Slow Crop Growth – Can I ‘Catch My Crop Up with Nitrogen?’

Key take-home messages: 

Thermal time drives growth: Crop development is primarily driven by thermal time, not solely early nitrogen application. Ensure adequate nutrition before considering additional nitrogen. 

Strategic Nitrogen application: Apply early nutrition judiciously based on actual needs or risk losing topdressing opportunities. Manage application rates for optimal return on investment. 

Use decision support tools: Tools like Yield Prophet® can estimate crop nitrogen needs and set realistic yield targets, crucial for late-start conditions. 

Understanding thermal time: Knowing the degree-day requirements for your crops aids in predicting growth stages and optimising management practices. 

Slow Crop Growth – Can I ‘Catch My Crop Up with Nitrogen?’

To assess the need for additional N of a crop and achieve yield potential, it is a matter of matching crop supply with crop demand. ‘Getting it right’ can achieve real efficiencies and therefore savings for a more profitable and sustainable farming system.  

There are a range of factors affecting N uptake and, ultimately, final yield. These include:  

• Starting soil N (crop rotation) 

• Mineralisation 

• N loss pathways (i.e. volatilisation) 

• Availability of N fertilizer 

• The economics 

• Seasonal conditions (rainfall and temperature) 

• Soil type 

• Your decision-making preferences  

Unless crops were sown early during a rain event, current low temperatures and the late break have crops experiencing slower growth than in previous seasons.  

So could applying more nitrogen help a crop ‘catch up’ ? 

Crop development and N uptake 

The short answer is, only if the crop needs it.  

Cereal crop N requirements at this time of year are lower than in spring and applying large amounts of nitrogen, will not necessarily speed up their growth and it won’t change it’s phenological development if there is adequate nutrition.  

Crop development is governed by sowing date and thermal time, or degree days. In the latest edition of the Technical Bulletin (a fortnightly growing season update, provided to BCG members), the thermal time required for crop establishment was outlined. Understanding thermal time is also useful for predicting crop development stages, aiding in management and planning. 

What can I do now? 

Estimate your current crop N supply. Set realistic yield targets and monitor weather forecasts for the opportunity to apply N if it’s needed.  

Crop models such as Yield Prophet®, are useful for managing in-crop N applications. Other options include French & Schultz or Sadras and Angus N calculation equations to estimate yield potential/crop demand.  

More recently, the N bank approach has been recognized as a way to better manage the nitrogen requirements of crops by setting a long-term average yield and the associated N needs. Instead of strictly responding to seasonal conditions, this approach considers the nitrogen already present in the system and ensures that the required N levels are consistently available to meet the intended outcomes. Over time, this method smooths out the variations in yield and, based on current research, can help set targets that are more financially viable in the long run. 

Does applying more N make a profit? 

Nitrogen rates are one part of the equation, but can represent a significant cost.  In a 5 year N bank study at Curyo, Hunt, Finger and Murray evaluated the potential for different N management systems to profitably close the yield gap. The study found that N decisions applying more N (63-74kg N/ha) that were based on 50 per cent Yield Prophet (R) or 125kg/ha N bank strategy were $128-179/ha per year more profitable than the district average N rate of 21-30kg N/ha. 

What is your N strategy this year? If you have some nitrogen trial ideas or would like be involved in BCG’s N research, please contact the BCG office on 03 5492 2787.