Ground Cover Supplement : GC Supplement - Summer weeds
9 Issue 137 | Nov – Dec 2018 | GRDC GROUNDCOVER SUPPLEMENT: SUMMER WEEDS GROUNDCOVER FEATHERTOP RHODES GRASS 012345678 1011 9 15 14 13 12 100 80 60 40 20 0 Soil surface 5cm 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. Viable seed (%) Time after burial (months) SOURCE: THE UNIVERSITY OF ADELAIDE 012345678 1011 9 15 14 13 12 100 80 60 40 20 0 Viable seed (%) Time after burial (months) 0 1 2 3 4 5 6 80 70 60 50 40 30 20 10 0 Seedling emergence (% of seed buried) Burial depth (cm) SOURCE: THE UNIVERSITY OF ADELAIDE 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. 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 ger mination 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. 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 ger minate 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 ger mination 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 dor mancy from seed shed and after this starts to lose dor mancy with all of the seed able to ger minate 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. FTR has a lower base temperature for ger mination 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 ter m. o GRDC Research Codes UA00149, UA00156 More information: Dr Christopher Preston, University of Adelaide, 08 8313 7237, firstname.lastname@example.org; Dr Bhagirath Chauhan, University of Queensland, 07 5460 1541, email@example.com Research through GRDC investment at the University of Adelaide and the University of Queensland conducted since 2013 has shown that FTR seed is relatively short-lived.
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