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Grazing management, sunlight, water and available nutrients are the factors determining grass growth. Soil temperatures are now around 14°C and, as we are now close to the longest day of the year in the UK, with long daylight hours our grass growth potential is very high.
Livestock producers applying N in a little and often approach in a paddock grazing system may have been tempted to reduce application rates in response to the seasons high fertiliser prices.
One method of doing this, would be reducing all applications by 15%, from the start of grazing in early spring until early autumn. However, the months with maximum growing potential are generally May and June, due to soil moisture and day light hours. Therefore, applications in these periods of maximum photosynthetic potential should not be reduced.
Instead, applications of N in early spring or late autumn should be reduced if a reduction in N application during the grazing season is going to be made.
AHDB’s ‘Forage for Knowledge’ seasonal grass growth data https://ahdb.org.uk/knowledge-library/forage-for-knowledge shows June generally has the highest grass growth rate (kg DM/ha).
Care must be taken so that livestock doesn’t waste metabolisable energy by urinating excess nitrogen, as this is uneconomic and has an environmental impact. Approximately 5 days post N applications swards will have a nitrogen spike within the leaf blade, which when digested by livestock can cause luxury uptake of nitrogen.
It is therefore best to apply fertiliser post grazing, around 14 days ahead of the cows moving back into the paddock.
Sulphur applications, alongside nitrogen, helps build more amino acids which increases the plants protein content, hence increasing Nitrogen Use Efficiency.
Applications of nitrogen and sulphur should be applied at a ratio of 12:1. Remember all sulphur recommendations are given as SO3 not S, and to convert S needs to be multiplied by 2.5. Therefore, the N:SO3 ratio is 12:2.5.
Grazed grass in a paddock system has a high sulphur requirement as the plant is undergoing heavy defoliation and it needs these nutrients to build back biomass.
There is a higher risk of sulphur deficiency through leaching after heavy rainfall, therefore through the season it is recommended to see if sulphur is deficient by taking tissue samples from the paddocks before introducing livestock.
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