BEST USE ON GRASS IS A PRECISION BUSINESS
Making the most of fertiliser on grassland requires careful management. Emma Penny takes a closer look at the policies on one Dorset farm
MANAGING grassland for 1400 dairy cows and 3000 sheep on two units – one of which receives only 34in (865mm) of rain a year, while the other receives 48in (1220mm) – can be a tricky task.
It is one in which fertiliser plays a key role. So says Richard Snow, farms director for farm management company Velcourt which manages 1460ha (3600 acres) for Ilchester Estates in Dorset.
The farm at Abbotsbury, on the south coast, is the drier of the two, receiving little summer rain. Evershot, which lies 16 miles further north, has heavier soils and gets more rain.
"Fertiliser use on both farms is dictated by a number of factors -soil type, soil analysis, particularly pH, and sward type," explains Mr Snow.
Acidity, which is a concern on the heavy soils at Evershot, will reduce the efficiency of nitrogen use by a quarter, he says. "We soil test every three years, and also test suspect areas such as clay caps and areas round trees more frequently."
Acidic areas are immediately limed, while P and K levels are topped up if the analysis shows indices are falling. Other pre-nitrogen application checks include calibrating the spinning disc fertiliser spreaders – a vital job, says Mr Snow – and testing the fertiliser.
Fertiliser is spot checked in an analytical lab to see whether it meets the stated specification. "Even a percentage drop, from 34.5% to 33.5%, in nitrogen content can make a difference. In the past, some ureas have been lower in N, and we have had to increase rates to compensate," he says.
"We use ammonium nitrate, and avoid imported products. Good quality urea has been shown to be very efficient, but we are using spinning discs, and they are tricky to calibrate for urea."
Better spreading precision is also possible because of the use of tramlines. This also reduces compaction, says Mr Snow. "Compaction leads to water logging, shallow rooting, and nutrients arent used as well."
Silage leys will receive a total of 350kgN/ha (280 units/acre) in splits. The first dressing – about 30-40kgN/ha (24-32 units/acre) – is applied as soon as T-sum 200 is reached and soil temperatures reach 3-5C.
"T-sum 200 generally ties in with soil temperature, but its pointless applying nitrogen when neither the soil nor the grass can use it," he says.
Application timing is site-specific; first nitrogen application at Abbotsbury, which is on the coast could easily be in late January, but its later at Evershot, just 16 miles north.
"Its important to get N on early – within reason – and we use a contractor to ensure we get round all the grassland at the right time."
The main fertiliser dressing is 60-120kgN/ha (48-96 units/acre) depending on the history of the ley, whether it has received slurry and cutting date. "Its vital to avoid high nitrate levels and butyric fermentation in silage, so well work back from the estimated cutting date and ensure all the N will be used by then."
Fresh grass samples are analysed before cutting for sugar and nitrate levels, with a requirement for nitrate levels to be below 800ppm. However, Mr Snow admits that if, for some reason, nitrates are higher than that, its a trade-off between leaving the grass for longer before cutting and the risk of poor weather. "If nitrogen applications are correctly timed, there shouldnt be a problem."
At Evershot, silage leys receive either a third dressing of about 100kgN/ha (80 units/acre) after second cut, or two applications of 60kgN/ha (48 units/acre) after second and third cuts, depending on the season.
"If grass growth potential looks good then well apply a third and fourth application – if its dry, and potential for the fourth cut is poor, then the third application suffices."
But at Abbotsbury, dry weather and poor summer grass growth mean silage leys receive only one application of 120kgN/ ha (96 units/acre) after first cut. "We have little summer rainfall, and so aim to have all fertiliser on by mid-May."
The lack of rain at Abbotsbury means grazing grass runs out of steam during the summer, which influences fertiliser policy says Mr Snow. "While the grass at Evershot receives 60kgN/ha every four to five weeks throughout the season until mid-August, at Evershot, the last application goes on in mid-June."
Thats because he is anxious to avoid large amounts of residual N on grassland to minimise potential environmental concerns.n
Check soil temperature as well as waiting for T-sum 200 to achieve the optimum timing for the first nitrogen dressing, says Velcourts Richard Snow.
MAKING THE MOST OF N
• Test soils every three years.
• Calibrate spreaders regularly.
• Use T-sum and soil temperature.
• Choose quality fertiliser.
• Basal dressing 30-40kgN/ha (24-32 units/acre) at T-sum 200 and soil temperatures 3-5C.
• Main dressing 60-120kgN/ha (48-96 units/acre) depending on ley history and slurry application.
• Silage leys receive either a third application of 100kgN/ha (80 units/acre), or a third and fourth dressing of 60kgN/ha (48 units/acre) depending on growth.
• Grazed grass receives 60kgN/ha (48 units/acre) applied every four to five weeks until mid-August.
Farms near the Dorset coast and 16 miles inland demand very different approaches to grass nutrition for Velcourt farm director Richard Snow.