By Kelly Angel, BCG
- Yields were significantly higher under narrow row spacing, but sowing direction had no influence on yield.
- Weeds had a significant effect on yield, but the scale of yield penalty (t/ha yield loss) did not alter with row spacing or sowing direction in 2015.
- Weeds established faster where row spacings were wider, however by late tillering, all treatments had similar weed numbers and biomass levels.
The reliance of farming systems on agrochemicals, particularly herbicides, has increased over time. As a result, the number of weed populations with some level of resistance to herbicides is increasing. This is motivating growers to seek alternatives to herbicides for weed management.
Previous experiments have shown that crop sowing direction and row spacing have an impact on weed growth and seed production. However, these experiments have only been conducted in environments that differ in many ways to that of the Mallee. The reported findings suggest narrow rows and sowing in an east-west direction better suppresses weeds. The aim of this experiment was to determine if this was also true in the Mallee, and is there any benefits from combining the two practices.
Aim: To determine if sowing direction and row spacing can be used to influence grass weed populations and growth, and their impact on crop performance in the Mallee.
One replicated trial was sown as a split plot design with sowing direction as the main plot and row spacing by weeds as the sub plot. A weed treatment was applied as tame oats broadcast prior to sowing, targeting a weed density of 75 plants/m².
The trial was located within a grower’s paddock of Mace wheat sown at the same time so that overall management could be carried out easily. The previous crop was brown manure peas, grazed over summer, and topdressing in 2015 was not required given the seasonal outlook and background nitrogen status.
Assessments carried out in crop included emergence counts of crop and weeds 40 days after sowing, biomass cuts at GS30 (end of tillering) and GS65 (flowering) and maturity crop head and weed panicle numbers. Plots were harvested and processed for standard yield and grain quality assessments.
Growing season rainfall in 2015 was decile 1 which resulted in a reasonably low yielding trial. Rainfall just before and after sowing resulted in good crop and weed establishment in all treatments, however by the end of the season, many wheat plants had failed to tiller, and around 30-50 per cent of the oats sown as weeds had also died while the remaining oats were short and only produced one to two panicles at best.
Sowing direction in the Jil Jil trial had no influence on crop yield, quality, weed biomass production or weed seed set. Given the seasonal conditions, and the observation that wheat plants largely did not tiller, this can most likely be attributed to the fact that the crop did not offer more significant competition for light and nutrients in one direction than the other that may otherwise occur in a more substantial canopy.
Row spacing did influence crop establishment. In plots sown on 22.5cm and 30.5cm (9 and 12 inch) rows, 10 and 12 more plants/m² established than in plots sown at 38 cm (15 inch) respectively (p=0.03,LSD=9.3, CV=11.3). The narrower spaced treatments also achieved a lower within-row (plants/m of row) density, suggesting there was less competition between neighboring plants.
As row spacing widens, the competition between crop plants increases as there are more seeds per meter of row. This can result in reduced plant stands. However, it was found that by maturity, this did not affect the number of heads/m² in any of the treatments, suggesting plants grown at a wider row spacing did compensate with greater tiller production.
The tillers that did form were very small, so although they did increase the head count, they did not increase yield proportionately to having more plants established to begin with.
Crops sown on narrow row spacings produced more biomass by anthesis, and resulted in higher yields in a weed free situation, with 38cm (15 inch) row spacing yielding 0.13t/ha less than the other two spacings which were not significantly different from each other (Figure 1).
It has long been known that narrower row spacing increases yield, however this was believed to be less important in low yielding environments. The more uniform pattern of crop present in 22.5cm and 30.5cm results in greater radiation interception, reducing evaporative losses and increasing dry matter production which leads to higher yields.
One of the arguments in favour of wider row spacing is that it can potentially increase grain yield in low yielding situations (Blackwell et al. 2006; Jones and O’Halloran 2006). This is believed to be because it takes time for the roots to grow and access the reserves in the inter-row area, meaning water is ‘rationed’ to crops at wider row spacings. The resulting reduced biomass production earlier in the season allows water to be conserved for use by the crop after anthesis, potentially increasing harvest index (Scott et. al., 2013).
Given the limited stored moisture and in-crop rainfall in 2015, the ability of wide row spacing to do this was restricted. Although there were differences in biomass production by anthesis, the work in 2015 showed no difference in greenness as measured by NDVI at mid-late grain fill, and there were no significant differences in screenings or protein at any row spacing.
Grain weight (GW) was also measured and revealed that 38cm spacings actually produced significantly smaller grain (GW=33mg) than 22.5cm (GW=34.1mg) or 30.5cm (GW=33.9mg) (P = 0.016, LSD 0.9, CV = 4.5%). To put this in perspective, at these grain sizes, one milligram difference in GW equates to approximately 26kg/ha difference in yield.
Given this result, differences in yield were attributed to a combination of grain size and grain number.
Presence of weeds in a dry season – what is the cost?
Row spacing influenced early weed emergence with wider row spacings having higher weed density when measured 40 days after sowing (Table 1). However by mid tillering weed populations and biomass production was similar in all treatments (data not shown).
Harvest assessments revealed that all row spacings and sowing directions produced a similar number of panicles/m², and oat grain yield in weedy plots was not significantly different regardless of row spacing or sowing direction.
Differences at emergence could be attributed to higher levels of soil throw into the inter-row which may have seen weeds buried a little deeper and taking a bit longer to emerge (in narrower rows). If this is repeatable it could be seen as an advantage in terms of the crop getting a head-start on the weeds.
The reasons behind the weed populations leveling out between treatments by harvest, as measured by panicle numbers, could be attributed to the dry conditions and compensation of lower density weeds producing more panicles.
Table 1. Weed density measured 40 days after sowing and panicle number at maturity as impacted by row spacing.
Although the aim was to try and establish 75 weed plants/m², average weed establishment was only 42/m², however this was enough to have significant impacts on the growth of the crop right through the growing season.
Crop biomass production was lower at the end of tillering in weedy plots and this
followed on to impact yield and some quality parameters. The severity of this impact late in the season however was not influenced by sowing direction or row spacing, but just whether or not the weeds were present, with all row spacings being equally affected by weeds.
The presence of weeds in 2015 resulted in a 0.23t/ha yield penalty, and also impacted on test weight due to oat seeds being present in the sample. Screenings were also 3.6 per cent higher than non-weedy plots (Table 2).
Table 2. Influence of weeds on yield and quality parameters.
The yield reduction in the case of weed presence could be attributed to both a reduction in grain number as a result of lower head numbers produced, as well as a reduction in grain weight (Table 2).
This illustrates well that competition for light and nutrient resources, particularly in a poor season, can have dramatic effects on crop yields, not to mention the carryover effects of weed seeds leading into the next crop.
Blackwell P, Pottier S, Bowden B, 2006, Agribusiness Crop Updates. Perth, Western Australia, ‘Response to winter drought by wheat on shallow soil with low seeding rate and wide row spacing, pp. 57-62.
French RJ, Schultz JE, 1984, Australian Journal of Agricultural Research, ‘Water use efficiency of wheat in a Mediterranean-type environment. I. The relationship between yield, water use and climate’, 35, pp. 743-764.
Jones B, O’Halloran N, 2006, Farming the Mallee with GPS guidance, ‘Wide rows and crop yield’, pp. 1-23.
Scott BJ, Martin M, Reithmuller, GP, 2013, Graham Centre Monograph No. 3. Edited by Toni Nugent & Catriona Nicholls, ‘Row spacing of winter crops in broad scale agriculture in southern Australia’.
The stubble project – maintaining profitable farming systems in Victoria and Tasmania with retained stubble (project number BWD00024) is funded by the GRDC.
A printer-friendly version of this trial report can be accessed here.