Sulphur boosts grass yields

Silage trials using a sulphur-containing true granular ammonium nitrate (AN) compound produced an extra 20% dry matter yield over a urea plus sulphur blend, which was worth £360 per ha in terms of milk production.

In grazing, the advantage to using AN plus sulphur over straight AN rose as high as 65%, says Galloway and Macleod’s Jim McMullan.

“Many producers across the country have had real problems producing enough forage to see them through the winter last year and a significant number had to buy in forages to make up the gap.

“Whilst not everything can be countered by better management, grassland nutrition is one of those areas that if you invest in it properly, you seldom lose out.

“There’s also growing evidence that over the years, it can not just make grassland more robust and improve productivity, it can also stretch the intervals between re-seeds," he said.

The silage trials were carried out in second and third cuts on two fields at Gartclush in Stirlingshire and compared urea plus sulphur with CF SingleTop (27N + 12SO3).

All plots received the same amount of nitrogen over the two cuts (140kg N/ha) from both sources, but due to the products analysis those receiving the urea+s blend got slightly more sulphur at 70kg SO3/ha, compared to 62kg SO3/ha from the SingleTop, he explains.

“As the sulphur was the same type in both products and at a similar rate, the difference in performance can only be due to the loss of nitrogen from the urea due to volatilisation to air, compared to the AN based true compound which contains N and S in every granule.”

At both locations, marginal gains from the NS compound were recorded in second cut, but, by the time third cut had been taken, these had become substantial, Jim McMullan points out. (See table 1.)

“In the first field, the compound produced a freshweight yield of 34.4t/ha at third cut compared to 33.2t/ha for the urea and this translated into dry matter yields of 6.2t/ha and 5.4t/ha respectively – a 14% advantage in favour of the NS compound.

“The difference was 20% in the second field with dry matter yields of 5.1t/ha for the NS compound compared to 4.2t/ha for the urea+s.”

According to Mr McMullan, you can look at the benefits of such improvements in several ways. In it’s simplest form, it's worth 20% more forage in the clamp in a difficult year, or if you wanted to sell it, it would be worth £107 per ha at £40t fresh weight.

Or, he said, the extra energy from the silage works out at approximately 10,000 MJME per ha, which could produce £372 of milk at 29p per litre or be used to replace concentrates and save £144 in concentrate energy.

“So the gains are very real and what is important is that these were being made in a difficult year for grass growth and silage production in general. A lot of producers would be in a better position entering the winter if they had managed to produce an extra 14-20% forage this year, even just in third cut.

“Up to 50% of the nitrogen in urea can be lost in dry and warm conditions, which were certainly prevalent for most this season.”

The sulphur factor should not be ignored, however, with grazing trials carried out by Galloway and Macleod at Hillowton near Castle Douglas, Dumfrieshire, showing a marked response to the same SingleTop NS compound over straight AN. (See table 2.)

In one mid season grazing rotation, SingleTop produced and extra 0.7t/ha of dry matter – 1.8t/ha compared to 1.1t/ha – giving a yield advantage of 62%.

The main issues in grazing are milk produced from grazed grass and minimising concentrate input, and this extra energy in one grazing round would be worth £375 if converted in to milk, and £145 if used to replace concentrate, he said.

Mark Garrett of CF Fertilisers says the results underline the importance of correct fertiliser management in grassland.

“Correct nutritional management and adding sulphur definitely make grassland more productive in the year they are being applied but they also build resilience into production on a long-term basis.

“Choice of nitrogen source is also critical with trials consistently showing urea is a false economy for grassland when you look at the bigger picture.”

It can be tempting to use because of the often lower purchase price per kg of nitrogen, but it almost always limits potential production, he points out.

He highlighted grassland trials at Reaseheath College in Cheshire, which have shown savings of £3000 from using urea over nitram ammonium nitrate on a 80ha dairy system. However, this also resulted in a large loss in grass yield, which would cost £13,500 to replace as supplementary concentrate.

Even with inhibited urea designed to make nitrogen loss to air less of a problem, a significant reduction in grass yield compared to the ammonium nitrate was seen.

In terms of the return on investment in sulphur, producers should expect to see an ROI of over 10 to 1, Mark Garrett says.

“If you use the 2.8t per ha dry matter yield lift in total for first and second cut silage yield that we saw in replicated trials in Cheshire in 2016 and 2017, at 11.5 MJME/kg DM and a silage utilisation rate of 70%, this would have saved £450 in concentrate energy feed costs per hectare.

“The cost of the sulphur fertiliser to achieve this would be £43/ha so this represents a return on investment of 10.5:1."

In addition, he said adding sulphur to the fertiliser programme can lift grass crude protein content significantly because sulphur is needed for protein production in the grass plant.

“We often see a crude protein lift by 2% points, for example, from 14% CP to 16%, and this can make a substantial difference to the cost of total mixed rations when the forage base has this crude protein improvement.”

Table 1. Galloway and Macleod 2018 silage trials (third cut) at Gartclush, Stirlingshire comparing SingleTop (27N + 12SO3) true granular NS compound with a urea+S blend

Location Fertiliser N applic. rate (kg/ha) S applic. rate (kg/ha) DM yield (t/ha) DM yield advantage (t/ha) Total energy advantage (MJ ME/ha) Milk value advantage (£/ha)

Field 1 NS compound 140 62 6.2 +0.8 (14%) +9169 +345

Urea+S 140 70 5.4

Field 2 NS compound 140 62 5.1 +0.9(20%) +9917 +372

Urea+S 140 70 4.2

Table 2. Galloway and Macleod 2018 grazing trials at Hillowton, Dumfieshire comparing SingleTop (27N + 12SO3) true granular NS compound to straight AN

Fertiliser Freshweight (FW) yield(t/ha) Value of extra FW yield (£) ME content (MJ ME/kg DM) DM yield (t/ha) DM yield advantage (t/ha) Total energy advantage (MJ ME/ha) Milk value advantage (£/ha)

NS compound 10.0 +90 11.3 1.8 +0.7(62%) +8224 +375

AN 6.8 11.0 1.1

Key fertiliser points to maximise grassland resilience

1. AN more reliable source for lifting grassland yields than urea

2. True granular NS compounds more effective than urea + s blends

3. Optimum fertiliser use can reduce bought-in feed bills by equivalent of £140/ha

4. Return on investment (ROI) of sulphur use over 10:1

5. Sulphur application can lift silage crude protein by 2% points i.e.14% to 16%