CSBP: Urea Sustain boosts yields and improves NUE

Jun 06,2024 Industry Article, 2024, Industry Update, Technical Article,

By Peter Rees, CSBP Senior Agronomist

Nitrogen (N) losses from ammonia (NH₃) volatilisation, denitrification and leaching have become an area of concern as growers look to decrease environmental impacts and improve profitability.

Increasing nitrogen use efficiency (NUE) through field research, product development and agronomic recommendations is a key focus for CSBP as it helps growers maximise yield and economic returns, while decreasing N losses.

Nitrogen fertiliser losses

Nitrogen fertiliser losses from volatilisation have been reported at up to 30% of applied N in field trials in WA¹. Losses are dependent on fertiliser type, environmental conditions (temperature, rainfall, wind speed) and soil properties (clay content, CEC and pH)². Losses can be high two to six days after application if there is insufficient rainfall to wash urea into the soil profile.

Leaching losses occur when N dissolved in water moves through the soil profile, draining below the root zone. Rates of loss are highly dependent on soil type, N application rate and rainfall.

Any applied fertiliser not converted to grain represents an economic loss as well as a potential environmental impact. With current NUE for N applications estimated at 30 – 60% it makes sense to investigate technologies and products that help crops take up more fertiliser N.

Urea Sustain trials and results

In 2023 CSBP introduced a new product to market, Urea Sustain, which is urea coated with NBPT (a urease inhibitor shown to reduce volatilisation) and DMPP (a nitrification inhibitor shown to reduce leaching). We also ran seven replicated field trials and three small microplot experiments in the 2022 and 2023 seasons, the results of which are summarised below.

**Table 1.** Locations, soil and seasonal details for field trials used in this study.

When all trials and N rates were aggregated and analysed, Urea Sustain boosted yields by an average of 5.6% over urea at the same rate (Figure 1). This higher yield was accompanied by slightly higher protein (average of 0.3% higher for Urea Sustain, results not shown).

This meant, on average, spending an extra $8/ha on Urea Sustain led to an extra $45/ha worth of grain (net return improved by $37/ha).

**Figure 1.** Relative yield benefits from Urea Sustain compared to urea when top-dressed at Z11–13. Coloured dots are means of three field replicates comparing equal N rates. Boxplots show the interquartile range and medians (vertical lines).

Where follow-up rainfall was received, urea and Urea Sustain behaved similarly, and the largest differences were found at medium to high N rates at sites with little or no rainfall in the week after application.

A great example of this was our trial at Cascade, which was previously featured in our November 2023 edition of Ag Insights. At this site, N rates ranged from 0 – 126 kg/ha and only minor rainfall events were received in the three weeks after topdressing. Across all rates of N, Urea Sustain yielded an average of 300 kg/ha more than urea, and protein increased by up to 2% (Figure 2).

In addition, post-harvest soil sampling showed that there was significantly higher soil residual N where Urea Sustain was used (Figure 3). This shows that not only did more N make it into the plant and the grain, generating more income, but more of the excess N that was applied could be found in the soil, indicating lower losses where Urea Sustain was applied.

**Figure 2.** Wheat yield (t/ha) and grain protein (%) responses to N rates supplied as Urea Sustain or urea top-dressed at Z11–13 in Cascade in 2023. Each dot represents the mean of three replicates.

**Figure 3.** Soil mineral N (mg/kg) (sum of NO₃– N and NH₄+ N in 0–10cm) at Cascade with increasing rates of applied N as urea and Urea Sustain in soil samples taken after harvest (left) and a comparison of the N sources across all rates of applied N for residual mineral N (right).

To better understand how Urea Sustain was performing in the field, the CSBP Agronomy Team used a special form of urea, labelled with a nitrogen isotope known as ¹⁵N, which contains an additional proton than standard nitrogen (¹⁴N). By analysing grain, above and below ground plant material and soil cores taken from the sites, the ¹⁵N allowed us to better trace where our fertiliser ended up.

At a site west of Mingenew, the equivalent of 42kg N/ha was applied either as ¹⁵N urea or ¹⁵N Urea Sustain at two different timings. After the first application, there was 11mm of rain, while after the second application, there was no significant rain in the subsequent six days.

When rainfall was received (Figure 4a, Mingenew 1), there was no difference in the amount of N recovered in the grain, above ground biomass, plant roots or the soil. For the later application of ¹⁵N (Figure 4b, Mingenew 2), where no follow-up rain was received for six days after application there was a significant increase in ¹⁵N recovery in the Urea Sustain plots compared to urea, with most of the difference being found in higher grain N recovery (10 vs 15kg N/ha) and soil recovery (8 vs 11kg N/ha). This improvement in grain N recovery represents a 50% improvement in N use efficiency under adverse conditions.

**Figure 4.** Nitrogen recovery (kg N/ha) in soil, plant, roots and grain material with two fertiliser application timings at Mingenew. Results are a mean of three replicates for the first application timing and two replicates for the second application timing. Vertical bars represent standard errors for total N recovery.

Trial findings and results

Our results show the use of inhibitors can boost grower returns and improve NUE. While our yield improvements averaged lower than the 10% reported in international reviews of NBPT efficiency³, six out of our seven trials were conducted in 2023, which was a drier season in WA and sites were less N responsive than in average years.

With rainfall becoming less predictable and farming operations generally growing in scale, inhibitors may also offer logistical advantages, allowing growers more certainty when topdressing crops ahead of forecast rains.

CSBP intends to continue research in the field of enhancing fertiliser efficiency. This work will include comparing inhibitor products and rates, and alternative uses and timings to help drive improved returns and better environmental outcomes for growers across Western Australia.

Urea Sustain is available at CSBP Albany, Esperance, Geraldton and Kwinana.

Talk to your local CSBP account manager to find out more and secure your Urea Sustain today.