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BALANCING SULPHUR AND NITROGEN FERTILISER APPLICATIONS BOOSTS SILAGE PRODUCTION EFFICIENCY IN SRUC TRIALS

New trials showing grass yield increases from Sulphur fertilisers of up to 34% and greater benefits with lower Nitrogen applications are making SRUC researchers look more closely at the impact of reduced atmospheric Sulphur levels in recent years.

Producers wanting to maximise grass production for silage should pay careful attention to balancing N and S fertiliser levels, new work from the SRUC is suggesting.

Whilst each element can individually lift production, striking the right combination of both can have a profound effect on grass output and cost effectiveness of fertiliser applications.

“The trials suggest Sulphur could have a Nitrogen saving effect allowing farmers to achieve similar production from using lower levels of Nitrogen supplemented with Sulphur,” says SRUC grassland researcher Dr. David Lawson.

“Although, it’s something we want to look at more, early indications are that Sulphur is much more important in grassland production and behaves in a more complex manner than previously recognised.”

Two Nitrogen application rates were used for the trials - a full rate of 210kg N/ha, as set by SRUC Technical Note No. 652 which gives Nitrogen recommendations for Scotland, and a lower rate of 90kg N/ha.

These were applied with two different Sulphur rates - the current recommendation of 40kg SO3/ha and also 120kg SO3/ha. Plots receiving no Sulphur input and the two different Nitrogen levels only, were also recorded.

Where no Sulphur was used 34.5% Ammonium Nitrate (AN) - as in CF Nitram – was applied. The lower rate Nitrogen used over two cuts of grass for silage produced a dry matter yield of 7.6t/ha and with full rate this increased to 11.7t/ha – a yield gain of 54%.

Adding the lower rate of Sulphur - in the form contained in CF SingleTop (27N +12SO3) - increased the lower rate Nitrogen plots’ dry matter yield by 0.5t/ha from 7.6t/ha to 8.1t/ha whilst adding the same lower rate S to the full rate Nitrogen application lifted dry matter yields from 11.7t/ha to 12.7t/ha – a full 1.0t/ha increase.

When Sulphur applications were lifted to the full 120kg/ha, the half rate Nitrogen plots increased by 2.1t/ha dry matter from 8.1t/ha to 10.2t/ha but with the full rate Nitrogen, full rate Sulphur made no significant difference. (see table)

The response to the higher than currently-recommended levels of Sulphur with low levels of Nitrogen presents some interesting options for grassland producers, Dr. Lawson believes.

“While we have to be cautious in interpreting the result from just one trial, there may be two possible explanations for the Sulphur effect at low Nitrogen rates. 

“Firstly, like all life, the crucial soil micro-organisms which occur naturally in all soils are unable to function to their full potential as they are also suffering from a deficiency of Sulphur, so feeding them with Sulphur helps the natural process of Nitrogen release from soil organic matter reserves.

“Secondly Sulphur, through its link with protein production, could also boost chlorophyll production, photosynthesis and plant growth, where the element is deficient. We hope to explore these issues in further work.”

Independent grassland consultant Dr. George Fishers says the SRUC work is part of a series of UK trials suggesting N and S fertiliser levels need to be carefully managed with significant silage quality benefits being possible in addition to yield gains.

“Most dairy and livestock producers have reported falling crude protein levels in silage over recent years with many not achieving the yields they would like to either,” he points out.

“Whilst there has been some work done pinpointing Sulphur as a means of lifting crude protein levels, the precise nature of the interaction of the element with Nitrogen is often over looked.”

Lifting outputs through Nitrogen use can dilute protein levels unless sufficient Sulphur is available to the plant, he explains.

“Equally, without sufficient Sulphur, Nitrogen utilisation is compromised as it is a vital contributor to protein production and therefore in the Nitrogen to be taken up by the plant. 

“It’s like a protein pipeline – if you don’t have the Sulphur, the plant doesn’t produce as much protein, so it doesn’t take up as much Nitrogen as it should.

“The two elements work very much in tandem and it’s only recently that we’ve needed to examine this in detail as there is simply less Sulphur available to plants from the atmosphere with the reduction on coal-fired power stations and the general clean-up that industry has achieved in recent decades.”

 

GROWING BODY OF EVIDENCE

Similar results have been seen in other trials across the UK in recent years.  Trials at Domvilles farm, Barthomley, Cheshire, have shown an average 29% lift in silage yields over two years (2016 and 2017) from the use of the combined NS compound SingleTop (27N +12SO3).

On the same reseeded pasture, using Sulphur for grazing also produced a 16% gain over the two years.

On a heavier clay loam at a trials site in Devon, response to Sulphur was 10% for the same levels of application as the Cheshire site.

There is clear evidence that Sulphur is required on the majority of grassland across the UK now, Dr Fisher says.

“Trials are showing significant responses to Sulphur from the very south of the country to the north with indications that even heavier soils, which tend to retain Sulphur better than lighter soils and have a greater Sulphur supply from soil organic matter, are starting to become deficient in the element.

“There is also a much sounder understanding of the way in which Sulphur and Nitrogen can be balanced to optimise both yield and crude protein levels in grassland.”

 

SULPHUR OPTIONS

Producers have options as to how they apply optimum levels of Sulphur but true granular compound like SingleTop, where N an S are in every granule, have proved to be the most reliable, says CF Fertilisers’ Northern Regional Manager, Mark Garrett.

“Plant tissue testing is the only real way of knowing where you are with the Sulphur requirements of your grassland.  But with light, medium and even heavier soils showing S-deficiency, many will choose to routinely add Sulphur to their fertiliser programme.

“Check the soil for pH, Phosphate, Potash and Magnesium status and establish the Nitrogen you need to use for optimum results in line with SRUC TN652 in Scotland, or the Fertiliser Manual RB209 in England and Wales, and then choose the right true granular product needed to balance this with the correct amount of Sulphur.”

Where slurry is used for both first and second cut on soils with adequate P and K, the main requirement is for N or NS like Singletop, he points out.

“NKS products as are particularly beneficial where P index is high and soil Potash levels need to be maintained. 

“Don’t forget that grass crops remove a lot of Potash from the system and slurries and manures do not contain much plant available Sulphur.”

MultiCut Sulphur (23-4-13 + 7SO3) can be used as an all season high Nitrogen/high potash product ideal for multiple cuts of grass silage with reduced Phosphate content to help maintain soil P and K status.

If the P index is high but the K index low, particularly if it’s on a lighter soil type, producers should consider using SingleTop (27N + 12SO3) in spring then KayNitro Sulphur (25-0-13 +7SO3) in the summer, he says.

 

THE TRUE COST OF CRUDE PROTEIN

Every £1 invested in Sulphur containing fertiliser to lift silage crude protein will save £2 in additional Soya meal required to achieve the same effect in rations, says Dr. George Fisher.

Plus you’ll get higher yields of silage with better energy content so the true return on investment is much higher, he adds.

“For a dairy cow requiring 10kg DM grass silage intake per day over a 185 day winter feed period, lifting the crude protein by 2% points – say from 14% to 16% - would cost an extra £14.77 per cow by replacing 250kg N/ha from straight N (34.5%) with SingleTop (27N + (12SO3)

“Doing the same thing by adding 45% crude protein soya meal to the diet at £305/t freshweight would cost £28.40 per cow – nearly twice as much.”

But this doesn’t take in the true value of Sulphur as it omits yield improvement, he says.

“If you use the 2.8t dry matter lift in total first and second cut silage yield that we saw in trials in Cheshire in 2016, at 11.0 MJ ME/kg DM and a utilisation rate of 70%, this would have saved £391 in concentrate energy feed costs per hectare.

“The cost of the sulphur fertiliser to achieve this would be £37/ha so this represents a return on investment of 10.5:1.”

Taking this a stage further, the £37/ha extra cost of the Sulphur containing fertiliser over straight AN would bring savings in concentrate feed and soya meal for the TMR of £391 plus an additional £71 – made up of the £28.40 multiplied by a typical 2.5 cows/ha stocking rate.

“This gives a total benefit of £462/ha and at this point, the return on investment from the £37/ha for the Sulphur fertiliser is 12.5:1,” George Fisher concludes.

 

Yields Of Grass DM/ha from two cuts – SRUC trials

 

Nitrogen applied over two cuts

Level of Sulphur Applied over two cuts

0

40kg SO3/ha

90kg SO3/ha

Nitrogen fertiliser applied – ½ rate – 90kg N/ha

7.6

8.1

10.2

Nitrogen fertiliser applied – normal rate – 210kg N/ha

11.7

12.7

12.5