Australian Mungbean Association

Australian-grown mungbeans have quality written all over them!

Row spacing to maximise yield

By Kerry McKenzie, PulseAg Consulting

 

To meet the full potential of your mungbeans crop getting the crop set up even before the seed is in the ground is imperative. There are a multitude of agronomic aspects to take into account such as herbicide history, weed spectrum, stored soil moisture, nutrition, and the list goes on. One aspect that can have a large influence of yield is your row spacing.

There are various reasons for your farming system to have the same row spacing for all summer crops such as stubble handling, ability for sorghum to forage for soil moisture late in the season, inter-row weed control and for some, planting their mungbeans at the same spacing as their sorghum is convenient and can have some of the benefits suggested. Recent research suggests that growers review the width that they plant mungbeans, and the closer together the better.

Pulse Agronomy Trial results

The recently completed Northern Pulse Agronomy Initiative, funded by the Grains Research and Development Corporation (GRDC), Queensland Alliance for Agriculture and Food Innovation (QAAFI) and Queensland Department of Agriculture and Fisheries (DAF), had a series of field trials that incorporated row spacings of 25, 50 and 100 cm (some sites had 75 cm treatment but not enough to report results).

The results showed that peak yields always occurred with the narrowest of row spacings. The 25 cm row spacing gave the best yield at most of the individual trial sites, however when all sites were statistically analysed together there was not a yield difference between the 25 cm and 50 cm row spacing, but yields dropped dramatically if planted any wider.

The results were the same regardless of the seasonal yield potential. There were sites that averaged less than 0.5 t/ha and others that averaged 2.2 t/ha, and 25 cm rows were the winner each time.

Comparing two years’ trials results from the Warra region; the 2013/14 season had a lower yield potential due to low in-crop rainfall, the following 2014/15 season with ideal rainfall through the growing season set it up for high yield potential (Figure 1). Both sets of data show the yields at the three row spacings, replicated three times and averaged across several varieties, Crystal and Jade-AU were common to both, 2013/14 included Celera II and 2014/15 had two pre-release varieties from the mungbean breeding program.

In both years the 25 cm row spacing had the highest yield when averaged across all the varieties, however statistically there was no difference to the 50 cm treatment. In 2013/14 only the narrowest treatment was statistically better than the 100 cm treatment, in the higher yielding site year of 2014/15 both the 25 and 50 cm treatments were better than the widest treatment.

When separated out by varieties the row spacing could have a further dramatic yield differences. As an example, the difference at the highest yielding replicated site at Warra in the 2014/15 season, Jade-AU yielded 1.6 t/ha when planted at 100 cm, but 2.5 t/ha at 25 cm.

When mungbeans are worth almost $1000/t that represents a potential loss of income of $900/ha on a gross margin basis if the crop was planted at 1 m spacing. All row spacings had the same target populations of 20–25 plants/m2 and same starter fertiliser rate of 50 kg/ha of a MAP 2% zinc product.

Figure 1 Mungbean row spacing trial in two consecutive summer seasons. Trial (a) in 2013/14 is a lower yield potential year and trial (b) in 2014/15 was a high yield potential year. In both instances 25 cm row spacing had the highest yields, statistically the same as 50 cm, 100 cm was significantly lower than 25 cm. (5% LSD 2013/14 197 kg/ha, 2014/15 320 kg/ha).

 

Even at the lowest yielding trial site at Billa Billa with a site average below 0.5 t/ha, the benefit could be $50/ha just by planting at the narrow row spacing.

Comparing the two extremes of row spacing the yield benefit of 25 cm over 100 cm was anywhere between 16% and 48% as can be seen in Figure 2.

Figure 2 Narrow row spacing benefits are related to seasonal conditions. Percent yield increase of 25 cm row spacing over 100 cm of 5 sites in southern Queensland

 

Trials were also conducted in the Emerald region by Doug Sands (DAF) and the results were not as consistent, yield benefits of narrow rows were observed but the yield potential had to be over 1 t/ha for the difference to be statistically relevant.

Biomass converts to yield

One of the reasons why mungbeans on narrow rows yield more is due to the measured differences in the amount of sunlight intercepted, as the narrow rows close over the inter-row space earlier they are able to intercept more of the incoming energy from the sun. The additional energy is able to be converted into plant biomass.

In all the Pulse Agronomy trials, plant biomass was measured in all treatments. The highest biomass were recorded in the narrow rows, similar to the grain yield results. Several trials results from the 2013/14 and 2014/15 seasons from both southern and central Queensland were combined (Figure 3) the data indicates that the Harvest Index (HI), the ratio of grain yield to total biomass, is consistently 0.3.

Figure 3 Relationship between mungbeans total plant biomass and grain yield has a straight line response with a Harvest Index of 0.3. (excludes CQ 2014/15 due to failed crop)

This relationship between biomass and grain yield is very strong (represented by the R2 value of 0.86). Therefore, anything that promotes maximum biomass production in a given season will almost certainly increase grain yield. Narrow row spacing has been proven as a tool to increase biomass.

Soil water extraction

The 25 cm rows were also able to explore a greater volume of the soil and extract more of the stored soil moisture. The measured difference was up to an additional 30 mm extracted by plants in the narrow row configurations. In the example from Warra in 2014/15 (Figure 4) the additional soil water extracted by 25 cm rows was approximately 18 mm. Not only is more water extracted but the plant converts this water more efficiently to grain. The calculated Water Use Efficiency (WUE) for this site was 8.1 kg grain/mm/ha at 25 cm while at 100 cm it was less at 6.9 kg grain/mm/ha.

Figure 4 Total water use for mungbeans at 25 and 100 cm as calculated from gravimetric starting Plant Available Water Capacity (PAWC) minus ending gravimetric PAWC plus in crop rainfall. The 25 cm treatment extracted more of the PAWC from the profile.

This goes against the common practice in cotton and sorghum where wider rows are used to save soil moisture for later in the season to ensure the crop finishes and yields. If this was the case for mungbeans the wider rows would yield better in the low potential years, but this theory doesn’t hold true for mungbeans as attested by the Pulse Agronomy trial results.

The tap-rooted mungbean plant it is able to extract more PAWC from the narrow row spacing and as previously mentioned, setting up the yield with early biomass is necessary to maximise the yield of mungbeans, which will continue to put roots to depth to maintain that yield potential. Mungbean roots have been recorded extracting soil moisture to below 1 m at the DAF Hermitage Research Facility when no subsoil constraints were noted.

Additional benefits – weed control and nitrogen fixation

Another major benefit of planting at narrower rows is the crop competition that reduces weed biomass and number of weed seeds set. Complementary trials conducted by Dr Bhagirath Chuahan (QAAFI) and Dr Michael Widderick (DAF) have confirmed the yield benefit of the narrowest of row spacings under weed pressure.

Depending on when weeds were introduced to the crop, i.e. weedy from planting or introduced at 3 and 6 weeks after planting (to simulate residual herbicide breakdown), Dr Chuahan demonstrated a reduction in weed biomass. If weeds were introduced at 6 weeks after planting, the 25 cm treatment could reduce weed biomass by almost 90% when compared to the widest treatment of 75 cm. Mungbean grain yield was also increased from 1.25 t/ha at 75 cm to just under 2 t/ha at 25 cm when weeds were introduced at 6 weeks after planting.

Due to the fact narrow rows produce higher biomass the amount of nitrogen fixed by this legume crop can also be increased. The amount of nitrogen fixed by legumes is directly related to the amount of biomass produced as more nitrogen is required to support this biomass. Mungbeans generally don’t fix excess amounts of nitrogen, however they are more than capable of meeting their own requirements in most circumstances.

Trials conducted by Dr Nikki Seymour (DAF) prior to the Pulse Agronomy project had already demonstrated the ability of mungbeans to fix more nitrogen at narrow row spacings (Figure 5).  When averaged across three varieties at two row spacings of 30 and 90 cm, the narrower row spacing was able to fix another 10 kg/ha of nitrogen. This may be the difference between the crop having a net benefit to the following crop in terms of residual nitrogen.

Figure 5 Total plant nitrogen fixed by three mungbeans varieties at two row spacings.  Satin II fixed more nitrogen than Crystal and Jade-AU when averaged over 30 cm and 90 cm row spacings. When all varieties were combined the 30cm row spacing fixed more nitrogen.

Summary

The additional benefit of reducing row spacing such as improved weed competition and increased nitrogen fixation may not be enough to consider reducing mungbeans row spacings. The major benefit comes from the additional yield potential of mungbeans when planted in rows closer together.

As mentioned from the outset there are many considerations when planning to grow a bumper mungbean crop. Serious consideration needs to be given to changing the planting configuration to have narrow rows of 50 cm or below.

Our current farming systems have been developed to store as much precious rainfall in our soil banks for withdrawal later, narrow row mungbeans will withdraw the maximum water out of this bank and convert it most efficiently into grain and dollars, with little to no extra cost to your operation.

 

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