H.F.M. Aarts

An evaluation of whole-farm nitrogen balances and related indices for efficient nitrogen use

Quantification of nitrogen (N) flows creates awareness among farmers, can help them to re-evaluate N management and may reduce nitrate loss to groundwater. Hence, whole-farm balances play a crucial role in legislation on N management in Netherlands. This paper reviews the strengths and weaknesses of whole-farm balances for assessment of the environmental impact of agriculture. The usefulness and reliability of a balance strongly depends on its completeness. The surplus per unit area indicates the environmental impact, provided that all relevant terms are included. However, the surplus per unit area, the surplus per unit output and the output per unit input, as derived from the balance, may not represent accurate indicators of the operational management skills of a farmer, as these estimates not only depend on the conversion of N within the farm, but also on the extent to which the farm relies on animal feed produced outside the farm and the extent to which processing of crops takes place outside the farm. Without additional information on the processes underlying the whole-farm level and N fluxes at spatial scales above the level of an individual farm, whole-farm balances do not reveal the nature and magnitude of losses, nor do they provide sufficient clues how to improve the efficiency of N use.

A farm level approach to define successful mitigation strategies for GHG emissions from ruminant livestock systems

Ruminant livestock systems are a significant source of greenhouse gases (GHGs). Thus far, mitigation options for GHG emissions mainly focused on a single gas, and are treated as isolated activities. The present paper proposes a framework for a farm level approach for the full accounting of GHG emissions. The methodology accounts for the relevant direct and indirect emissions of methane, nitrous oxide and carbon dioxide, including carbon sequestration. Furthermore, the potential trade-off with ammonia volatilisation and nitrate leaching are taken into account. A ruminant livestock farm is represented with a conceptual model consisting of five pools: animal, manure, soil, crop and feed. The carbon and nitrogen inputs, throughputs and outputs are described, and the direct emissions are related to the carbon and nitrogen flows. The indirect emissions included in the methodology are mainly carbon dioxide emissions from energy use and nitrous oxide emissions related to imported resources and nitrogen losses. The whole farm approach is illustrated with a case of two dairy farms with contrasting livestock density and grassland management. It is shown that the inclusion of carbon sequestration and all indirect emissions have a major impact on the GHG budget of the farm. For one farm, the effect of four mitigation options on the GHG emissions was quantified. It was concluded that a whole farm approach of full accounting contributes to a better insight in the interactions between the carbon and nitrogen flows and the resulting emissions, within and outside the farm boundaries. Consequently, the methodology can be used to develop efficient and effective mitigation strategies.

Nitrogen (N) management in the ‘De Marke’ dairy farming system

In the sandy regions of The Netherlands, high losses of N from intensified dairy farms are threatening the environment. Therefore, government defined decreasing maximum levy-free N surplusses for the period 1998–2008. On most dairy farms, the current N surplus has to be reduced by half at least. Farmers fear that realizing these surplusses will be expensive, because it limits application of animal manure, which then has to be exported or additional land has to be bought. Moreover, farmers are worried about the impact on soil fertility. To explore the possibilities for reducing surplusses of average intensive farms by improved nutrient management, farming systems research is carried out at prototype farm ‘De Marke’. Results are compared with results of a commercial farm in the mid-1980s, the moment that systems research started and introduction of the milk quota system put a halt to further intensification. Results indicate that average intensive farms can realise a reduction in N surplus to a level below the defined final maximum, without the need to buy land or to export slurry. Inputs of N in purchased feeds and fertilisers decreased by 56 and 78%, respectively. Important factors are reduced feed intake per unit milk, as a result of a higher milk yield per cow, less young stock and judicious feeding, an improved utilization of ‘home-made’ manure and a considered balance between the grassland and maize area. Changed soil fertility status did not constrain crop production. Nitrate concentration in the upper groundwater decreased from 200 to 50 mg l-1, within a few years.

Soil-plant-animal relations in nutrient cycling: the case of dairy farming system 'De Marke'.

Forage and ruminant production in Western Europe have increased significantly since World War II. However, in the last decade the livestock production sector has come under increasing pressure as the European Union introduced the milk quota system, effectively curbing total national and individual farm production volume, and national governments increasingly took measures to reduce the losses of nutrients from these systems to the environment. In the late 1980’s in the Netherlands a project was initiated with the objective to design, test and further develop a farming system that can serve as a starting point for the development of dairy farms on dry sandy soils with average milk production (system ‘De Marke’) using the method of ‘prototyping’. First, a number of farming systems have been identified that, in theory, meet the formulated objectives. From this theoretically acceptable set one of the technically and economically most attractive and, from a research point of view, most interesting was implemented at the experimental farm ‘De Marke’ in 1992. The functioning of this system is monitored in quantitative terms by measuring flows of dry matter and nutrients. The results sofar suggest that on dry sandy soils strict environmental standards for losses of nitrogen and accumulation of phosphorus can be attained within a short time while maintaining the current milk quota. Grass, maize and fodder beets could be grown in such a way that the norms for nitrate losses could be met. Improved utilization of animal manure and lower manuring levels allowed a reduction in the use of fertilizer-N of 74% compared to current practice. The input of P in feed and the output in milk and meat could be more or less balanced: P-surplus was only 18% of that on the current farm. To obtain reliable results the research will be continued for a longer period of time because the system has to stabilize, the soil nutrient and organic matter stores and soil fertility react slowly to changes in management, and weather conditions are always variable.

Phosphorus (P) management in the 'De Marke' dairy farming system.

In the sandy regions of the Netherlands water quality is threatened by high losses of nutrients from intensive dairy farms. About 67% (32 kg ha-1yr-1) of farm inputs of P in purchased feeds and fertilisers do not leave in milk or cattle. The Dutch government defined decreasing maximum permitted nutrient surplusses for the period 1998–2008, at 9 kg ha-1yr-1 for P. Farmers suppose that reducing the surplusses will be costly, because it limits application of slurry, which then has to be either exported or additional land has to be purchased. Moreover, farmers are worried about the impact on soil fertility and crop growth. To explore the possibilities of reducing surplusses by improved management, farming systems research is carried out at prototype farm ‘De Marke’. Results indicate that average intensive dairy farms can reduce P surplus sufficiently, without the need to buy land or to export slurry. Key factors are reductions in purchased feeds (by reduced needs per kg milk as a result of a higher milk yield per cow, less young stock and judicious feeding) and fertilisers (by improved management of ‘home-made’ manure and an increased maize area). Initially, P fertility status of the fields of ‘De Marke’ decreased, but stabilised in the seventh year at a level not restrictive to crop production.