Can we reduce nitrogen inputs for wheat sown onto high-input crop stubbles and still achieve high yields?

By Heather Cosgriff (SFS)

Key messages:

  • 2015 trial results coverSoil nitrogen availability was very high prior to sowing, 321.6kg/ha available
  • Incorporating poppy stubble led to high potentially mineralisable nitrogen levels
  • There was no yield or protein response to additional urea applications
  • Trojan generally had higher yields under any treatment, but not significantly so.
  • Powdery mildew infections may have reduced grain size and increased screenings in Trojan plots

From nitrogen (N) response work done in 2014, we discovered that wheat yields were not responding to urea applications above 100kg/ha on some of our trial sites. Soil cores taken to 90cm and tested for N levels showed that there was potentially 185kg of N already present at depth in the soil.

In 2015 we surmised that the situation would be similar, with the trial sown into an 8t/ha poppy stubble that had been disced prior to sowing for minimal incorporation. In Victorian trials residues that were incorporated by disc had increased microbial activity and conversion of organic N into plant available forms (Angus and Peoples, 2012) so disced poppy stubble theoretically should supply high levels of N over the growing season.

Wheat, on average, uses 20-25 kg of N per tonne of grain yield, so an 8t/ha crop would require around 180kg N/ha for the grain alone, not including crop biomass development. Therefore, in theory, if N supply following an intensive crop such as poppies is greater than crop N demand, growers could minimise the amount of extra urea applied and/or apply only at key timings, and still achieve good yields

Trial design

Manning and Trojan wheats were selected for the Cressy site, as both are gaining in popularity in the region.

On April 4 topsoil samples (0-10cm) were taken at the Cressy site for complete soil testing to ascertain nutrient requirements and/or limitations and to establish organic soil carbon percentage. For soil N assessment soil cores were taken across the site and sampled to three depths, (0-30cm, 30-60cm and 60-90cm) prior to sowing and again after harvest to assess the amount of available N in the soil, and the amount utilised by the wheat during the growing season.

Urea applications were made throughout the season as per the treatment list, with each treatment being replicated four times.

Varieties: Trojan and Manning wheat

Nitrogen treatments:

  1. Nil additional N
  2. 100 kg urea/ha one application mid-tiller
  3. 2 x 100 kg urea/ha application mid-tiller and before first node
  4. 2 x 150 kg urea/ha application mid-tiller and before first node
  5. 3 x 100kg urea/ha applied early tiller, late tiller, first node
Table 1. Crop inputs
Date Plots Product Rate Timing Method
Fertiliser 28/04/15 All 14-16-11 + flutriafol 150kg sowing seeder
Herbicide 17/04/15 All Glyphosate 2L knockdown Self-propelled sprayer
29/04/15 All Boxer Gold 1.3 pre-em Tow-behind sprayer
18/08/15 All Triathlon 750 post em hand boom
Fungicide 1/09/15 All Epoxiconazole 500mL GS 32 Trojan hand boom
Activator 200mL GS 30-31 Manning
7/10/15 Trojan Amistar Xtra 400mL GS 39 hand boom
20/10/15 Manning Anistar Xtra 400mL GS 39 hand boom
Trojan Prosaro 300mL GS59 hand boom
Manning Prosaro 300mL GS59 hand boom
Irrigation Late Oct to Dec All 25mm/week


Nitrogen at depth

Table 2. Deep N results for the Cressy site.
  Pre-sowing Post-harvest nil N plots
Sample depth (cm) 0-30 30-60 60-90 0-30 30-60 60-90
Nitrate Nitrogen (NO³) mg/kg 22 40 9 8.5 6.4 5.9
Ammonium Nitrogen mg/kg 2 2 5 4.9 1.3 1.3
Bulk density (g/cc) 1.2 1.4 1.4 1.2 1.4 1.4
Total N available (kg/ha) 86.4 176.4 58.8 48.2 64.7 90.7
Total N (kg/ha) 321.6 110.8
Soil Organic Carbon % 4.21 From topsoil 0-10 cm
50% potentially mineralisable N (kg/ha) 142.09 If soil conditions allow for mineralisation of N from organic matter in the soil
Total potential available N 463.69kg/ha
Yield potential with nil additional N – lower 11.49t/ha
Yield potential with nil additional N – upper 16.56t/ha

Available nitrogen and potentially mineralisable nitrogen (PMN)

Soil tests taken prior to sowing showed that this site had high levels of available soil N to a depth of 90cm, with more than 300kg/ha available (Table 2), 86kg/ha of this was found to be available within the early root zone of the plant (top 30cm).

The predicted yield is based on the nitrogen removal per tonne of grain and straw, and assumes no other factors limit grain production. Potential production is certainly not limited by N availability in this circumstance, with potential predicted grain yield in excess of 11t/ha. After harvest we measured residual N of nearly 111kg, meaning that the 10+t/Ha crop effectively removed somewhere between 220-353 kg/ha of N or 22-35 kg N /t of yield. Which would fit within expected nutrient removal rates.

Potential mineralisable nitrogen (PMN) is calculated from the organic matter in the soil and assumes spring temperature and rainfall conditions will allow for microbial breakdown of stubble to release mineral nitrogen.

In clay soils such as those at this trial site, soil N levels that provide nutrition for the crop, but do not lead to excessive leaching are thought to be around 105-175 kg/ha. (Carson J & Phillips L, 2014). If this research holds true for Tasmania, we could expect some leaching of nitrate from this site, and applied urea could be minimised.

Grain yield and quality

Figure 1: Average wheat yield (Trojan & Manning combined) per urea treatment. Urea treatment not significant, p=0.05, LSD 2.06 t/ha.

Figure 1: Average wheat yield (Trojan & Manning combined) per urea treatment. Urea treatment not significant, p=0.05, LSD 2.06 t/ha.

Figure 2: Yield response (t/ha) of Wheat variety to urea treatment. Not significantly different, p=0.05 LSD=3.42.

Figure 2: Yield response (t/ha) of Wheat variety to urea treatment. Not significantly different, p=0.05 LSD=3.42.

Grain yield for each treatment are shown in Figures 1 &2. Figure 1 displays the average yield response to urea application, while Figure 2 breaks this down into a varietal response. Site average yield was 10.33 t/ha.

There was no significant effect on grain yield from any of the urea application treatments at the 95 per cent probability level.

The highest yield of 11.9 t/ha was achieved by applying 100kg urea at GS25 to Trojan wheat. There was a trend for Trojan wheat out-yielding Manning under every treatment except for two applications of 150kg of urea.

The large variation in yields was most likely due to differences in plant populations per plot caused by poor emergence and bird damage after sowing.

Powdery mildew infection

Powdery mildew infection is favoured by cool moist conditions, high humidity (>70%) and high nitrogen nutrition.

Visual disease ratings taken in late September showed Trojan to have high levels of powdery mildew. Plots were rated on a scale of 1-5, with 1 = disease present on stem only 5 = disease throughout canopy and present on heads.

Trojan plots rated 2.9 for powdery mildew, significantly greater than Manning (Table 3).

Plots of Trojan that had two applications of urea at 150kg/ha had the highest infection rate of 3.5. This corresponded to the lowest grain yield and highest percentage of screenings (Table 4).

Table 3. Powdery mildew infection. P=0.05, LSD 0.7.

Mildew infection scale 1-5





LSD (P=0.05)


Table 4. Urea treatment v. screenings (%). Significant P=0.05.
Urea application

Screenings (%)

Nil N


100kg/ha GS25


100kg/ha GS25 + 100kg/ha GS30


150kg/ha GS25 + 150kg/ha GS30


100kg/ha GS25, GS29, GS30



As expected, high rates of soil residual N prior to sowing reduced the effectiveness of applied urea for the plant populations of Trojan and Manning wheat at this trial site.

It is likely that high nitrogen nutrition influenced late powdery mildew infection at the site, particularly Trojan plots, which affected yields and grain quality.

Growers who are planning to follow a high input crop such as poppies with wheat should consider taking soil samples at depth to ascertain nitrogen content. They may be able to reduce urea inputs and improve disease control.

GRDCLogoStacked_TM_CMYKThe stubble project – maintaining profitable farming systems in Victoria and Tasmania with retained stubble (project number BWD00024) is funded by the GRDC.


Nutrient Management in Broadacre Cropping, SARDI/CropCo. Accessed at:

Growing eight tonnes a hectare of irrigated wheat in Southern NSW, NSW DPI Primefact No. 197, 1996. Accessed at:

Angus JF and Peoples MB (2012) Nitrogen from Australian dryland pastures, Crop and Pasture Science 63: 746–758. Accessed at:

Carson J & Phillips L. Soil Nitrogen Supply Fact Sheet. Soil Biology Initiative, 2014. Accessed at:

Southern Region Barley & Wheat Powdery Mildew fact sheet , GRDC Resources, 2014. Accessed at:

About BCG

Birchip Cropping Group Inc. (BCG) is a not-for-profit agricultural research and extension organisation led by farmers in the Victorian Wimmera and Mallee.
This entry was posted in Crop nutrition, Disease management, Research results and tagged , , , , , , , , , , , . Bookmark the permalink.

2 Responses to Can we reduce nitrogen inputs for wheat sown onto high-input crop stubbles and still achieve high yields?

  1. Pingback: Altering the timing of clethodim application and using angled nozzles can improve annual ryegrass control in canola | The stubble project: Victoria and Tasmania

  2. Pingback: 2015 field trials | The stubble project: Victoria and Tasmania

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s