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Nitrate losses still weather dependent

18 May 2001
Nitrate losses still weather dependent

By Andrew Blake

COVER crops, minimal cultivations and early establishment do help reduce nitrate leaching into drainage water, a 10-year research project concludes.

But nothing is foolproof when it comes to reducing the risk, admits ADAS scientist Mark Shepherd who was involved in the 500,000 project.

If it starts raining in the autumn and then, like last year, doesnt stop there is nothing much you can do to avoid leaching, said Dr Shepherd.

Along with colleague Paddy Johnson, he monitored levels at Peter Smiths farm at Oasby, Lincolnshire for the Ministry of Agriculture funded research.

The experiment, within the Aswarby Nitrate Sensitive Area, was set up to explore ways of controlling nitrate losses when NSAs first came in.

Three management systems — standard, protective and intermediate — were tested over two complete five-course all arable rotations.

These included peas, winter milling wheat, winter barley, winter oilseed rape and winter feed wheat on shallow soil over limestone.

The standard approach involved ploughing before all crops.

The protective policy introduced minimal cultivations and cover crops were used to mop up residual nitrogen before spring sowings.

Bridging the gap between these two schemes was the intermediate system explained Dr Shepherd.

In both five-year periods we showed we could reduce nitrate losses by over 40%, but there are pitfalls, he said.

The average annual nitrogen loss in drainage water under the standard approach was about 56kg/ha (45 units/acre).

Intermediate measures trimmed that to about 40kg/ha (32 units/acre), and the protective policy cut the figure to about 30kg/ha (24 units/acre).

A key message is that whichever management system was used it had no effect on the yields of peas or of milling wheat after peas.

It also looks as though minimal cultivations are useful in reducing N leaching, said Dr Shepherd.

But where we used them we eventually got a build up of grass weeds and ran into problems.

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Nitrate losses still weather dependent

18 May 2001

Nitrate losses still weather dependent

By Andrew Blake

THERE are no guaranteed ways to cut nitrate leaching. But cover crops, minimal cultivations and early establishment do help reduce losses into drainage water, a decade of MAFF-funded work costing £0.5m confirms.

"Nothing is foolproof when it comes to reducing the risk of nitrate leaching," says Mark Shepherd, ADAS scientist involved with colleague Paddy Johnson in monitoring a 10-year experiment on Peter Smiths farm at Oasby, Lincs.

"If it starts raining in the autumn and then, like last year, doesnt stop there is nothing much you can do to avoid leaching."

The experiment, within the Aswarby Nitrate Sensit-ive Area, and near the southern boundary of the Lincs Nitrate Vulnerable Zone, was set up to explore ways of controlling nitrate losses when NSAs first came in.

Three management systems – standard, protective and intermediate – were tested over two complete five-course all arable rotations of peas, winter milling wheat, winter barley, winter oilseed rape and winter feed wheat on shallow soil over limestone.

The standard approach involved ploughing before all crops. The protective policy introduced minimal cultivations as well as cover crops to mop up residual nitrogen before spring sowings. The intermediate system was a compromise between the two extremes, explains Dr Shepherd.

"In both five-year periods we showed we could reduce nitrate losses by over 40%, but there are pitfalls."

The average annual nitrogen loss in drainage water under the standard approach was about 56kg/ha (45 units/acre). Intermediate measures trimmed that to about 40kg/ha (32 units/acre), and the protective policy cut the figure to about 30kg/ha (24 units/acre).

A key message is that whichever management system was used it had no effect on the yields of peas or of milling wheat after peas.

"It also looks as though minimal cultivations are useful in reducing N leaching. But where we used them we eventually got a build up of grass weeds and ran into problems."

In the first five-year period oilseed rape output was best under the standard ploughed approach. But the picture was reversed in the second spell where the protective minimally cultivated method gave best yields.

After some very heavy September rainfalls during the first rotation, autumn N was delayed until the crop was established to reduce losses.

"That offset the negative effects of minimum cultivation. Although there is no really good reason why the actual yields should have been better, the extra N did help get the roots down." That in turn helps mop up any residual N, he adds.

In the initial stages, rye was sown as the over-winter cover crop ahead of peas. "But it was expensive, so using the experience of NSA farmers we changed to cheaper winter barley to supplement existing cereal volunteers.

"In our work cover crops were the most effective method of controlling leaching. They do increase the cost of growing spring crops, which is maybe why they are not mandatory in the NVZs." &#42

ADAS scientist Mark Shepherd says 10 years of trials have shown how the weather has the final say on nitrate leaching.

Mean yields in Oasby NSA experiment 1996-99 (t/ha)


Husbandry system

Standard Intermediate Protective

Peas 4.10 4.46 4.30

W wheat 8.49 9.21 8.86

(milling)

W barley 7.34 7.20 5.78

W OSR 2.87 3.13 3.35

W wheat 9.27 10.02 7.47

(feed)

Harvest 2000 results omitted because no overall nitrogen applied.

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