12th September 2022

High cell wall digestibility – The must have for AD productivity

Tim Richmond, Maize Product Manager for Limagrain UK says that originally the maize for AD market was dominated by very late maturing, high yielding but generally lower quality varieties, but growers are becoming more interested in the attributes of the varieties grown such as dry matter and energy content and their ability to produce more methane.

“Many growers have focussed solely on the yield of the crop without paying sufficient attention to quality attributes.  For optimum efficiency you don’t want a large quantity of low dry matter, low energy feed, yet this is exactly what growers get when they focus on varieties selected solely on yield.

“It is also important to select early maturing varieties as they have significant environmental benefits, helping meet the terms of the biogas.org.uk voluntary code of practice.  In particular, by being harvested earlier and in better conditions, there is less chance of damaging soil structure and a greater opportunity to establish successor crops.

“Early varieties were a particularly good fit this year, where the late spring sowing in many parts of the country has resulted in a shorter available growing period that will impact ripening at harvest.”

Mr Richmond says there has been a move towards varieties with better quality attributes, principally ME content and cell wall digestibility.  With 50% of the total energy being in the vegetative part of the plant, increasing cell wall digestibility is becoming increasingly important.

“A key driver of efficiency is the rate of fermentation. The faster and more completely feedstock is fermented, the faster the gas is generated and the greater the total output.

“A low ME, less digestible plant will take longer to ferment. It may produce the same gas yield but it will take more time to get it.  Higher cell wall digestibility helps increase the energy output per day spent in the digester.

“The ideal variety for AD is early maturing with high dry matter yield, combined with excellent ME content and superior cell wall digestibility.”

Mr Richmond says the BSPB/NIAB Maize Descriptive Lists which contains data to assess the quality characteristics of varieties that will influence their suitability for AD allows AD operators to make a more balanced judgement regarding potential varieties.

“The system tests the very latest varieties in UK conditions against other well-known forage maize varieties. Results include all the key criteria affecting quality including dry matter content, dry matter yield, ME content and ME Yield.  Growers can now assess varieties based on the characteristics that will influence their potential to produce methane.

“While well-proven varieties such as Atrium, Asgaard and Fieldstar continued to rank highly, the 2018 list contained two new varieties which show exceptional potential delivering exactly the attributes required to increase methane yield and AD plant efficiency.”

Gatsby is an impressive variety producing high yields of energy rich silage, scoring highly on ME yield and relative ME yield thanks to a combination of high dry matter yields and excellent ME content, a result of high starch content and excellent cell wall digestibility.  Being earlier maturing it can fit seamlessly into arable rotations, enabling the establishment of successor crops.

LG31.211 is highly digestible with massive yields combined with excellent quality – meaning it will support excellent gas yields

“On favourable sites these varieties produce over a tonne DM/ha more than the average of the evaluated varieties, and over 2.5t DM/ha more than the lower performing varieties.  This equates to 350 cubic metres more methane per hectare than the average, worth £260/ha for no increase in growing costs.  At the same time the improved digestibility will increase the rate of digestion, increasing overall plant throughput.  For a 500kW digester requiring around 220ha of maize this could add up to over 77,000 cubic metres more methane than if an average performing variety is grown, worth £59,000 and so having a huge impact on performance and financial returns.”

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