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Grazing management to increase N use efficiency on Irish dairy farms


Nitrogen use efficiency (NUE;%) is a key indicator of agricultural system efficiency. It is defined as the proportion of the total N inputs, in feed, fertiliser, organic manures etc. that are recovered in milk & meat exports from the farm.

The current average NUE is estimated to be 25% on Irish dairy farms based on National Farm Survey (NFS) results. Recent research indicates that NUE can be increased to 35%, resulting in both reduced fertiliser costs and a reduced risk of N losses to both water and air. Researchers Michael O’Donovan, Elodie Ruelle and Brendan Horan give more information from their research on increasing NUE on dairy farms.

Opportunities to increase NUE on dairy farms

In principle, any management change that increases milk production from existing N use levels or maintains current levels of milk production using smaller N inputs will improve NUE on the farm. In practice, the following are the key strategies for Irish dairy farms to increase farm gate NUE:

Better use of organic manures

By applying slurry and soiled water in spring using low emission slurry spreading (LESS) methods, increased quantities of N can be recovered, and thereby reducing chemical N fertiliser requirements. Currently, Teagasc recommend that up to 30% of the farm should receive slurry in February instead of chemical N fertiliser.

Increase soil fertility  

Suboptimum soil fertility is one the key factors that limits pasture production on many dairy farms because of reduced soil nutrient availability, and it also inhibits the establishment and growth of clover within grazing swards. By soil testing all paddocks on the farm every 2 years and correcting soil fertility, up to 1.5 t of additional pasture DM can be utilised annually while reducing fertiliser N requirements.

Incorporate white clover into grazing swards

The incorporation and refined management of white clover in grazing swards can significantly increase individual animal performance while also reducing requirements for chemical N application during the grazing season. While a 75%:25% ryegrass-white clover sward will biologically fix up to 150 kg of N, the reduction in chemical N requirements coupled with increased animal performance from pasture can increase NUE within Irish dairy systems.

Reduce concentrate level and N contents

As the crude protein (CP) content of grazed grass is normally in excess of 20% during summer, grazed pasture adequately meets the dietary CP requirements of the dairy herd. Consequently, improving pasture management and reducing both the quantity of concentrate fed and the CP content of the concentrate during the main grazing season is a significant opportunity to reduce N inputs and increase NUE within Irish dairy systems.