Kinetics of methane fermentation yield in biogas reactors: Genetic variation and association with chemical composition in maize

Abstract

Maize (Zea mays L.) is the most competitive crop for methane production in Germany. Methane fermentation yield per unit of dry matter (MFY) is a determinant of methane yield, but little information is available on this trait. Our objectives were to investigate the kinetics of MFY during fermentation of maize, estimate quantitative-genetic parameters for different traits related to MFY and examine the relationship of MFY with chemical composition and silage quality. Whole-plant material of 16 inbreds and their 32 testcrosses was analyzed for MFY over 35 days of fermentation using a discontinuous laboratory assay. Data were also generated on chemical composition and in vitro digestible organic matter (IVDOM). Significant genotypic variances and high heritabilities were observed for MFY at early fermentation stages (up to 5 days) probably due to different concentrations of easily degradable chemical components. However, genotypic variances and heritability of MFY reduced as fermentation progressed, because of complete or partial degradation of all chemical components. Further, there were strong correlations of MFY with chemical components at early fermentation stages but not at later stages. Therefore, MFY at later stages, which is closer to potential MFY, does not seem to be amenable to selection. High heritability of IVDOM and its strong correlation with MFY in testcrosses indicated its possible use for preliminary or indirect selection. Keeping in view the magnitude of genetic variance that was low for MFY and high for dry matter yield (DMY), the other component of methane yield, more emphasis on breeding for DMY seems appropriate.

Introduction

Biogas production from energy crops is gaining importance in many European countries. Under the agroclimatic conditions of Central Europe, maize (Zea mays L.) is currently the most competitive crop for biogas production due to its high yield of methane, easy integration into existing farming systems, and good suitability for storage of biomass [1]. Thus, maize is currently the most widely used crop for biogas production in Germany [2], with an area of more than 500,000 ha in 2010 [3]. To make cultivation of maize as an energy crop even more economical, it is important to breed cultivars with high yield of methane and rapid methane production during fermentation. In this context, the study of the temporal release of methane during fermentation, i.e., its kinetics, is essential.

The main aim in breeding maize as a biogas substrate is to develop cultivars with a high energy yield per unit of planted area, i.e., methane yield per unit area (MY). Besides dry matter yield per unit area (DMY), MY is determined by methane fermentation yield (MFY), which is the volume of methane produced per kg of dry matter. Thus, the breeder can focus on improving MFY and/or DMY. It is expected that DMY in maize can be increased by breeding from the present level of 15–18 t ha⁻¹ to 30 t ha⁻¹ [4]. This can be achieved through prolonging the vegetative growth period by using late maturing germplasm, combining late maturity with cold tolerance, introgressing short day genes from exotic populations and improving drought tolerance [4]. Since late maturing hybrids are expected not to attain complete ear fill [1], they may have a low MFY due to low concentrations of easily degradable components like starch, protein and fat that are mainly stored in the ear. However, only limited knowledge is available on MFY in maize.

By definition, MFY is the product of methane concentration in biogas (MC) and biogas fermentation yield (BFY), i.e., the volume of biogas produced per kg of dry matter. Based on stoichiometric equations, both BFY and MC are expected to vary among different chemical components [5], [6] and this will be reflected in MFY. In previous studies, the correlations between MFY and chemical composition in maize were inconsistent and ranged from strong [7] to very weak [1], [8]. Thus, further research is warranted to explore this association as it may also impact the kinetics of MFY and vice versa.

Quality traits like digestibility of organic dry matter and concentration of metabolizable energy (ME) of the feedstuffs have been recognized to be important in silage maize [9]. Therefore, selection is practiced to simultaneously improve DMY and quality using selection indices [10]. MFY can be seen as analog to the quality traits in silage maize. Thus, there is a need to examine if similar indices may be used in biogas maize breeding.

In animal nutrition, different in vitro assays have been developed to determine digestibility and ME of feedstuffs by measuring gas production from fermentation after incubation with rumen liquor [11], [12], [13], [14]. Cumulative gas production after 24 h in vitro has been found to be strongly correlated with in vivo digestibility and ME [11]. To assess the potential MFY in biogas reactors, different discontinuous laboratory assays based on the same principle as those for determination of digestibility and ME in vitro, have been developed. Some examples are the bioassay proposed by Owen et al. [15], the Hohenheim Biogas Yield Test [16] and the Weihenstephan Batch System [17]. These methods differ from the digestibility trials used in animal nutrition for the inoculum and incubation time. Thus, it would be of interest to examine the association of the kinetics of MFY with silage quality traits in breeding maize for MFY. However, to the best of our knowledge, no report is available in the literature on this topic.

In this study, we examined a set of 48 diverse maize genotypes, which is a larger set of genotypes than those used in previous studies [1], [7], [18]. Our objectives were to (1) investigate the kinetics of MFY during fermentation of dried whole-plant material by applying different mathematical models, (2) estimate quantitative-genetic parameters for various MFY-related traits relevant for maize breeding, and (3) examine the relationship of MFY with plant chemical composition and silage quality traits.