Field biomasses in Biomass Atlas: Main yields and side stream biomasses
Main yield means the yield of the main product: grain and seed yield, tuber and root yield and forage biomass yield. The main yield can be used for food, forage or bioenergy. “Side stream biomasses” are stem, top or straw biomasses of seed, grain, tuber, bulb or root crops. The amounts of side stream biomasses can be estimated according to main yield and known harvest indices of the crops. Side stream biomasses and biomasses that are not harvested for food or forage (e.g. riparian zone and fallow biomasses, surplus biomasses of grasslands or green manure biomasses) could be collected and used for bioenergy production. Reject liquids from biogas production and ashes from straw burning could be returned to the fields as fertilizer products. Future plans for usage of side stream biomasses include separation of valuable fractions such as proteins from the biomasses.
Utilization of field crop side streams for bioenergy is not common at present. Most straw, stem, top and green manure biomasses are chopped and ploughed in the fields. Some straw biomasses are used as bedding for livestock or as cover material in horticultural soils. Some stem, top and straw biomasses are used as forage and roughage for livestock.
Farms with animal husbandry often secure their forage production by managing grasslands according to the worst scenario yield. In bad years they produce just enough forage, but in good years this strategy results in surplus grass biomasses which cannot be used for forage and may not even be harvested. If farmers were compensated for their inputs by satisfactory prices also for the surplus biomasses, they could be harvested, baled and used for bioenergy.
Qualities of field biomasses
Quality and price of the field crop main yields varies according to the target usage. As a rule, the quality, price and weight by volume of side streams are much lower than those of the main yield. Thus costs of transport to a bioenergy plant are relatively higher for side streams than the main yield. The low weight by volume of side streams also decreases the potential emission savings through relatively higher emissions from harvesting and transport. However, the biggest obstacle to the usage of straw or other side streams for energy in Finland is the autumn climatic conditions. Even if grain or seed could be harvested at moisture levels of under 20%, the straw and stem moisture can be 30-60% at harvest. For preservation of straw, it should have less than 25% moisture before baling.
Grass biomasses are usually moist, and they have to be silaged or dried and baled for preservation. Some field energy crops such as reed canary grass can be harvested dry in the spring. Moist biomasses can be used for fermentation processes for bioenergy, especially together with manure, while dry straw or stem biomasses can be burned with turf in CHP plants. After fermentation the nutrients contained in the biomasses are left in the reject liquid, which can be used as organic fertilization product. Ashes from the CHP plant could be used for fertilization of fields and forests.
Calculation of field crop side stream potential
Cereals and other seed crops: Side stream biomass potential can be estimated according to crop harvest index (HI, main yield/total plant weight). The harvest index of cereals is normally about 0.4-0.5. A good rule of thumb is that cereals produce as much straw as grain yield. After harvest, about one third of the straw or stems is left in the field as stubble, and only 60-70% of the theoretical straw or stem yield can be harvested. This means that about 1700 (barley and rapeseed) to 2800 kg (rye) of dry biomass could be collected per ha/year in Finland. To preserve good soil quality, it is recommended that straw is harvested only once in two years from the same field (Hakala et al. 2016, Biomass & Bioenergy 95: 8-18). Thus, depending on the crop, 900-1500 kg of dry straw or stem biomasses could be collected per ha/year.
Root crops: Tops of some root crops could be harvested for biogas production. For many root crops, the harvest index is known (Hakala et al. 2009, Agricultural and Food Science 18: 347-365). E.g. for sugar beet, 40% of total biomass is top biomass, the moisture content of which is as low as 20% (Juurikassarka 2005, 1: 12). Sugar beet tops could be used for biogas fermentation, but they are also used as forage.
Field crop areas and yield information in Biomass Atlas
The field areas under different crops can be studied in detail in Biomass Atlas. The field area data are retrieved from the statistics of the Agency for Rural Affairs (http://www.mavi.fi/en/Pages/default.aspx). The data can be used e.g. for planning of field contract work, estimating markets for agricultural chemicals or potential usage areas of organic fertilizers (from biogas plants).
For studies of crop main yield and side stream potentials, the crops have been sorted according to crop and side stream type. The main yield data are retrieved from the statistics of Natural Resources Institute Finland (Luke) (http://stat.luke.fi/en/). The data are arranged according to Centres for Economic Development, Transport and the Environment (https://www.ely-keskus.fi/en/web/ely-en/). Yield data for grasses can be found only for hay, silage and pre-wilted silage. Riparian zone, fallow and green manure biomasses are not estimated in the Luke statistics. Estimated yields of these areas in Biomass Atlas are based on average hay yields.
The information on field areas and yields of crops is updated yearly in Biomass Atlas. Information on manure is updated when needed, and information on forest biomasses every five years.