Grassland Blog

Fertiliser management for swards in dry conditions

Tom Oates, Nutrition Agronomist

This spring has been exceptionally dry, making managing grass and livestock very challenging. Furthermore, with Fertiliser and Feed prices at all time highs, there are many difficult management decisions on farm to be made.

AHDB has produced a cost benefit calculator for nitrogen fertiliser use on grassland. The calculator compares the cost of nitrogen applications with the cost of the grasses feed value, helping to conclude if its more cost effective to apply nitrogen fertiliser to grassland or purchase feed. The tool also helps calculate cost benefit ratios to aid management choices regarding N application rates.

However, there are limitations to this tool as it doesn’t consider the effects of sulphur on NUE and the importance of other essential nutrients.



Therefore, maintaining the correct K:Na ratios <20:1 is of vital importance not only to mitigate grass staggers, but also to increase sward palatability through converting sugars in grass into more soluble carbohydrates. This conversion and increase in grass sugars will increase DMI. Grass needs to receive high N and SO3 applications to increase DM yield, but N utilisation and conversion to protein can be low in a poor growing season with dry and cloudy weather conditions. The buffering capacity can be in increased, causing the fermentation process to slow down when nitrate levels are above 0.10% in a fresh grass analysis. High sugar levels aided by sodium applications will help increase the speed of fermentation process as the higher the sugar content, the more food for the bacteria, the quicker the pH drops.

When deciding if to apply N due to dry conditions, remember even during dry weather overnight dews can deposit moisture which is sufficient to start dissolving fertiliser granules.

As the grass sward experiences stress through drought, the uptake of potassium increases to alleviate this stress, as potassium helps regulate the opening and closing of the plant’s stomata, protecting the plants cell turgor, therefore reducing wilting.

This high potassium uptake can increase the risk of acute or chronic hypomagnesemia (grass staggers). Sodium is an important nutrient needed to reduce this risk, as most livestock’s saliva is produced from sodium which is secreted into the rumen to neutralise acids and maintain a constant pH. If sodium levels are low, potassium substitutes sodium in the saliva and once secreted into the rumen it will inhibit magnesium absorption through the rumen wall.


Where silage fields have had their first cut, remember to apply slurry first then inorganic fertilisers within a nutrient management plan to increase second cut yields. The reason for this is the risk of denitrification. When covering N inorganic fertiliser in slurry, it creates anaerobic conditions which means N is lost to the atmosphere as Nitrous Oxide. Try and leave a week between applications.

Need more detail?

Contact your Origin Fertiliser merchant or us directly by completing the form.