Getting the most out of winter nitrogen: practical tips for pasture productivity
Author: Lee Menhenett — Technical Agronomist Pastures and Cropping
Learn how to assess your pasture’s nitrogen (N) readiness this winter and how to maximise response rates through species selection, grazing strategy, and moisture timing.
Key messages
- Nitrogen response requires adequate soil moisture – timing is critical
- Winter mineralisation slows; reserves may be low after a dry autumn
- Strategic applications can drive a large share of total winter growth
- Match N timing to the seasonal growth curve of pasture species
- Smart grazing is essential to unlock N benefits.
Why soil nitrogen is low this winter
After a year of dry conditions and soaring fodder prices, every kilogram of pasture matters. With winter growth lagging and feed budgets under pressure, smart N application could be the difference between holding feed quality or falling behind. But applying N in winter is not a silver bullet – success depends on timing, grazing decisions, and understanding what your pasture is telling you.
N fertiliser is a fantastic tool to manipulate pasture dry matter, however the degree of success will be dictated by understanding the key response influencers and executing the application to favour a positive result.
The N toolbox: where supply comes from
N is available to pastures from different sources
- Mineralisation
- Existing soil mineral nitrogen
- Animal dung and urine
- Fertiliser applications
Why mineralisation slows in winter
Soil mineral N is a legacy of organic carbon mineralising into plant available inorganic forms – ammonium and nitrate. For peak mineralisation to occur, soils need to be warm and moist, favouring bacterial activity. Conditions that will limit mineralisation are:
- Periods with small rain events and high evaporative conditions – which minimise the time soils remain moist, limiting the build-up and activity of soil bacteria and the amount of nitrogen mineralised.
- Dry summer and autumn conditions – with limited mineralisation events leading to low soil nitrogen levels.
- Winter – when soils cool and mineralisation rates decrease.
Modelling based on 20 years of climate data and findings from field work by Suter et al. (2020), shows monthly mineralisation of N from a Western District irrigated pasture system. Figure 1 highlights higher median mineralisation rates in warmer months, while over winter these rates decrease significantly with smaller ranges in variability. In dryland systems it is anticipated a greater amount of N to be mineralised at the start of the autumn break due to lack of pasture growth and N utilisation over the summer.
Figure 1: Predicted annual dynamics of mineralisation of soil organic nitrogen for the irrigated system at Allansford.
Source: Suter et al. (2020).
Pasture species matter: growth habits and N response
The seasonal growth habits of pasture species vary, which is an important consideration when applying N.
N application strategies should target species that have greater winter growth activity.
Grazing timing: don’t undercut your N investment
Pasture grazing management and responses to applied N are closely linked. The temptation to graze pastures prematurely during winter is understandable as this is the time of year when feed reserves are at their lowest. Coupled with limited fodder availability, there will be a desire to graze any green shoots.
Allowing ryegrass to express 3 leaves/tiller and Phalaris or Cocksfoot 4 leaves, will maximise dry matter accumulation and allow the plant to replenish its carbohydrate reserves. Grazing at 1 or 2 leaf growth stages decreases plant densities, root development and limits subsequent dry matter growth rates. Leaf 1 will produce approximately 20% of total grazing rotation dry matter, leaf 2 another 30% and leaf 3 50%.
While the temptation might be to allow stock to enter a paddock that is showing some reasonable feed, waiting an extra couple of weeks may return greater dividends. When dry matter in a rotation is compromised through grazing management, so too is any potential response to N applications. The added disbenefit of premature rotation grazing is the fact that N responses will not carry over into subsequent grazings.
Regional growth rates and soil fertility
In addition to species seasonal growth habits, winter growth rates will vary across geographical areas in the southern pasture districts. Daily winter growth rates are generally between 10-30kgDM/day. Specific regional growth rates can be sourced from the EverGraze website – cited below. Growth rates, and responses to applied N, will also vary based on paddock soil fertility.
How much N to apply, and when
N increases the size of the plant, driving greater dry matter accumulation above normal growth rates. The response to N is described as kgDM/kgN, with winter responses generally in the range of 5-10kgDM/kgN. Rates of N should be applied in the range of 30-50kgN/ha (65-110kg/ha urea).
N should be applied during active pasture growth and prior to a significant rainfall event (i.e. >10mm). In circumstances where ground cover is limited and the reliability of a significant rainfall outlook is poor, Green Urea NV® should be considered to reduce volatilisation losses – retaining more N in the system to produce dry matter or allowing N to be immobilised by soil bacteria to cycle back into the system from future plant uptake.
Final takeaways
N applications to pastures during winter are a sound strategy to alleviate feed deficits, however, the degree of additional feed grown above normal growth rates, are dictated to by many factors. The primary factor is actively growing pastures with adequate stored moisture.
Further information
For more information feel free to contact me at lee.menhenett@incitecpivot.com.au
Further reading
Regional pasture growth rates | EverGraze More livestock from perennials
Standard report template with sample content
Pasture productivity benefits from strategic urease and nitrification inhibitor use are limited.pdf
References
Suter, H., Belyaeva, O., Ward, G. Pandey, A. and Li, Y (2020), The University of Melbourn, Rural R&D For Profit More Profit from Nitrogen project RRDP1715 Final report Improving dairy farm nitrogen efficiency using advanced technologies.
