Take home messages

• It is critical to understand the crop growth stage during waterlogging to determine potential yield impacts.
• In these trials, nutrition post-waterlogging did not influence grain yield, except in the trial at Hagley, where yields increased with higher rates of applied nitrogen.

Background

Waterlogging issues and drainage solutions are a high priority for farming systems in the high rainfall zones (HRZ) of Victoria and Tasmania. In 2021, these areas experienced a higher-than-average rainfall season, particularly through the winter months of June and July.

Waterlogging creates a stressful environment for plants to grow in and can result in reduced yields, and in severe cases, plant death. The growth stage of the crop during a waterlogged period is essential to understanding the effects it can have on final grain yields. Waterlogging close to sowing will affect germinating seeds and young seedlings, and as these plants do not have well-established root systems, the effects can be severe. If a soil is waterlogged during June–July in south-west Victoria or Tasmania and the crop is well established, final yields may not be severely impacted, as soils are cold, the demand for oxygen is low, and plant growth is slow (Waterlogging Factsheet). Established plants will be most affected when they are rapidly growing, as such, prolonged waterlogging during the warmer spring period is when yield penalties may be most severe. The HRZ of Victoria and Tasmania is particularly prone to waterlogging conditions, with high rainfall and sodic (dispersive) subsoils.

Under waterlogged conditions, nitrogen is lost from soils through denitrification and leaching, and during this period, plants also have a limited ability to uptake nutrients. Providing the crop with adequate nutrition following a waterlogged period is therefore of utmost importance to help the plants recover from this stress.

Method

Four trials were established in locations that had experienced some degree of waterlogging throughout the season. This report focuses on three of those four trials. The trials were located at Vite Vite North and Streatham in Victoria and Hagley in Tasmania (Table 1). Vite Vite North and Hagley were small plot trials, while the Streatham trial was located in part of a grower’s paddock.

Table 1: Trial location, crop type, variety, sowing date and starting fertiliser.

The treatments varied across the locations, with the main products used including urea, sulphate of ammonia (SOA) and trace elements. Nitrogen in SOA is in a more readily available form for plant uptake than urea. This was applied in combination with urea to determine if this immediate uptake was beneficial when recovering a waterlogged crop. The trace elements applied across the canola was a product called Maximise (Zn, Cu, Mo, and B). In the wheat, the product Awaken (N, K2O, B, Cu, Fe, Mn, Mo, and Zn) was applied. These were used to ensure crops were not nutrient limited. In the wheat, they were applied at least two weeks following the initial nitrogen recovery application when the crop was actively growing again, to improve efficiency of plant uptake.

Streatham, VIC

This trial looked at three methods of nutrition recovery. Applications were made at GS67, flowering declining (Table 2).

Table 2: Recovery canola at Streatham.

Rainfall at the Streatham site was above average in May, June, and July, with a total of 206mm falling across the three months. A moisture probe at a nearby site at Westmere showed plant available water at its peak at 97% in August, up from 90% in July. It then began to decline from September to the end of the season. The location of this trial was in a low-lying area of the paddock and the plants displayed visible symptoms of waterlogging, such as reduced plant growth and yellowing, particularly compared with other areas of the paddock.

Vite Vite North, VIC

The faba bean trial at Vite Vite North had five rates of urea applied at 10% flowering, which occurred in mid-August (Table 3). The application rates ranged from 0 to 240kg/ha urea. Monoammonium phosphate (MAP) was applied at sowing across the whole trial at a rate of 100kg/ha.

Table 3: Recovery faba beans at Vite Vite North.

Rainfall at the Vite Vite North site was well above average in May, June, and July, with a total of 254mm of rain falling across the three months. A moisture probe at Vite Vite showed plant available water was at 95% from mid-June to mid-July and then moved to 100% until mid-August, after which  it started to decline.

Hagley, TAS

The recovery wheat in Tasmania had seven treatments, as outlined in Tables 4 and 5. Each treatment was replicated at a full and reduced nitrogen rate. The urea was applied over three timings, 10 October when the crop was at GS32 (flag minus 2), 28 October at GS39 (flag leaf fully emerged), and 12 November at GS45 (boots swollen). The SOA was applied on 17 October at GS33, and the trace elements eleven days later on 28October. No nutrition had been applied to the trial prior to this, except 100kg/ha MAP at sowing. Hagley received above average rainfall through July and August, with a total rainfall of 218mm across the two months.

Table 4: Summary of treatments of the recovery wheat at Hagley.

Table 5: Detailed treatment list of products, rates, and timings of applications at Hagley.

Results and Discussion

Streatham, VIC

Peak nitrogen requirements in canola are from the start of flowering to the end of pod formation. The application timing, towards the end of flowering for this trial, was very late in-season compared to standard practice.

The applications in this trial did not result in any differences in grain yield or oil content (Table 6). A target biomass for optimal yield potential in canola is considered around 5t/ha by the start of flowering. An average biomass of 5.3t/ha was measured towards the end of flowering, prior to nutrition applications being made. The critical period for yield determination in canola is 300-degree days from the start of flowering, which is approximately 30 days in the south-west Victorian environment. Moisture probes in the area showed plant available water to be at its peak in August. Hence, the timing of waterlogging in this situation may have reduced negative effects on grain yield, as despite appearing visually impacted by waterlogged soils, the trial overall yielded well, with an average yield of 4.1t/ha.

Table 6: Canola recovery yield and oil content Streatham.

Post-harvest soil tests were taken in late February to determine if the additional nitrogen applied would remain available to the crop for the following season. The soil test results showed no significant differences in total nitrogen between any of the treatments, including the nil.

Vite Vite North, VIC

At Vite Vite North, the waterlogging occurred in the very early stages of flowering in the faba beans. Applying nitrogen did not result in any significant differences in grain yield (Table 7). Pulses are typically not well suited to waterlogged conditions. Of all the pulses, faba beans however are considered the most tolerant (Faba beans Grownote). The trial reached full flower towards the end of August. Yields achieved in this trial would indicate that, although soils were at field capacity through July and August and some visual signs of waterlogging were evident, the impact of waterlogging on final grain yield was not severe.

Table 7: Recovery faba beans at Vite Vite North.

Post-harvest soil tests indicated that nitrogen available in the soil in the 0–60cm zone was above 120kg N/ha across all treatments (Figure 1). On average, the nil treatment had 22kg N/ha more nitrogen compared with treatments where applications had been made. It could be that the plants were more efficient at fixing their own nitrogen where no nitrogen had been applied.

Figure 1. Post-harvest nitrogen soil test results across the five treatments at Vite Vite North. Results are from 0–30cm and 30–60cm zones.

Hagley, TAS

The average yield for wheat in Tasmania was 9.4t/ha and this average included a nil treatment which was almost 2t/ha lower than the nitrogen treatments (Table 8). Where higher rates of nitrogen were applied, a significant increase in grain yield was achieved in two of the three treatments. Grain protein significantly increased between nil, reduced rate, and full rate treatments. The average protein value across all treatments of 7.7% indicates that nitrogen may have been a limiting factor to grain yield. Provided it is not applied too late, a grain protein of 10.8% or higher can be a good indicator that nitrogen has not been limiting.

Soil tests taken close to sowing showed a starting soil nitrogen of 120kg N/ha in the 0–60cm zone, and an estimated mineralisation of 60kg N/ha. Using the 40kg N per tonne of grain yield rule, these results indicated there was enough nitrogen in the soil for 4.5t/ha prior to the nitrogen being applied. In response to the waterlogging, conservative yield targets of 9t/ha for the full rate and 7t/ha for the reduced rate were selected. The low protein results suggest yield potentials could have been higher and higher rates of nitrogen could have been applied. In this trial, total nitrogen had a larger influence on grain yield and protein compared with product type.

Table 8: Grain yield and quality results for the wheat trial at Hagley.

Conclusion

Understanding a plant’s growth stage during a waterlogging period is critical in determining potential yield impacts. Despite presenting some waterlogging symptoms, faba beans and canola yielded exceptionally well in these trials. Average yields of 8t/ha were recorded for the faba beans and 4.1t/ha in canola, noting again that the faba bean trial was in small plots and the canola an area from a grower’s paddock. The wheat at Hagley also yielded well, with an average grain yield of 9.4t/ha. These yields are likely attributed to the timing and severity of waterlogging in relation to the crop’s physiological development stage. The type of product used did not influence grain yield in any of the trials. The Hagley site was the only trial to present a yield response by rate of nitrogen, with higher rates of nitrogen applied improving grain yield.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. SFS would also like to thank the host growers and staff for their contributions.

References

Waterlogging

GRDC Grownotes, Faba Bean, Section 14, Environmental Issues

Fact Sheets Waterlogging

Nutrition Strategies to Mitigate Yield Losses Following Waterlogging

Contact details

Greta Duff
Southern Farming Systems
23 High St, Inverleigh VIC 3321
0428 871 900
gduff@sfs.org.au
@gduff_sfs