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Are grassland farmers missing a fertiliser trick?

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What valuable additions are missing in your grassland toolbox?

Many grassland farmers could be missing a number of tricks when it comes to the application of fertiliser, according to Philip Cosgrave, country agronomist with Yara. Among other things, he says the use of sulphur and selenium are valuable additions to the grassland farmers’ toolbox, and it is important to supplement the use of slurry with NPKs.


Mr Cosgrave says sulphur has a key role to play in conjunction with nitrogen and will provide a return on investment. Despite good evidence for the use of sulphur, research suggests the minority of grassland farmers are taking advantage of it. Results from the British Survey of Fertiliser Practice for 2016 show less than one-tenth of grassland – 9% – received a sulphur dressing, although this figure rose slightly to 16% for grassland cut for silage. Even then, according to Mr Cosgrave, the applications recorded are about half what they need to be across the year. Wynnstay fertiliser manager Dave Mitchell agrees too few farmers are using sulphur as part of their fertiliser regime and he encourages his clients to use grass analysis to understand the value of the forage and make adjustments, either before the second cut or for the following season.


Absence of sulphur limits nitrogen effectiveness


As Mr Cosgrave says, applying nitrogen without sulphur will limit the effectiveness of the nitrogen.


“When it comes to grass the sulphur helps the nitrogen to do its job and to be successfully converted to protein. The more nitrogen you apply, the more sulphur you should apply. On grassland for grazing, you should be applying 2-2.5kg of SO3 for every 10kg of applied nitrogen.


“For first cut silage you would be looking at about 120kg nitrogen with 35-45kg SO3 and for second cut a SO3 rate of 35-35kg. The British Survey figures showed even on farms which are using sulphur the application rates are only about 35kg/hectare over the two cuts.


“Our research shows one-tonne of ensiled grass silage is costing £27.48 to produce, with sulphur making up only 21p of this overall cost.”


Sulphur plays a key role in enabling the assimilation of nitrogen and its conversion into protein. Lack of sulphur can result in a number of undesirable consequences, including a reduction in the yield potential and grass protein; high nitrate levels reducing palatability; increased leaching of nitrates – a real environmental concern at the present time; and lower silage quality. In some conditions this may affect the timing of harvesting, as the grass analysis may show it is high in nitrates at the point when you want to cut. In turn this can affect the preservation quality of the silage as a result of an increase in the ‘non-protein’ in the plant, including nitrates and amino acids.


Ultimately, this will affect the performance of your animals. Mr Cosgrave says the role played by sulphur in organic manures is negligible as it needs to be mineralised, and it will not be mineralised in time to make a difference to this year’s first cut grass crop.

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Use NPKs with your organic manure


Mr Cosgrave’s second area of concern is the use of NPKs where farmers are using organic manure or slurry.


He says: “Typically, where farmers are using organic manure or slurry they would use a nitrogen product or nitrogen and sulphur to balance the nutrient input. This isn’t necessarily best practice as it assumes the organic nutrients in the slurry are going to become available when the plant needs them, and unfortunately that’s not the case.


“There is a great deal of research which shows there is poor correlation between the soil index and dry matter yields. This is because nutrient availability varies according to soil characteristics.


“It’s not just whether you have a light soil or a heavy soil, but other variables such as soil temperature which can influence the mineralisation of applied manures into plant available nutrients.


All of this adds a layer of complexity when it comes to deciding exactly what is available to the crop.


“By applying an NPK fertiliser as well as the organic manures, you are reducing the risk of not having the P and K you need for the grass and ensuring it is 100% available. The cost of providing this ‘insurance policy’ compared to using straight nitrogen or nitrogen and sulphur is minimal.


“Trials and farmer experience have proven the benefits of using NPKs to insure against the unpredictable nature of slurries and if we could measure grass yields routinely we would see this.”


Health benefits from selenium


Selenium is clearly an important element with benefits in the important area of reproductive health, during the dry period, for the cow’s immune system and also for the vigour of calves. An option now available to farmers is to use a fertiliser which has had selenium added which can boost normal grass selenium levels.


Mr Cosgrave says: “A cow requires 0.3mg of selenium for every 1kg of DM consumed. Data from our analytical services shows more than 90% of grass silage samples tested have below 0.1mg/kg. By using a fertiliser with selenium, you are insuring the grass forage component of the diet is meeting more of the cows’ daily requirement with highly effective organic selenium. It’s something which should be considered, particularly in producing dry cow silages.


“Sodium selenate is the selenium source which Yara uses to fortify its products, because it’s proven to be a fast and reliable means of increasing the selenium levels in grass”


Sources of nitrogen


His final point is with regards to different forms of nitrogen.


He says: “What we’ve found with regards to ammonium nitrate and calcium ammonium nitrate is that they are consistent in delivering yields in comparison to urea, particularly in second cut silages. This makes sense because we know urea is prone to volatilisation and applications to second cuts tends to be at warmer and dryer periods.


“For first cut silage we need to look at optimum nitrogen application rates of between 120 and 130kg/ha. Some people might be tempted to use lower application rates, but there is a clear positive correlation between optimum nitrogen rates and protein in grass. By optimising your nitrogen application rates, you are optimising the protein levels in your silage.


“Furthermore, depending on how you manage the crop, it is possible to achieve both quality and quantity. It shouldn’t be a matter of balancing one with the other. Regardless of weather conditions if you get the management right you will make better silage. “It’s a matter of managing the critical areas and continually monitoring the crop, rather than just setting a date for cutting and aiming for it. It’s important to continually assess and walk the fields, exactly as you would with an arable crop.”

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