Current ForestrySA best practice plantation establishment weed control in the Green Triangle Region of South Australia and Western Victoria.

Transcription

1 Session A4: Management Plantations Current ForestrySA best practice plantation establishment weed control in the Green Triangle Region of South Australia and Western Victoria. Wayne Richardson Coordinator Research ForestrySA PO Box 162, Mt Gambier, South Australia Biography Wayne Richardson has been employed in the plantation forest industry since 1981 and with ForestrySA since He has worked in harvesting, resource planning, operations and for the past 23 years in forest research specialising in weed control and herbicide research. Richardson is a member of the steering committee of the Australian Plantation Forest Industry Herbicide Research Consortium. He co-authored the APVMA Guidelines for Efficacy Evaluation of Herbicides Weeds in Australian Forests as well as the Field Guide to Forest Weeds in the Green Triangle and Ranges regions of South Australia and Western Victoria and the creator of Weedchem. Abstract Soil moisture availability is the single largest limiting factor to plantation survival and growth in southern Australia. Key to optimising soil moisture availability is minimising the competion impacts of weeds. Selecting the wrong herbicide prescription for any specific site can have rotation length impacts on productivity. ForestrySA uses a range of establishment weed control strategies based upon proven research trial results to formulate best practice. On second rotations sites weed control begins prior to final harvesting with the aim of removing woody weeds and reducing weed seed loadings before site preparation commences. Under canopy chemical application is standard practice which significantly reduces the potential for off site chemical movement and reduces the replant time. Specific weed control to target hard to kill or very competitive weeds is kept as a separate operation where practical to the general residual weed control applications with the target species being treated when they are most receptive to control. Herbicide prescriptions are very site specific and are selected to match the weed species present or likely to be present. The maximum chemical effort is put into first year weed control with the aim of reducing the chemical foot print in the second year of establishment. Introduction Soil moisture availability is the single largest limiting factor to plantation survival and growth in southern Australia. The factors that determine the amount of soil moisture available on any plantation site are climate (rainfall being the largest factor), soil characteristics (especially effective rooting depth, drainage and water holding capacity), topography (aspect significantly impacts on evaporation, slope increases runoff) and the competition impacts of weeds. Forest managers cannot make it rain, they cannot flatten mountains or significantly increase soil depth but they can manage the available soil moisture resource by their silvicultural practices. In the Green Triangle Region of South Australia and Western Victoria (GT) ForestrySA uses three integrated strategies to optimise soil moisture management. Firstly low residue logging and chopper rolling of logging debris is standard practice to maintain as much organic matter as practical on site following harvesting. This retains valuable nutrients in situ, aids in the decomposing of logging slash by crushing and incorporating it into the soil, reduces evaporation losses of moisture, significantly reduces the need to heap and burn logging debris and provides mechanical weed control of pine wildlings, eucalypt coppice and other woody weeds. Secondly site preparation (spot 179

2 cultivation, ripping, mounding or ploughing) is tailored to the topographic and soil conditions of individual planting areas with the aim of managing soil moisture conditions. Thirdly ForestrySA aims to maximise tree survival and early tree growth by minimising weed competition during the first two years following planting (ForestrySA 2006). To achieve this aim an integrated approach to weed control employing mechanical and chemical control options is utilised. The most practical and cost effective method of weed control is via the use of herbicides. This paper has been written to provide an insight into the current best practice use of establishment herbicides by ForestrySA to maximise tree survival and optimise plantation productivity. ForestrySA Chemical Approval Process All herbicide prescriptions and individual chemicals (including generics of ForestrySA approved active ingredients) have to pass the ForestrySA risk assessment process for operator, customer and neighbour safety as well as potential environmental impacts before being considered for efficacy testing. If a product is assessed as safe for operational use it will be evaluated in research trials for efficacy, chemical compatibility with other products, tank mixing (how well does it disperse, does it clog filters or nozzles) and packaging prior to possibly being approved for operational use. The trialling process includes investigating optimal application rates for products with forestry specific registrations, comparing the performance of generic products with the same active constituent and investigating both experimental chemicals and registered products without a forestry specific label. If a product without a forestry registration has a potential effective economic or environmental fit for forestry use ForestrySA will negotiate with the manufacture to seek a forestry specific label change. Selecting Site Specific Herbicide Prescriptions In February March of each year an establishment weed control tour is conducted of every compartment which is scheduled to receive either first or second year weed control. Forest operations managers and the research section make up the decision group which sets the herbicide prescription to be used on a compartment and if required partial compartment basis. Prescriptions are based upon the weeds present and the weeds likely to be present, environmental or operational restrictions, soil type and characteristics (fine sand 1 st rotation sites tend to blow away if bared off with broadcast spraying resulting in top soil erosion and sand blasting of seedlings), topography, site preparation method (spot cultivation, mounding, line marking only), preferred application method (aerial or ground based) and for second year weed control sites the growth already achieved since planting. Under advice from the research section, where practical, expensive and high application rate herbicides are substituted with low rate, lower risk and lower cost products. The most appropriate chemical prescriptions for environmentally sensitive areas are selected based upon research results and chemical label restrictions and where practical second year herbicide applications are omitted. The prime considerations when selecting alternative herbicide prescriptions are human safety, potential environmental and neighbour impacts, label and buffer restrictions, chemical efficacy and lastly product cost. Selecting the wrong herbicide prescription for any specific site can have rotation length impacts on productivity. ForestrySA research trials and growth monitoring plots have shown that the impacts of poor competition control during the first two years for radiata pine and the first year for Tasmanian blue gums continues on after weed competition has been eliminated by canopy closure of the tree crop. This was demonstrated in the ForestrySA research trial LT227 (Richardson 2007) which investigated the efficacy of pre-plant glyphosate +/- metsulfuron methyl applied alone or in combination with postplant sulfometuron-methyl, amitrole or hexazinone. Measurements of tree volume at planting plus 74 months (Figure 1) shows the continuation of the growth trend set by the establishment weed control. 180

3 Total Stem Volume (m3) nil control Forest Mix granules (May) Forest Mix WDH (May) Roundup Biactive (June) Roundup Biactive (June)+ Amitrol T (Oct) Roundup Biactive (June)+ Oust (Oct) Roundup Biactive (June)+ Velmac G (Oct) Trounce (May) Trounce (May)+ Amitrole T (Oct) Trounce (May)+ Oust (Oct) Trounce (May)+ Velmac G (Oct) 0 19/04/2001 1/09/ /01/ /05/ /10/ /02/2008 Measurement Date Figure 1: LT227A total stem volume production (m 3 planting plus 74 months showing the continuation of the growth trend set by the level of weed control achieved at establishment. Operational Practice On 2nd rotation radiata pine sites establishment weed control begins prior to final harvest. Woody weeds such as bracken fern (Pteridum esculentum), sollya (montpellier) broom (Genista monspessulana), gorse (Ulex europaeus), wattle species and radiata pine wildlings are sprayed under the canopy of the standing crop preferably a minimum of three and a maximum of eighteen months prior to harvesting. For spring to early summer operations 36 g/ha a.i (60 g/ha of product) with a polyether modified polysiloxane surfactant (Pulse 0.5% is used. For autumn applications the combination of 1440 g/ha a.i (4 L/ha of 360 glyphosate) plus 18 g/ha a.i (30 g/ha of product) with Pulse 0.5% of tank mix. Chemical application is via a Cassotti boomless sprayer mounted on a logging skidder using an output rate of 540 L/ha. The reciprocating boom in combination with the forward momentum of the skidder effectively allows the chemical application to be applied around the shadow of the standing crop trees ensuring effective ground coverage. The spray head of the Cassotti is mounted on a steel pole at the rear of the machine which provides both physical protection to the machine and allows it to be raised as high as necessary to spray over the top of the understorey. Metsulfuron-methyl is used alone in spring to early summer as soil moisture and temperature conditions allow for a longer effective growing period for the chemical to impact on the weed crop, which maximises the opportunity for effective control. The combination of glyphosate and metsulfuronmethyl is used in autumn because the available window of optimal weed receptivity (when there is adequate soil moisture following the break of season rainfall but prior to the onset of cold winter soil conditions) is shorter and the glyphosate is required to ensure knockdown occurs rapidly. Control of woody weeds such as bracken is generally more effective in spring than autumn (Richardson, McGuire & Grey 2006). If spring control is not effective the opportunity exists to follow up with a second autumn application prior to planting. Pre clear felling / post thinning woody weed control significantly reduces weed seed loads prior to reestablishment, significantly reduces off site chemical movement potential and reduces fallow periods 181

4 required to precondition woody weeds such as bracken prior to chemical or mechanical control. Mechanical control via the use of a chopper roller is a key component for second rotation pine and eucalypt establishment and the second phase of pre plant weed control. The weight and steel blades of the twin offset chopper roller not only crush and incorporate logging debris into the soil hastening nutrient cycling but also provide effective control of pine wildlings and other woody weeds. In second rotation eucalypt plantations chopper rolling is used following the chemical treatment of coppice growth to remove dead coppice stems, crush and break up the stumps and to aid in the removal of any live coppice stems that were either not controlled by the chemical application or emerged after spraying. Specific weed control targeting hard to kill and perennial weeds is the third step of the establishment weed control process. Couch grass (Cynodon dactylon), bracken fern, pine wildlings, blackberry and other spring to summer receptive species are targeted with glyphosate and metsulfuron-methyl either applied alone or in combination when they are most receptive to chemical uptake. Areas of specific weeds to be treated are firstly mapped and where practical chemical application is restricted to a minimum target area. Application rates vary depending upon the species being targeted with phalaris (Phalaris aquatica) being treated with glyphosate at a minimum of 2160 g/ha a.i (6 L/ha) with Pulse % tank mix dependant upon its size and the prevailing soil moisture conditions. Couch grass will receive the maximum label rate of glyphosate for ground based application 3240 g/ha a.i (9 L/ha) with Pulse 0.5% of tank mix due to the level of difficulty in control. Woody weeds such as bracken fern will be treated with 36 g/ha a.i and Pulse 0.5%. Where there are a combination of weeds to target such as woody weeds and grass species in the same area combinations of both glyphosate and metsulfuronmethyl are employed with the rates of both being varied. The spring specific weed control program has the advantages of reducing weed seed loads by removing a range of annual and perennial weeds before they can set seed for the following year and allows for a second opportunity in autumn to target species such as bracken if the spring control program is not successful. The other key component to spring specific weed control is the observation of non disturbance periods (slashing, ploughing, mounding and spot cultivation). The general rule of thumb is to observe a minimum non disturbance period of six weeks following chemical application of woody weeds and perennial grasses to ensure that the chemical action is not disrupted. Specific weed control in autumn targets primarily perennial woody weeds and broadleaf species such as flatweed (Hypochoeris radicata) and sorrel (Acetosella vulgaris). Autumn applications are predominantly a combination of glyphosate and metsulfuron-methyl with rates ranging from 720 g/ha a.i plus 12 g/ha a.i plus Pulse 0.2% to 2880 g/ha a.i plus 36 g/ha a.i. plus Pulse 0.5%. One of the key drivers of the metsulfuronmethyl rate used is the need to observe a 1 day per gram of product applied withholding period prior to planting. This rule of thumb applies to both pine and eucalypt species on the acid sand soils of the GT region (ForestrySA 2006). In order to manage this need it is practice to work backwards from the desired planting date and adjust the metsulfuron-methyl rate accordingly. In order to compensate for any reduction in metsulfuron-methyl the rate of glyphosate is increased. On first rotation sites the risk of baring the site off with a broadcast application leading to sand blasting of the planting stock can be a significant risk. Operational practice to reduce this risk while optimising the control of perennial broadleaf weeds is to broadcast spray with metsulfuron-methyl and strip spray the planting lines with glyphosate to remove the remaining weed species. Annual grasses which will die during the summer drought months are left in place inter row to hold the soil and reduce the impact of wind blow. To minimise the number of operations required the glyphosate application can occur with the strip applied residual weed control operation. The control of eucalypt coppice for either the replanting of eucalypt seedlings or the conversion to radiata pine falls under the category of specific weed control. The preferred option is to kill the eucalypt stump at the time of harvesting by applying chemical to the cut stump immediately following felling, preferably with a chemical applicator attached to the harvesting machinery. Trial work has proven the effectiveness of this approach but it has yet to become operational practice but is being actively pursued by the plantation industry. Current best practice as used by ForestrySA is to use a combination of chemical and mechanical control. If the coppice crop is taller than 2.0 meters in height it is hydro-axed to stump height and then allowed to regrow until it reaches a preferred target height for chemical application of meters tall. This operation produces an even crop of coppice growth for spraying. If the coppice crop is in the target height range it is treated with the maximum label rate of glyphosate (3240 g/ha a.i) and metsulfuron-methyl (36 g/ha a.i) plus Pulse 0.5% of tank mix applied with a minimum water rate of 400 L/ha sprayed as a double pass operation from both sides of the coppice. 182

5 This can only be done as a ground based operation with the spray boom travelling up and down the same row from opposite directions. Chemical coverage of all coppice stems is essential to optimise control potential. If an individual coppice stem is not sprayed it will survive and the stump will produce new growth. Following spraying non disturbance must be observed for as long as practical and be followed up if possible with chopper rolling or another mechanical operation to remove any surviving coppice or new growth that emerges following spraying. Additional glyphosate application with the general weed control may be required to catch any late emerging coppice. The fourth stage of establishment weed control is the application of the general or residual chemical prescriptions. Where no environmental, legislative or operational impediments are in place the combination of 1.5 kg/ha a.i plus 4.5 kg/ha a.i is the preferred first choice prescription for radiata pine on the majority of sites. This is applied with a non ionic surfactant (alcohol 0.2% of tank mix pre planting and without a spray adjuvant if applied post planting to minimise any potential phytotoxic impact in the form of needle burn from the hexazinone. If there is no operational advantage in using atrazine, sulfometuron-methyl ( g/ha a.i) can be substituted to gain superior flatweed or sorrel control, and or clopyralid ( g/ha a.i) can be applied with hexazinone to control thistles and legumes such as broom and gorse. If the specific pre plant weed control has not been as successful as required or new germination has occurred glyphosate ( g/ha) is added for extra knockdown. If the first choice prescription cannot be used an alternative prescription built around optimising the weed control possible given the environmental or operational restriction to best practice is selected. However moving away from the proven best efficacy prescriptions will compromise weed control and site productivity as demonstrated in the ForestrySA research trial LT268 (Richardson 2009) which was designed to compare the efficacy of weed control, survival, tree growth and chemical costs of the range of standard and experimental pre plant herbicide prescriptions currently used and likely to be used by ForestrySA for radiata pine establishment (Figure 2). Only data from the planting plus 12 month measurement was available at the time of the production of this paper but the trend in growth in response to the level of weed control achieved is evident with total stem volume growth potential being reduced by a minimum of 26% when the preferred atrazine plus hexazinone prescription was not utilised. Total Stem Volume (m3/ha) Nil Control Hexazinone + Atrazine Sulfometuron methyl + Atrazine + Pulse Sulfometuron + Atrazine + Glyphosate + Pulse + Liase A123 + Hexazinone A123 + Atrazine Sulfometuron + A123 + Pulse + Liase Sulfometuron + Glyphosate + Pulse A123 + Glyphosate + Pulse Glyphosate + Clopyralid + Pulse Metsulfuron + Glyphosate + Pulse Terbuthylazine + Hexazinone Terbuthylazine + Sulfometuron + Pulse Sulfometuron + Glyphosate + Clopyralid + Pulse /08/2008 1/10/2008 1/12/2008 1/02/2009 1/04/2009 1/06/2009 Measurement Date Figure 2: LT268A total stem volume production (m 3 planting plus 12 months. Note: A123 is a commercial in confidence experimental treatment. 183

6 For Tasmanian blue gums the preferred first year herbicide prescription is 4.5 kg/ha a.i plus 52.5 g/ha a.i plus 300 g/ha a.i plus g/ha plus Pulse 0.2% plus ammonium sulphate (e.g. Spraymate 1 L/100L of tank mix to act as an anti antagonist between the atrazine and the glyphosate. As the range of second year / post plant chemical options that can be safely used with eucalypts is significantly smaller than with pine species the maximum effort is placed in reducing weed competition in year one with the view that catch up weed control options may not be available. As with radiata pine if the first choice prescription cannot be used an alternative prescription built around the environmental or operational restriction to best practice is selected. If the first year weed control is optimal the opportunity then exists to reduce the chemical footprint and cost in year two without impacting upon productivity (Shaw et al 2002). Choices include applying no second year weed control but this option is only utilised in a small minority of cases with radiata pine but more frequently with Tasmanian blue gums due to their more vigorous initial growth rate. The standard radiata pine second year herbicide prescription is 4.5 kg/ha plus sulfometuronmethyl at g/ha a.i which is applied with Pulse 0.2%. As with first year weed control if this cannot be used alternative prescription built around the environmental or operational restriction to best practice are selected. If weed loadings by specific species warrant, hexazinone and atrazine will be used two years in a row though the trend is to utilise hexazinone and sulfometuron-methyl to increase the spectrum of control. No fixed standard for Tasmanian blue gum is in place with the prescription being targeted at the specific weed spectrum present with the choice generally being an atrazine plus clopyralid or haloxyfop and clopyralid combination. Conclusion To maximise crop survival and early growth ForestrySA uses the establishment weed control strategies described here. All mechanical and chemical weed control options used are based on proven research trial results and no chemical is approved for operational use until it has been vetted for safety and operational efficacy by the research section. Establishment weed control utilises both mechanical and chemical methods and is aimed at optimising the reduction in weed competition while taking into account environmental and operational restrictions. On second rotation sites planning and operational practice begin prior to final harvesting with the aim of removing woody weeds and reducing weed seed loadings before site preparation in the form of site clearing and cultivation commences. Under-canopy chemical application is standard practice which significantly reduces the potential for off-site chemical movement and reduces the replant time. Specific weed control to target hard-to-kill or very competitive weeds is kept as a separate operation where practical to the general residual weed control applications with the target species being treated when they are most receptive to control. Herbicide prescriptions are very site-specific and are selected to match the weed species present or likely to be present. The maximum chemical effort is put into first year weed control with the aim of reducing the chemical foot print in the second year of establishment. References Boardman, R. (1988). Living on the edge; the development of silviculture in SA pine plantations. Australian Forestry 51 (3), ForestrySA Plantation Forestry Manual (2006). Chapter 3 Competition control. (ForestrySA: Mount Gambier, Australia). Richardson, W. McGuire, D and Grey, N (2006) Woody weed control using metsulfuron-methyl and glyphosate applied by boomless spray technology prior to clear fell of radiata pine (Pinus radiata). (Proceedings of the 15th Australian Weeds Conference, eds Preston, C, Watts, J.H and N.D Crossman, pp (Weed Management Society of South Australia, Adelaide). Richardson, W. (2006) LT227 Pre-Plant and Post-Plant Herbicide Products x Timing. (ForestrySA Research: Mount Gambier, Australia). Richardson, W (2009) LT268ARadiata pine establishment herbicide prescription weed control, tree growth and cost benefit modelling trial (ForestrySA Research: Mount Gambier, Australia). Shaw, S.M, Mattay, J.P and P. Adams (2002) Longterm growth response to patterns of weed control in radiata pine. (Proceedings of the 13 th Australian Weeds Conference, eds Spafford Jacob, H, Dodd, J and J.H.Moore, pg (Plant Protection Society of WA, Perth) 184