Open access publications in the SLU publication database

Abstract

Among the solid biofuels that can be produced in agriculture, straw is often considered to have the greatest potential. However, there is some uncertainty regarding the quantities available, e.g. due to decreasing straw length as a result of modern plant breeding. This study had four main objectives: a) to investigate the cultivation areas and yields of different straw crops in the counties of Sweden, b) to present straw:grain-ratios that can be used to estimate the quantities of straw available with respect to the varieties used today, c) to estimate the quantities of straw available in each county for fuel purposes after deducting the quantities used in animal husbandry, and d) to investigate the time of crop harvest. As expected, the largest areas of straw crop cultivation were located in the plain counties (Skåne, Västra Götaland, Östergötland, Uppsala and Södermanland). During the period 1989-2008, the cultivation of winter wheat and spring wheat increased in most counties, while the cultivation of winter rye decreased markedly. The cultivation areas of barley and particularly of oats also decreased. The area of oilseed crops decreased considerably during the same period, although it has increased again in recent years. Yields per hectare increased somewhat for most crops, e.g. that of winter wheat increased by 0.5% per year in the county of Skåne. The quantities of straw available for harvest are usually estimated by means of the straw:grain ratio. In comparison to earlier studies, this study showed that straw:grain ratio has declined considerably during recent decades. The composition of a total of 147 samples taken during the harvest seasons 2007-08 in the counties of Skåne, Blekinge, Halland, Västra Götaland and Uppsala in terms of quantity of straw in relation to quantity of grain was as shown in the table below. The figures are weighted with respect to the cultivation areas of different varieties, and they are valid for a stubble height of 20 cm (40 cm for the oilseed crops). It was concluded that the ratios varied greatly depending on variety, conventional/organic cultivation, weather conditions, etc. The straw:grain ratios obtained were used to estimate the quantities of fuel straw in all Swedish counties. The physical quantities were reduced by multiplying them by a harvest coefficient that accounted for restrictions regarding rain during harvest, low soil organic matter contents, etc. The quantities used for fodder and bedding were then deducted. Surplus straw was found in the counties of Skåne, Östergötland, Västra Götaland, Uppsala, Västmanland, Södermanland, Örebro and Stockholm. The total annual surplus was nearly one million tonne, corresponding to 3-4 TWh. The median dates for harvest of winter wheat, winter rye, spring wheat, spring barley and oats were estimated using data from the Swedish objective crop yield surveys for the period 1980-92 and data on yellow ripeness times for the varieties used at that time and today. A deeper analysis was also carried out regarding the starting date and duration of winter wheat harvest in the counties of Malmöhus, Skaraborg and Uppsala. The results showed that on average, the harvest period in e.g. Malmöhus had started on 13 August (the most common date was 3 August), that half the area was combined by 21 August, and that harvesting lasted for about 13 days suitable for combining (these figures were valid for the county as a whole; locally the harvest period, for example, may be considerably shorter).