How to address grassland sulphur to improve silage quality

Falling levels of atmospheric sulphur could be having a bigger impact on silage crude protein levels than originally thought, new trials suggest.

With producers struggling to balance low protein silages with costly bought-in feeds last year, the results indicate sulphur is just as important as nitrogen in producing optimum forage quality.

See also: More articles on grassland management

George Fisher, grassland specialist

Dr George Fisher

Overall analysis of forage samples in the trials carried out by CF Fertilisers shows sulphur actually accounts for about 30% of the variation in silage crude protein, explains independent grassland specialist George Fisher.

“Sulphur is essential in the production of two vital amino acids – methionine and cysteine – and without it proteins cannot be synthesised efficiently.

“In one trial, where sulphur levels were restored to optimum, proteins increased by 7% and yields lifted by nearly 2.0t/ha, and at another site a 1.0t dry matter/ha yield lift over the farm’s standard practice urea regime was recorded while proteins increased by 5%.”

Addressing sulphur needs could benefit many farms struggling with poor forage quality, he believes.

“For a really good dairy ration you ideally need grass silage of about 18% crude protein. For intensive sheep systems you can’t really go below 16%, but for many producers that has been very difficult to achieve recently.

“It’s not that long ago that we got 70 to 80kg/ha of sulphate simply falling out of the sky every year, but this has now fallen to below 10kg/ha and in many areas below 5kg/ha.

“It’s not unrealistic, therefore, that atmospheric sulphur levels being 10% of what they were in 1980 are having just as much of an effect on crude protein as applied nitrogen levels being 50% less than they were,” says Dr Fisher.

Balancing rations

According to independent dairy nutritionist Martin Attwell, Trouw Nutrition’s analysis of crude protein levels in UK first and second cut silages averaged little more than 14% last year.

“This covered significant inconsistency with levels ranging from 7% to 22%, with many of the higher levels being from silage samples that included red clover in the ley,” he says.

“In dairy cow diets, particularly high yielders, we would try to balance the total ration at around 16 to 17% crude protein.

“With protein continuing to be a very expensive component of diets, the more protein in the forage the better this is, although there are complications of having too much.”

Get your application right

Mark Garrett, CF Fertilisers

Mark Garrett

Mark Garrett of CF Fertilisers says a good policy for ensuring adequate sulphur is applied to grassland is to start by checking the nitrogen requirement.

“Check the nitrogen you need to use for optimum results in line with RB209 and then choose the right sulphur-based product from this.

“Where slurry is used after both first and second cut the main requirement is for N or NS, with only a modest requirement for NPKS compounds. NKS products are particularly beneficial where the 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.”

If the P index is high but the K index low, particularly if it’s on a lighter soil type, producers should consider using a fertiliser with a high-quality True Granular Compound product for little-and-often applications of nitrogen and sulphur (27N + 12SO3) in spring, then one with more sulphur (25-0-13 +7SO3) in the summer, he says.

Reducing feed costs with sulphur

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 Fisher.

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 percentage points – say from 14% to 16% – would cost an extra £14.77 per cow from replacing 250kg N/ha of straight N (34.5%) with a high-quality granular compound product for little-and-often applications of nitrogen and sulphur (27N + 12SO3), says Dr Fisher.

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

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

“If you use the 2.8t DM lift in total first and second cut silage yield that we saw in trials in Cheshire in 2016 using additional sulphur, 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 ammonium nitrate 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,” Dr Fisher concludes.