Sulphur (S) is one of at least 16 elements essential for plant growth. It is a major constituent of some amino acids, which are building blocks of proteins. It is also essential for plant functions, including photosynthesis and nodule formation and N fixation in clover.

When grassland becomes S deficient, herbage yield and quality suffer. Sulphur-deficient plants are characteristically small and spindly with younger leaves turning pale green to yellow. Such plants are often similar in appearance to those suffering from nitrogen deficiency.

S deficiency can result in sizeable reductions in grass yield before visible symptoms emerge, and hence farmers may be unaware that the problem exists. Research is highlighting changes to the likelihood and timing of S deficiency problems on grassland soils.

Effects of sulphur on herbage yield

When swards are well supplied with S, more than 80% of N in shoot tissue will be present as protein. When there is not enough S, the proportion may be less than 50%, necessitating the purchase of expensive protein-containing feeds to supplement ruminant diets. Concentrations of sugars in plant shoots also decline under S-deficiency conditions, impairing the digestibility and feeding value of herbage.

Excessive use of S-containing fertilisers on grassland, however, can be detrimental to animal health. High concentrations of sulphur in ruminant diets can inhibit the absorption and utilisation of copper, leading to copper deficiency in both cattle and sheep. Such problems with copper utilisation are particularly likely in areas where soil molybdenum concentrations are also high. Excess S can also depress selenium uptake by herbage and impair animal health. However, neither copper nor selenium deficiencies are likely to be triggered by S fertilisation unless S-enriched fertilisers, eg ammonium sulphate, are used and crop S requirements are appreciably exceeded. But, regardless of the likelihood of copper or selenium deficiencies, S should be applied to optimise pasture growth, since grazing animals can be supplied with trace elements by alternative means, eg via injection or oral supplementation.

Sulphur availability and behaviour

Sulphur is available to plants through mineralisation of organic matter, weathering of S-containing minerals, atmospheric SO2 deposition, applications of mineral fertiliser and organic manure and direct deposits of livestock excreta by grazing animal. In the last few decades, however, S availability to crops has declined in Ireland partly as a result of declining atmospheric deposition, but primarily because of increased usage of fertilisers containing little or no S.

Increases in crop yields have also led to greater removal of S from soils and contributed to the decline in soil S reserves. At the same time, because sulphate is mobile in soil, it has been leached out in land drainage water, with anything up to 50kg S/ha/year lost from intensively managed grassland. Research in Northern Ireland has quantified net S balances in soils since the 1940s and has shown that soil S reserves are declining.

More recent research based on farm survey data has shown that S deficiency is now more widespread across all soil types in Northern Ireland, particularly in the early part of the growing season. Out of 67 dairy farms surveyed each year between 2004 and 2006, 49 farms had silage swards testing as deficient in S at first cut in April/May (Figure 1). While lighter soils with low organic matter levels are generally most prone to S deficiency, S deficient swards are now occurring on all soil types, including heavier textured clay and clay loam soils.

It is recommended that at 14kg SO3/ha should be applied routinely as fertiliser to all silage swards in spring. This moderate dressing of S should not be detrimental to livestock and has the potential to prevent yield losses which can be worth up to €100/ha. Sulphur-containing fertilisers should also be applied routinely for second- and third-cut silage crops on land that has received little or no slurry or where soils are shallow or sandy in texture.

Sulphur from slurry

Previous research suggests that although animal manures contain considerable amounts of S, it is largely unavailable for crop uptake in either short or longer terms. The results of the present survey uphold this conclusion. As shown in Figure 2, 54% of swards receiving no slurry or fertiliser S at first cut were S-deficient.

While applying slurry S alone reduced the incidence of S deficiency to 38%, this was still an unacceptably high level of incidence. In contrast, applying both slurry and fertiliser S (and indeed fertiliser S alone) reduced the incidence of S deficiency to almost zero, indicating that slurry alone will be unlikely to overcome S deficiency in the grass.

Rate of sulphur application

Advice based on research work in Ireland has indicated that between 25kg and 50kg S/ha should be applied in spring to maximise grass production on S deficient soils throughout the entire growing season, or 10kg S/ha/cut of silage. The results from the recent farm survey in NI suggest that an application of 14kg S/ha at first cut should prevent swards becoming S deficient on either moderate or heavy-textured soils, but that higher applications could give rise to excessive levels of S in shoot tissue with implications for animal health.

Applying S fertiliser to second-cut crops or for grazing in summer is more likely to be required on lighter soils. Heavier textured soils that have received S in spring are less prone to S deficiency in the summer.

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