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English Issue Biogas Journal | Autumn_2017 Grain maize stover – A substrate that inspires hope Cultivating grain maize yields grain maize stover without any extra effort. Because the stover is waste, there are many good reasons to use it as a biogas substrate. But how is maize stover harvested and what kind of yield is produced? Can the substrate actually be silaged? Are reasonable methane yields obtained from fermentation and is using stover worthwhile from an economic perspective? The Bavarian State Research Center for Agriculture (LfL) is answering these questions in a research project extending over several years. The results so far definitely inspire hope. By Monika Fleschhut (M.Sc.) and Martin Strobl (Dipl.-Ing. agr.) Photos: Monika Fleschhut In Germany, the cultivation of grain maize results in 3.8 million tonnes of maize stover dry matter each year, which have not been harvested up to now. Instead, the dry matter remains on the fields for humus production and to return nutrients to the soil. In comparison, 12 to 14 million tonnes of silo maize dry matter are used in German biogas plants. Suitable amounts of maize stover are evidently available, offering true potential for substitution in terms of quantity. As a waste product, the substrate is obtained without using any land area and, as a result, is not associated with any land use competition. Because no effort is required up until harvest time and because the substrate does not compete with any other use, grain maize stover is per se very inexpensive. Whereas the recovery of the stover is often associated with cultivation problems, there are also many advantages: Particularly in crop rotations with a large percentage of grain maize, removing grain stover can make stover management and soil cultivation for the next crop rotation easier, while reducing the risk of infection with Fusarium fungi or European corn borers, for example. If the digestate is spread out again after fermentation in the biogas plant, the cycle is closed, to a large extent. Because maize stover is also not included in the “maize and grain cap” (Sec. 39h, German Renewable Energy Act [EEG] 2017), which limits future use of maize (the entire plant, maize kernel/ spindel mixture, grain maize and husk-cob-meal) and grain to 50 percent by mass – in subsequent years to as low as 44 percent by mass – at least initially, no legal restrictions on using maize stover in biogas plants are expected. 12
Biogas Journal | Autumn_2017 English Issue So overall, there are many good reasons for using maize stover for biogas production. But it only makes sense if the substrate is suitable. Important criteria in this respect include the highest possible harvest amounts and methane yields, suitability as silage and unproblematic fermentation, and finally, consistency with regard to economics. Analyses performed by the Bavarian State Research Center for Agriculture (LfL) In order to specify the amount and quality of the maize stover obtained in the grain harvest, from 2014 through 2016 standardized plant cultivation trials with grain maize was carried out at the Freising location and the yield structure of grain and residual plant parts (= maize stover) was determined. Also tested were the effects of variety selection (four/five varieties) and harvest date (three harvest dates in a period from the beginning of October to the beginning of November). All of the varieties were tested with three repetitions in a block design. The maize stover yields determined in this way indicate “maize stover potential” and are equivalent to the maize stover left after threshing: theoretically, the amount of harvestable maize stover. The amount of stover that can actually be recovered was systematically investigated in practical harvest technology trials carried out over three years at Grub, an LfL experiment station. In addition, on large plots consisting of at least 630 square metres, eight harvesting processing (four types of windrowing technology in combination with two recovery methods) were tested and analysed with four repetitions. The windrowing technology used included a BioChipper (BioG GmbH), a Schwadhäcksler UP-6400 (Uidl Biogas GmbH/Agrinz Technologies GmbH), a Merge Maxx 900/902 (Kuhn S.A.) and a Mais Star* Collect (Carl Geringhoff Vertriebsgesellschaft mbH & Co.KG). The BioChipper and the Schwadhäcksler UP-6400 are modified shredders that have a windrowing function with a working width of 6 and 6.4 m, respectively. After threshing, the maize stubble was mulched and, at the same time, the maize stover is picked up via the air suction produced by the flail shaft, chopped, and deposited to the side into a windrow. With the Merge Maxx, which has a working width of nine metres, the maize stover is also picked up in a separate step, without additional chopping, and transported onto a cross conveyor belt. The Mais Star* Collect is a modified harvester used with combines. Below the harvesting unit is a collection bin, which allows the maize stover to be windrowed as threshing occurs. Field choppers (with a pick-up attachment) and loader wagons were tested in a comparison for the subsequent recovery of the windrowed maize stover. In addition to determining the maize stover potential, the “windrowed stover yield” and the “removed stover yield” and the dry matter and crude ash contents established (to measure contamination). For various samples from both the standardized plant cultivation trial and the practical harvest technology trial, the silaging characteristics were tested based on silage trials at a laboratory scale and an initial silage trial at the larger scale in a silage tunnel. To evaluate the maize stover quality, the material composition was investigated with wet chemistry methods using the Weender/Van Soest analysis, and the specific methane yields were determined at a laboratory scale with batch trials according to the Association of German Engineers [VDI] 4630 (2006). Maize stover potential and methane yields Previous trials have shown that maize stover potential averaged 11.0 tonnes dry matter per hectare and the 13
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