Eleven years of narrow row spacing

Reduced row spacing improves the competitive ability of the crop and so reduces weed growth (Scott et al. 2013). It will also generally increase yield in the absence of weeds (Scott et al. 2013). This increased crop yield easily offsets the minor cost of this technique (from increased fuel usage and a slight increase to the time of sowing), making narrow row spacing economically desirable even in the absence of weeds (Scott et al. 2013). Harvest weed seed destruction may be achieved using a Harrington seed destructor or chaff cart (Walsh et al. 2013). The cost includes an initial investment in machinery, mechanical operation, delayed harvest, fire risk (if burning chaff dumps) and lost nutrients if chaff is removed or burnt (Jones and Monjardino, 2006). Alternatively, weed seeds may be destroyed by burning all residue, or windrow burning (Walsh et al. 2013). The cost of burning residue includes labour, management of the fire risk and lost nutrients (Storrie, 2014).

A study was conducted over 27 years, from 1987 to 2013, to determine the long term impacts of crop row spacing and crop residue burning on yield. Within this trial, herbicides were applied according to regional practices, to control broad-leaved weeds (mainly Sisymbrium orientale L., Indian hedge mustard) and annual ryegrass (Lolium rigidum L. Gaud.). A chaff cart collection system was also utilised in some years. While broad-leaved weed control was successful, annual ryegrass was still found in the trial. The current research utilised data from the final 11 years of this trial (2003 to 2013) to determine the long term impact of crop row spacing and crop residue burning on annual ryegrass seed production.

Method

A trial was run from 1987 to 2013, at the Department of Agriculture and Food Western Australia Merredin Research Station, in a red clay-loam sandy salmon gum/gimlet soil. Treatments included crop row spacings (9, 18, 27 and 36cm) and crop residue management (residue burnt prior to crop seeding or unburnt). The trial was established as a randomised block design with six replications (plot size of 5m by 30m), but treatments were not re-randomised in each subsequent year. To implement the burnt residue treatment, crop residue from the prior year was burnt over the entire plot in autumn. 

The current study utilised data from 2003 to 2013, as this was the period of the trial during which annual ryegrass seed production was measured. Crops were sown using a no tillage seeding system (knife points and press wheels), and fertiliser applied at seeding (Agras^®, Double Phos^® or Double Super^®). In 2008, the field was a chemical fallow due to low rainfall. All herbicides used to control annual ryegrass are listed in Table 1. Broad leaf selective herbicides applied in the trial are not included, as they successfully controlled Indian hedge mustard and did not affect the annual ryegrass or the crop. Herbicide control of annual ryegrass was inadequate, and by 2003 there was dense annual ryegrass in the trial, particularly in the wide row spacing, unburnt plots. Harvests were conducted in November or December, using crop lifters. A Rytec^TM chaff system was utilised at harvest in all plots in the trial from 2003 to 2006.

Table 1. The sowing date, crop cultivar, sowing rate, herbicide use for annual ryegrass control, annual rainfall for the Merredin Research Station and growing season (May to October) rainfall from 2003 to 2013 (from Merredin Research Station weather station number 010093). Note that + is used to indicate two herbicide products that were combined into a single tank mixture at the time of application.

Measurements and statistical analysis

Crop plant density was measured using two 46cm by 108cm quadrats per plot, four to six weeks after crop emergence. Crop yield was assessed by harvesting the centre of each plot (1.62m by 30m). Annual ryegrass seed production was assessed annually, although the method of assessment varied over the 11 year period. Full details of annual ryegrass seed production assessment can be found in Borger et al. (2016).

The crop density, crop yield and annual ryegrass seed production variates were analysed with a linear mixed model for repeated measurements (restricted maximum likelihood (REML) procedure, GenStat 16th edition, VSN International, 2012). Full details of the analysis can be found in Borger et al. (2016). Least significant difference (Lsd) was used to separate means of crop yield and standard error (SE) of the difference was used to separate annual ryegrass seed data.

Results

Crop plant density and yield

Crop density varied significantly between years, as different crops were sown at different rates, but was not affected by residue or spacing (data not presented). Average yield was greater in the unburnt plots (1.64 and 1.53t/ha in the unburnt and burnt plots, P <0.001, Lsd: 41). This was due to increased yield in the unburnt plots in 2003, 2007, 2011 and 2013, although yield was reduced in the unburnt plots in 2005 (Table 2). Average yield also significantly increased at narrower row spacing, although the relationship was not linear in every year (1.66, 1.64, 1.55 and 1.49t/ha in the 90, 180, 270 and 360cm row spacing treatments, P <0.001, Lsd: 589). Exceptions to this trend included 2012 and 2013, where the difference was not significant from 9cm to 36cm. The interaction between crop residue and row spacing treatments, and the interaction between residue, spacing and year, were not significant.

Table 2. Average crop yield (t/ha) in the burnt and unburnt residue treatments (P <0.001, Lsd: 92) and the row spacing treatments (P <0.001, Lsd: 130), from 2003 to 2013.

Annual ryegrass seed at harvest

Average annual ryegrass seed production was lower in the burnt plots, with 56 and 296 seeds/m² in the burnt plots and unburnt plots (P: 0.005, SE: 1.5). Seed density was greater in the wide row spacing treatments, with 57, 77, 222 and 332 seeds/m² in the 9, 18, 27 and 36cm row spacing treatments (P: 0.014, SE: 2.9). There was a significant interaction between year, crop residue and row spacing. The annual ryegrass seed density was generally greater in wide row spacing treatments, although the relationship between seed density and row spacing was not linear in every year (Table 3). However, in the burnt plots, there was no significant difference between row spacing treatments by 2010, as the annual residue burning had reduced seed production to very low levels.

Annual ryegrass seed production was exceptionally high in 2009. Pre-seeding and non-selective herbicides were applied at seeding, and successfully killed annual ryegrass (Table 1). However, a very dense annual ryegrass cohort emerged late in the season. The canola crop was already flowering, and so it was too late to allow use of a selective herbicide.

Table 3. Annual ryegrass seeds/m² in the burnt and unburnt treatments, at a row spacing of 9cm to 36cm, from 2003 to 2013 (P <0.001, SE: 20.9). 

Conclusion

Burning stubble was a highly effective method of weed control, reducing annual ryegrass seed production to close to zero by the end of the 11 year management period. Removing the crop residue through burning is also likely to have improved the efficacy of the pre-emergent herbicide. Where crop residue is left on the soil surface, the pre-emergent herbicide may bind to the residue and the minimal soil disturbance in the no tillage system ensures that the residue and herbicide are not fully incorporated into the soil (Chauhan et al. 2006). As stated in the methods, burning was performed over the entire plot area, and caused a yield reduction (possibly due to reduced soil moisture retention). However, harvest weed seed destruction has made significant progress since 1987. Alternative methods are now available that will destroy annual ryegrass seed more effectively than burning the entire field, without causing a reduction to yield (Walsh et al. 2013).

This research demonstrates the long term benefits of narrow row spacing to reduce annual ryegrass seed production. While narrow row spacing is generally accepted as a method to increase the competitive ability of crops and suppress weed growth, the benefits of narrow row spacing have not been demonstrated in all studies (reviewed by Scott et al. 2013). However, most row spacing trials occur in a single year, on a site with an evenly distributed weed seed bank, and prior studies have generally assessed annual ryegrass biomass rather than seed production. 

The major difference in the current study is that the row spacing treatments had been established for 16 years prior to 2003 when annual ryegrass seed production was first assessed. The trial demonstrates that higher weed seed production occurs in the wide row spacing plots, leading to a higher weed seed bank in sequential years. The reduction in weed seed at narrow row spacing likely resulted from improved crop competition (Scott et al. 2013). However, narrow row spacing would also result in increased soil disturbance at seeding compared to wide row spacing, which would improve the performance of pre-emergent herbicides (Chauhan et al. 2006).

Crop yield was reliably increased at narrow row spacing. This is partially due to reduced weed competition, but narrow row spacing increases crop yield in the absence of weeds, due to canopy closure at an earlier stage, increased light interception, reduced evaporation and reduced intra-species competition for resources (Scott et al. 2013). Prior research has indicated that crops with wide row spacing may have more soil moisture available at maturity and so have higher yields in dry seasons (Blackwell et al. 2006; Scott et al. 2013). In the current trial, a significant yield increase was not observed in the dry years (notably 2007 and 2010). However, the increased annual ryegrass density in the wide rows may have negated any advantage of increased soil moisture as the weeds would compete with the crop and utilise stored soil moisture prior to crop maturity. Very narrow row spacing is not viable in high yielding areas, where the resulting crop residue is difficult to manage during the subsequent seeding operation. However, modern machinery has improved the ability of growers to achieve narrow rows, and growers can reassess their chosen crop spacing. While growers may not want to use 9cm spacing, any reduction in their current spacing will increase crop yield and reduce weed seed production. For row spacing, every centimetre counts.

References

Blackwell P, Pottier S & Bowden B (2006). Response to winter drought by wheat on shallow soil with low seeding rate and wide row spacing. In: Agribusiness Crop Updates, 57-62. Department of Agriculture and Food WA, Perth, Western Australia.

Borger CPD, Riethmuller G & D’Antuono M (2016). Eleven years of integrated weed management: long-term impacts of row spacing and harvest weed seed destruction on Lolium rigidum control. Weed Research, doi: 10.1111/wre.12220.

Chauhan BS, Gill GS & Preston C (2006). Tillage system effects on weed ecology, herbicide activity and persistence: a review. Australian Journal of Experimental Agriculture 46, 1557-1570.

Jones R & Monjardino M (2006). Economic benefits of IWM. In: Integrated weed management in Australian cropping systems – A training resource for farm advisors. (eds T McGillion & A Storrie), 1-10. CRC for Australian Weed Management, Adelaide, South Australia.

Scott BJ, Martin P & Riethmuller G (2013). Graham Centre Monograph No. 3: Row spacing of winter crops in broad scale agriculture in southern Australia. NSW Department of Primary Industries, Orange.

Storrie AE ed. (2014). Integrated weed management in Australian cropping systems. Grains Research and Development Corporation, Australia.

VSN International (2012). GenStat for Windows 15th edition. VSN International, Hemel Hempstead, UK.

Walsh M, Newman P & Powles S (2013). Targeting weed seeds in-crop: a new weed control paradigm for global agriculture. Weed Technology 27, 431-436.

Acknowledgments

Thanks are due to the managers and staff at the Department of Agriculture and Food Western Australia Merredin Research Station, who have assisted with this trial from 1987 onwards, particularly for the careful burning. Thanks are also due to Mr Cameron Wild, for processing annual ryegrass seed samples from 2011 to 2013. This paper was kindly reviewed by Peter Newman and Sally Peltzer.

Contact details

Catherine Borger
catherine.borger@agric.wa.gov.au

Peter Newman
petern@planfarm.com.au