Second and third cut nutrients

Published 23 May 14

In too many cases, second and third silage cuts will suffer in yield and quality due to poor nutrient management, says Chris Duller, independent soil and grassland management specialist.

P and K requirements

A typical 17t/ha (7t/acre) crop of first cut physically removes 29kg P and 102kg/K/ha; all carried off your field in the plant cell structures and cell contents. Even at a relatively high soil index of 3, there is still a requirement for applied potash (40kg/ha) for second cut. If you fail to provide enough potash, the response to nitrogen will be limited, protein levels will be lower and grass is likely to lose turgidity. Basically, it tends to fall flat, reducing the leaf area capturing sunlight – and upsetting the man on the mower!

Nitrogen rates

As soils warm up, they produce nitrogen through the breakdown of organic matter – a process termed mineralisation. The healthier the soils are (in terms of structure, worm numbers, etc.) the more nitrogen they produce. Second cut nitrogen application needs to reflect this mineralisation and the lower grass growth rates which are typical for the time of year. First cuts in May generally produce around 40% of your annual grass growth and second cuts 30-35%. Nitrogen rates need to come back to around 80-100kg/N/ha (64-80units/acre) – including any slurry N.

Nitrogen products, containing sulphur for second cuts, should be a must for most people. Some high rainfall areas may not find sulphur limiting, but 95% of samples I have seen (even in wet Wales) showed a need for sulphur applications.

Soils can’t store sulphur and light sandy soils are particularly deficient.  25kg/ha (20units/acre) of sulphur as SOȝ is the recommendation in order to ensure that the grass utilises nitrogen efficiently and builds proteins.  Previous research measured over 30% yield improvement on second cuts when using sulphur on light soils.

Slurry use

To reduce leaf contamination, keep slurry rates low (no more than 27m3/ha – 2,500gallons/acre) and ideally use a trailing shoe or injector.  This will widen your window for application.

This time of year, there is a strong economic incentive to use these types of machine to reduce nitrogen losses through ammonia volatilisation – you could easily lose £17/ha - £7/acre worth of nitrogen with a splash plate spreader. 

For more information on how to better assess the fertiliser required for the range of crops you plan to grow, see the Fertiliser manual (RB209)