Making best use of fertiliser within a grazing system

Artificial fertilisers remain a significant cost on the dairy farm, but their input is essential if high quality grass is to be provided for the grazing herd through the year.

Total fertiliser input is now capped by legislation, however the level used on individual farms will be very much dependent on the intensity of the grazing system. Nevertheless, 170 to 220 kg Nitrogen per hectare (136 to 176 units per acre) is a reasonable target for most farms, and this equates to approximately 5 to 6.5 bags of CAN (27%N) applied per acre over the season. On farms where early grazing is possible, then a February application of a half bag of urea per acre could see total fertiliser application increase to approximately 200 to 250 kg N per hectare. Generally the level of fertiliser (CAN, 27%N) applied after each grazing declines as the season progresses, with fertiliser applications during April and May being between one and one and a half bags per acre, and declining to under a bag per acre by August and September.    

Common practise on most farms is for the first fertiliser application of the season to be ‘blanket spread’ across the main grazing area, with the remaining fertiliser applied routinely after each field is grazed. While the ‘little and often’ approach of weekly or even twice weekly spreading is widely used, two recent studies at AFBI-Hillsborough have shown that continuing to blanket spread fertiliser through the grazing season once per grazing rotation had no detrimental impact on sward or animal performance.

The response to fertiliser depends on the quality of the grass species in the sward. High proportions of the best recommended varieties will give higher responses to nitrogen fertiliser. Response rates should average approximately 25 kg of grass dry matter utilised for each 1.0 kg of N fertiliser spread over the growing season up to 150 kg N/ha. Response rates from applications of 150 to 300 kg N/ha will still give a response of approximately 20 kg of grass dry matter per 1.0 kg of fertiliser. This represents a return on investment of up to 3:1 over a six month period. Where the grass grown replaces concentrates at over twice the price per tonne of dry matter, financial returns from fertiliser application can be doubled.

Within the growing season, responses to nitrogen fertiliser are best during the periods of most rapid grass growth as shown on the GrassCheck growth curve. Responses per kg of fertiliser used should be similar for urea (46% N) and calcium ammonium nitrate (CAN) when timed correctly. Care should be taken to ensure that urea is only used in damp conditions.

The grazing swards on most dairy farms usually have the highest soil analysis indices for phosphate (P) and potash (K). Where grazing land soil analysis shows Index three or above for both P and K, neither slurry or compound fertiliser are needed. Straight nitrogen will be sufficient all season. At a P Index of two, a dressing of 1500 gallons of slurry per year will supply all the soil phosphate requirements.

Cattle slurry contains significant quantities of nitrogen, phosphate and potash (27 kg N, 11 kg P and 29 kg K per 2,000 gallons). While the first priority should always be to return these nutrients to silage fields (to replace those removed in the silage crop), slurry can also be applied to grazing ground to reduce fertiliser costs.  Using the inverted splash plate, this is most commonly achieved by applying dilute slurry in February, or by applying dilute slurry to 30-40 percent of the grazing area following grazing in early March. However, during the main grazing season splash plate spreading is problematic, but using trailing-shoe type slurry systems can help to overcome some of these problems. For example, recent research at AFBI-Hillsborough has demonstrated that slurry can be applied twice during the grazing season using a trailing-shoe system (2,800 and 1,700 gallons per acre applied) without any detrimental effects on animal performance, while reducing fertiliser N input by 80 kg per hectare (2.4 bags CAN per acre).

Although artificial fertiliser is an expensive input, it has the potential to yield a significant return on investment, especially if the grazed grass produced is efficiently utilised and replaces concentrates in the diet. Application rate and timing, sward quality and soil analysis can all affect the response to fertiliser N inputs.