Methane yield performance of amaranth (Amaranthus hypochondriacus L.) and its suitability for legume intercropping in comparison to maize (Zea mays L.)

The use of amaranth as a new crop in biogas crop rotations has raised scientific interest Europe-wide over the past decade. However, the results of studies on its overall performance are somewhat contradictory. This study aims to investigate both the performance of amaranth (Amaranthus hypochondriacus L.) as a biogas crop in comparison to maize (Zea mays L.) and to test its suitability for intercropping with two different legume species. For this purpose, field trials were conducted in southwest Germany in 2014 and 2015. The two legumes common bean (Phaseolus vulgaris L.) and white clover (Trifolium repens L.) were chosen for intercropping with the main crops amaranth and maize. Each year, aboveground fresh matter yield, dry matter content (DMC), ash content and specific methane yield (SMY) were determined. The average methane yield per hectare (MYH) of all amaranth treatments was 3030.6 ± 187.3 m3N ha−1 in 2014 and 2265.6 ± 243.4 m3N ha−1 in 2015, approximately half that of maize each year. In 2015, the low MYHs stemmed from low dry matter yields (DMYs) caused by drought conditions. This effect was much stronger for maize than for amaranth. The average SMY over the two years was much lower for amaranth (266.0 ± 1.7 lN kg−1 of volatile solids (VS)) than for maize (330.0 ± 1.5 lN kg−1 of VS) due to higher contents of both ash (>13% of VS) and lignin (>6% of VS). Intercropping with common bean did not significantly affect the DMY and SMY of either main crop in either year. By contrast, the establishment of white clover was successful in both main crops and years. However, white clover also significantly decreased the DMY of both main crops in 2015. Overall, it was shown that amaranth and maize appear equally suitable for legume intercropping, but new amaranth genotypes and optimized cultivation techniques are required to render it a more productive biogas crop.