Summer Weeds: Counting the costs for a climate-changed future

Transcription

1 Summer Weeds: Counting the costs for a climate-changed future

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3 Summer weeds: counting the costs for a climate-changed future by Harm van Rees, Jackson Davis and Kaylene Nuske Birchip Cropping Group May 2011 RIRDC Publication No 11/031 RIRDC Project No 08-86

4 2011 Rural Industries Research and Development Corporation All rights reserved. ISBN ISSN Summer weeds: counting the costs for a climate changed future Publication No. 11/031 Project No. AWRC The information contained in this publication is intended for general use to assist public knowledge and discussion and to help improve the development of sustainable regions. You must not rely on any information contained in this publication without taking specialist advice relevant to your particular circumstances. While reasonable care has been taken in preparing this publication to ensure that information is true and correct, the Commonwealth of Australia gives no assurance as to the accuracy of any information in this publication. The Commonwealth of Australia, the Rural Industries Research and Development Corporation and the authors or contributors expressly disclaim, to the maximum extent permitted by law, all responsibility and liability to any person arising directly or indirectly from any act or omission or for any consequences of any such act or omission made in reliance on the contents of this publication, whether or not caused by any negligence on the part of the Commonwealth of Australia, RIRDC, the authors or contributors. The Commonwealth of Australia does not necessarily endorse the views in this publication. This publication is copyright. Apart from any use as permitted under the Copyright Act 1968, all other rights are reserved. Wide dissemination is, however, encouraged. Requests and inquiries concerning reproduction and rights should be addressed to the RIRDC Publications Manager on phone Researcher contact details Harm van Rees Birchip Cropping Group, PO Box 85, Birchip VIC 3483 Ph: (03) Fax: (03) info@bcg.org.au In submitting this report, the researchers have agreed to RIRDC publishing this material in its edited form. RIRDC contact details Rural Industries Research and Development Corporation Level 2, 15 National Circuit BARTON ACT 2600 PO Box 4776 KINGSTON ACT 2604 Phone: Fax: rirdc@rirdc.gov.au. Web: Electronically published by RIRDC in May 2011 Print-on-demand by Union Offset Printing, Canberra at or phone ii

5 Foreword Summer weeds have an adverse effect on farm viability in southern Australian cropping regions. They use water and nutrients that could otherwise be used by ensuing crops. For example, the research outlined in this report project found that summer weeds such as heliotrope growing at a high density used 50 millimetres of stored soil water that would otherwise have been available to the following crop. In dry-land crop production regions such as the Victorian Mallee, this is equivalent to potential additional wheat production of 1 tonne per hectare. In the future the importance of summer weeds may increase, since climate change scenarios foresee a greater proportion of annual rainfall occurring during summer. This project makes a significant important contribution to understanding the economics of summer weeds and their management. It is important to be able to quantify the costs and benefits of summer weed control, both for the current climate and under changed climatic conditions. This information will support targeted research and extension aimed at mitigating the impact of summer weeds on production and farm viability. The research also provided valuable information about water use at different densities of summer weeds and enabled the development and validation of a summer weeds module for APSIM, the Agricultural Production Systems Simulator. The module will be incorporated in Yield Prophet so that farmers will have easy access to the information. This project was funded in Phase 1 of the National Weeds and Productivity Research Program, which was managed by the Australian Government Department of Agriculture, Fisheries and Forestry (DAFF) from 2008 to The Rural Industries Research and Development Corporation (RIRDC) is now publishing the final reports of these projects. Phase 2 of the Program, which is funded to 30 June 2012 by the Australian Government, is being managed by RIRDC with the goal of reducing the impact of invasive weeds on farm and forestry productivity as well as on biodiversity. RIRDC is commissioning some 50 projects that both extends on the research undertaken in Phase 1 and moves into new areas. These reports will be published in the second half of This report is an addition to RIRDC s diverse range of over 2000 research publications which can be viewed and freely downloaded from our website Information on the Weeds Program is available online at Most of RIRDC s publications are available for viewing, free downloading or purchasing online at Purchases can also be made by phoning Craig Burns Managing Director Rural Industries Research and Development Corporation iii

6 Contents Foreword... iii Summary... vi Introduction... 1 Methods... 2 Trial design... 2 Heliotrope... 2 Melons... 3 Soil type... 4 Summer rainfall... 4 Soil water content... 5 Results... 6 Soil water use... 6 Heliotrope... 6 Melons... 7 Soil water change... 7 Heliotrope... 7 Melons... 8 Weed dry matter production and growth characteristics... 9 Dry matter production... 9 Plant height The Normalised Vegetation Difference Index The development and use of a summer weeds module for APSIM Model development Impact of summer weeds on soil water at sowing Impact of summer weeds on crop yield Impact of climate change on soil water and crop yield Conclusion Implications References iv

7 Tables Table 1 Soil description at summer weed site... 4 Table 2 Gravimetric soil water depletion for four densities of heliotrope at the middle stage of the trial (February) and the end of the trial (March) relative to the start of the trial (December)... 6 Table 3 Average soil water and crop yield modelled for Berriwillock, 1960 to Table 4 Average soil water and crop yield modelled for Berriwillock in a changed climate scenario, 1960 to Figures Figure 1 Heliotrope and Camel melon... vii Figure 2 Heliotrope treatments, early February Figure 3 Melon treatments, early February Figure 4 Summer rainfall, Figure 5 Change in gravimetric soil water attributed to three densities of heliotrope: summer Figure 6 Summer rainfall and soil water depletion for heliotrope at 55 plants per square metre... 7 Figure 7 Relative soil water depletion for four densities of heliotrope at 30 centimetres soil depth... 8 Figure 8 Relative soil water depletion for four densities of melon at 55 centimetres soil depth... 8 Figure 9 Heliotrope dry matter production at three densities during the summer... 9 Figure 10 Melon dry matter production at three densities during the summer... 9 Figure 11 Average height of heliotrope at three densities Figure 12 Normalised Vegetation Difference Index as assessed with the Greenseeker on four densities of heliotrope during the summer Figure 13 Normalised Vegetation Difference Index as assessed with the Greenseeker on four densities of melon during the summer Figure 14 Simulated water use by heliotrope at a density of 55 plants per square metre Figure 15 Simulated water use by heliotrope at a density of 10 plants per square metre Figure 16 Simulated water use by heliotrope at a density of 20 plants per square metre Figure 17 Modelled plant available water on 1 April for a weed-free situation and with heliotrope, 1960 to Figure 18 Modelled wheat yield for a weed-free situation and a situation in which heliotrope was not controlled, 1960 to Figure 19 Modelled plant available water on 1 April in a changed climate scenario for a weed-free situation and one with heliotrope, 1960 to Figure 20 Modelled crop yield in a changed climate scenario for a weed-free situation and one with heliotrope, 1960 to v

8 Executive Summary What the report is about This project provided valuable information about water use at different densities of summer weeds and enabled the development and validation of a summer weeds module for APSIM, the Agricultural Production Systems Simulator. The module will be incorporated in Yield Prophet so that farmers will have easy access to the information. Who is the report targeted at? This research will help farmers make decisions about the profitability of controlling summer weeds. Further, it will assist with determining the impact of summer weeds during a time of climate change, when it is expected there will be a reduction in winter spring rainfall and an increase in summer rainfall. The summer weeds module models soil water loss as a result of weed growth and clearly demonstrates the adverse effect of summer weeds on potential wheat yield. Using southern Mallee meteorological data and running the model over 60 years (1950 to 2009), it was estimated that uncontrolled summer weeds resulted in a loss of 1.0 tonnes a hectare in wheat yield (wheat yields where summer weeds were controlled, 3.8 tonnes a hectare; summer weeds not controlled, 2.8 tonnes a hectare). The expected consequences of a changed climate in north-western Victoria are reduced winter and spring rain and increased summer rain, so controlling summer weeds will only become more important. Where are the relevant industries located in Australia Southern Australian cropping regions. Background Summer weeds have an adverse effect on farm viability in southern Australian cropping regions. They use water and nutrients that could otherwise be used by ensuing crops. In the future the importance of summer weeds will increase, since climate change scenarios foresee a greater proportion of annual rainfall occurring during summer. At present our understanding of the economics of summer weeds and their management is inadequate. We need to create the tools necessary for quantifying the costs and benefits of summer weed control, both for the current climate and under changed climatic conditions. This will allow the development of a framework for targeted research and extension aimed at mitigating summer weeds impact on production and farm viability. vi

9 Heliotrope (Heliotropium europaeum) Camel melon (Citrellus lanatus) Figure 1 Heliotrope and Camel melon The water use of heliotrope and camel melons was monitored during the summer in Victoria s southern Mallee. In November and early December 2009 close to 100 millimetres of rain fell in the region, and weed growth was prolific following the rain. The debate about whether it was worth controlling these weeds was robust. This project assessed summer weeds impact on the amount of stored soil water and, through modelling, looked at the benefits or otherwise of controlling those weeds. It was found that summer weeds such as heliotrope, at high density, used 50 millimetres of stored soil water that would otherwise have been available to the following crop. This project provided valuable information about water use at different densities of summer weeds and enabled the development and validation of a summer weeds module for APSIM, the Agricultural Production Systems simulator. The module will be incorporated in Yield Prophet so that farmers will have easy access to the information. The summer weeds module models soil water loss as a result of weed growth and clearly demonstrates the adverse effect of summer weeds on potential wheat yield. Using southern Mallee meteorological data and running the model over 60 years (1950 to 2009), it was estimated that uncontrolled summer weeds resulted in a loss of 1.0 tonnes a hectare in wheat yield (wheat yields where summer weeds were controlled, 3.8 tonnes a hectare; summer weeds not controlled, 2.8 tonnes a hectare). The expected consequences of a changed climate in north-western Victoria are reduced winter and spring rain and increased summer rain, so controlling summer weeds will only become more important. Aims/objectives collecting detailed data on the growth and water and nutrient use of two of the main summer weeds in Victoria s southern Mallee heliotrope (Heliotropium europaeum) and camel melon (Citrellus lanatus) development of parameters and validation of modules for these summer weeds in the farming systems model APSIM, the Agricultural Production Systems simulator using the summer weeds module in conjunction with both historical meteorological data and down-scaled climate change scenarios to conduct analyses of the impacts of summer weeds on subsequent crops over multiple sites and seasons and under different climate change scenarios. vii

10 Methods used In summary, the methods used for the project were as follows: site located on a sandy clay loam (calcarasol) at JilJil, 32 kilometres north of Birchip in Victoria s southern Mallee soil water initial and final gravimetric soil water measured in November 2009 and March 2010; weekly neutron probe readings taken over the summer of ; and calibration of the neutron probe for the specific soil type, including bulk density and soil DUL (drained upper limit) and CLL (crop lower limit) measurement weed growth summer weeds ( heliotrope and camel melon) monitored; plots with different densities of each of these weeds established, with four densities for each weed (heliotrope at 0, 10, 20 and 55 plants per square metre and melon at 0, 1, 2 and 4 plants per square metre); dry matter production and other plant growth characteristics of these weeds assessed until plant senescence model requirements soil classification for model initialisation; records of daily summer rainfall during trial; summer weed plant water use (from soil water extraction) model outcomes operational for summer soil water use; model outcome of water use in different years; model impact of summer weeds water use on subsequent cereal crop production; model impact of shifts in rainfall patterns, from dominant winter rain to increased summer rain. Results/key findings The summer weeds module models soil water loss as a result of weed growth and clearly demonstrates the adverse effect of summer weeds on potential wheat yield. Over 60 years (1950 to 2009) the model estimated uncontrolled summer weeds resulted in a loss of 1 tonne a hectares in wheat yield (wheat yields where summer weeds were controlled, 3.8 tonnes a hectare; summer weeds not controlled, 2.8 tonnes a hectare). The return on investment (the cost of herbicide plus application compared with the return in extra grain yield) was between one in five and one in nine depending on the price of wheat. This clearly demonstrates the importance of early and effective summer weed control. The model will be further tested in the Grains Research and Development Corporation water use efficiency project and in the weed field trials carried out by the Birchip Cropping Group during the summer of Implications for relevant stakeholders for: This project clearly demonstrates the importance of farmers controlling summer weeds. In the summer of summer weeds such as heliotrope used 50 millimetres of soil water that would otherwise have been available to the ensuing crop. In dry-land crop production regions such as the Victorian Mallee this is equivalent to potential additional wheat production of 1 tonne a hectare. The summer weeds module developed for APSIM makes a significant contribution to our ability to model soil water change over summer resulting from uncontrolled summer weeds and what impact this has on modelled yields over time. In addition, taking into account a changing climate, the summer weeds module demonstrates the increased importance of controlling weeds during summer. viii

11 Introduction Our research approach involved three steps: collecting detailed data on the growth and water and nutrient use of two of the main summer weeds in Victoria s southern Mallee heliotrope (Heliotropium europaeum) and camel melon (Citrellus lanatus) see Figure1 development of parameters and validation of modules for these summer weeds in the farming systems model APSIM, the Agricultural Production Systems simulator using the summer weeds module in conjunction with both historical meteorological data and down-scaled climate change scenarios to conduct analyses of the impacts of summer weeds on subsequent crops over multiple sites and seasons and under different climate change scenarios. This project was linked to the project entitled Yielding benefits through partnerships, which was funded by the Grains Research and Development Corporation. The summer weeds research was done at sites with soil types similar to those already established. This provided instrumentation and an existing treatment framework that increased our understanding summer weeds impacts on potential grain production. In summary, the method used for the project was as follows: site located on a sandy clay loam (calcarasol) at JilJil, 32 kilometres north of Birchip in Victoria s southern Mallee soil water initial and final gravimetric soil water measured in November 2009 and March 2010; weekly neutron probe readings taken over the summer of ; and calibration of the neutron probe for the specific soil type, including bulk density and soil DUL (drained upper limit) and CLL (crop lower limit) measurement weed growth summer weeds ( heliotrope and camel melon) monitored; plots with different densities of each of these weeds established, with four densities for each weed (heliotrope at 0, 10, 20 and 55 plants per square metre and melon at 0, 1, 2 and 4 plants per square metre); dry matter production and other plant growth characteristics of these weeds assessed until plant senescence model requirements soil classification for model initialisation; records of daily summer rainfall during trial; summer weed plant water use (from soil water extraction) model outcomes operational for summer soil water use; model outcome of water use in different years; model impact of summer weeds water use on subsequent cereal crop production; model impact of shifts in rainfall patterns, from dominant winter rain to increased summer rain. 1

12 Methods Trial design Heliotrope Plots were established with four densities of heliotrope (Heliotropium europaeum) 0, 10, 20 and 55 plants per square metre, the different densities being achieved through hand weeding (see Figure 1). Each density treatment was replicated three times and each treatment plot was 10 square metres. At the centre of each plot a neutron probe access tube was located for fortnightly readings of soil water content. The plots were divided into subplots of 1 square metre, which were used for dry matter cuts during the season. Gravimetric soil water content was determined for each treatment plot at the start, the middle and the end of the monitoring trial. 0 plants/m 2 10 plants/m 2 20 plants/m 2 55 plants/m 2 Note: The heliotrope plants in the high-density plots (55/m 2 ) are very small. The neutron probe access tube is visible in the centre of each plot. Figure 2 Heliotrope treatments, early February

13 Melons Plots with melons (Citrellus lanatus) were established at four densities 0, 1, 2 and 4 plants per square metre (see Figure 3). Each treatment plot was 10 square metres. Two replicate plots were established for each density, the number of replicates was limited to two because it was difficult to find even stands of melons located relatively close together. The assessments and other observations were carried out in the same manner as for the heliotrope plots. 0 plants/m 2 1 plant/m 2 2 plants/m 2 4 plants/m 2 Note: The neutron probe access tube is visible in the centre of each plot. Figure 3 Melon treatments, early February

14 Soil type The trial site was 32 kilometres north of Birchip in Victoria s southern Mallee ( S, E), in the north-west corner of a paddock on a slight rise to the north. The soil is classified as a calcarasol, with a sandy topsoil and a light clay subsoil (containing free calcium carbonate). For APSIM simulations the soil profile was described in full, with detailed information on soil water characteristics and chemical constraints to plant root growth (see Table 1). Table 1 S oil des cription at s ummer weed s ite Depth Texture / (clay) PAW DUL CLL BD EC ph (cm) (mm) (vol%) (vol%) (ml/cm 3 ) (ds/m) (water) 0 10 Fine sandy loam (10 20%) Light clay (35 40%) Light medium clay (35 40%) Light medium clay (35 40%) Light medium clay (35 40%) Notes: Description applies to the heliotrope plots; the plots with melons were on deeper sand. Depth of the sand ranged from 40 to 60 cm, overlying a light clay. PAW = plant available water; DUL = drained upper limit, also known as field capacity; CLL = crop lower limit, also known as wilting point; BD = bulk density; EC = electrical conductivity. Summer rainfall Ninety millimetres of rain fell in November 2009, before the trial began. The summer of was relatively dry, there being only one large rainfall event, on 8 March 2010 (see Figure 4). The trial was completed on 27 March Rainfall (mm) Summer weed trial (JilJil) rainfall /11/ /11/ /11/2009 6/12/ /12/ /12/ /12/2009 3/01/ /01/ /01/ /01/ /01/2010 7/02/ /02/ /02/ /02/2010 7/03/ /03/ /03/ /03/2010 4/04/ /04/2010 Note: Arrows show the start and finish for the trial. Figure 4 S ummer rainfall,

15 Soil water content Gravimetric soil samples were taken at the start of the trial (on 7 December 2009), in the middle of the trial (on 26 February 2010) and at the end of the trial (on 26 March 2010). In each treatment plot cores were taken to a depth of 1 metre in five soil layers (0 10, 10 20, 20 40, and centimetres). In the two weeks before the start of the trial the site had received 90 millimetres of rain and the soils were relatively wet to a depth of about 40 centimetres. As noted, during most of the trial there was minimal rain. On 8 March 2010, however, 37 millimetres fell, and the top layer of soil became quite wet. The average gravimetric soil water content for each treatment (based on three replicates) was calculated for each soil layer. On a sandy soil such as this the top 20 centimetres of soil is susceptible to evaporation, so the topsoil layer was not used for the calculation of soil water use of the different weed densities. The gravimetric soil water readings were recalculated to volumetric soil water content using the measured bulk density of the soil. 5

16 Results Soil water use Heliotrope Table 2 shows the soil water changes between the December and February measurements and the December and March measurements. Table 2 Gravimetric soil water depletion for four densities of heliotrope at the middle stage of the trial (February) and the end of the trial (March) relative to the start of the trial (December) Soil water depletion (mm) 2 Heliotrope density /m December to February December to March Note: Measurements taken at cm depth. The data on soil water depletion were used to calculate how much soil water was used by the heliotrope treatments of different densities. The amount of soil water depleted in the control plot (zero heliotrope) was subtracted from each of the different density treatments. The lowest density heliotrope (10 plants per square metre) used less soil water than the medium-density treatment (20 plants per square metre), which in turn used less water than the high-density treatment (55 plants per square metre) (see Figure 5). Change in gravimetric soil water (mm) Dec to Feb Dec to Mar Heliotrope density - soil water change H 10 H 20 H 55 Heliotrope plants per square metre Figure 5 Change in gravimetric soil water attributed to three densities of heliotrope: summer The volumetric soil water content for each of the treatments was calculated using the bulk density specific for this soil type (see Table 1). From the start of the trial in early November until the end of the trial in late March the soil dried out considerably: 19, 38, 51 and 70 millimetres of soil water was lost over the 20 to 100 centimetres soil layer for heliotrope at 0, 10, 20 and 55 plants per square metre respectively (note that 37 millimetres of rain fell at the beginning of March) (see Figure 6). If it is 6

17 assumed that the zero heliotrope treatment reflects losses through evaporation, the actual heliotrope treatment losses were 19, 32 and 52 millimetres for 10, 20 and 55 plants per square metre respectively. Rainfall (mm) /11/ /11/2009 Rainfall and soil water depletion by Heliotrope (at 55 plants/m 2 ) Note: Measurements taken at cm depth. Figure 6 29/11/2009 6/12/ /12/ /12/ /12/2009 3/01/ /01/ /01/ /01/ /01/2010 rainfall (mm) Soil water content 7/02/ /02/ /02/ /02/2010 7/03/ /03/ /03/ /03/2010 4/04/ /04/2010 S ummer rainfall and soil water depletion for heliotrope at 55 plants per square metre A loss of 20 to 50 millimetres of stored summer rainfall to weeds is a major loss for the ensuing cereal crop. Using WUE (water use efficiency) calculations, a 20 or 50 millimetres loss in stored water is equivalent to a reduction in potential wheat yield of 400 kilograms per hectare and 1 tonne per hectare respectively (based on wheat producing grain at 20 kilograms per hectare per millimetre of stored water). Melons The melon plots were on deeper sands and the sand depth was more variable, ranging from 40 to 60 centimetres. There was also water extraction during the season because of the melons, but the data was? not as clear, possibly as a result of the variation in the depth of the sandy layer or because the position of melons in each treatment plot was not the same. Taking a soil core immediately adjacent to a melon, as opposed to 50 centimetres from the plant, will make a large difference in the water content of the soil. For gravimetric sampling, the location of the soil core relative to the plant is crucial since melons have a tap root rather than a more branching root system. The same caution applies to the position of neutron probes. Soil water change A neutron probe is an excellent tool for assessing relative differences in soil water change between different treatments. At the project site a neutron probe was used weekly to monitor relative water use between the four treatments and to gauge how water was being depleted in the soil profile as the summer progressed. Heliotrope Heliotrope extracted water until late January early February, presumably because the soil had then reached the lower limit of extraction by heliotrope. In March there was, as noted, a significant rainfall event and the counts increased (see Figure 7). The water extraction rate between early December and Soil water content (Vol. mm) 7

18 late January was steady, indicative of relatively stable weekly water use (note that there was little or no rain during this period). Count rate Heliotrope at four densities and change in soil water content at 30cm depth Heliotrope 0/m2 Heliotrope 10/m2 Heliotrope 20/m2 Heliotrope 55/m /12/ /12/ /12/ /12/2009 6/01/ /01/ /01/ /01/2010 3/02/ /02/ /02/ /02/2010 3/03/ /03/ /03/ /03/2010 Note: A high reading reflects a wetter soil; a low reading reflects a drier soil. Figure 7 R elative soil water depletion for four densities of heliotrope at 30 centimetres soil depth Soil water loss appeared to be relatively steady week by week from mid-december until late January. In the preceding section we calculated that the heliotrope at 55 plants per square metre had used 50 millimetres of water during that period. Thus, in the five weeks during which heliotrope extracted the greatest amount of water (mid-december to late January) the weekly water use was 10 millimetres. Melons Soil water extraction by the melons was not as clear as for heliotrope. There was no clear water extraction profile for any density of melons, although the zero density always had a relatively higher water content (see Figure 8). As was the case with the gravimetric soil samples, the lack of a trend in the melons water use could be a result of the varying depths of sand in the topsoil (the sandy topsoil varied between 40 and 60 centimetres in depth) or the position of melons within a plot and the distance of these plants from the probe Melon at four densities and change in soil water content at 55cm depth Count rate D0,55 D1,55 D2,55 D4, /12/ /12/ /12/ /12/2009 6/01/ /01/ /01/ /01/2010 3/02/ /02/ /02/ /02/2010 3/03/ /03/ /03/ /03/2010 Note: A high reading reflects a wetter soil; a low reading reflects a drier soil. Figure 8 R elative soil water depletion for four densities of melon at 55 centimetres soil depth 8

19 Weed dry matter production and growth characteristics Dry matter weights and growth characteristics were determined fortnightly for each density treatment of both heliotrope and melons. Dry matter production The growth of both weed types was rapid from the start of the trial until early February (see Figures 9 and 9). During this time there was a negligible amount of rain and we suspect that growth had stopped, and in fact dry matter decreased, during February and early March. On 8 March, 37 millimetres of rain fell and the weeds recovered and growth began again. At the end of the trial in late March some of the weeds were still green and on 14 April these were cut, dried and weighed. Average dry matter weight (g/m 2 ) /01/ /01/2010 Dry matter weights (g/m 2 ) for three densities of Heliotrope 29/01/ /02/ /02/ /03/2010 heliotrope 10/m2 heliotrope 20/m2 heliotrope 55/m2 26/03/2010 9/04/2010 Figure 9 Heliotrope dry matter production at three densities during the s ummer Average dry matter weight (g/m 2 ) /01/ /01/2010 Dry matter weights (g/m 2 ) for three densities of Melon 29/01/ /02/ /02/ /03/2010 melon 1/m2 melon 2/m2 melon 4/m2 26/03/2010 9/04/2010 Figure 10 Melon dry matter production at three densities during the s ummer 9

20 Plant height The heliotrope plants increased in height until early February; in early March there was some senescence and plants shortened. The lower density heliotrope plants were more robust and taller compared with the higher density plants (see Figure 11). 14 Heliotrope average plant height at three densities 12 Plant height (cm) Heliotrope 10/m2 Heliotrope 20/m2 Heliotrope 55/m2 6/01/ /01/ /01/ /01/2010 3/02/ /02/ /02/ /02/2010 3/03/ /03/ /03/2010 Figure 11 Average height of heliotrope at three densities The number of heliotrope leaves, nodes and flowers per plant was recorded fortnightly and showed the same trend as total biomass and average plant heights. The plants in the lower density plots had more leaves, nodes and flowers compared with the higher density populations. Heliotrope flowered throughout the summer period; peak flowering occurred in late January to early February, with 12, nine and four flowers per plant for the 10, 20 and 55 plants per square metre treatments respectively. The Normalised Vegetation Difference Index The NDVI of the weed treatments was monitored every 10 days with the Greenseeker. The Greenseeker assesses the greenness of a canopy, which is expressed as an NDVI value. Heliotrope NDVI values were low throughout the season, there being a short peak in early January, when the heliotrope was still quite green. As heliotrope matures the colour changes from a deep green to more grey green, which would also reduce the NDVI of the stand (see Figure 12). For this reason it is unlikely that the NDVI is useful for assessing the stand density of heliotrope later in the summer. 10

21 Heliotrope NDVI 0.2 Heliotrope 0/m2 Heliotrope 10/m2 Heliotrope 20/m2 Heliotrope 55/m2 NDVI /01/ /01/ /01/2010 1/02/ /02/ /02/2010 3/03/ /03/ /03/2010 Note: A high value indicates an abundance of green vegetation. Figure 12 Normalised Vegetation Difference Index as assessed with the Greenseeker on four densities of heliotrope during the s ummer Melons remain much greener during the summer, and the NDVI readings clearly reflect the different densities of melons in the trial (Figure 13). Melon NDVI 0.6 Melon 0/m2 Melon 1/m2 Melon 2/m2 Melon 4/m2 NDVI /01/ /01/ /01/2010 1/02/ /02/ /02/2010 3/03/ /03/ /03/2010 Note: A high value indicates an abundance of green vegetation. Figure 13 Normalised Vegetation Difference Index as assessed with the Greenseeker on four densities of melon during the s ummer 11

22 The development and use of a summer weeds module for APSIM The Birchip Cropping Group studied the impact of summer weeds during the summer of , as discussed in the preceding section. The main outcome of the work was to use the data collected, primarily on biomass production and soil water use by weeds, to develop a summer weeds module for APSIM, the Agricultural Production Systems Simulator (Keating et al. 2003) APSIM is a simulation model that incorporates physiological and agronomic assumptions for wheat crop production and is thus an appropriate tool for determining potential yield. It can be used to calculate yields under ideal as well as actual management conditions. The current version of APSIM does not contain a summer weeds module, and for the successful simulation of crop yields it is essential to measure soil water content immediately before sowing. If soil water content following summer rain could be accurately modelled by taking into account soil water use by weeds, it would alleviate the need to measure soil water, which is an expensive and time-consuming procedure. The module will also help farmers make decisions about the profitability of controlling summer weeds. Further, it will assist with determining the impact of summer weeds during a time of climate change, when it is expected there will be a reduction in winter spring rainfall and an increase in summer rainfall. In summers with higher than usual rainfall weeds can have a big impact on soil water content at the time of sowing crops. A rainfall event large enough to germinate summer weeds does not, however, occur in all years. If the soil is completely dry a 25-millimetre rainfall event is required to germinate summer weeds such as heliotrope (Hunt 2005). Long-term rainfall records for the southern Mallee, using the Berriwillock meteorological data set from 1900 to 2010 (Berriwillock being in the heart of the southern Mallee, at 35.6 o S and o E), show that such an event occurs over three consecutive summer days in 54 per cent of years. For a soil with some existing soil moisture the rainfall event required to germinate and grow summer weeds might well be less: a 15-millimetre rainfall event might be sufficient, and this occurs in 77 per cent of years. Model development The summer weeds module for APSIM was developed using the data collected for the highest weed population, at 55 heliotrope plants per square metre. Leaf and node appearance was used to ensure that plant growth was similar to the observed data. Leaf area was adjusted to check the ability of the model to use water at the rate observed. Figure 14 shows the simulated soil water content plotted against the measured soil water content. 12

23 Note: Simulated water use is represented by the line; the dots represent the observed data. Figure 14 S imulated water use by heliotrope at a density of 55 plants per square metre To check the reliability of the soil water use by heliotrope, the module parameters developed for heliotrope at 55 plants per square metre were checked against the measured data for heliotrope densities of 10 and 20 plants per square metre (see Figures 15 and 16). Note: Simulated water use is represented by the line; the dots represent the observed data. Figure 15 S imulated water use by heliotrope at a density of 10 plants per square metre 13

24 Note: Simulated water use is represented by the line; the dots represent the observed data. Figure 16 S imulated water use by heliotrope at a density of 20 plants per square metre 14

25 Impact of summer weeds on soil water at sowing The summer weeds module was included in APSIM and the model was run for plant available soil water using Berriwillock meteorological data from 1960 to The scenarios began in 1960 with a starting soil water content of 113 millimetres of plant available water, and the soil water content in subsequent years was modelled (see Figure 17). There were two scenarios: a no-weed situation (that is, heliotrope was controlled) and a situation in which heliotrope had germinated in response to 25 millimetres of rain over three days and was not controlled. In years with significant summer rain, such as , controlling summer weeds resulted in about 100 millimetres of summer rain being stored and available for the subsequent crop (plant available water with summer weeds controlled, 133 millimetres; plant available water with summer weeds not controlled, 31 millimetres) Impact of summer weeds on Plant Available Water (1st of April) Plant available water (mm) Note: Heliotrope germinated only in those seasons in which 25 mm of rain fell over three days. Figure 17 No weeds With Heliotrope Modelled plant available water on 1 April for a weed-free situation and with heliotrope, 1960 to 2009 The average plant available water over the period is shown in Table 3 for the summer weed-free situation and the situation in which weeds were not controlled and germinated in response to 15 or 25 millimetres of rain over three days. Table 3 Average soil water and crop yield modelled for B erriwillock, 1960 to 2009 Weed free Heliotrope emerging after 15 mm rain 2009 Heliotrope emerging after 25 mm rain Modelled soil water on 1 April (mm) Modelled wheat yield (t/ha)

26 Impact of summer weeds on crop yield The yield of the wheat variety Yitpi was modelled using the Berriwillock meteorological data set in a non-limiting N environment. The wheat was sown after 1 May every year, regardless of rainfall in autumn (in some years the crop was dry sown). Two scenarios were modelled: summer weeds controlled and summer weeds not controlled (see Figure 18). Wheat yield (t/ha) Impact of summer weeds on following crop yield Note: Heliotrope germinated only in those seasons with 25 mm of rain over three days. Figure 18 No weeds With Heliotrope Modelled wheat yield for a weed-free situation and a situation in which heliotrope was not controlled, 1960 to 2009 The results showed that, on average, an extra 30 millimetres of plant available water was stored when summer weeds were controlled, and this resulted in an average yield increase of 1 tonne a hectare. Considering that heliotrope control (two applications of herbicide) costs about $30 a hectare (including application costs) and a tonne of wheat is worth anywhere between $150 and $280 on farm (depending on the year) the return on investment is considerable (minimum one to five; maximum one to nine). 16

27 Impact of climate change on soil water and crop yield The impact of summer weeds on crop production was modelled using projected climate scenarios from OzClim ( OzClim models patterns of regional change in temperature and rainfall as obtained from a selection of global climate models run by CSIRO and other research centres (Page & Jones 2001). The climate change scenario was run using the Berriwillock meteorological data set and compared situations in which summer weeds were controlled with ones in which they were not controlled (see Figure 19). The worst-case scenario of climate change, which is based on high emissions and high climate variability, was used for the modelling (Hochman et al. 2009) Impact of summer weeds on Plant Available Water (1st of April) in a changing climate Plant available water (mm) Note: Heliotrope germinated only in those seasons in which 25 mm of rain fell over three days. Figure 19 No weeds With Heliotrope Modelled plant available water on 1 April in a changed climate scenario for a weedfree s ituation and one with heliotrope, 1960 to 2009 Generally, the modelled yields for a changed climate are lower than current yields (see Figure 20). Wheat yield (t/ha) Impact of summer weeds on following crop yield in a changing climate Note: Heliotrope germinated only in those seasons in which 25 mm of rain fell over three days. Figure 20 No weeds With Heliotrope Modelled crop yield in a changed climate scenario for a weed-free s ituation and one with heliotrope, 1960 to 2009 In a changed climate scenario the modelled yield for a weed-free situation was significantly higher compared with a situation where weeds were not controlled (see Table 4)

28 Table 4 Average soil water and crop yield modelled for B erriwillock in a changed climate s cenario, 1960 to 2009 Weed free Heliotrope emerging after 15 mm of summer rain Heliotrope emerging after 25 mm of summer rain Modelled soil water on 1 April (mm) Modelled wheat yield (t/ha) When the current situation is compared with a changed climate situation (comparing Tables 3 and 4) it is clear that there is a loss in yield of about 0.3 tonnes a hectare as a result of a changed climate (3.77 compared with 3.45 tonnes a hectare for a weed-free situation in the current climate compared with the changed climate scenario). Conclusion The measurements of biomass and soil water depletion caused by summer weeds, as obtained during the summer, enabled the development of a summer weeds module in APSIM. It was found that at the highest density of heliotrope (55 plants per square metre) the weeds used about 50 millimetres of soil water that would otherwise have been available for an ensuing crop. The summer weeds module models soil water loss as a result of weed growth and clearly demonstrates the adverse effect of summer weeds on potential wheat yield. Over 60 years (1950 to 2009) the model estimated uncontrolled summer weeds resulted in a loss of 1 tonne a hectares in wheat yield (wheat yields where summer weeds were controlled, 3.8 tonnes a hectare; summer weeds not controlled, 2.8 tonnes a hectare). The return on investment (the cost of herbicide plus application compared with the return in extra grain yield) was between one in five and one in nine depending on the price of wheat. This clearly demonstrates the importance of early and effective summer weed control. The model will be further tested in the Grains Research and Development Corporation water use efficiency project and in the weed field trials carried out by the Birchip Cropping Group during the summer of Implications This project clearly demonstrates the importance of farmers controlling summer weeds. In the summer of summer weeds such as heliotrope used 50 millimetres of soil water that would otherwise have been available to the ensuing crop. In dry-land crop production regions such as the Victorian Mallee this is equivalent to potential additional wheat production of 1 tonne a hectare. The summer weeds module developed for APSIM makes a significant contribution to our ability to model soil water change over summer resulting from uncontrolled summer weeds and what impact this has on modelled yields over time. In addition, taking into account a changing climate, the summer weeds module demonstrates the increased importance of controlling weeds during summer. 18

29 References Hunt, JR 2005, The ecology of common heliotrope (Heliotropium europaeum L.) in a Mediterranean dry-land cropping system, PhD dissertation, University of Melbourne. Hochman, Z, van Rees, H, Carberry, PS, Hunt, JR, McCown, RL, Gartmann, A et al. 2009, Re-inventing model-based decision support with Australian dryland farmers. 4. Yield Prophet helps farmers monitor and manage crops in a variable climate, Crop & Pasture Science, vol. 60, pp Keating, BA, Carberry, PS, Hammer, GL, Probert, ME, Robertson, MJ, Holzworth D et al. 2003, The Agricultural Production Systems simulator (APSIM): its history and current capability, European Journal of Agronomy, vol. 18, pp Page, CM & Jones, RN 2001, OzClim: the development of a climate scenario generator for Australia, in F Ghassemi et al. Proceedings MODSIM 2001: International Congress on Modelling and Simulation, Australian National University, Canberra, Modelling and Simulation Society of Australia and New Zealand, Australian National University, Canberra, pp

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32 Summer Weeds: Counting the costs for a climate-changed future by Harm van Rees, Jackson Davis and Kaylene Nuske, Birchip Cropping Group. Pub. No. 11/031 Summer weeds have an adverse effect on farm viability in southern Australian cropping regions. They use water and nutrients that could otherwise be used by ensuing crops. In the future the importance of summer weeds will increase, since climate change scenarios foresee a greater proportion of annual rainfall occurring during summer. At present our understanding of the economics of summer weeds and their management is inadequate. We need to create the tools necessary for quantifying the costs and benefits of summer weed control, both for the current climate and under changed climatic conditions. This will allow the development of a framework for targeted research and extension aimed at mitigating summer weeds impact on production and farm viability. It was found that summer weeds such as heliotrope, at high density, used 50 millimetres of stored soil water that would otherwise have been available to the following crop. This research provided valuable information about water use at different densities of summer weeds and enabled the development and validation of a summer weeds module for APSIM, the Agricultural Production Systems simulator. The module will be incorporated in Yield Prophet so that farmers will have easy access to the information. This project was funded in Phase 1 of the National Weeds and Productivity Research Program, which was managed by the Australian Government Department of Agriculture, Fisheries and Forestry (DAFF) from 2008 to The Rural Industries Research and Development Corporation (RIRDC) is now publishing the final reports of these projects. This report is an addition to RIRDC s diverse range of over 2000 research publications which can be viewed and freely downloaded from our website Information on the Weeds Program is available online at Most of RIRDC s publications are available for viewing, free downloading or purchasing online at Purchases can also be made by phoning Cover photos: Sourced from this report. Left: Heliotrop; Right: Camel melon This publication can be viewed at our website All RIRDC books can be purchased from: Contact RIRDC: Level 2, 15 National Circuit Barton ACT 2600 PO Box 4776 Kingston ACT 2604 Ph: Fax: rirdc@rirdc.gov.au web: