The recent publication by HGCA of its new sulphur information sheet has underlined the importance of sulphur and its status as a key nutrient. HGCA also took the opportunity to highlight what it described as the strong evidence from the 2012 HGCA/Defra yield plateau project that S application in many wheat and oilseed rape crops could be improved.
Most farmers are aware that free sulphur deposits from the atmosphere have declined significantly, comments GrowHow UK arable agronomist, Allison Grundy (left). Indeed this latest HGCA document confirms that current deposition is only around 10 per cent of 1980s levels. Deficiency is now so widespread that our advice for the past six or seven years has been to apply sulphur routinely, unless you have good cause to believe your soil is sulphur sufficient.
This is particularly true for oilseed rape,which was the first widely grown crop to exhibit signs of sulphur deficiency. With current sulphur deposition virtually at zero over much of the country, there has been a feeling among many advisers that sulphur rates may need to be increased for OSR, especially where modern high yield potential hybrid varieties are being grown. Three seasons ago GrowHow set out to test this theory, working with seed breeders Monsanto and commissioning ADAS to conduct the research.
Growers dont need me to remind them that the past three seasons have been pretty challenging, says Allison. The bad weather limited yields and even meant that some of the trials sites had to be abandoned. This made it difficult to comprehensively test the theory that higher yield might require higher sulphur rates. However farming is full of such challenges and the results we have to date do provide some guidance. We have found that optimum S rates are in line with current RB209 and GrowHow recommendations of between 50 and 120kgSO3/ha.
Response to sulphur Two varieties were grown, DK Expower and DK Extrovert. The experiment was designed to compare the crops response to sulphur with a range of rates from zero up to 150kg SO3/ha and at two N rates. The lower N rate was sufficient to produce a 3.5t/ha crop with an additional 90kg N/ha for the higher rate, targeting a 5t/ha crop.
For harvest 2013, the average yield at Rosemaund was a disappointing 2.71t/ha and the optimum S rate for both varieties was 82kg SO3/ha at the high N rate. Given that the yields at Rosemaund did not hit the target of 5t/ha it is surprising how high the optimum S rate was, she adds. It is possible therefore that the optimum S rate would have been greater at this site if yields had been higher.
Previously at Rosemaund, an optimum sulphur rate of 112kg SO3/ha was measured. This is precisely the amount of S03 delivered from the standard recommended rate of 3 bags/ha of DoubleTop. It increased yield from 3.93 to 4.1t/ha. So a 25/haspend on sulphur delivered an extra 70/ha worth of yield.
Sulphur rates did not affect oilcontent but increasing S significantly increased seed glucosinolates from 13.616.3 umol/g in response to the highest rate of 150kgSO3/ha. While this is still less than the 35 umol/kg allowed in rape meal, it is clear that any benefits from increases in S rates must be considered in the context of the effects on glucosinolate concentration, says Allison.
But it is not just S that is important in OSR. Earlier work, again by ADAS, has shown that it is essential to manage the OSR canopy. This holds true whether you are growing a conventional variety, a so-called low biomass variety or a hybrid, Allison says.
The trials compared conventional N management, as defined in RB209, with a canopy managed approach. The key conclusion was that managing nitrogen inputs to target a Green Area Index (GAI) of 3.5 by flowering increases oilseedrape yields by up to 0.36t/ha.
The biggest response came from delaying nitrogen in situations where the crop would have produced an over-large and lodging-prone canopy in other words, the bigger the crop in February, the greater the benefit from delaying N applications. Producing a smaller canopy enables more light to penetrate the crop thus increasing photosynthesis and producing more seeds per podand more pods per square metre.
The conventional RB209 advice isto apply N in two splits, the first in late February/early March and the second at the end of March/early April.
Under the managed approach the fertiliser required to achieve the optimum canopy size should be applied at the start of stem extension (the second conventional timing). The final split goes on at early yellow bud to early flowering with the rate adjusted to the yield potential of the crop. Allison explains; For every 0.5t/ha of anticipated yield above 3.5t/ha an extra 30kg N/ha should be applied.