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Increasing grass yield by draining land in the right areas and right way

The decision to drain a particular field or area of the farm will incur substantial capital investment. It should, therefore, be based on the potential return on the investment. What are the benefits of improved drainage to a grassland farmer? Tom Coll, Teagasc Drystock Advisor has the answers

The decision to drain a particular field or area of the farm will incur substantial capital investment. The decision therefore, should be based on the potential for a return on the investment. What are the benefits of improved drainage to a grassland farmer especially to the farmer in the west of Ireland on difficult land in a high rainfall area?  Primarily, land drainage can increase yield and lower production costs and can result in an extended grazing season. Surface damage by livestock can be reduced and trafficability/accessibility for machinery improved. Drainage should result in a lower reliance on supplementary feedstuffs, reduced disease risk to livestock and increase the utilisation and availability of N in soil.

Return on investment: Draining silage ground

To generate a return on investment the areas of the farm with best potential for improvement and production should be targeted. Targeting the silage area on the farm for land drainage and reseeding, where required, can in the long term substantially reduce production costs on drystock farms. Growing silage on the driest well drained fields on the farm facilitates the making of good quality silage and the ability to extend the grazing season. Silage fields are usually the areas that can be targeted for grazing at the shoulders of the year. Where these fields have been properly drained, reseeded and subsequently well managed the ryegrass content is normally quite high. This will result in more grass available for grazing in spring and autumn and extended grazing can be achieved due to better underfoot conditions and driers fields. When silage ground is grazed in spring yield may be slightly reduced however quality can be increased dramatically when dead leaf material is removed from the sward. Land drainage can also facilitate the targeted early application of slurry to silage ground which can be a quite a substantial saving on chemical fertiliser requirements and also improve yields.

Ground conditions often dictate how early silage can be cut.                                                                          This is evident in the range of cutting dates as we move from the south east to the north west of the country.

In general the earlier the cutting date the better the silage quality.

To compare a 75 DMD silage to a more common 67 DMD silage found on many drystock farms this equates to an additional 360 kgs of concentrates or €100 required for the average weanling over a 120 day housing period to achieve 0.6kgs of growth rate per day. Every day at grass in spring will give almost twice the equivalent winter weight gain and a saving of almost €1 per day per weanling where silage is below average quality. Land drainage must result in a longer grazing season and lead to a substantial improvement in silage quality to be cost effective. It should also result in higher yields so that a lot more grass is produced from the area drained to justify the investment.

Increased stock-carrying capacity

Targeting the areas on the farm with the best potential for production can also save on drainage costs. Some fields may only require a small bit of remedial drainage work and not a full scale drainage plan. For many lower to medium-stocked farms improving the drainage and yields on a proportion of the farm will increase its stock carrying capacity. The same animals can now be carried on a lesser well managed area at a higher stocking rate. This can free up less productive areas on the farm for forestry or future environmental schemes such as peatland rewetting and add to overall farm income. Drier more aerated soils have less losses as result of denitrification and also have an increased rate of nitrogen mineralisation increasing the availability of soil organic nitrogen. Well-limed soils at optimum pH have the potential to release around 150kgs of nitrogen per Ha per year for plant growth. Drainage can also increase the utilisation and availability of nitrogen in the soil. The losses from surface runoff can be reduced by reducing soil compaction as part of the drainage plan.

Drainage Systems and Design

Two types of drainage system exist: a groundwater drainage system and a shallow drainage system.

The Teagasc Land Drainage Manual (PDF) outlines the planning, implementation and costs associated with land drainage. In summary it states that no drainage work should be carried out before the drainage characteristics of the soil are established by a site and soil test pit investigation.

  1. Two types of drainage system exist: a groundwater drainage system and a shallow drainage system.
  2. The design of the system depends entirely on the drainage characteristics of the soil.
  3. Distinguishing between the two types of drainage systems essentially comes down to whether or not a permeable layer is present (at a workable depth) that will allow the flow of water with relative ease.
  • If such a layer is evident, a piped drain system at that depth is likely to be effective.
  • If no such layer is found during soil test pit investigations, it will be necessary to improve the drainage capacity of the soil.

Absence of permeable layer = groundwater drainage system

This involves a disruption technique such as moling, gravel moling or subsoiling in tandem with collector drains. Drains are not effective unless they are placed in a free draining soil layer or complimentary measures (mole drainage, subsoiling) are used to improve soil drainage capacity. If water is not moving through the soil in one or other of these two ways, the water table will not be lowered. Outfall level must not dictate the drainage system depth.

Permeable Layer present = shallow drainage system

If a free draining layer is present, it must be utilised.  Drain pipes should always be used for drains longer than 30 m. If these get blocked it is a drainage stone and not a drainage pipe issue. Drainage stone should not be filled to the top of the field trench except for very limited conditions (the bottom of an obvious hollow). Otherwise it is an extremely expensive way of collecting little water. Most of the stone being used for land drainage today is too big. Clean aggregate in the 10–40 mm (0.4 to 1.5 inch approx) grading band should be used. Generally you get what you pay for. Subsoiling is not effective unless a shallow impermeable layer is being broken or field drains have been installed prior to the operation. Otherwise it will not have any long-term effect and may do more harm than good.


Most land drainage systems are poorly maintained. Open drains should be clean and as deep as possible and field drains feeding into them should be regularly rodded or jetted. In a lot of cases the blockage may be not that far from the outlet and may be as a result of a smearing or pipe bending at the outlet. This sometimes occurs when maintenance works to exiting open drains are being carried out.

The cost of drainage can range from €400 to €1400 per acre for a shallow drainage system depending on the particular site and in most cases there are additional reseeding costs. Long term this cost even though substantial can be cost effective when compared to land purchase or rental costs. However, it must results in cost savings and increased production from the area drained.

Prior to undertaking any on-farm drainage, the Environmental Impact Assessment Regulations guide for farmers Here (doc) should be consulted. The EIA Summary Information Leaflet can be read Here (PDF) .

If you intend to undertake land drainage works that (a) exceed 15 hectares, (b) the works are to be carried out within (or may effect) a proposed NHA or a nature reserve or (c) the proposed works may have a significant effect on the environment, screening by DAFM is required.