Making best use of Slurry

Slurry from housed livestock in N. Ireland contains valuable sources of plant nutrients in the form of Nitrogen, Potash and Phosphate. Recent increases in fertilizer costs has increased the value of  a typical 1500 gallon load of slurry to almost £45.

The method and timing of slurry application are two factors that determine the utilization efficiency of these nutrients by the growing crop, whether grass or arable. Spreading slurry during the growing season, using systems such as the Trailing Shoe can improve the utilization of slurry nitrogen and maintain high rates of grass growth. This is particularly important given the recent substantial rise in the cost of inorganic fertilizer and on heavily stocked farms where fertilizer application levels are limited under the Nitrates Directive.

Recent research at AFBI, Hillsborough has demonstrated that spreading slurry using a Trailing Shoe slurry tanker increases the utilization of slurry nitrogen by up to 25 percent compared to conventional splash plate application systems. The resultant saving in nitrogen fertilizer costs is worth up to £27 per hectare per crop of grass silage, depending on the slurry application rate. Trailing Shoe spreading technology has been adopted as management policy on the Greenmount Campus dairy unit to increase nitrogen utilization from slurry and reduce fertilizer costs. Fertiliser savings in 2008, compared with conventional splash plate slurry application are expected to be over 8 tonnes of 27 percent Nitrogen at a cost of over £2000.

Additional benefits of the technology include:

Research trials at AFBI, Hillsborough indicate yield benefits from trailing shoe slurry application up to three weeks after the silage harvest, without any detrimental impacts on the quality of silage made from the subsequently ensiled grass regrowth. This wider window of opportunity also applies prior to first cut silage in March or early April when farmers would traditionally have been reluctant to apply late slurry applications to silage swards with appreciable grass cover.

Most trial work has been carried out at relatively high application rates, typically 50 m³/ha (4500 gal/acre). With conventional splash plate application, most dairy farmers would restrict application rates to less than 50 percent of these levels between silage cuts. The trailing shoe application system therefore presents heavily stocked farms with the opportunity to apply greater quantities of slurry during the growing season resulting in improved nutrient utilization and savings in fertilizer costs.

Dairy cow slurry has a total NPK analysis of 3.0/1.2/3.5 per m³ of 6 percent dry matter material.

Given the high rate of removal of potash from the soil by heavy crops of grass harvested for silage, the recommended CAFRE practice is to apply the majority of slurry on swards destined for silage harvesting to replace the potash removed by the crop.

Trailing shoe slurry spreading equipment deposits slurry on the soil surface in narrow bands of approximately 50mm width. By comparison, with conventional splash plate spreading the slurry is spread across the complete sward surface. Trailing shoe spreading reduces the exposure of slurry to the air, reducing ammonia volatilization and resulting in the improved slurry nitrogen utilization. This reduction in ammonia volatilization also means a reduction in the level of smell associated with slurry spreading. Combined with the slurry deposition on the soil surface, the reduced visibility and smell from slurry spreading operations should help improve relations between farmers and other rural dwellers!

Research studies to date have proven that the Trailing Shoe slurry spreading technology has many benefits to offer in terms of improving slurry nitrogen utilization and widening the spreading window during the growing season. Ongoing research studies into the technology at AFBI, Hillsborough indicate that repeat applications of slurry by trailing shoe do not have any detrimental impacts sward life and grass quality.

Slurry being spread by Trailing Shoe at Greenmount Campus following first cut silage harvested at the beginning of May 2007