MMN January 2025 – Spring Sulphur Applications to Boost Grass Growth
10 January 2025Sulphur is an essential nutrient often overlooked by livestock farmers. It plays a vital role in nitrogen use efficiency and protein formation. Although light sandy soils are most vulnerable to sulphur losses, research suggests that deficiencies can also occur in heavy textured soils in early spring. The application of sulphur can boost dry matter yields by 10-15% (AHDB), but more notably, will improve grazing and silage quality.
Where are sulphur deficiencies most likely to occur?
- Light textured soils.
- Areas with high rainfall.
- Multi-cut silage systems and rotational grazing systems.
- Farms applying large amounts of nitrogen fertiliser.
The leachable nature of sulphur means that soils with low levels of organic matter are most at risk of losses occurring. However, significant winter rainfall will wash sulphur from the soil before it can be utilised by the crop.
Sulphur works in conjunction with nitrogen in the formation of plant protein. Therefore farms using high levels of nitrogen fertiliser and/or achieving above-average DM yields will require additional sulphur applications.
Source: FAS Technical note TN685, 2017
Tissue analysis
Sulphur deficiency can often be mistaken for nitrogen deficiency. The yellowing of young leaves is typically seen in a sulphur-deficient crop, whereas the yellowing of mature leaves is seen where there is a shortage of nitrogen.
Due to varying levels of sulphur in the soil throughout the year, plant tissue analysis is preferred over soil sampling to determine sulphur deficiencies. It is advisable to carry out tissue sampling less than 10 days before cutting for silage. Critical levels of sulphur occur when the N:S ratio is greater than 13:1, or the total S in the dry matter is less than 0.25%.
How much sulphur should be applied?
Where a deficiency is expected, RB209 recommends:
- Silage – 40kg SO3 before each cut.
- Grazing – 20-30kg SO3 for each 100kg N/ha that is applied.
The most common form of inorganic sulphur is ammonium sulphate (60% SO3). Care should be taken to avoid over-applying sulphur during the growing season to prevent copper and selenium deficiencies in livestock. Small and frequent applications are advised on grazing fields.
Sulphur from manure
Manure is a valuable source of sulphur. However, due to the slow mineralisation of S within organic matter during the spring, it is unlikely that it would supply enough sulphur to meet crop demand. RB209 states that 45% of SO3 would be available to grassland after a spring application of cattle slurry. If 30m3 per hectare of 6% DM cattle slurry was applied to a field, approximately 9.6kg SO3 would be available.
Conclusion
Mineral soils with multi-cut silage systems are likely to require sulphur applications to improve forage quality and yield, particularly after an unseasonally wet winter. This is worth bearing in mind for this year’s silage season, especially if protein levels in grass silage were lower than expected last year. Sulphur fertiliser should be applied in addition to slurry and FYM where deficiencies are expected.
marcus.fox@sac.co.uk; 01539 769059
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