Legumes supporting biomass crops

Cropping is often about producing as much biomass as possible for as little cost as possible but this is normally strongly constrained by the quality parameters demanded by the end users. In many cases high value specialist crops (as grain) is rejected due to substandard quality, possibly going to a lower value market with considerable loss of profit or even as waste. Biomass crops for feed or anaerobic digestion are at the other end of the spectrum. Some quality standards are required still but the thresholds are lower and high yield can compensate for shortcomings in quality. Therefore, the emphasis is on reducing costs, and agronomic input use efficiency is high on the agenda.

In this case study we are looking at the options for producing biomass suitable for animal feed or anaerobic digestion and how legumes in particular can help. So taking into consideration our climate and soils, we are field trialling combinations of several cereals with different legume species in our Scottish Government-funded work in collaboration with this TRUE study. We are focussing on winter crops, i.e. sown in the autumn and harvested in the following summer to make best use of the full growing season.

Why cereals and legumes together? Because these mixtures / intercrops / plant teams produce more biomass when grown together than separately on the same land area. It is all about exploiting diversity, and in general the more diverse and complementary, the greater the synergies in terms of biomass production. Cereals and legumes exhibit contrasting behaviour biologically, and while their growth habits and capacities for nutrient capture differ, these differences can render them highly complementary and therefore strong candidates for developing very efficient biomass crops.

Winter crops generally have higher yields in our environment but of course the varieties used must be winter-hardy. So far we have looked at wheat, barley, oats, rye and triticale, singly and in combinations, sown with field beans or peas or vetch. And we are growing them under low or minimal inputs of fertiliser, crop protectants and herbicides. The crops are cut as wholecrop, i.e. before they ripen. This is achieve a balance between maximum biomass and digestibility. So it is all about balancing or compromises. The cereals are all well adapted to our climate so generally grow well, although some have quite high demands for fertiliser so careful variety selection may be more critical than in intensive production. Choice of legume variety is even harder with many proving too competitive or non-competitive. So at the moment we are looking at what works and what needs adapting.