Yield is presented by season, fraction and regrowth age in Fig. 1. Overall biomass yield corresponds to the upper limit of the piled graphic. Leaf yield is shown at the base of the figure, in order to draw attention to the low relative variability in leaf yield across both seasons. When regrowth occurred under limiting weather conditions, leaf yield did not surpass 4 Mg ha−1. However, during the wet season, when higher soil moisture and higher temperature were available to promote regrowth, leaf yield reached 5 Mg ha−1. In growth cycles 154 days long, despite the leaf accumulation showing a biologic limit, stem accumulated 16 Mg ha−1 in the wet season or 10 Mg ha−1 in the dry season, whereas leaf proportion meant only 20% of the available biomass by day 154 in either season. Similar data for leaf proportion and leaf yield have been reported for elephant grass subjected to a single harvest per year22. However, management under long growth cycles implies reducing the annual harvest of green leaves across the year, as noticed in a previous work23.
Decisions on the utilization of elephant grass CT115 intended for ethanol production should focus on increasing the annual harvest of green leaves, in order to improve the yearly harvest of ethanol from a given field17,23. Cutting intervals under 70 days of regrowth might be established in order to prevent excessive stem accumulation. That in turn, according to a previous work, might increase both the leaf yield per harvest as well as the biomass yield per year23. The continuous stem accumulation, at relatively unvarying offer of green leaves, coincides with a previous study in which elephant grass is kept under undisturbed growth6. Furthermore, higher annual biomass yield has been reported for cutting intervals under three months, which also achieved a higher harvest of green leaves through the year23.
Cutting intervals around 70 days might prevent useless stem accumulation and reduce the fiber content of the harvested biomass, therefore promoting a higher biodegradability24. Longer cutting intervals have been associated to a higher stem growth, a higher plant lignification, and lower biodegradability16.
Fiber partition as affected by season and fraction
Neutral detergent fiber (NDF) and acid detergent fiber (ADF) contents are shown in Fig. 2, whereas cellulose, hemicellulose and acid detergent lignin (ADL) contents are shown in Fig. 3, both organized by morphological component and season. Across season, the season-fraction interaction was not significant for NDF (P = 0.99), ADF (P = 0.94), hemicellulose (P = 0.97), cellulose (P = 0.42), holocellulose (P = 0.71), ADL (P = 0.17) or ashes (P = 0.92) contents. In consequence, differences between seasons remain true within each fraction and differences between fractions remain true within each season. Leaf had 34 ± 10.8 g kg−1 less NDF (mean ± sed; P = 0.002), 85 ± 9.5 g kg−1 less ADF (P < 0.0001), 51 ± 7.1 g kg−1 more hemicellulose, 61 ± 5.1 g kg−1 less cellulose, 24 ± 2.7 g kg−1 less ADL, and 28 ± 5.9 g kg−1 more ashes, as compared to stem (P < 0.0001). In addition, the wet season regrowth showed similar NDF (P = 0.31) and ashes (P = 0.40) contents, but 71 ± 6.2 g kg−1 less ADF (P < 0.001), 60 ± 7.1 g kg−1 more hemicellulose, 66 ± 5.1 g kg−1 less cellulose (P < 0.0001) and 5.3 ± 2.7 g kg−1 less ADL (P < 0.05) than the dry season regrowth. For information about the adjustment and significance of each explicative variable on the model, refer to Supplementary Tables.
Green leaf meant higher hemicellulose, but lower cellulose and lignin contents than stems. In fact, a high biodigestibility of the dry matter has been reported for leaves, as compared to stems, for the grasses Cynodon sp., Arundo donax, and Cenchrus purpureus5. In addition, higher digestibility and higher protein content have been reported for the leaves of Andropogon gayanus18. A higher hemicellulose concurs with a lower lignification and higher content of non-fiber soluble components, which could be converted to ethanol. Furthermore, a great number of research works had been addressed to the conversion of hemicellulose to ethanol25.
The higher content of biodegradable compounds as well as the lower cellulose and lignin contents recorded for the wet season regrowth, coincides with a previous study where elephant grass was managed at a cutting interval of 8 weeks throughout the year8. In addition, a higher in vitro digestibility, which is related to higher ethanol production5, was reported for the grass Andropogon gayanus grown in the wet season, as compared to the dry season regrowth18, which might imply a lower cell wall content (NDF). On the other hand, a study about variations in the chemical constitution of elephant grass between seasons found higher quality for the dry season regrowth7. This finding, which diverges from the current study, may be due to the important differences in the rainfall distribution throughout the year, since Indonesia is located in the Equator, and rainfall occurs to some extent in every month.
Holocellulose content was similar for the leaf and stem fractions (663 ± 7.1 and 673 ± 7.5, P = 0.35). Likewise, it was similar for the wet and dry seasons (665 ± 7.3, 671 ± 7.3, P = 0.58). The given similarities occurred despite the wide inverse variations in cellulose and hemicellulose contents both between fractions and between seasons (Fig. 3). The leaf from the wet season averaged 138 g kg−1 more hemicellulose than cellulose, and the stem from the dry season showed 100 g kg−1 more cellulose than hemicellulose. Surprisingly, hemicellulose and cellulose contents were similar between the stem from the wet season and the leaf from the dry season (Fig. 3).
Fiber partition as affected by plant age
Fluctuations in NDF, ADF, ADL and ashes contents across each season are shown in Table 1, by morphological fraction: leaf and stem. In both seasons leaf fraction recorded less ADF since day 42 (except contents were alike on day 56 of the wet season), less ADL since day 70, and more ashes since day 56 or 42 of the wet and dry seasons, respectively. During the first 98 days of regrowth, NDF and ADF contents followed increasing trends in either season or either fraction; while they remained constant afterwards. ADL content increased through day 98 for the stem fraction in either season, whereas it remained constant for the leaf fraction, in both seasons, with one exception. (Table 1). Ash content declined across the two seasons, but decreased slower in leaf fraction.
Variations in hemicellulose, cellulose and ADL contents within each season are presented in Fig. 4, ordered by season, morphological fraction and age. Actual means and statistical differences for the visual information of such figure, are presented in Table 2. The higher hemicellulose and lower cellulose contents recorded for the leaf fraction across each season (Fig. 3) remained true virtually on every age in either season.
For the leaf fraction, hemicellulose content increased through day 70 of the wet season or through day 98 of the dry season, whereas the cellulose content increased through day 42 for the leaf fraction, in either season, and kept on similar records from then onwards. Regarding stem fraction, hemicellulose content increased only during the wet season, through day 56, then decreased, but it reached a second maximum on day 126. Cellulose content was relatively constant in either season for the stem fraction, but reached a maximum on day 98 in both seasons.
The similar holocellulose content found between leaf and stem fractions on average across seasons (Fig. 3), remained true in ten out of the eleven ages, in either season. This was especially interesting, given that hemicellulose and cellulose contents differed between leaf and stem, virtually on every age class (Table 2).
A higher hemicellulose content for the leaf of elephant grass has been previously reported for the dry season, while a higher cellulose content has only been reported for the wet season; both results in a study in Thailand26, as an average for eight varieties of elephant grass. Climate and variety differences explain the discrepancies with the current work.
Published data are consistent with the fact that grass age and the content of most of the fiber constituents are directly related19,27,28; nonetheless, just a few age classes are usually included. Hemicellulose content has been reported to decrease for the whole plant of elephant grass in long-lasting growth cycles19. The current study gives rationale for such fact, since along regrowth, an increment of the stem proportion (Fig. 1), whose hemicellulose content was lower (Table 2), will lead to a lower hemicellulose content for the whole plant (see Supplementary Tables).
All seven variables describing chemical constitution in the current research work showed similar records from day 98 onwards, in each season and for each morphologic fraction.
Correlation between fiber fractions
Hemicellulose and cellulose contents were inversely correlated across the whole data (r = − 0.58, P < 0.001), while such inverse correlation remained true within the leaf (r = − 0.34, P < 0.026) or stem (r = − 0.48, P = 0.001), as well as for the wet season alone (r = − 0.33, P = 0.033); while a similar trend occurred for the dry season (r = − 0.28, P < 0.071).
ADL and cellulose contents were directly correlated for the whole data (r = 0.57, P < 0.001) and for the stem fraction (r = 0.387, P = 0.015), but not for the leaf fraction (r = 0.12, P < 0.42). Finally, ADL was inversely correlated with hemicellulose content for the whole data (r = − 0.47, P < 0.001), or for the leaf fraction (r = − 0.54, P < 0.001), but not for to stem fraction (r = − 0.09, P < 0.54).
Biomass quality of elephant grass CT115 can be improved, by means of increasing both the share of green leaves and the share of the wet season regrowth, in the biomass harvested along the year. A higher biomass quality, will in turn increase the annual yield of ethanol per area unit.
Strategies to accomplish a higher quality of the harvested biomass, as proposed above, may involve (1) cutting intervals of around 56 days during the wet season or around day 70 during the dry season, and (2) reduce cutting intensity. The latter implies cutting to a greater height, so that the fodder left uncut in the field will facilitate a faster restoration of the grass photosynthetic structures27. Nonetheless, such strategies may require further validation according to the wide diversity in climate conditions and crop management systems.