Grazing needs slurry break

2 February 2001

Grazing needs slurry break

By Simon Wragg

PRODUCERS should avoid applying further slurry to grassland designated for grazing or risk contaminating swards and exposing cattle to magnesium deficiency, or grass staggers, warn specialists.

According to Paul Beckwith, soil scientist at Harper Adams University College, Shropshire, grazing needs eight weeks clear of slurry before cattle should be allowed to graze.

"The relatively high amount of potassium in cow slurry can lock-up available magnesium in swards. Ideally, after the start of February slurry ought to go on silage ground.

"If you have switched from applying muck in the back end to going on in the new year due to the poor weather, more nutrients will be available to the crop as less is leached away."

Typically, the level of nitrogen available to grass doubles for cow slurry spread in the new year compared with October applications (see table). "As well as increasing nitrogen availability, a typical slurry application of 50cu m would provide about 150kg/ha of potassium, sufficient to cover much of the grass crops needs, depending on the soil index at the time.

"The greatest effect of more nutrients being made available will be seen on lighter soils. If in doubt do a soil mineral N test. But the level of artificial fertiliser required this season could be reduced; not a bad thing when prices are rising," adds Dr Beckwith.

According to Paul Macer of Kite Consulting, producers should consider getting slurry analysed to assess its value. "There are a number of labs specialising in sampling which can be used.

"Normally, producers can expect 3kg N, 1.2kg P and 3.5kg of K from 1cu m of cow slurry, equivalent to 90gal. But not all that will be available to the grass immediately. N availability is dependent on when it is applied, but expect roughly a third for post-new year applications. About half the P and most of the K will be available," he says.

Producers should avoid spreading more than recommended rates. "When the lagoon is full there can be a temptation to put more on, but it can affect the palatability of grass."

In all areas the risk of run-off must be considered, especially where grassland is adjacent to a watercourse, adds Dr Beckwith. "Soils are still very wet beneath the surface and it does not take much for surface flooding to occur. We are not out of the run-off risk yet and applications need to be kept down despite the cost and time element of using owned or rented machinery. Be especially wary if more rain is forecast." &#42

Grass requires eight weeks clear of slurry before cattle are allowed to graze it.

Slurry value to grassland

kg N/ha N/acre

Oct 20 16

Jan/Feb 50 40

March 60 50

NB: Cow slurry at 6%DM applied at MAFF max rate of 50 cubic m/ha (4500gal/acre)