Farmers could be losing thousands of pounds by choosing the wrong type of fertiliser, according to new research on urea, sulphur and ammonium nitrate (AN).
At a time when incomes are under immense pressure, the temptation to cut back and use cheaper alternatives is considerable, but it could be an extremely damaging false economy.
Independent grassland consultant Dr George Fisher warns ‘the chances of reducing your grass growth are surprisingly high’. Trials organised by CF Fertilisers and run independently at Reaseheath College and elsewhere over 2015 and 2016 found using AN rather than urea boosted silage yields by 18% and grazed grass yields by 7%. This was due to losses of urea nitrogen to the atmosphere, even though the weather in 2016 was relatively suitable for urea use.
“The effectiveness of urea is very weather-dependant: There has to be enough rain within three days of application to wash it into the soil, and low temperatures are required,” says Dr Fisher. “If you have high temperatures and, most importantly, no rain, then you will get increased volatilisation loss of nitrogen as ammonia into the atmosphere, with the resulting impact on yield.”
In a damp, cold spring, urea can be just as effective as AN, but it is important to have accurate weather forecasting and to only use urea when confident the conditions are right, he warns.
“You certainly don’t want to use it throughout the growing season as it will be desperately inefficient.”
For example, if the air temperature is 0degC you will need 5mm of rain within three days to wash the urea sufficiently into the soil.
“Over a five-year period in western Britain the chances of that happening in spring are only one-in-three – so there’s a two-in-three chance of losing nitrogen and yield,” says Dr Fisher.
“If it’s 5degC you’ll need 7mm of rain within three days, which only has a one-in-four chance of occurring. In terms of risk management within a business taking these chances is hard to justify.”
Trial results at Reaseheath showed using AN instead of urea boosted silage dry matter yields by an average of 1.74 tonnes/hectare, to 11.26t/ha over two cuts a year in 2015 and 2016. Grazed grass yields increased by 0.41t/ha to 6.32t/ ha over the first three rounds of grazing.
“Even though the weather occasionally allows urea to be as effective as AN for grass growth, it’s not worth taking the risk of losing valuable home-grown energy for milk production. What you lose in production potential is far more than you save in costs.”
Sulphur deficiency is not always visible. Paling and then yellowing of the leaves (similar to that observed in nitrogen deficiency) may be seen, although it is in the younger leaves first, whereas nitrogen deficiency is seen in older leaves.
It used to be thought only lighter soils responded to sulphur fertilisers and mostly in mid and late season. But as time passes with low sulphur deposition from the atmosphere and the sulphur in soil organic matter depleting, sulphur deficiency is now being seen in medium and heavier soils, and responses are gained in early season as well.
Sulphur status is normally assessed by herbage analysis. It is possible to obtain a soil analysis for sulphate content, but this is very variable and so plant tissue testing is more reliable.
The amount of sulphur in grass normally ranges from 0.2% to 0.4% in the dry matter. The most recent advice suggests it should be at least 0.25% to avoid deficiency.
Obtaining the nitrogen: sulphur (N:S) ratio of herbage is the usual way to assess whether sulphur is required. Samples should be taken when the grass is growing rapidly in spring, or during aftermath growth following silage cuts. A ratio greater than 13:1 is considered deficient, but a response may be seen down to 10:1. Care must be taken in interpreting results as very high or very low nitrogen contents can provide confusion.
For example, a nitrogen concentration of 4.6% and a sulphur concentration of 0.35% gives a ratio of 13:1, which appears sulphur deficient, but the need for sulphur is borderline as the sulphur content is relatively high. Similarly, a nitrogen concentration of 2% and a sulphur concentration of 0.21% gives a ratio of 9.5:1, which appears sulphur sufficient, but the sulphur percentage is low and the sward would benefit from sulphur applications.
A malate:sulphate analysis of fresh herbage may also be used to assess sulphur deficiency.
This approach measures an important metabolite (malate) which is used in energy production in the grass plant and is dependent on sulphur to work efficiently. The test is arguably more accurate than the N:S ratio and can be obtained from CF Fertilisers.
Another area where farmers could boost grass yields and quality for profit is by adding sulphur – something which very few people currently do even though a lack of sulphur produces similar results in the crop to nitrogen deficiency, says Dr Fisher.
“Grass needs sulphur to produce amino acids; the building blocks of protein which is needed for grass growth; without sulphur it won’t grow as well.”
In the past, sulphur deposition from the atmosphere in the form of acid rain was sufficient to meet the crops’ needs. “But deposition has been reducing since the 1970s and an increasing number of soils and crops are now deficient,” he adds. Advice to only apply sulphur to lighter soils and later in the season is now out-of-date.
“According to the British Survey of Fertiliser Practice, only 11% of grassland farmers applied sulphur to their silage ground in 2015, and 7% to their grazing swards,” says Dr Fisher. “But in replicated trials in 2016 adding sulphur to silage leys on a medium loam in Cheshire boosted yields by 29%, to 12.45t/ha over the first two cuts.”
The silage trial used 190kg/ ha of AN plus 84kg/ha of sulphate applied as the granular compound SingleTop on a field reseeded in 2015. Replicated on grazing leys, at rates of 170kg/ha of AN plus 76kg/ha of SO3, yields across the first four rounds of grazing increased by 14%, to 9.32t/ha.
So what is that extra 2.8t/ha DM of silage worth?
“At £25/tonne fresh weight for selling silage, that equates to £280/ha,” explains Dr Fisher. But when converted into megajoules of energy (MJ ME), and again into litres of milk, the returns are even greater.
“At 11MJ ME/kg DM and 70% utilisation, 2.8t/ha of silage is worth 21,560MJ to the cow.” It takes 5.3MJ to produce one litre of milk, so if milk is valued at 22ppl the extra yield produces a return of £895/ha.
It’s also interesting to compare the benefit of feeding that forage to the cows against substituting the energy with concentrates, he adds. “To provide 21,560MJ of energy in a concentrate containing 13MJ ME/kg, and priced at £210/t, would cost £391/ha.”
When it comes to an extra 1.2t/ha DM of grazed grass, at 12MJ ME/kg and 85% utilisation, the value equates to £508/ha as energy for milk production. To replace that energy with concentrates would cost £223/ha.
“It’s more cost-effective to make the best use of the energy you are growing onfarm. By adding value to it in the form of milk you’ll always make more money than you would by selling the extra forage or replacing it with concentrates.”
Data from the National Farm Survey in Ireland shows that for every additional tonne of grazed grass utilised farmers could increase net profits by €184. In addition, it shows net profits decreased from £1,300/ha to £1,083/ha as supplement fed per cow increased from 0.35t DM/cow to 1.7t DM/cow, even though milk solids rose from 359kg to 419kg a cow.