By Alison Clark, senior agricultural consultant, SAC Consulting

With just shy of 700 dairy cows across the Scotland’s Rural College dairy farms in Dumfries, slurry collection, management and utilisation plays a major part in the agronomy of the grass and cereal crops.

Farm manager Hugh McClymont has long been an advocate of maximising the use of the nutrients available in slurry and there has been no purchased phosphate or potash going on at Crichton Royal Farm for the past 20 years.

The awareness of the value of slurry has partly been due to being located in the Lower Nithsdale NVZ, which imposes restrictions on the amount of inorganic N that can be applied.

Also, housed cows fed on a high-quality ration produce a lot of slurry which needs to be managed properly to reduce the risk of diffuse pollution. This valuable fertiliser can increase the sustainability of the farm by reducing the reliance on inorganic fertilisers.

The first step to managing nutrient applications on the farm is to determine soil status. Crichton Royal Farm is precision soil sampled every five years to assess the nutrient and pH status of the soils.

Once the soil nutrient status is known, crop requirements can be determined using Farm Advisory Service Technical Notes which can be found at (TN726 Fertiliser Recommendations for Grassland and TN731 Nitrogen Recommendations for Cereals, Oilseed Rape and Potatoes.)

To complete the picture, slurry is analysed regularly, both through the SRUC analytical lab and also through John Deere “HarvestLab” equipment which is available on the contractor’s spreading equipment. Typical slurry analysis for Crichton is shown below against the analysis for SRUC Barony Farm, highlighting the importance of slurry analysis.

Crichton Barony

Dry Matter 5.4 % 3.4%

Nitrogen 2.9kg/m3 2.4kg/m3

Phosphate 1.2kg/m3 0.7kg/m3

Potash 2.6kg/m3 1.6kg/m3

Assuming 45% of the nitrogen is available to the growing crop, when applied at 30m3/ha, Crichton slurry is supplying around 39kg/ha of nitrogen. Availability drops to around 35% when spread in the summer months equating to 30kg/ha nitrogen.

The combined slurry and soil analysis data allows the targeted application of slurry, with identified nutrient shortfalls made up with purchased fertiliser. Crichton adopts a multi-cut system for making silage, which lends itself to making maximum utilisation of slurry. The table below demonstrates the typical application pattern for the silage growing season.

Due to the high content of P and K in the slurry and good soil indexes for these nutrients, all crop requirements are supplied by slurry, meaning only supplementary nitrogen needs to be bought in. All slurry is applied to the land using a dribble bar which reduces ammonia losses by around 30%.

Slurry Available Nitrogen Bagged fertiliser Purchased fertiliser cost

1st cut 30m3/ha slurry 39kg/ha 65kg/ha N from protected urea £50/ha*

2nd cut 30m3/ha slurry 30kg/ha 50kg/ha Nitrogen £40/ha**

3rd cut 30m3/ha slurry 30kg/ha 50kg/ha Nitrogen £40/ha**

4th cut 30m3/ha slurry 30kg/ha No inorganic fertiliser

5th cut 30m3/ha slurry 30kg/ha No Inorganic fertiliser

Total Nitrogen applied 159kg/ha 165kg/ha £130/ha

SRUC Farm Management Handbook Fertiliser Cost for three cut silage £444/ha

*based on protected urea at £350/t **Based on ammonium nitrate at £280/t

Prices correct at 6/5/21

Protected urea is one of the tools farmers now have available to reduce greenhouse gas emissions. Compared with standard urea, protected urea reduces ammonia emissions by around 78%. The urea is protected by being coated in a urease inhibitor and while giving similar yield benefits to standard urea, there is a 71% reduction in Nitrous Oxide emissions compared with CAN.

The use of protected urea provides more nitrogen to the soil for crop use and reduces the overall greenhouse gas emissions from fertiliser applications.

Slurry is also widely used in the arable crops at Crichton, including maize and winter cereals. Slurry is applied to maize ground before ploughing and then again on top of the ploughing immediately before the maize is drilled to give a highly fertile seed bed to allow rapid germination of the seed.

Winter cereals, destined for wholecrop, are top dressed with slurry in early spring to boost growth. Application with a dribble bar and when ground conditions are optimal means that there is little crop damage.

Winter wheat after maize Slurry Available nitrogen Bagged fertiliser Purchased fertiliser cost

March 50kg/ha N from protected urea £38/ha*

April 30m3/ha slurry 39kg/ha 0 0

Late April 0 0 50kg/ha Nitrogen £40/ha**

Total Nitrogen applied 39kg/ha 100kg/ha £78/ha

SRUC Farm Management Handbook Fertiliser Cost for Winter Wheat £238/ha

By making a conscious effort to target slurry where it can be most used to the benefit of the crop, SRUC Crichton Royal Farm is making significant cost savings on purchased fertiliser, while also actively reducing environmental impact by adopting low-emission slurry spreading techniques and using organic manures to replace purchased fertiliser.

Adopting the use of protected urea in 2021 has further reduced the greenhouse gas emissions from the farm, by reducing nitrous oxide emissions.