germinated at 3 mo after maturity, whereas seeds from the cropping field at the same site (SA-1C) showed little germination (< 3%) even at 8 mo after maturity. These highly dormant ripgut brome populations from cropping fields were responsive to cold stratification, with germination increasing significantly after 2 to 14 d of exposure. Germination of dormant ripgut brome populations increased with addition of gibberellic acid (0.001 M GA3), particularly when lemma and palea had been removed. Ripgut brome populations from cropping fields (VIC-2C and SA-1C) showed strong inhibition of seed germination when exposed to light. These differences in seed dormancy among ripgut brome populations were also expressed in seedling emergence pattern in the field. The nondormant populations collected from fence lines showed high seedling establishment (> 80%) during autumn, which coincided with the planting time of winter crops in southern Australia. In contrast, five populations from cropping fields showed much lower seedling establishment (3 to 17%) before the time of crop planting. Delayed seedling establishment in populations from cropping fields could lead to less effective preseeding weed control and higher weed infestations in field crops. Results of this study also showed that the seedbank of these highly dormant ripgut brome populations can readily persist from one year to the next. Effective management of ripgut brome populations with long seed dormancy and increased seedbank persistence would require a major change in cropping systems used by the growers in southern Australia. Nomenclature: Ripgut brome, Bromus diandrus Roth.

Influence of row spacing on water use and yield of rain-fed wheat (Triticum aestivum L.) in a no-till system with stubble retention.

A 3-year field study was undertaken to investigate the effect of row spacing on vegetative growth, grain yield and water-use efficiency of wheat. All 3 years of the study experienced 21–51% below-average rainfall for the growing season. Widening row spacing led to reduced biomass and tillers on per plant basis which could be related to the reduction in light interception by the wheat canopy in the wide rows which in turn could have reduced assimilate production. Reduction in vegetative growth in 54-cm rows translated into a significant reduction in grain yield which was strongly associated (r2 = 0.71) with the loss of spike density. The pattern of crop water use (evapotranspiration, ET) during the growing season was very similar for the three row-spacing treatments. However, there was some evidence for slightly lower ET (~5%) in 54-cm rows in two growing seasons. More importantly, there was no evidence for increased ET during the post-anthesis phase in wide rows as has been speculated by some researchers. Over the 3 years of the study, grain yield declined by 5–8% as row spacing increased from 18 to 36 cm and by a further 12–20% as row spacing increased from 36 to 54 cm. There was a consistent decline in water-use efficiency for grain (WUEG) with increasing row spacing over the 3 years. WUEG declined by 6–11% as crop spacing increased from 18 to 36 cm and declined further by 12–15% as row spacing increased to 54 cm. Lower light interception at wider row spacing could have reduced assimilate production by wheat as well as increased soil evaporation due to lower shading of the soil surface in more open canopies. Growers adopting wider row spacing on these relatively heavy textured soils are likely to experience some reduction in grain yield and WUEG. However, some growers may be prepared to accept a small yield penalty from intermediate row spacing as a trade-off for increased stubble retention and soil health.

Differential tolerance in wheat (Triticum aestivum L.) genotypes to metribuzin

Field and glasshouse experiments were undertaken at 2 locations in South Australia to evaluate wheat (Triticum aestivum L.) tolerance to metribuzin. Sloop SA barley (Hordeum vulgare L.) was used as a standard in the evaluation. Linear and logistic regression models were used to describe the response of wheat genotypes to metribuzin. Parameter estimates of B (slope) and ED50 (dose required for 50% inhibition) estimated by the models were used to compare the responses of the genotypes with that of the known sensitive cv. Spear. In the field, wheat cvv. Blade, Kite, EGA Eagle Rock, and Sloop SA barley showed tolerance to metribuzin, exhibiting little change in seedling density and anthesis dry matter (DM) when treated with metribuzin (187.5 and 375 g/ha). In contrast, wheat cv. Spear, which has 50% common parentage with Blade, showed a strong negative response to metribuzin for both seedling density (B = –0.308) and anthesis DM (B = –0.482), indicating sensitivity to the herbicide. Sonora 64 and Tezanos Pinto Precoz, parental lines of Blade, were also sensitive to metribuzin (>187.5 g/ha), showing similar negative responses to that of Spear for anthesis dry matter (B = –0.307 and –0.387). However, Kite, which is another parent of Blade, showed excellent tolerance to this herbicide, exhibiting only a small response (B = –0.076), which was statistically (P < 0.05) different from that of Spear. In the second field study, cvv. Blade, EGA Eagle Rock, and Sloop SA barley were again far more tolerant to metribuzin than the sensitive cv. Spear, requiring almost double the dose of metribuzin to induce 50% inhibition (ED50). Glasshouse studies, where metribuzin was highly active in a light sandy potting soil at what would be considered low rates in the field (50 and 100 g/ha), confirmed the sensitivity differences among wheat genotypes observed in the field. Wheat genotypes Blade, Kite, EGA Eagle Rock, and daughter line RAC 0824 were consistently tolerant to metribuzin. Sensitivity to metribuzin (50 g/ha) was observed in Spear, and most parental lines of Blade with the exception of Kite. Interestingly, Kite showed equivalent tolerance to its progeny, Blade and EGA Eagle Rock, exhibiting similar ED50 and B (slope) values. It is noteworthy that EGA Eagle Rock has recently been specifically bred for tolerance to metribuzin using Blade as a parent. Considering Kite was the only parent to show tolerance to metribuzin in these field and glasshouse studies, it would appear to be the major contributor to metribuzin tolerance in cv. Blade. At present, Kite is not favoured by wheat breeders due to the presence of a rust-resistant gene (SR26) linked to yield penalty. Further research is required to determine alleles responsible for metribuzin tolerance in wheat and to identify alternative sources of metribuzin tolerance.

Influence of Management on Long-Term Seedbank Dynamics of Rigid Ryegrass (Lolium rigidum) in Cropping Systems of Southern Australia

Abstract A field study was undertaken to investigate the influence of different management strategies on rigid ryegrass plant density and seedbank dynamics over 4 yr. Even though weed seedbank declined by 86% after oaten hay in year 1, the residual seedbank enabled rigid ryegrass to reinfest field peas the next year, and the population rebounded sharply when weed control relied solely on PPI trifluralin. However, use of POST clethodim followed by crop-topping for seed-set prevention of rigid ryegrass in field pea was highly effective and caused a further decline in the weed seedbank. Integration of effective management tactics over 3 yr significantly reduced rigid ryegrass weed and spike density (90 and 81%) in the final year of the 4-yr cropping sequence. Use of oaten hay in year 1, followed by effective weed control in field pea and wheat crops, depleted the high initial seedbank (4,820 seeds m−2) to moderate levels (< 200 seeds m−2) within 3 yr. Effective weed-management treatments depleted the rigid ryegrass seedbank, reduced in-crop weed infestation, and returned higher grain yields and profitability. The results of this study clearly show that large rigid ryegrass populations can be managed effectively without reducing crop productivity and profitability provided multiyear weed-management programs are implemented effectively. Nomenclature: Clethodim, trifluralin, rigid ryegrass, Lolium rigidum Gaudin LOLRI, field pea, Pisum sativum L, oat, Avena sativa L.; wheat; Triticum aestivum L.

Alternative Herbicides for the Management of Clethodim-Resistant Rigid Ryegrass (Lolium rigidum) in Faba Bean (Vicia faba L.) in Southern Australia

Abstract Two field experiments were conducted during 2012 and 2013 at Roseworthy, South Australia to identify effective herbicide options for the management of clethodim-resistant rigid ryegrass in faba bean. Dose–response experiments confirmed resistance in both field populations (B3, 2012 and E2, 2013) to clethodim and butroxydim. Sequencing of the target site of acetyl coenzyme A carboxylase gene in both populations identified an aspartate-2078-glycine mutation. Although resistance of B3 and E2 populations to clethodim was similar (16.5- and 21.4-fold more resistant than the susceptible control SLR4), the B3 population was much more resistant to butroxydim (7.13-fold) than E2 (2.24-fold). Addition of butroxydim to clethodim reduced rigid ryegrass plant density 60 to 80% and seed production 71 to 88% compared with the standard grower practice of simazine PPI plus clethodim POST. Clethodim + butroxydim combination had the highest grain yield of faba bean (980 to 2,400 kg ha−1). Although propyzamide and pyroxasulfone plus triallate PPI provided the next highest levels of rigid ryegrass control (< 60%), these treatments were more variable and unable to reduce seed production (6,354 to 13,570 seeds m−2) to levels acceptable for continuous cropping systems. Nomenclature: Clethodim; rigid ryegrass, Lolium rigidum Gaudin; faba bean, Vicia faba L. Resumen En 2012 y 2013, se realizaron dos experimentos de campo en Roseworthy, en el sur de Australia, para identificar opciones de herbicidas efectivos para el manejo de Lolium rigidum resistente a clethodim en campos de haba. Experimentos de respuesta a dosis confirmaron la presencia de resistencia a clethodim y butroxydim en ambas poblaciones de campo (B3, 2012 y E2, 2013). La secuenciación del sitio activo del gen de acetyl coenzyme A carboxylase identificó la mutación aspartate-2078-glycine en ambas poblaciones. Aunque la resistencia a clethodim de B3 y E2 fue similar (16.5 y 21.4 veces más resistentes que el control susceptible SLR4), la población B3 fue mucho más resistente a butroxydim (7.13 veces) que E2 (2.24 veces). La adición de butroxydim a clethodim redujo la densidad de L. rigidum 60 a 80% y la producción de semilla 71 a 88%, al compararse con la práctica estándar de los productores de aplicar simazine PPI más clethodim POST. La combinación de clethodim + butroxydim tuvo el mayor rendimiento de grano de haba (980 a 2,400 kg ha−1). Aunque propyzamide y pyroxasulfone más triallate PPI brindaron los segundos niveles de control de L. rigidum más altos (< 60%), estos tratamientos fueron más variables e incapaces de reducir la producción de semillas (6,354 a 13,570 semillas m−2) a niveles aceptables para sistemas de cultivo continuo.

Herbicide Application Strategies for the Control of Rigid Ryegrass (Lolium rigidum) in Wide-Row Faba Bean (Vicia faba) in Southern Australia

Abstract Two field experiments were undertaken at Roseworthy, South Australia from 2006 to 2007 to evaluate the performance of herbicide application strategies for the control of herbicide-resistant rigid ryegrass in faba bean grown in wide rows (WR). The standard farmer practice of applying postsowing PRE (PSPE) simazine followed by POST clethodim to faba bean grown in WR provided consistent and high levels of rigid ryegrass control (≥ 96%) and caused a large reduction (P < 0.05) in spike production (≤ 20 spikes m−2) as compared with nontreated control (560 to 722 spikes m−2). Furthermore, this herbicide combination resulted in greatest yield benefits for WR faba bean (723 to 1,046 kg ha−1). Although PSPE propyzamide used in combination with shielded interrow applications of glyphosate or paraquat provided high levels of rigid ryegrass control (≥ 93%), these treatments were unable to reduce ryegrass spike density within the crop row (20 to 54 spikes m−2) to levels acceptable for continued cropping. Furthermore, a yield reduction (13 to 29%) was observed for faba bean in treatments with shielded application of nonselective herbicides and could be related to spray drift onto lower leaves. These findings highlight that shielded interrow spraying in WR faba bean could play an important role in the management of rigid ryegrass in southern Australia. However, timing of shielded interrow applications on weed control, crop safety, and issues concerning integration with more effective early-season control strategies require attention. Nomenclature: Clethodim; glyphosate; paraquat; propyzamide; simazine; rigid ryegrass, Lolium rigidum Gaud LORI; faba bean, Vicia faba (L.).

Applications of Metribuzin for The Control of Rigid Brome (Bromus rigidus) in No-Till Barley Crops of Southern Australia

Field experiments were conducted at Warooka and Rudall on the Yorke and Eyre Peninsula of South Australia during 2004 and 2005 to investigate the effectiveness of metribuzin for the selective control of rigid brome in no-till sown barley. Metribuzin (135 to 203 g ai/ha) incorporated by sowing (IBS) was more effective in controlling rigid brome (> 67%) than the same herbicide dose applied POST. Although IBS metribuzin at the highest rate (270 g/ha) provided effective weed control (82 to 90%), it was more phytotoxic to barley, reducing crop density by 23% relative to the nontreated control at Rudall. Soil at Rudall had low clay and organic matter content. In contrast, tank mixtures of metribuzin (203 g/ha) with pendimethalin, applied IBS, provided reliable rigid brome control (89 to 93%) and resulted in little crop damage (< 5%). Over the site-by-year combinations studied, all herbicide-treated barley at Warooka and Rudall yielded 6 to 50% more grain than the nontreated crop. Although metribuzin provides growers with an opportunity to selectively control rigid brome in no-till barley, high rates (≥ 203 g/ha) of this herbicide on sandy textured soils can result in significant crop damage. Nomenclature: Metribuzin, rigid brome, Bromus rigidus Roth BRORI, barley, Hordeum vulgare L