Summary
Summer perennial weeds are a major cost to animal production in SE Australian mixed farming systems. Predicted climate change towards warmer, moister summers is expected to increase the spread and impact of summer weeds on pasture systems. A previous project (B.WEE.0135) funded by MLA has identified an integrated approach for silverleaf nightshade (SLN) management by targeting both the seedbank and the rootbank. This one-year project has focused on the production of a best management practice (BMP) guide and the delivery of updated research and management information to growers, ag-advisors and other stakeholders. In addition, progress has also been made in better understanding the biology and ecology of the weeds.
The large multicellular trichomes on mature leaves of SLN have been identified to be physical barriers to the uptake of herbicides. These trichomes, even those with intrusive bases, do not appear to provide a pathway for herbicides to directly enter the vascular system. Lower trichome density on the adaxial/upper surface indicates that spraying the top of the leaves may be more effective than spraying the undersides of the leaves. The stellate trichomes, coated with a thin layer of cuticle, are water-repellent, highlighting the need of further research to screen a large range of surfactants/adjuvants in order to overcome the hydrophobic trichome cuticle and to improve herbicide uptake. The techniques of using fluorescent dyes developed in this study could be used to determine the most effective adjuvant and spraying volume, droplet size on herbicide uptake.
The seed burial and persistence studies at 2 sites (Ganmain and Culcairn) were completed. The burial depth, duration of burial, and presence of seed pod had significant impact on the viability of SLN and PGC seeds. Seeds buried at deeper depths persisted longer as compared to seeds on soil surface. After 3-years of burial, the SLN seed viability ranged between 10 and 60% for the bare seeds and 20-65% for seeds with an intact pod, depending on the depths of burial. For prairie ground cherry (PGC), seed viability ranged between 0 and 20% for the bare seeds and 0-70% for seeds with an intact pod. PGC seeds or pods placed on soil surface completely lost viability after three years of burial. This research suggests that any soil incorporation to bury seed pods will significantly increase the persistence of the seeds of SLN and PGC in the soil. A long-term management strategy is needed to effectively manage these two intractable perennial weeds due to the persistent seedbank.
A pasture field site established at Wellington in 2008/2009 season was used to study the interactive effect of pasture competition and herbicides for improved control of SLN. Many summer active pasture species, such as lucerne, Strickland finger grass, digit grass, Rhodes grass, and Bambatsi panic grass, were highly effective in suppressing SLN stem emergence. Results showed that application of 2 L/ha Roundup PowerMax + 1 L/ha Grazon Extra + 0.5 L/ha Uptake in March 2011 did not significantly affect the growth of many pastures, with the exception of bambatsi panic. The herbicide treatment caused less SLN stem numbers as compared to the untreated control plots except the digit grass and Rhode grass pasture. The results indicated that joint action of herbicides and pasture competition could have synergistic effect on reducing SLN population. Further trials on grazing pastures before spraying with various rates of Tordon 75D or Grazon Extra at different soil types would provide useful information for subsequent adoption of this integrated approach combining pasture competition and a late application of picloram-based products for rootbank control.
Continuous monitoring of two herbicide trial sites established previously at Leeton and Culcairn further confirmed the synergism between pasture competition and herbicide application. Same set of herbicide treatments were applied in both sites prior to April 2010. At Culcairn site, a natural population of annual ryegrass was established in 2010, while limited vegetation was established at Leeton site. Consequently the SLN numbers was much lower at Culcairn site than at Leeton site. The initial use of herbicides is necessary to rapidly run down SLN stem numbers. The follow-up pasture will not only provide feeds to livestock, but also provide competition to further suppress SLN emergence.
In April 2010, a new field trial was established at Corowa, NSW to examine the effect of herbicide rate, adjuvant and spray timing on the efficacy of SLN rootbank control. Results showed that Tordon 75D at a lower rate (< 1L/ha) did not achieve good results. For both glyphosate at 4L/ha and Tordon 75D at 2L/ha, there was no difference between the late applications in April and May, indicating the wide window opportunity for the late application to target the rootbank. Adjuvant Sprinta seems to have achieved better results than Uptake when used in conjunction with 4L glyphosate or with 2 L Tordon 75D. Although this one-year trial was not able to draw firm conclusion on the control of this perennial weed, the research does indicate that evaluation of a range of adjuvants could further improve the herbicide uptake and repeated use of picloram-based products over a number of seasons is necessary for better control of SLN rootbank.
A BMP guide was produced after thorough consultation with local agronomists and noxious weeds officers. It was printed in 2000 hardcopies in September 2010. About 1800 copies have so far been distributed to growers, ad-advisors, noxious weed officers and NRM officers in NSW, VIC and SA. The BMP has also been widely disseminated by a range of communicative channels including websites, email distribution list, radio interviews, workshops, grower groups as well as by direct contact with growers.
A series of 15 workshops were successfully conducted in NSW, Victoria and South Australia in 2010 and 2011. The workshops were run closely with local grower groups and noxious weeds or NRM networks. The positive feedbacks from workshop participants have been overwhelming and encouraging. Each workshop started with a baseline survey at the beginning of the workshop and finished with an evaluation survey at the end of the workshop. In total there were 438 participants attending the workshops.
• 206 participants (47.5%) returned the workshop evaluation form;
• Among them, 165 growers (80%) were motivated and willing to try the BMP;
• 78 growers (38%) were willing to host a demonstration site;
• 118 growers (57) wanted to try the BMP on a total of 28,707 ha, excluding the isolated infestation;
• 12 growers wanted to try the BMP on more than 1,000 ha.
• 47 growers (23%) indicated that they were willing to try the BMP but did not mention the trial area.
The baseline survey at the beginning of the workshop has showed that SLN not only caused economic loss due to direct control cost and lost production, but also significantly affected the social wellbeing of the growers. There was strong social frustration among growers seeing the weed coming back year after year despite of their time and efforts committed in spot spraying in hot summer each year. In general, the knowledge of the weed and its control is limited among the workshop participants. Poor awareness of the potential impact of the weed has been identified as a big barrier for the adoption. Raising awareness of the weed problem and its impact and providing management solutions are much needed to drive the adoption.
There were huge differences in the knowledge and management of SLN between growers and between different groups of growers across different states. There is a need to facilitate the information sharing between growers across different geographic regions in order to improve adoption.
Growers currently relied heavily on herbicides to manage SLN. Glyphosate has often been used in tank mix with suitable Group I herbicides for broadacre boom spraying to target summer weeds, as well as SLN (although it was not specially targeted). Tordon 75D has been a popular product for spot spraying. There are a number of issues associated with control. Timing of application is a big issue and often only one spraying is carried out to control the first emergence. The timing of the herbicide application, grazing or slashing is often too late to achieve 100% seedset control. In addition, growers have little knowledge of the importance of the rootbank control.
The baseline survey has also identified a number of knowledge gaps for further research:
• Differences in chemicals, rates and spraying volumes between boom spraying and spot spraying.
• Other chemicals or mixtures and suitable adjuvants for effective broadacre boom spray.
• The plant back for picloram-based products at different soil types.
• The current distribution and infestation at the national level is largely unknown.
• The survival of SLN root system in the soil.
The general lack of understanding and awareness of the weed, its impact and control identified at the workshops has strongly indicated that many more growers will need to be covered through further extension activities. This project has adopted a bottom-up approach and identified many advocate growers who are willing to try the BMP. Further funding support would enable these motivated farmers to continue to the next step of the adoption process – implementation of the BMP. The successful implementation of this first cohort of growers will encourage the 2nd and 3rd cohorts of growers to adopt the BMPs.
