WET soils have reduced grass growth and can have lower load-bearing capacity and unavoidable grazing damage. This can reduce grass production and utilisation by up to 50%.
On north Kerry farms over the last 10 years annual rainfall has ranged between 1,000 mm and 1,336mm. Wet years cause higher costs and lower milk/liveweight sales due to lower intakes of grass.
Difficult grazing conditions increase the length of time that livestock need to be fed indoors, on relatively expensive silage and concentrates. This also negatively impacts on animal performance . After turnout, difficult grazing conditions and wet grass negatively impact on animal performance.
Wet conditions impede silage making, delaying harvest and generally result in poorer nutritive value silage.
Wet soils also increase the incidence of fluke, worms and other diseases, even where good control procedures are in place.
The problem of wet down into the ground, moves underground and comes to the surface in low-lying areas of the farm as springs. A system of deep (2m - 3m) underground drains need to be put in place to intercept the rising water.
The low permeability of heavy soils remains a problem, particularly under high rainfall. Drainage to alleviate soil wetness promotes deeper rooting, allowing the sward to draw nutrients from a deeper volume of soil, which improves sward productivity. It also improves load bearing capacity.
Soils with drainage problems account for 40% of soils in Ireland.
Before any drains are installed trial holes of 2m â?“ 3m deep should be dug to investigate the problem.
In many situations a farmer can have both a rising water table and impervious subsoil.the solution would involve both a system of deep drains and shallow drains. This situation can be overcome by having shallow drains with deep sumps at regular intervals along the drain.
Most drainage solutions involve a combination of stone filled drains and either mole drains or gravel filled moles. The process of mole ploughing loosens up the soil, which improves the rate of percolation through the soil and the channel formed by the foot of the mole plough provides a route for the water to exit the soil into collector drains. The moles or gravel-filled moles are usually 40 â?“ 55cm.deep at 1m â?“ 2m spacing and the collector drains are usually 0.8 â?“ 1m deep at 10m â?“ 20m spacing.
Mole drains should last for three/four years and with the advent of fourwheel drive tractors they can be done often at relatively low cost. Gravel filled moles are much more expensive to install but should last for many years if installed in good weather conditions.the gravel in gravel filled moles should be 10mm â?“ 20mm.
Another issue is whether or not it is beneficial to include a corrugated plastic drainage pipe in the stone filled trench. Plastic pipes and the gravel used to surround them in the bottom of the trench are expensive. It is becoming increasingly common practice on farms not to include them and simply fill the drain with stone. The stone in the collector drains can be limestone,sandstone or good quality hard shale and this should be brought up to the level of the topsoil.the collector drains should have an even slope to prevent silt build up and should be marked where they drain into the watercourse.