Latest research on brome grass and susceptibility of emerging weed species to harvest weed seed capture

Take-home messages

• Increasing incidence of brome grass in cropping paddocks in southern Australia appears to be associated with selection of biotypes with greater seed dormancy by crop management practices used by the growers.
• Higher levels of seed dormancy allow brome to germinate and establish after pre-sowing weed management, resulting in greater in-crop weed establishment. This change in weed behaviour also appears to be associated with high seedbank persistence from one year to the next (~25%). Therefore, multiyear control strategies are required to exhaust brome seedbanks to low levels.
• Weed seed dispersal prior to crop harvest differed greatly between weed species. Bedstraw, statice, turnip weed and Indian hedge mustard showed no pre-harvest dispersal (100% retained); whereas barley grass had shed 94% of its seeds at harvest time. Seed retention in brome grass varied from 50-75% and was influenced by growing season rainfall and crop being grown. These results indicate harvest weed seed control (HWSC) tactics are unlikely to improve barley grass management but may have some benefits for brome control.
• A preliminary kiln study showed that temperatures in excess of 450 degrees Celsius for at least 40s were required to guarantee the complete kill of brome grass seed. As burning standing stubbles are unlikely to provide the required level of heat exposure, narrow windrow burning should be considered to improve weed seed kill.

Brome grass ecology and management

Rigid brome (Bromus rigidus) tends to be the main brome species present on farms around Lock and many other areas of the Eyre Peninsula (EP). This species tends to have high seed dormancy, which usually breaks around late May/June when most of the crops on the EP have already been sown. This species was investigated extensively in a previous GRDC project UA00060 (2003-07). Germination in B. rigidus was found to be strongly inhibited by exposure to light and 15-30% of its seeds persisted from one year to the next. This weed species also occurs extensively in high rainfall areas around Warooka on the York Peninsula.

Great brome (Bromus diandrus) tends to be more common in the lower and upper North and in the Mallee. This species has increased in prevalence in the last 10 years, which appears to be related to increased adoption of no-till farming and intensification of cereal-based cropping systems (i.e. wheat on wheat), where less herbicide options are available for its control. Most attributes of B. rigidus and in-crop populations of B. diandrus are very similar. Germination in both species is inhibited by light and a large number of plants emerge after the crops have been sown. Some of the increase in abundance in crops of these brome species can also be explained by the adoption of earlier sowing or even dry sowing. In situations where brome grass infestations are high, it can reduce wheat yields by 30-50%.

On-farm selection for increased seed dormancy and delayed seedling emergence after the opening rains appears to be responsible for the increasing dominance of this weed species. Our research has clearly shown higher levels of seed dormancy in brome grass populations collected from cropping fields than those from non-crop situations such as fence-lines or roadsides (Figure 1). Populations collected from intensively cropped situations in 2015 were much slower to emerge and reach 50% of final emergence (t50) than those sourced from the fence-line and other non-crop habitats (cropped t50 ~40 d; fence-line t50 ~20 d). This two-fold difference in seedling emergence time between brome populations was related to the variation in seed dormancy. A similar trend was observed in populations of brome grass collected from the paddocks and fence-line situations in 2016.

Figure 1. Differences in germination and seedling emergence pattern between cropped (closed symbols; solid line) and adjacent fence-line (open symbols; broken line) populations of great brome (B. diandrus) collected in 2015 across south-eastern Australia. A similar trend was observed in populations collected in 2016.

These results clearly indicate that management practices used by growers to control brome in cropping paddocks have caused a shift in weed population behaviour. This increase in seed dormancy has been caused by selection of individuals in these populations that possess genes for greater seed dormancy that enables them to escape pre-sowing weed control tactics such as tillage or knockdown herbicides. The process of selection for increased seed dormancy would work similarly to the selection for herbicide resistance. Over time weed management practices in cropping paddocks would select for biotypes that possess higher dormancy and select against or remove those with low dormancy. Such selection pressure would not occur on the fence-line or non-crop areas or pastures.

Seeds of highly dormant populations of brome grass were responsive to chilling (i.e. exposure to 5°C), a process which has been shown to increase gibberellic acid production within the seed, a hormone known to break seed dormancy and stimulate germination. In the field this means that the dormant brome grass seed requires not only moisture, but also a period of colder temperatures to germinate. Therefore, germination of most of the seedbank of brome would not occur until cooler-moist conditions in late autumn-early winter, thus allowing it to evade early season weed control tactics (e.g. knockdown herbicides) and survive pre-emergence herbicides.

Another biological mechanism that appears to delay seedling emergence in the field is the strong inhibitory effect of light on seed germination in rigid and great brome. Strong inhibition of germination by light is likely to aid brome infestation in the field in no-till systems by enabling seeds to remain ungerminated on the soil surface even after adequate rainfall until after the sowing of the crop, thus preventing seedlings from being killed by knockdown herbicides. This feature of brome grass ecology helps in explaining why it has proliferated under no-till, where seeds remain on the soil surface until being buried by the sowing pass, which would remove the inhibitory effect of light.

Greater seed dormancy in rigid brome grass populations from cropping fields could have also contributed to the development of a more persistent seedbank. A field study undertaken at Lock showed that 20% of the seedbank of rigid brome persisted from one season to the next, with seeds remaining viable in the soil for up to three years (Figure 2). Similar levels of seedbank persistence in great brome were shown in the long-term study at Balaklava, where more than 25% of seedbank persisted from one season to the next. Seedbank carryover of this magnitude could be an important factor in the proliferation of brome grass where crop rotations have often provided effective control just for one year (i.e. pasture-wheat rotation) or under cereal monoculture.

Figure 2. Longevity of rigid brome (B. rigidus) seed in the field at Lock from 2003 to 2006. In some populations seedbank persistence can be as high as 30%.

Given the high level of seedbank persistence of brome grass, long-term control of brome would need an effective multi-year management program. Fortunately, the introduction of imidazolinone-tolerant wheat (Clearfield™) has widened grower’s options for the management of brome in the wheat-phase. Use of break crops such as a legume or canola in combination with Clearfield™ cereals can provide a range of herbicide options for brome control and can be included in a rotation to prevent crop competition and weed seed-set. However, brome is a prolific seed producer even when growing in crop competition (80 to 270seeds/plant) and weed populations can rebound sharply if management tactics used do not provide a high level of weed control.

Seed dormancy in wild turnip and barley grass

Wild turnip seeds were found to have a high level of seed dormancy and even after nine months after maturity, germination in different populations ranged from 3 to 40%. This high level of seed dormancy was reflected in long-term persistence of the seedbank with some populations showing seedling emergence even in the third season of the study. The results of this research show that presence of physiological dormancy regulated within the embryo is the main mechanism controlling wild turnip germination and this high level of seed dormancy leads to long-term persistence of its seedbank. Since wild turnip plants can set a huge amount of seed, weed seed production needs to be managed every year.

Barley grass has been generally considered to have low seed dormancy and little or no seedbank persistence from one year to the next. Our research has shown this to be not true at least for the in-crop populations of barley grass. Surprisingly, some barley grass populations showed only 20-30% seedling establishment in 2016 and there was some concern that they had low seed viability. However, populations with low seedling establishment in 2016 showed much higher seedling emergence in 2017. Therefore, high seed dormancy in some cropping populations of barley grass appears to cause much greater seedbank persistence and establishment in the second year after seed set.

Selection pressures imposed by weed-control tactics used in crops appears to have selected for more persistent barley grass populations in southern Australia. Similarly, Shergill et al. (2015) reported higher persistence for cropped populations of barley grass, whereas previous studies (Peltzer & Matson 2002; Popay 1981; Powles et al. 1992) showed that seeds of barley grass have a short-lived seedbank and very few seeds are likely to persist beyond one year. Seedbank carry-over could be an important factor in the observed increase of barley grass in crops in the southern region.

Figure 3. Seedling recruitment (2016 and 2017) of cropped & fence-line populations of barley grass from seedling trays carried-over from 2016.

Weed seed dispersal and susceptibility of emerging weeds to harvest weed seed capture and control

At present there is a high level of interest in the grains industry in harvest weed seed control (HWSC) including weed seed catchers, weed seed destructor technologies and narrow windrow burning. The effectiveness of these practices depends on the amount of weed seed retained on the plant and present above the cutter bar height at crop harvest. Field studies conducted over the past two seasons at Roseworthy have investigated the seed shedding behaviour of several emerging weeds until the crop was harvest-ready (≤12% moisture content). The pattern of seed shedding was determined by regularly collecting seeds from the seed traps placed on the soil surface in the crop canopy.

Barley grass (H. glaucum) was particularly prone to early seed dispersal and <6% of seeds produced were retained in panicles at the harvest-ready stage of wheat (Figure 4). Barley grass was also the first weed species to reach maturity, producing viable seeds and initiating seed shed at 43-45 days before crop harvest. Relative to barley grass, bifora and brome grass (B. diandrus) were slower to reach physiological maturity and initiated seed shed 21-25 days before crop harvest. Weed seed retention was much higher for bifora (50%)and brome grass (75%) and bedstraw showed no seed dispersal prior to crop harvest (100% retained). Even though brome grass had high seed retention (75%) until harvest in 2016, many panicles (30-80%) had lodged below the crop harvest height of 15cm (Table 1). The severity of lodging in brome grass increased with weed density, which could be related to weaker stems at its higher density and this could be an important escape mechanism from HWSC for this species.

Table 1. Effect of brome and barley grass density (low, medium and high) on panicle lodging at the harvest-ready stage of wheat (≤12% moisture content). Panicles found ≤15 cm crop harvest height were scored as lodged.

Figure 4. Seed retention of brome grass relative to barley grass bifora and bedstraw in relation to wheat maturity (≤12% grain moisture content) at Roseworthy in 2016. Bars show ± standard errors.

Based on the level of seed dispersal observed in this study, bedstraw, statice, turnip weed, and Indian hedge mustard were the most suitable weed species for harvest weed seed capture. Despite bifora and brome grass shedding some seed prior to harvest, >50% of seeds were retained for HWSC. In contrast, sowthistle and barley grass appear to be the least suitable weed species for HWSC and show a high level of shed seed prior to crop harvest.

Results from a preliminary kiln study showed that both temperature and duration of exposure were important factors for killing weed seeds (only brome grass data presented). Temperatures in excess of 450^oC for at least 40s were required to achieve complete kill of brome grass seeds (Table 2). Even at these high temperatures (450^oC), short exposure (20s) failed to completely kill brome seeds with more than 68% remaining viable after the treatment. These preliminary results clearly suggest that short hot burns associated with burning standing stubbles are likely to only achieve partial kill of weed seeds. Therefore, narrow windrows of high biomass are required to generate the temperatures and exposure times needed for killing brome grass seed.

Table 2. Effect of temperature and duration of exposure on the percentage (%) germination (survival) of brome grass seed. Values in mean column with different letters are significantly different (P = 0.05). SAGIT funded project (S416).

Even though our studies have shown that several weed species retain most of their seed until crop harvest, little is known about the proportion of weed seed that subsequently exits in the grain, straw and chaff fractions under commercial harvest conditions. An important factor in many HWSC systems (i.e. chaff carts, chaff lining and HSD) is that they only target the portion of weed seed exiting the harvester in the chaff fraction. Narrow windrow burning is the exception and will control weed seeds exiting both in the straw and chaff fractions provided a hot, and long burn is achieved. Further research is therefore required to clarify this aspect of weed seed collection to determine the relative effectiveness of each HWSC system in the long-term management of these emerging weeds.

Acknowledgments

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the authors would like to thank them for their continued support.

We are grateful to GRDC for providing project funding (UA00156), and Jerome Martin and Malinee Thongmee from University of Adelaide for providing technical assistance. We also acknowledge the South Australian Grains Industry Trust Fund (SAGIT) for funding the project (S416) – Burning of weed seeds in the low rainfall systems project.

Contact details

Gurjeet Gill
Waite Campus, PMB 1, Glen Osmond, SA, 5064
(08) 8313 7744
gurjeet.gill@adelaide.edu.au