Riverine Plains Inc Research Update

Take home messages

• Riverine Plains Inc conducts a range of research activities, with investment from GRDC and other partners to provide local information to members.
• Large farm scale trials provide grower-relevant information.
• Decisions about stubble management require consideration of the whole system across a range of seasonal conditions.
• Stubble management can manipulate crop development.

Background

Riverine Plains Inc is a progressive grower group dedicated to improving the productivity of broadacre farming systems in north-east Victoria (VIC) and southern New South Wales (NSW). The group provides relevant and unbiased research and information to its members and acts as a conduit for information flow from credible research sources to its membership.

During 2017, Riverine Plains Inc contributed to a range of research projects, however this report will focus on the ‘Maintaining profitable farming systems with retained stubble in the Riverine Plains region’ project and a subset of work within this project focused on frost risk in stubble retained systems.

Results and discussion

The GRDC Stubble Project

The Stubble Project, ‘Maintaining profitable farming systems with retained stubble in the Riverine Plains region’ is funded by GRDC as part of a National Initiative (RPI0009) and conducted in partnership with the Foundation for Arable Research (FAR) Australia. It is looking at ways to improve and maintain profit and sustainability in stubble retention cropping systems across the region. Four large, commercial scale field trials (‘Focus Farms’) have been established across the region, which are comparing different stubble management practices and plant establishment, growth and yield (Table 1). Smaller trials are also evaluating the importance of timing of nitrogen (N) application, plant growth regulators, row spacing and variety selection in optimising production in stubble retained systems.

Table 1. Different stubble management strategies for focus farms as part of the ‘Maintaining profitable farming systems with retained stubble in the Riverine Plains region’ project.

Comparative yields across seasons

The large plot field trials are always placed into a cereal stubble. Therefore, the sites do not continue in the same location every year, but are placed in different paddocks every year to maintain the same rotation position, with the trial crop being sown into wheat stubble. While the trial results cannot be directly compared across seasons (2014 to 2017), the effect of different stubble management techniques can be reviewed across years to determine if any single approach appears to consistently yield better (Tables 2 to 5).

Table 2. Comparative yields and protein contents for large plot field trials located at Coreen/Corowa over the 2014 to 2017 seasons.

Table 3. Comparative yields and protein contents for large plot field trials located at Yarrawonga over the 2014 to 2017 seasons.

Table 4. Comparative yields and protein contents for large plot field trials located at Dookie over the 2014 to 2017 seasons.

Table 5. Comparative yields and protein contents for large plot field trials located at Henty over the 2014 to 2016 seasons (discontinued in 2017).

Summary of yields under different stubble management techniques from 2014 to 2017

Coreen/Corowa: There were no differences in cereal yields due to stubble management techniques in 2014 to 2016. In 2017, the retained stubble treatment resulted in a yield increase compared to all other treatments.

Yarrawonga: In 2016, the addition of 40kg N/ha at sowing resulted in yield increases in both the stubble retained and cultivated treatments, while the burnt treatment increased yield in 2017.

Dookie: The only difference in yield was seen in 2014 when the long stubble incurred a yield penalty compared to all other treatments.

Henty: The mulched treatment with the addition of N at sowing had a higher canola yield in 2014.

Generally, across the past four seasons (2014 to 2017) stubble management has not been a key driver of yield, except for stubble height at Dookie, and addition of N at sowing at Yarrawonga and Henty in 2014. This general lack of effect may be largely due to extreme weather through some of the growing seasons (heat stress in October 2015, waterlogging and high cloud cover in winter and spring 2016), which would have overridden any effects of stubble management on yields.

Key reason for yield differences in 2017

The two key factors driving yields in 2017 were:

1. Lack of rain in spring 2017.
2. Frost damage.

The timing of flowering had a strong impact on the degree to which these two factors influenced final crop yields. Wheat varieties that flowered and filled grain earlier may have done better in the drier spring, which also means they may have been harvested before the harvest rains (e.g. Yarrawonga site, burnt treatment). The frost damage was also dependent upon the flowering date, although the odds were not great this year, due to the high number of frosts.

Therefore, the flowering date is obviously important. We know we can select different varieties with different flowering dates. Can we also use stubble management to manipulate flowering date?

Background — biomass lag with retained stubble

Over the 2014 to 2015 seasons, it was observed that the stubble retained treatments had lower dry matter contents during the earlier growth stages, however by flowering, there was little difference between plants grown in NTSR systems or where the stubble had been burnt or cultivated. The impact of this delay in biomass on the physiological development of the plant was unknown.

Were the plants growing in NTSR systems at the same growth stage, with less biomass, or was there a difference in growth stage? If there was a difference in physiological development of plants in NTSR and burnt or cultivated systems, this could relate to differences in how the crops handle frost and heat stress, based on a shift in the flowering window.

In order to understand if this biomass lag was simply a difference in the rate of dry matter accumulation or if it related to differences in growth stage, a series of growth stage assessments were done at the Coreen, Yarrawonga and Dookie sites in the 2016 season to understand what was happening.

At the Yarrawonga site, stubble management did not influence plant growth and development until stem elongation (11 July 2016) at which point all treatments except the tall stubble treatment began stem elongation. The tall stubble treatment took an extra three weeks to move into stem elongation.

The Dookie site showed differences in growth stage from seedling stage, with treatments separating from 27 June 2016. Plants in the burnt treatment were slightly more developed than the other treatment, with this increase largely maintained over the next four weeks. In comparison, while plants in the tall stubble treatment lagged in development from 27 June 2016, and showed minimal development at the next reading on 4 July 2016, by 18 July they had caught up to the short stubble and disced treatments, while the burnt treatment was still slightly ahead.

In comparison, there were no significant differences in growth stage development at the Coreen site, with plants in the stubble standing, disced and burnt treatments all developing at the same rate.

This work demonstrated that stubble management could have an impact on physiological maturity, which means that stubble could change the flowering date. But, what exactlycaused this change? Was it the actual physical presence of the stubble, or was it something that changed in the environment in the presence or absence of stubble?

The three factors most likely to influence wheat growth in the presence or absence of stubble were considered to be:

- In-canopy temperature.

- In-crop N supply.

- Light availability to the crop.

These three factors will be discussed in detail in the presentation.

The impact of stubble on canopy temperature

From the start of the Stubble Project, there was a lot of interest from growers about the interaction between retaining stubble and frost risk – were crops more likely to suffer frost damage under retained stubble?

As the large field trial plots used within the Stubble Projectare ideal for measuring temperature due to minimal ‘edge effects’, extra investment from the GRDC enabled three of the field sites to be instrumented for temperature monitoring (in-crop temperature loggers and a weather station next to the field site).

In-crop monitoring of canopy temperatures began in 2015, with field sites again instrumented in 2016 and 2017.

The three sites which were instrumented in 2015, 2016 and 2017 were:

Yarrawonga:

• Treatments monitored: Long stubble, short stubble, cultivate, burn.
• In-canopy data loggers at 300mm height, moved to 600mm height in September.
• In-canopy data loggers at 50mm height above the soil.

Dookie:

• Treatments monitored: Long stubble, short stubble, cultivate, burn.
• In-canopy data loggers at 300mm height, moved to 600mm height in September.
• In-canopy data loggers at 50mm height above the soil.
• Data loggers buried 50mm under the soil surface.

Coreen:

• Treatments monitored: NTSR, cultivate, burn.
• In-canopy data loggers at 300mm height, moved to 600mm height in September.
• In-canopy data loggers at 50mm height above the soil.

Figure 1. The number of hours that each stubble treatment spent below each temperature threshold at the Yarrawonga site, as monitored at the loggers placed 300mm above the soil surface, which were moved to 600mm height in September 2017. Bars are measures of standard error.

Figure 2. The number of hours that each stubble treatment spent below each temperature threshold at the Dookie site, as monitored at the loggers placed 300mm above the soil surface, which were moved to 600mm height in September 2017 (high canola crop). Note, this site was on a hill and experienced very little frost. Bars are measures of standard error.

Figure 3. The number of hours that each stubble treatment spent below each temperature threshold at the Coreen site, as monitored at the loggers placed 300mm above the soil surface, which were moved to 600mm height in September 2017. Bars are measures of standard error.

Frost damage was detected at the Yarrawonga and Coreen sites (Figures 1 and 3). While some variation can be seen at each temperature threshold, there were no significant differences in duration and intensity of cold experienced by each of the treatments.

The 2017 Dookie site (Figure 2) was located on the side of a hill, with treatments overlain in a randomised block design up the hill. This meant that each replicate of the stubble treatments experienced variation in altitude, which confounded any effects of treatment on in-canopy temperature. This explains the larger variation measured at the Dookie site. Generally, the Dookie site experienced very little time below zero (< 100 hours over the season), compared to the other sites.

A key point to remember with the data logger readings is that the temperatures recorded within the crop canopy may be up to 5°C less than that recorded at a weather station. Therefore, when these loggers record temperatures less than zero, the adjacent weather station may still read above zero.

As frost damage tends to occur when the weather station reports temperatures less than zero, it is possible that the crop can withstand slightly negative temperatures (as measured in the crop canopy) without damage.

Considering there were about 500 hours below zero measured at the Yarrawonga and Coreen sites during the 2017 season (2016 had 160 to 270 hrs; 2015 had 230 to 270 hrs), there was only a small amount of variability measured within each treatment (as evident by the small error bars). If the stubble treatments were a large driver for in-canopy temperatures, this should have been clearly seen in the 2017 season. It was not. Therefore, it appears that in very cold winters, stubble management has very little effect on in-canopy temperature. So, changes in in-canopy temperature during the flowering period per se did not drive the difference in frost damage between burnt and stubble retained crops in the 2017 season.

But, we know that stubble management can alter the flowering window. Therefore, it is highly likely that this shift in flowering window is the reason why frost damage differed between stubble retained and burnt paddocks.

Some more detail on how the stubble may alter the flowering window will be discussed in the presentation.

Conclusion

The impact of stubble management has differed according to site and season over the past four years. It is important to view these results within the context of the seasonal conditions. Therefore, the most productive and profitable approach to stubble management may change according to the season (wet start to dry finish, dry start to wet finish, etc.).

While the GRDC Stubble Project will finish in June 2018, Riverine Plains Inc continues to conduct research on behalf of its members into 2018. To broaden the delivery of research to members, Riverine Plains Inc also collaborates with other organisations on a range of research projects, from weed seed management to soil constraints. This supports the ongoing establishment of on-farm trials within the region, ensuring that the results obtained are immediately relevant to local growers.

Contact details

Cassandra Schefe
Riverine Plains Inc
PO Box 214 Mulwala NSW 2647
(03) 5744 1713
cassandra@riverineplains.org.au