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How Much Nitrogen is Enough For Intensively Grazed Pastures?


A research review of two Nitrogen studies noted the following:1,2


  • A survey of Taranaki dairy farms showed that over 40% of the groundwater samples had nitrate-N concentrations exceeding 11mg/litre (considered the maximum safe limit).


  • After the application of 50kg of N it took 7 weeks to be fully utilised by the grass and maximise DM production, but at which stage the protein and Metabolisable energy levels have reduced. Consequently, the pasture is grazed sooner when not all the applied N has been taken up by the grass. 


  • Clover content of pasture declined from 17% to 11% when 200kgN/yr was applied as fertiliser and declined to 2% when 400kgN/yr was applied. Current elevated levels of Nitrogen fertiliser applications will eliminate clover from pasture and with it the “free” 150Kg N/yr supplied by clover/rhizobia fixing of atmospheric Nitrogen. 


  • A response of 0.2 to 0.8 kg milk solids (MS)/Kg N were obtained (Homes and Bryant 1982) 


  • The concept of using lesser amounts of N fertiliser in spring and times of herbage deficit increased milk solids (MS) by 15% over the year. 


  • Trials showing the association between milk solids (Kg/ha/yr) and N fertiliser (kg/ha/yr) showed the response between 80 to 100kgN/ha/yr is viable. 


  • The economic benefits Economic Farm Surplus ($/ha/yr) ranged from minus $260 to plus $350/ha after the application of 80 – 150 kg N/ha/yr (1990’s values). 


  • Nitrogen applications gave an average increase of $100/ha/yr on well managed DRC farms. As this value was based on 1990’s returns it could be assumed that the return would have increased to about $300/ha. 


  • There was no correlation between the amount of N applied and the Economic Farm Surplus, as other variable were equally important.


  • In the WA trials pasture utilisation maximised by applying 1 kg N/ha/day between grazing cycles, during the growing season with total N applied at 80kgN/ha/yr. This equated to 176 kg of N fertiliser/ha/year (80 kgsN) 


  • Milk production per cow was similar across all farmlets in terms of milk fat and protein.


  • Pasture grazed at 1.5 leaves had insufficient fermentable sugars to allow rumen microbes to use pasture proteins, the excess protein is converted to ammonia and then to urea, excreted in the urine. (The process is energy dependent on the cow impacting production, body condition and fertility). The excess urea hits the pasture in concentrated spots and is a major contributor to the total Nitrate leached.


  • Pasture grazed at the 3-leaf stage did not impact pasture nutritional quality, but increased pasture utilisation by 20% in annual and 7% in perennial ryegrass pasture.


  • A trial involving 115 WA dairy farms between 2005 and 2008 showed a poor relationship between N fertilisation and annual operating profit (EFS). 


  • While pasture is relatively efficient in taking up fertiliser Nitrogen, dairy cows and grazing animals are highly inefficient in using dietary Nitrogen and typically excrete 75 to 85% of total Nitrogen consumed in urine and manure. Therefore, less Nitrogen is required to maximise pasture utilisation and milk production than would be needed to maximise pasture growth.


  • The application of more than 100 to 200kg of Nitrogen per ha per year cannot be justified on either economic or environmental grounds and the data suggests that 0.5 to 1.0 Kg N/ha/day between grazing during the growing season is likely to be most profitable. 

 

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References:

1. GROUNDWATER NITRATE LEVELS UNDER GRAZED DAIRY PASTURES RECEIVING DIFFERENT RATES OF NITROGEN FERTILISER S. F. Ledgard1, N. Selvarajah2, D. Jenkinson2, and M.S. Sprosen1 1AgResearch Ruakura Research Centre, Private Bag 3123. Hamilton. 2Environment Waikato. P O Box 4010. Hamilton. Download available here. Date accessed, August 2025.

 

2. Staines, M, Morris, R, Casson, T, Bolland, M, Russell, B, Guthridge, I, Lucey, J, and Bennett, D. (2011), Greener pastures 2 - Nitrogen for intensively grazed dairy pastures. Department of Primary Industries and Regional Development, Western Australia, Perth. Bulletin 4815.https://library.dpird.wa.gov.au/bulletins/125. Download available here. Date accessed, August 2025.

 

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