Address sulphur and improve nitrogen use efficiency

Getting sulphur levels right can improve nitrogen uptake by forage and produce greater yields from less N, according to independent consultant, Charlie Morgan.

“And as a result, the amount of N leached from the system can be reduced by up to 75%, conveying huge financial and environmental benefits.”

However, in recent years, because the amount of sulphur gas produced by burning of fossil fuels has reduced, forage sulphur deficiencies are more likely to be a problem.

Low sulphur

Forage sulphur deficiencies are definitely becoming more prevalent, says Mole Valley Forage Service’s, Matt Rance.

“We are seeing more leaf and soil tests highlighting low levels of sulphur in grass crops, combined in many cases with physical indications of sulphur deficiencies, such as yellowing of the younger leaves, and poor response to nitrogen application.

“At this stage, grass production will be compromised and milk production reduced. As a result, leaf analysis is essential.

And do not assume yellow grass means a shortage of nitrogen – it could be a number of other factors, including sulphur deficiency, he says.

Forage analysis 

“It is essential to confirm sulphur is the primary factor limiting forage performance. When applying unnecessary elements, these substances will not be used efficiently, potentially creating environmental issues and unnecessary cost.”

Everything else must be right before applying sulphur, agrees Mr Morgan. “Firstly consider pH and NPK levels – these will be more restrictive to grass yields than sulphur levels. When these factors are not right, you won’t get the yield response from sulphur application.”

And when sulphur levels are identified as being too low, the first step is to think how you can use farmyard manure and slurry to raise levels, says Mr Rance.

Artificial fertilisers including sulphur are also a good way of raising sulphur levels, however, it is essential to look at other factors affecting soil health. “Low PH, compaction and water logged soils can affect availability, so focusing on soil condition is vital.”

Before applying sulphur, it is important to balance major elements in the soil, says independent consultant Jo Scamell. “When key elements are inadequate, particularly calcium, sulphur can be detrimental to trace element availability. And when copper availability is reduced, livestock fertility may be affected.”

Sulphur must only be applied when needed, agrees DairyCo extension officer, Piers Badnell.

“Fertilisers containing sulphur, such as ammonium sulphate, are more expensive and can potentially affect pH levels, so they must only be used when necessary.”

Ideally, grass should be analysed for sulphur at or before first cut silage, he says. “Grass should have a nitrogen:sulphur ration no greater than 13:1. The critical level of sulphur is 0.25% total S. When levels are below this, it is worth applying 25-40kg of sulphate/ha.”

And by addressing low sulphur levels, grass production can be significantly improved.

Grass yields

Research by IGER identified up to a 30% yield increase on sulphur deficient sandy soils and an 11% increase on clay ground, says Mr Morgan.

“The response will depend on the severity of deficiency. When soils are severely lacking, a greater response may be found.”

Trials showed applying sulphur to sandy soils with a nitrogen:sulphur ratio of 12:1, improved the ratio to 7.6:1. “This represents a significant improvement to nitrogen use efficiency and resulted in an increase in grass yields of 1.85t/ha DM (0.75t/acre DM).

“The cost of producing 1kg DM last year was 6.5p, so this equates to £48.75 more yield for the same inputs.”

And this extra forage will mean a reduction in concentrate feeding. “Silage feed quality will also be improved, with increased protein levels and improved dry matter intakes.”

And as a rule of thumb, when sulphur is identified as a limiting factor one year, there is a 95% chance you will need to apply again the following year, adds Mr Badnell.

Milk protein

Sulphur will increase nitrogen use in forage and can be beneficial to milk production, says Mr Rance.

“By improving nitrogen use and providing sulphur, the plant can produce sulphur containing amino-acids (the building blocks for proteins), such as lysine and methionine which have been highlighted as being beneficial to milk production.”

Sulphur itself is used by the rumen bugs to make methionine – an amino acid used in milk protein synthesis, says SAC nutritionist, Colin Morgan.

“And providing actual methionine can also be useful. A small proportion of methionine in grass will not be broken down by the rumen and this can pass into the intestine to be digested. This can then be used to make milk protein.”

CASE STUDY: Gary Hawker, Clandon Farm, Dorchester

Producing top quality grass is essential at Clandon Farm, Dorchester where the herd of 375 dairy cows are out at grass for 10 months of the year, explains herd manager Gary Hawker.

“After moving to a more extensive system 10 years ago, we changed our emphasis towards improving soil performance.”

And about six years ago, soil analysis identified sulphur as one of the key limiting factors to soil performance. “Since then we have been applying 200-300kg/ha of a fertiliser blend containing kieserite (magnesium sulphate) and ammonium sulphate, along with NPK and sodium, 6-7 times a year.”

Applying sulphur to soils of low pH or where calcium is limiting may result in detrimental affects on availability of some trace nutrients, explains Mrs Scamell.

“However, on the farm’s chalk downland soil, applying sulphate to land is fine, as calcium levels are abundant.”

And as a result we have seen marked improvements in sward nitrogen use efficiency which has enabled us to reduce nitrogen use significantly, says Mr Hawker. Milk proteins have also improved from 3.2% to 3.6-4%.

“Before we addressed soil performance we were experiencing problems with bloat, however now we have completely eliminated the problem through a range of soil improvements.”

And grass palatability has also improved. “Cows are now grazing down to a residual of 1400-1500kg/ha DM, where as before we were finding it difficult to meet these targets.”

A contributing factor to this is improved nitrogen efficiency, says Mrs Scamell. “More efficient N use, results in improved production of ‘true protein’ in grass, which is more nutritionally balanced than non protein nitrogen.”

As a result, grass has a higher nutrient content per kilo of dry matter. “This well structured grass encourages healthy gut function and thus reduces problems with bloat.”

And this improvement in grass quality has also resulted in less cases of mastitis. “Because grass is better quality, dung is of a more stable consistency and cows are less stressed, reducing the incidence of mastitis,” explains Mrs Scamell.

According to Mr Hawker, fertility has also shown marked improvement. “We aim to calve in a tight 10 week block so getting cows in calf is essential. Before, we were getting empty rates of 20% where as now we are achieving 6%.

“There are other factors influencing improvements, including a change in breeding policy from Holstein Friesians to Jersey cross Friesians, but there is no doubt soil improvements have played a key role,” he says.

All these results are a consequence of adopting a whole combination approach, stresses Mrs Scamell. “Sulphur is a key contributor to the benefits seen, but we would not have seen these improvements without addressing magnesium, sodium and sulphur levels together.”