Short-lived seed provides opportunity to beat feathertop Rhodes grass

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Image of Feathertop Rhodes grass in sorghum
Feathertop Rhodes grass in sorghum. PHOTO: Adam McKiernan, Queensland DAF

Feathertop Rhodes grass has small seeds, abundant seed production and wind-assisted dissemination that help it to spread quickly; however, research into the biology of this weed has identified managing the seedbank as a key opportunity for growers

KEY POINTS

  • Feathertop Rhodes grass has relatively short-lived seed and preventing seed-set can deplete seedbanks rapidly
  • Competitive winter crops and harvest weed seed control in summer crops also provide opportunities for management

Feathertop Rhodes grass (Chloris virgata) is a summer grass distributed throughout Australia. It is favoured by no-till systems and has some natural tolerance to glyphosate but, in addition, a few populations have now evolved resistance (see page 15).

In the north it is a major weed in fallow and can cause significant reductions in summer crop yield due to its competitiveness. For example, field research at the University of Queensland, Gatton, found that 22 feathertop Rhodes grass (FTR) plants per square metre reduced mungbean grain yield by 50 per cent and 49 plants/m2 reduced yield by 73 per cent.

Short-lived

Research at the University of Adelaide and the University of Queensland with GRDC investment conducted since 2013 has shown that FTR seed is relatively short-lived.

Field trials at Roseworthy, South Australia, found that seed on the soil surface typically lost viability within 12 months from shedding (Figure 1). Where rainfall occurs over summer, seed buried at five centimetres also has a short life. These rainfall events encourage germination of FTR on both the soil surface and at depth, reducing persistence of the seedbank. However, in dry years buried seed can persist for longer, depending on the ability of moisture to penetrate the soil surface.

Graphs showing the persistence of feathertop Rhodes grass
FIGURE 1 Persistence of viable feathertop Rhodes grass seed on the soil surface and at 5cm depth in the field at Roseworthy, SA, in a normal season 2014-15 (left) and a dry summer 2015-16 (right). Error bars represent the standard error of the mean for eight replicates. SOURCE: The University of Adelaide

Seedbank persistence was slightly higher for buried seed than seed left on the surface at Gatton, Queensland. After 12 months there was about 16 per cent and 10 per cent viable seeds present at two and 10cm depths, respectively, compared with none on the surface.

FTR prefers to germinate from close to the soil surface. Burial inhibits emergence of seedlings, yet 10 per cent of buried seed can still emerge from a depth of 5cm (Figure 2). This makes strategic tillage a potential option for reducing populations. Dark conditions reduce germination of FTR but do not prevent it. The losses of buried seed under good soil moisture conditions are likely due to failed emergence, which would explain why there is greater persistence at depth under dry soil conditions. This has implications for the choice of management options. For instance, seed burial in dry seasons could be less effective at depleting the seed bank.

As the seedbank on the surface persists for only 12 months, prevention of seed-set can deplete FTR seedbanks rapidly.

Preventing seed-set

FTR typically has about four months dormancy from seed shed and after this starts to lose dormancy with all of the seed able to germinate 10 months after shedding, provided conditions are favourable. Populations collected from crop fields and fencelines in Queensland had similar emergence patterns, suggesting crop management practices have not changed dormancy behaviour.

Graph showing feathertop Rhodes grass seedling emergence
FIGURE 2 Feathertop Rhodes grass seedlings are more likely to emerge from seed on or near the surface than when seed is buried 5cm deep (glasshouse trials in 2013 and 2014). Error bars represent the standard error of the mean for 16 replicates. SOURCE: The University of Adelaide

FTR has a lower base temperature for germination than many other summer grasses and can start to germinate in early spring if moisture is available. Therefore, control in winter crops can be a useful tactic for reducing weed numbers in fallows.

In dryland situations in SA, FTR takes about four months to produce mature seed; however, this time is halved under irrigated situations. When first mature, the seed remains attached to the plants providing the opportunity for harvest weed seed control in summer crops. Research at Gatton has indicated that up to 97 per cent of seed can still be retained at harvest in mungbean and about 80 per cent in sorghum. However, once seed is mature, it easily disperses from the seed head.

Under ideal conditions FTR can produce up to 40,000 seeds per plant, making it essential to control isolated plants in fallows before the seed matures through tactics including patch cultivation, hand roguing, chipping or burning. The most effective management is to prevent seed-set and drive down the existing seedbank through aggressive management in the short term.

More information

Dr Christopher Preston, University of Adelaide
08 8313 7237
christopher.preston@adelaide.edu.au

Dr Bhagirath Chauhan, University of Queensland
07 5460 1541
b.chauhan@uq.edu.au