Pastures could face nitrogen drought this spring

What you should think about before applying nitrogen

Tablelands Telegraph - September 2021

Phil Cranney, Senior Land Services Officer, Pastures

Several factors will lead to a major nitrogen deficiency in many pastures throughout the Central Tablelands this spring. Here are the top five reasons for this nitrogen drought perfect storm:

  1. Leaching of nitrogen - above average rainfall this year has moved mobile nutrients downwards, beyond the root zone.
  2. Denitrification in water-logged soils is caused by the soil microbes using nitrogen for energy, instead of oxygen. Compacted soils can also amplify this process.
  3. Reduction in sub-clover establishment this year due to the lack of sunlight and bare soil late summer to early autumn, means less nitrogen being fixed by annual legumes. Low pH and high aluminium levels are also a cause of poor nitrogen fixation in legumes.
  4. High growth rates in pastures over summer and early autumn utilising soil nitrogen.
  5. Nitrogen immobilisation - high levels of dead and unutilised grass leading to higher-than-normal C:N ratio (ideal for soil microbial function and soil health is approx. 24:1). Soil microbes then use higher rates of nitrogen from the soil to help break down the excess carbon and the nitrogen becomes tied up in the microbes themselves.

How to identify this nitrogen deficiency?

Faecal and urine patches in your pasture giving rise to a darker green leaf growth (especially grasses) and higher growth rates, is a simple way to identify nutrient deficiency in your soil. However, a more effective way of identifying nitrogen deficiency is to buy a small bag of urea and do a small test strip.

The two big questions you need to ask yourself before applying more nitrogen to pastures are:

  1. Have you done a pasture feed budget? You need to do a pasture feed budget to objectively measure feed supply vs feed demand.
  2. Do you have enough mouths to eat the extra feed you grow as a result of applying nitrogen? Utilisation is a key profit driver and any livestock system.

Failure to utilise extra pasture grown with urea costing $1,050/tonne, it can be a large cost to your system. The difference between 60% and 80% utilisation rates at 15kg DM Pasture response for every 1kg nitrogen applied is equivalent to approx. $40/tonne of DM grown.

After doing the numbers, and you are confident of a positive return from the nitrogen application, remember to minimise risk of nitrogen poisoning by moving the livestock onto pastures in the afternoon when the stock are full. Never move stock onto recently fertilised pastures empty or in the morning.

Depending on soil temperatures, weather conditions and pasture growth rates, waiting 3-4 weeks after application is the sweet spot between optimal utilisation and safe grazing. If you are unsure about managing livestock health risk, please talk to Brett Littler or your local district vet.

If you can find the livestock, it comes down to the cost of pasture grown vs grain prices – 2.3 cents per Mj will be the cost of the extra pasture grown for $950/tonne urea price at a response of 15kg DM pasture grown per 1kg of nitrogen applied at a conservative 60% utilisation rate.

Oats at $250/tonne on farm will cost 2.65cents/Mj. When considering the oats as an option to compare, don’t forget to add the costs of storage, handling and feed out. The other fringe benefit we should consider when correcting a soil nutrient deficiency, is the potential increase in feed quality (Me & protein) of the pasture which leads to production benefits and quicker turn-off rates.

The key message for spring pasture management is: Don’t panic buy urea without first doing a feed budget and assessing the return on investment of buying extra stock to eat the extra feed this spring. Remember that pasture utilisation is a key profit driver!

Key tools and calculators:


NSW DPI feed cost calculator

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