Bigger sugar beet yields need not mean bigger fertiliser spend growers attending this year’s BBRO winter meetings heard. Abby Kellett reports.
Since the mid 1980s, UK sugar beet yields have nearly doubled from about 40 tonnes per hectare (16t/acre) to 75t/ha (30t/acre), yet the crop’s nitrogen demand has remained constant.
This is according to Prof Debbie Sparkes, Nottingham University, who told a meeting at Newark the current 90-120kg N/ha recommendation for sugar beet crops is as relevant now as it was decades ago, despite the use of modern varieties with greater yield potential.
Prof Sparkes shared results from six years’ worth of trials, all of which supported her assertion growers need not apply more than 120kg N/ha to sugar beet in a single season.
In fact, in 2012, high soil mineral nitrogen levels meant there was no response to nitrogen application at all, she said.
Growers should aim to apply enough nitrogen to enable the canopy to reach a leaf area index (LAI) of 3, but no more.
“Sugar yield is directly related to the amount of light intercepted by the canopy. Realistically, the maximum amount of light the canopy can intercept is about 90 per cent.
“In order to do this, our trials have shown the canopy has to have a LAI of about 3. To achieve the magic LAI, the crop needs 90-120kg N/ha.”
She warned by applying too much N, growers risk suffering a yield penalty.
“If the crop takes up much more than this in early crop growth, more of the plant’s energy is targeted towards leaf production at the expense of root growth, so roots are effectively robbed of nutrients and energy.
“The last thing we want is to build N in the root. You do not want high amino nitrogen levels and we have seen the detrimental effect of partitioning if there is excessive N.”
Even during the latter part of the season, because nitrogen in senescing leaves is recycled there is no need to top up with further N once the optimum canopy size has been reached, she said.
In order to push yields forward, growers need to focus on achieving the optimum canopy size as early as possible.
“We really want to focus on maximising light interception in the early part of the year. Anything you can do to increase light interception, particularly in May and June, is going to promote high yields.”
As a means of accelerating canopy expansion, the British Beet Research Organisation (BBRO) has begun work looking at whether nitrogen placement has an effect on canopy development, something which has already been investigated by Scandinavia’s equivalent organisation, Nordic Sugar.
Nordic Sugar found the yield increase associated with N placement ranged from 0 to 8 per cent, depending largely on soil moisture.
Prof Sparkes said: “In a wet spring, there was often no benefit and in a dry spring there tended to be the greatest benefit as the nutrients were placed in the rooting zone and crops could access nitrogen more easily.”
Applying nitrogen alongside phosphorus, potassium and sodium led to the greatest yield response, however the research flagged up logistical issues in placing numerous products close to the seed.
Seed placement was also found to be an important factor.
“They had to ensure nutrients were not right on top of the seed, but both below and to the side of seed to avoid inhibiting seedling growth and establishment.”
This work has been largely driven by a need to use nitrogen more efficiently as environmental regulations in Scandinavia limit the amount of N growers can apply.
Prof Sparkes said: “Their emphasis is on getting more from less. They are limited by the amount of N they can apply because of environmental regulations, but they have found they can get by using less nitrogen when the seed is placed as uptake is much better.”
In the first year of UK trials, results suggested there was no yield improvement to be had from placing nitrogen alongside seed at drilling.
However, researchers believe high levels of mineralisation caused largely by wet weather in early summer, was likely to have masked the difference between placed nitrogen and non-placed nitrogen and work is set to continue in this area.