1. Introduction
Fossil fuels are not renewable and contribute to the increment of CO
2 concentration in the atmosphere. Biomass from agricultural residues could contribute to a sustainable or circular bioeconomy as a feedstock for bioenergy (biofeedstock) or high value bio-based products in integrated biorefineries [
1,
2]. In Europe, a large amount of residual biomass is generated in the cultivation of cereals, maize, and oilseed crops [
3]. The most efficient use of crop residues depends on the energetic goal, the characteristics of the environment, the crop varieties, and the interaction among these [
4]. The exploitation of residues could stimulate the economy in rural areas, which are being depopulated, by increasing the value of crops and by generating new jobs in biorefineries [
5]. Generally, local crop varieties have large genetic variability, which reduces the vulnerability to new stresses and could provide alleles to improve crop adaptation [
6]. These local varieties can be a source of genes to increase adaptation to environmental change of elite germplasm [
7] and contribute to a more sustainable agriculture. In low-input or organic farming, there is also an interest in local varieties, whose seeds can be saved by farmers for the next growing season, something that cannot be done with elite varieties, as for example maize hybrids, that must be purchased each growing season [
8,
9].
Enhancing the energy content of crops by increasing cellulosic biomass, while more efficient biochemical technologies [
10,
11,
12] to convert biomass to ethanol are being developed, should be the focus to reduce unit cost of biofuels. In maize, exploring alternative traits for high-quality crop varieties with improved fiber digestibility would provide a complementary value for otherwise unused cellulosic energy of residues, converting the plant to a dual-purpose crop, and may reduce risk for growers. For instance, maize stover is a residue and a potential source of for cellulosic ethanol production [
13,
14,
15]. In the stover, plant cell wall components include lignin, hemicellulose, cellulose, and other organic components, with the least digestible plant components being cellulose and lignin [
16]. These cell wall components can be measured as three fiber quality parameters [
17]—neutral detergent fiber (NDF), which is composed of cellulose, hemicellulose, and lignin; acid detergent fiber (ADF), which is composed of cellulose and lignin; and acid detergent lignin (ADL). Lignin is an undigestible component that has no energy value as animal feed [
18,
19], nor value for energy conversion to methane or ethanol, and restricts digestibility of other fiber elements. Lorenz et al. [
20] suggested that fodder fiber quality and composition may be used to predict cellulosic ethanol yield due to positive correlations with NDF. However, qualitative changes in fiber quality should not penalize agronomic traits to be considered a high-quality bioenergy crop. Albrecht et al. [
21], in research with three cycles of recurrent selection of maize (Iowa synthetic#1), reported that traits associated with stalk lodging resistance did not have a negative impact on digestibility or substantially alter fiber composition or concentration of the maize stalks. Smith et al. [
22] showed that qualitative changes of biosynthesis substrates (e.g., feruloyl-CoA) of the cell wall seems to increase the digestibility of the cell wall and may provide higher stover yields and superior biofuels substrates without changes in agronomic traits. Although, greater biomass may be inclined to lodging or may be antagonistic to grain yield and moisture traits [
23]. Simultaneous improvement of whole plant fiber characteristics and corn grain yield has the potential for second generation biofuel production.
The ideal moisture concentration of crop residue depends on storage facilities and energy conversion goals. Low moisture concentration is necessary if the residue is used for combustion, but not when residue is fermented for bioethanol or biogas production. For biogas production in Europe, whole-plant maize is usually ensiled at 65–72% according to Grieder et al. [
24]. Shinners et al. [
25] analyzed different systems of storage and concluded that crop residues can be stored for long periods when dried to below 20% moisture. Or, after wilting in the field to 40% to 55% moisture, stover can be stored, without chopping, as silage in wrapped square or round bales. Depending on storage structure, e.g., bags, bunkers, or piles, chopped forms of residue ranging in moisture from 35% to 67% can be stored as silage.
Maize varieties have shown genetic diversity for stover yield [
15,
26,
27]. A few maize varieties from a common pedigree, Corn Belt dent, including Lancaster, Minnesota 13, and Reid, are the predominant contributors to maize hybrids cultivated in temperate areas. Within Reid, a synthetic comprised of 18 inbreds (Stiff Stalk Synthetic, BSSS), and an early strain, Iodent Reid, are ancestors of many elite inbred lines [
28]. One of the most common heterotic patterns in temperate maize is BSSS x no BSSS. Commercial breeding was based on recycling closely related inbred lines within heterotic groups [
29,
30], resulting in many elite lines being derived from only a few ancestors, i.e., B73 and B14 from BSSS, Mo17 from Lancaster, etc. [
31]. Maize was introduced to Europe soon after discovery of America and there are three groups of germplasm in Europe: central-northern European Flints related to eastern US Flints, Mediterranean European varieties close to Caribbean or northern South American varieties, and Atlantic European varieties (Northern Spain and France), which are not close to any known American variety [
32,
33]. Landraces in Spain were extensively studied at the National Research Institutes in Spain for agronomic and morphological traits [
34,
35,
36]. Similar to what has been seen in US, only a fraction of the variability of the European maize landraces is present in the European elite lines [
37].
In this research, we compared a set of maize landraces from Spain to a representation of flint and dent varieties of other origins, Corn Belt and commercial hybrids, to evaluate the potential bioenergy suitability and agronomic trait performance. We studied fiber parameters to assess whether adaptation predisposes certain materials for dual use or not.
4. Discussion
Elite maize varieties selected for grain yield may not be ideal for dual exploitation, as selection for grain may negatively affect some relevant characteristics for a dual purpose biofuel resource [
23,
39]. To reduce the cost per unit of biofuels, biomass yield, residue use, and the efficiency in plant fiber conversion digestibility are key factors. We evaluated several agronomic and compositional characteristics relevant to the dual exploitation in open pollinated varieties that did not undergo intense selection and could have favorable characteristics for dual exploitation not present in the elite varieties. We used open pollinated varieties from three important heterotic groups—Atlantic Europe, Mediterranean Europe, and the US Corn Belt. Three of the four US Corn Belt varieties—Lancaster, Minnesota 13, and BSSS—have historical relevance in maize breeding that are not in the background of the European varieties.
Results showed that population origin presents a reservoir of genetic diversity for breeding to improve biomass quality to efficiently convert maize stover into fermentable sugars for bioenergy [
15,
26,
27]. Multivariate analysis clearly separated the Atlantic Europe, the Mediterranean Europe, and the commercial hybrid group, indicating that the clusters differed for the variables under study. Landraces from Mediterranean Europe cluster independently and had, on average, lower NDF, ADF, and ADL values (
Table 4). This difference from the other origins agrees with Mir et al. [
45], who suggested they were introduced from northern South America to Europe via Spain. Likewise, the EU Atlantic populations may share common US Corn Belt origin, based on the PCA cluster (
Supplementary Figure S1) and the similarity of latitudes from the northern US flint landrace.
The varieties of the US Corn Belt origin, especially Minnesota 13, grouped separately from the others in the first axis of the PC analysis (
Figure 3). Late-flowering positively correlated with stover yield and grain yield [
46], and Minnesota13 differed from the others because it had the earliest flowering and the lowest grain and stover moisture, although considering that earliness, it still had a relatively high grain yield. This variety was very important to adapt maize for environments with shorter growing seasons, such as the northern Corn Belt and Central Europe, and contributed to the development of one of the most important Iodent lines, PH207 [
47]. Furthermore, BSSS was close to the group of commercial hybrids, which was also expected given that BSSS has contributed to elite germplasm, with such outstanding lines as B37, B14, and B73. Specifically, one of the parents of the hybrids with open formula is B73 and one of the parents of the other open formula hybrid is A632, which is derived from B14. The other hybrids probably also have a parent of BSSS origin. In the PCA, the loading contributions of fiber composition variables (i.e., NDF, ADF, ADL) were similar from the stover and the cob, only cob ADL seems to contribute more than stover ADL, which was the lowest loading contribution of the analysis (
Figure 4). Agronomic variables, on average, contributed slightly more to the loadings of fiber components (
Figure 4). This may be because agronomic traits have shown larger phenotypic variability than the compositional traits.
As expected, our results showed that successful commercial maize breeding focused on grain yield [
48], because the hybrids had higher grain yield than landraces, even though the later had higher grain moisture on average (
Supplementary Table S2). There was a substantial difference in grain yield between the most productive modern hybrid, PR34G13, and the most productive landrace, Faro, of 2000 kg/ha (
Supplementary Figure S2). However, Faro is a late genotype and its productivity was at the cost of higher stover moisture. Within the varieties with lower grain moisture, Posada de Llanera had the highest grain yield, although 3000 kg/ha lower than PR34G13. This large difference in grain yield could be a limitation to introducing favorable features of the landraces to elite material, and prebreeding using genomic multistage or index selection is needed [
7]. In the case of stover yield there is not a gap between local varieties and commercial hybrids. For example, Faro had higher stover yield than PR34G13 and Posada de Llanera had only 800 kg/ha difference in spite of its lower grain moisture compared to the hybrid PR34G13 (
Supplementary Tables S1 and S2). We observed that the gap between selected and unselected material was greater for grain yield, which was the direct target of selection, than for stover yield, which was only indirectly selected (
Supplementary Figure S2). As a consequence of this, the harvest index (HI) of the local varieties was lower (0.42–0.52) than the HI of the hybrids (0.53–0.57). In fact, two of EU Mediterranean landraces (BastoxBlanco and RastrojoC3) showed greater amounts of stover than grain yield. In contrast, Lorenz et al. [
49], in a literature survey, concluded that the HI did not change over time in US Corn Belt germplasm. Within the landraces, Faro from the Mediterranean area and Lancaster from the Corn Belt had high stover yield in addition to acceptable grain yield, which suggests potential for dual-purpose use. However, these varieties have late flowering, which contribute to high grain moisture at harvest. A breeding goal would be to reduce grain moisture without affecting the grain and stover yield for a dual purpose of grain and residue production. For instance, Posada de Llanera has better agronomic characteristics for dual exploitation because it was among the five best landraces for stover, cob, and grain production and had the second lowest grain moisture after Minnesota 13. This variety seems to have characteristics that are valuable for dual exploitation, such as a relatively late flowering line that allows large accumulation of vegetative biomass and rapid kernel growth and dry down, which explains the relatively high grain yield and low grain moisture in spite of the short grain filling and dry down period [
50]. The moisture of the residues of all varieties would be satisfactory for storage in silos [
25,
51], although, data in more environments, for example in Atlantic Europe, would be needed to confirm this hypothesis.Regarding the composition of the residues, fiber amounts studied in cobs were significantly different based on the origin, whereas the fiber studied in stover did not differ among origins (
Table 4). We found higher values of ADF, ADL, and NDF in the cobs than in the stover. The cellulose content can be approximated as ADF-ADL, while the hemicellulose as NDF-ADF. The average quotient of hemicellulose in stover divided by hemicellulose cob was 0.78 in landrace varieties, while the quotient of stover cellulose dived by cob cellulose was 0.92. These values are similar to the values (0.76 and 0.98) reported by Lorenz et al. [
52], in spite of the different origin and characteristics of the germplasm, suggesting the potential for bioethanol production is greater in cob than in stover [
20].
Fiber composition differences for stover, based on origins, were not detected. This may be due to it not being a breeding target, or due to a greater environmental effect on stover measurements. At the variety level, there were significant differences in fiber composition. Only a few varieties were different from the rest, for example, Norteño Largo had high ADL, which would be detrimental for conversion to ethanol or biogas, and Minnesota 13 had a high concentration of NDF, which could mean greater concentration of hemicellulose. This lack of variation among such a diverse sample of germplasm, including local varieties from Mediterranean Spain that are the most variable in Europe [
53], suggests that the selection for stover fiber could be not very effective in temperate maize [
52]. We found more variation in fiber composition of cobs than in fiber composition of the stover, suggesting that may be a better breeding target for biofuel production. The Atlantic European group had a favorable composition of cobs for bioenergy with more NDF and ADF and less ADL, and, within this group, Aranga stood out due to its high concentration of ADF and Lazcano for high concentration of NDF. The general trend of our results suggest that EU Atlantic landraces have evolved towards a lower value of ADL, which is an indicator of suitability in the degradation process of energy production for biofuels. The lower the ADL the more suitable for processing residue to biofuel.
Previous studies reported that lodging reduces grain and stover yield [
23], although, fiber improvement may not affect lodging [
21]. Despite the high values of heritability for each trait, correlations between agronomic traits and stover composition were less relevant; this may be due to the low variability detected for stover fibers. Only the correlation between stover ADF and lodging was greater than 0.7 and positive, suggesting that increased ADF in the stover, favorable for bioenergy production, may lead to an undesirable lodging effect (
Table 6). Interestingly, as stover ADL and ADF increase, the stalks tend to lodge more, which is the opposite of what one would expect, but is consistent with results reported by Albrecht et al. [
21]. For cob composition, there were also high correlations with lodging, however, the negative relationship of ADF and lodging may be beneficial if negative selection occurred for ADF, which would drive positive agronomic effect due to lodging reduction [
21]. However, there were negative correlations between cob ADL and lodging, indicating that decreasing ADL in the cob in order to favor its digestibility may have an undesirable effect on lodging. How a characteristic of the cob influences lodging, which depends on the roots and stalks, deserves more research. In general, genotypic correlations between fiber composition and agronomic trait were not of such magnitude that they may hamper the simultaneous selection for these two traits, in agreement with Lorenz et al. [
54] and Lewis [
23].
According to these trends, silage maize maybe more appropriate for dual exploitation when the final use is bioethanol or biogas because the digestibility of the whole plant is an important target in silage breeding. However, Barrière et al. [
55] observed than the degradability of the modern silage hybrids in Europe is worse than the old ones and pointed out that new investigation of genetic resources is needed. Furthermore, harvest of silage hybrids is performed before the complete maturity of the grain, while in dual purpose hybrids harvest is performed at grain maturity. The ideal composition, yield, and moisture of both types of hybrids could be different, for example, in dual purpose hybrids the moisture of the grain should be as low as possible at harvest. The ideal biomass structure and composition is system dependent and in the case of combustion the degradability is not an issue [
39].