Nitrogen fertiliser use, rotations and soil management are key to breaking through the wheat yield barrier, according to NIAB TAG. Andrew Blake reports

Growers seeking a magic bullet to boost crop output are bound to be disappointed. But closer attention to the wide range of yield limiting factors operating over several seasons should help them make progress.

That has been the underlying message from two researchers at NIAB TAG’S open days this summer.

Stuart Knight points out that the national average wheat yield, which rose rapidly from 1980 to the mid-1990s, has since stagnated. “It’s interesting that changes in EU policies and the dramatic falls in crop prices in the mid-1990s coincided with the start of that stagnation,” he says.

The oilseed rape picture is rather different, complicated by the switch from single to double low varieties. “But in the last five years we’ve seen consistent improvement with the highest ever national average of 3.9t/ha in 2011.”

Many factors, including changes in varieties and weather, affect yields, but an analysis, based on Recommended List and National List trials has suggested that 90% of the improvement in yield between 1982 and 2007 in winter wheat, barley and oilseed rape came through access to better varieties, says Mr Knight.

“That contrasts with sugar beet and maize in which the yield improvement has come equally from varieties and agronomy combined with the environment.”

So how might growers do better? Mr Knight suggests three key areas merit attention: nitrogen fertiliser use, rotations and soil management.

Analysis of 54 NIAB TAG winter wheat N experiments between 1997 and 2009 on long-term arable soils following cereals or break crops (including sugar beet) showed the average optimum dose was about 240kg/ha of N for a break-even ratio (N to grain price) of 3:1.

The average RB209 recommendation for most of that period (based on the Field Assessment Method) was only 190kg/ha of N – some 50kg/ha of N below the economic optimum, he notes.

“Not all growers would have based their N usage on RB209 which has since been revised (noe the DEFRA Fertiliser Manual); however, some loss of yield in the late 1990s and 2000s due to under-fertilisation with N seems likely.”

A survey of 228 oilseed rape crops grown by NIAB TAG Network members in 2009-10 revealed a strong link between N fertiliser use and yield, he adds.

“Fields receiving between 150 and 179kg/ha opf N averaged seed yields of 3.76 t/ha, those receiving between 180 and 209kg/ha of N averaged 3.96 t/ha and fields receiving between 210 and 239kg/ha of N averaged 4.12 t/ha.”

Mr Knight says the trend to tighter oilseed rape rotations has almost certainly checked yields, and the extent to which trafficking and deep soil compaction have limited national average output is unclear. “It needs further investigation.” In one study at Morley in Norfolk a zero-traffic approach produced a 14% yield gain, he reports.

One way to boost yields while lowering energy costs and pollution risks may be to employ cover crops and soil amendments such as green waste compost, believes Ron Stobart.

Under its New Farming Systems project NIAB TAG is comparing the impact of three cover crop types – clover, fodder radish and a legume species mix – within a range of rotations including spring and winter crops and a mixture of both. The trials are overlaid with three nitrogen strategies: zero N plus 50% and 100% of the standard N dose.

The highest average yield response – about 9% – was linked to using the legume species mix.

Overall, in 2009 and 2010, cover crops raised the margins over N by £70-80/ha compared with the standard practice, notes Mr Stobart. “The average was £77/ha, similar to current seed and establishment costs.

“Unfortunately for now that only covers their costs. But several factors may eventually make them more attractive.”

New techniques could make cover crops cheaper and their benefits might increase over time. Fertiliser costs will probably increase, and extra income from cover crops might come via environmental schemes, he explains.

Four years’ work exploring the effects of applying 35t/ha a year of green waste compost have confirmed that it lifts organic content of the soil and eases its water absorption. “In most seasons it’s also improved yields – generally by about 7%.”

When it comes to managing the soil, seven years’ work examining rotations and cultivation practices under the STAR project, Mr Stobart says it is encouraging that the so-called “managed approach” has produced the highest margins.

Decisions based on that approach depend on the weather, soil conditions, cropping, weed burden, soil assessments and what is regarded as best local practice. “The techniques have ranged from single-pass drilling through to ploughing,” he says.