Earlier scientific work emphasises that agricultural practise contributes significantly to the global nutrient cycle, eutrophication and N release to soil and atmosphere (Jarvis, 1993).
Although the nutrient input with fertilizer and feed into agricultural farms in kilogram per hectare and year has increased crucially during the past decades, the nutrient use efficiency still remains low on a global scale (Bussink and Oenema, 1998). At the same time, the total annual amount of nutrients leached and volatilized is considerably high on intensively managed farms. N use efficiencies have been calculated in different ways, but the overall conclusion is that imported as well as internally produced N is converted into product N only to a little amount (Børsting et al., 2003; Groot et al., 2003; Schröder, 2005). Several studies indicate that a linear relationship exists between the N input on a grassland farm and the calculated surplus (Bleken et al., 2005). Moreover, even if the overall farm nutrient budget is in balance, N turnover from plant biomass to beef and milk and, in parallel, to excreta can be very high and so can be the mobile N fraction that is exposed to leaching and volatilization.
Grassland forage is preferably utilized by ruminants bringing about two implications on nutrient cycle, surplus and handling, and all of it is relevant in precision agriculture. Firstly, ingested N is excreted for the most part and converted into mobile N fractions in faeces and urine. Additionally, recycling of ingested surplus N through blood, lever and saliva is an immanent characteristic of the rumino-hepathic cycle, leading to considerable N excretion rates mainly with urine when N is available in excess. In a two years study with suckler cow herds on permanent grassland, Schellberg et al. (2006) calculated that most of N ingested was excreted as urine, the most labile N fraction on farm. This fraction contributed annually between 20.3 and 27.5 % to the total N in circulation.
Secondly, due to the recycling of excreted N, slurry and farmyard manure are the major fertilizer types on grassland that require special attention in terms of storage, specification of nutrient content, as well as rate and technique of application. On the other hand, chemical-synthetic fertilizers play only a minor role or can even be neglected when the total farm budget is sustained by N2 fixation through legumes and N deposition.
To underpin the importance of internal recycling of nutrients, an outline of the N flow on dairy farms is given in figure 1 for two situations, (i) in grassland regions where arable cropping is not suitable for climatic, topographic, and soil structural reasons and where grassland forage is the only resource for production compared to (ii) regions where arable cropping is practicable and advantageous. The existence of arable crops on a dairy farm is decisive in terms of the unloading of the farm N budget because nutrients in slurry can be converted into marketable products and so be exported in appropriate form. For example, with a cereal crop DM yield of 8 t ha-1 and a grain N content of 2.5 %, the total N export would be 200 kg ha-2. If not marketed but internally converted to milk, the same harvest would substitute the equivalent amount of N in alternatively imported feed concentrates, provided that the energy concentration and related intake and milk production per unit of either feed would be the same. That way, internally converted self-produced feed concentrates can substantially donate to closed nutrient cycling within the dairy farm and to the N discharging of the environment.
On modern dairy farms, slurry is the main source of fertilizer nutrients containing significant amounts of primary and essential nutritive elements. From a total of 107 manure samples, Van Kessel and Reeves (2000) determined 4.5, 1.8, 0.9 and 2.9 kg m-3 of total N, NH4+
, P and K in slurry, respectively. The excellent fertilizer value of slurry on grassland has been confirmed in earlier agricultural studies (e.g. Schröder, 2004). On the other hand, in many experiments nutrients in slurry have been identified as a potential source of losses to the environment, especially if excess amounts are applied beyond the use capacity of the soil or if improperly applied (Newton et al., 2003, Thome et al., 1993). But, even if applied properly, part of the N in slurry circulating across the dairy farm is inevitably lost to the environment, often referred to as unavoidable losses.
Slurry nutrients that are under control either in the cow house or in the container can be more efficiently handled by means of precision agriculture techniques as compared to current conventional management. In Figure 1, four tracks have been identified that allow control.
The advantage of site-specific application is firstly to be seen in the release of a dosage of slurry N that matches the local requirements as precisely as possible. In contrast, the unawareness of site-specific requirements of N, P and K may lead to either local nutrient
Figure 1. Generalized scheme of the nitrogen (N) flow in a dairy system without (upper) and with (lower) cropland. Dashed lines: flow of excretal N, dotted lines: path of N losses. Number 1 – 4 indicate paths of N that can be influenced by near-ground site-specific slurry application on either grassland or cropped land.
deficiency or surplus. Secondly, a side-effect of near-ground precision application techniques is that volatilization of NH3 would be considerably reduced. Thirdly, if local surplus N is minimized by precise application procedure, N losses throughout the growing season that might appear due to soil N saturation will be reduced. It should be mentioned here that not all N flows can be encountered with precision agriculture techniques, although recent work has demonstrated that even faeces and urine spot distribution with cattle grazing can be controlled. Additionally, the correct calculation of aliquots of slurry applied to individual field plots by means of commercial PC software would already improve precision of nutrient management. Here it becomes obvious, that precision agriculture and conventional techniques should be linked and should not be separated from each other.
One approach to handle recycled N on farms in an environmentally friendly way is to calculate precisely the required N substitution after harvest on a per plot basis. With respect to uneven distribution of nutrients within farms, there is a need to monitor soil nutrient content from time to time. However, such approach does currently not consider heterogeneity within fields. The authors therefore hypothesize that site-specific application of nutrients can potentially better adapt to the local demand and so minimize losses. The question of precise management of nutrients on farms is not a new issue, but with site-specific application technology developing rapidly, the spatial dimension is increasingly recognised. Following best practise for nutrient management as recommended by Goulding et al. (2007) (“apply fertilizers evenly, and well away from watercourses”), site-specific slurry application is seen as a significant contribution to environmentally friendly and efficient nutrient use on grassland.