Arbejdsrapport fra Miljøstyrelsen, 5/2006 – Miljøvurdering af konventionel og økologisk avl af grøntsager

Miljøvurdering af konventionel og økologisk avl af grøntsager

Summary and conclusions

Information about the environmental impact of consumer goods is being increasingly demanded both by public authorities and consumers and among producers, distributors and retailers in the production chain. Food is an important component of the overall environmental impact on a family, and earlier assessments have demonstrated large differences in the environmental impact generated per kg product of different foods (see, for example, www.lcafood.dk). This is both because different products, such as milk and potatoes, obviously require different production processes, and because a particular product can be produced and processed in several different ways. Documentation of a product's environmental impact is expected to become one of the competitive parametres in the marketing of a product in the future.

The use of life cycle assessments (LCA) to document the environmental impact of Danish vegetables is therefore an important contribution to this end. LCAs are used to calculate the overall resource use in a production flow and the associated emissions to the environment per kg product retailed for a manageable number of environmental impact categories.

This report presents the LCA of greenhouse-grown tomatoes and cucumbers and field-grown carrots and onions, based on information from a selection of growers in combination with information from the Danish Association of Horticultural Producers and expert statements. Models have thus been produced for the conventional production of tomatoes and cucumbers in a) modern greenhouses with combined heat and power facilities, b) on mineral wool mats, and c) with and without recycling of the nutrient solution. A model has also been produced for organic tomato greenhouse production with a similar heat input, but with compost added to the soil.

The combined heat and power production in a greenhouse plant is relatively efficient, and also benefits from the fact that with the assistance of a catalyser the CO₂ from the flue gas can be used for CO₂ fertilisation, which apparently increases plant production. Considerable fossil fuel consumption is, however, required to maintain the temperature in the greenhouse at the start of the season, which results in greenhouse gas emissions of more than 3 kg CO₂ equivalents per kilo conventional tomatoes and nearly 5 kg CO₂ equivalents per kg organic tomatoes.

In a conventional tomato and cucumber production more than 50 per cent of the standard Nitrogen and Phosphorus fed to plants is lost if the drainage water is not recycled. Information on how much this loss could be reduced if recycling facilities were installed, is very limited. A few results from growers and model calculations show an increase from 40 to 66 per cent of the N and P use efficiency by tomatoes when feeding water was recycled. The nutrient loss from organic tomatoes was lower from conventional production, but probably at the level of conventional systems with recycling of water. The lower yield and relatively higher energy consumption per tomato results in a nutrient load per kilo organic tomato that is higher or on par with the conventional production, depending on whether the leaching from the animal manure-based compost is attributed to the tomatoes or to the livestock production on which the compost is based.

Model comparisons were made for conventional and organic carrots for late harvesting and storage in either cold store or under straw matting in the field. The consumption of diesel in carrot production is far larger than for most other agricultural crops. This is mainly due to the distances involved between production sites and packing house – a consequence of growers having to rotate their carrot sites to ensure a cropping interval of at least five years. A comparison of carrots kept under straw and in cold store until 15 January revealed that greenhouse gas emissions are lower when carrots are covered by straw, despite the relatively large fossil fuel consumption involved in the pressing, transporting and spreading of straw. The nutrient load was similar for the two storage methods, but straw covering made the largest contribution to acidification and photochemical smog.

Generally, conventional carrot production resulted in smaller emissions of environmentally harmful substances than organic production, although the models do not include the use of pesticides. This was due to methodological problems. The lack of pesticides in the environmental assessment should be considered if comparing the two systems, because of the potential environmental advantage offered in the organic system in this aspect, especially for carrots and onions. Two strategies were used in the modelling of organic carrot production. One strategy included a large application of animal manure (50 t/ha) and yields of 53 t/ha, and a second strategy included a smaller application of animal manure (25 t/ha) and yields of 40 t/ha. The comparison shows that the low-yield strategy had the smallest nutrient load per kg carrot, but the largest contribution to the greenhouse effect and photochemical smog.

Conventionally produced onions contributed more to the greenhouse effect, acidification and nutrient load than carrots compared per kg product, which is due partly to the large consumption of energy during the drying and packing stages. The environmental impact of field-grown vegetables is generally lower than other – particularly livestock – products. Greenhouse productions of vegetables produce as much CO₂ per kg product as the production of one kg pork. Comparisons of a number of different typical dinner meals for a Danish family demonstrate that reducing meat consumption from around 200 gram per person to 100, and increasing vegetable consumption, could reduce the total environmental load from the meal by 25-30 per cent.