Catch Crops Planted in the Summer Months Can Complement Energy Grains and the Combination

New Energy, 1/2008

Light snack

Catch crops planted in the summer months can complement energy grains and the combination with them be an alternative to energy maize. But they’re not cheap. Regional climate has to be considered to grow them.

By Martin Bensmann, Photo: Silke Reents

Times are tough for the operators of biogas power plants. Price increases of up to 30% for grains are impacting the purchase prices of energy crops as a basic commodity. This affects not only biogas producers but also planners and makers of methane power plants. A glance at Germany shows the consequences. The German Biogas Association, based in Freising, Bavaria, reports that last year only 200 new biogas plants were built, 75% fewer than in 2006.

How many are going to be built this year depends decisively on how grain prices develop. Even if they were to drop, potential investors might hold back until the revised Renewable Energy Sources Act (EEG), which regulates feed-in rates for renewable energies in Germany, is in force. The bill passed by the cabinet in December improves the terms for biogas producers.

The new legislation is to take effect at the start of 2009, possibly already on 1 July this year. That means installation planners and biogas producers are in for some hard months. While the manufacturers are trying to keep afloat with foreign orders, the operators of biogas installations are trying to find alternative energy crops. They’re looking for ways to increase the yields of their installations and operate with lower costs overall.

Now being broadly discussed is the cultivation of summer catch crops. One possibility is sowing after the regular grain harvest (winter barley) in mid-July. But this makes sense only in regions with enough summer precipitation and temperatures that enable adequate biomass growth by autumn.

Not every catch crop is suitable

Experience with growing summer catch crops has been gained in cattle farming. There special crops are grown as fodder to keep the stock fed until autumn when pastures are sparser. The crops have so far been little used to produce biogas. So the questions are what technology is needed and which varieties really deliver additional yield. Experts argue that not all biogas installations by a long chalk can work with summer catch crops. The loading and stirring techniques are key. So in many cases there has to be quite costly retrofitting.

According to Jörg Michael Greef, head of the Institute for Crop and Grassland Farming of the Federal Agricultural Research Centre (FAL) in Brunswick, several crops are suitable: summer grain, maize, sunflowers, oil-radish, forage rape and annual ryegrass (see chart). But a closer look reveals that there’s no easy decision and each plant has its advantages and disadvantages.

The drawback with summer grain, for example, says Greef, is grain following on grain. That requires chemical plant protection and increases environmental burdens and costs. The advantage of leafy crops like sorghum, sunflower or grasses is that they loosen the crop rotation. Since summer grain does not ripen far enough, it has to be mown and start wilting on the field to raise the dry substance content. “With maize only the quick-maturing varieties come into consideration. But these varieties also don’t develop enough dry mass until autumn so that the plants contain relatively much water, which seeps from the silo and has to be caught.” That causes extra costs, Greef notes. Too little dry mass is also delivered by bird rape, oil-radish and forage rape. “In the final analysis, only water is driven around at harvest and no energy,” the crop expert sums up.

The scientist sees better prospects in early-ripening sunflowers. He recommends varieties that form many small clusters of blooms. One advantage of this is the maturing from outside to inside, which forms more dry mass, another is that the oil synthesis starts quickly. Moreover, in warm temperatures the sunflower begins to grow relatively early. A disadvantage is the expectable feeding by birds on the seeds, which can already be a problem when sowing.

“The advantage of annual ryegrass is that a substantial amount of biomass grows in eight to 10 weeks,” Greef describes another quick snack for the biogas fermenter. But the grass has to start wilting on the field, he adds, and that “could become a problem in wet years”.

Underseeds can bring better yields

That can be avoided by sowing catch crops earlier – say after harvesting grain whole plant silage already early or mid-June. That gains four to six weeks time compared with coupling to the grain harvest. “Well suited are early-ripening maize and sunflower varieties. Early maturing sorghum or annual ryegrass can also be used.” But sunflowers need to be harvested earlier to avoid fungal decay.

Greef sees another option in simultaneous sowing of various crops. Thus Italian ryegrass can be sown as an underseed in autumn across the seeding direction of the winter grain. In the further progress of the vegetation the underseed keeps growing slowly under the grain. After the grain is harvested, the underseed grows fully and sprouts above the grain stubble. Greef stresses that the sowing date is decisive to the success of the underseed. If sowing is too late, the grain grows over the grass. The result is that the underseed doesn’t establish satisfactorily, its growth is stunted or it doesn’t germinate at all.

Frank Trockels, product manager for maize and grasses with Deutsche Saatveredlung AG in Lippstadt, favours sowing underseed in spring. He names as suitable grasses Italian ryegrass and meadow fescue. “Certainly best for whole plant silage of grain is winter barley. When harvested early it delivers high nutritional and energy yields and vacates the field so early that a long growing period remains for the underseed,” says the seed expert.

Experience has shown, he says, that after the early application of reduced amounts of soil-effective chemical weedicides in autumn, underseeding in spring is problem-free. Special grass underseeds are resistant to a number of crop protection agents. However, no more than necessary should be sprayed, he says. On flat land the most favourable sowing time is the first half of March while the second half of the month is better for low mountain range locations.

Especially in regions with summer dryness underseeds are superior to sowing after ploughing. This is because the additional soil tillage after the silage harvest is costly and increases water evaporation. Another advantage of the grasses is their productiveness. For example the crop-option of grain silage plus underseed depending on the grain yield is even superior in some areas to conventional maize cultivation (see chart).

“Such a cultivation technique also has ecological advantages because it especially loosens crop rotations with high maize content while protecting the soil from erosion at the same time,” Trockels explains. Grasses make better use of available nutrients by turning them into biomass and protecting groundwater from nitrate pollution. The roots of the catch crop enrich the soil with biomass and drain it naturally so that it can absorb more rain.

Use efficiency potentials first

Ecological brownie points are good but does the in-between snack also pay off economically for biogas power plant operators? With underseeding, costs of seeding and harvesting of around EUR 300 per hectare must be expected. Annual ryegrass sown after ploughing, with two plant protection applications, is expensive: EUR 470 per hectare. The costs per tonne of fresh mass, depending on yield per hectare (15, 20 or 25 tonnes) range between EUR 20 and 30. With underseed it is EUR 10 to 20.

For Walter Danner, spokesman of the Lower Bavaria regional group of the biogas association, the talk now doing the rounds of affordable catch crops is eyewash. “In our Bavarian Forest area we’re having to pay more than EUR 28 per tonne for grass silage off grassland,” the practitioner reports. He clearly rejects summer catch crops like mustard, oil radish and forage rape because with them “too much water has to be transported”. He says the transport distances would also get far too big to collect the biomass amounts needed because the low hectare yields make much more growing area necessary. And so Danner consistently follows the path of improving the biological process in the fermenters. He sees an economic imperative in more intensive exploitation of the fermentation substrates. The test outcomes of the three-phase fermentation process he developed in a biogas installation in practical operation are impressive. With his process Danner gets about 30% more biogas out of the biomass used. Moreover, installations producing not just power but also using the heat are more efficient.

For Ulrich Keymer, biogas expert at the Bavarian State Institute for Agriculture, transport distances of around 20 kilometres and more are economically questionable. He has calculated that per tonne of chopped grass it costs EUR 2.30 per tonne for two kilometres, EUR 3.50 per tonne for five km, EUR 5.50 per tonne for 10 km and EUR 7.50 per tonne for 15 km. And under certain conditions, if wet silage is delivered the so-called dry fermentation bonus (two cents per kilowatt-hour) is in danger.

In summary, under certain conditions grasses, millet or sunflowers now seem to be the most suitable plants for summer catch crops. But despite that, catch crops are not necessarily cost reducers for the plant operators. Success also depends on the technical equipment of the biogas installation.

In principle it is good to try to achieve higher area productivity by planting summer catch crops. Under economic considerations field areas should not remain unused after the grain or silage harvest. Those farmers with access to enough self-owned fields have it easier to plan and establish the summer catch cropping. Those who have to chase land have greater expense.