The discrepancy between plot and field yields: Harvest and storage losses of switchgrass

Abstract

Three separate experiments were planned with the aim of assessing storage losses and discrepancies in biomass yield between plot and field, the latter being poorly studied in spite of the relevant management scale. The results show that storability of switchgrass is remarkable, irrespective of harvest time (summer or post-frost harvest) and bale type (rectangular, soft- or hard-core round bales). Bale weight significantly decreased over time, yet it was almost entirely attributed to a decline in moisture content. Microbial processes appeared trivial and this was corroborated by temperature inset and pH trends. In contrast, significant biomass losses were ascertained during the harvest, which accounted from 35% to 45% of potential harvestable biomass. Biomass not picked-up by the baler machine was up to 17%, while the uncut biomass due to the mower swinging averaged 29%, and it was also significantly affected by the field slope. Because of the lower ash content of basal stems, the uncut biomass penalized biofuel quality and quantity, at the same time. Potential harvestable biomass was similar to that achieved with hand-harvested plots thus revealing that the two considered sources of biomass loss (not recovered and uncut biomass) are mostly responsible of the discrepancy between plot and field yields.

Introduction

Biofuels are still not competitive against fossil fuels because of the energy dissipation from sun to crop and low-yield conversions that occur from harvest till the final processes. This makes bioenergy chains an overall low efficiency process. Moreover, the energy content per unit of dry biomass, namely gross calorific value, is less than the half of oil fuel, thus making bioenergy generally uneconomic compared to oil or alternative agricultural uses for cropland. Reducing energy losses would be therefore imperative to enhance the competitiveness of bioenergies against non-renewable sources. More to that, increasing energy yield could allow achieving the break even point in marginal lands thus preserving the more fertile soils for food crops.

Basically, energy dissipation of bioenergy chains can be schematized in three phases: (i) from sun to productive Earth's areas; (ii) conversion of solar radiation into biomass; (iii) from harvest to final conversion processes.

About 50% of potentially available solar energy cannot be used by crops because of Earth's albedo and atmosphere absorption, and of this, about 50% is photosynthetic active radiation (PAR). This huge energy dissipation can be hardly reduced by man as it depends on atmosphere, seasonal conditions and latitude.

The conversion efficiency of solar radiation into biomass is also very low [1], [2]. For example, that of switchgrass was found to range from 0.9 to 1.1 g MJ⁻¹ [3]. Again, this energy dissipation can be scarcely controlled by man in short term as it mostly depends on intrinsic crop characteristics.

On the other hand, the third phase (from harvest to conversion plants) has strictly to do with man as it mostly depends on harvest, storage and efficiency of power plants. As such, it could be reduced in short term with adopting appropriate techniques.

Several studies addressed biological and thermo-chemical technologies (combustion, pyrolysis, gasification etc.) for bioethanol or electricity/heat production [4]; conversely, very little is known on biomass losses during harvest and storing of switchgrass. Sanderson et al. [5] showed that, due to the difficulties of baler machines in recovering biomass, almost 6% of the standing switchgrass remained on the stubble after baling. Vleeshouwers [6] pointed out that the extent of not recovered biomass can be much higher than this, a result that was corroborated by other experiments on switchgrass (H.W. Elbersen, personal communication). In addition, biomass losses may derive from compositional changes of non-structural components and physical losses that may alter the convertibility of biomass into biofuel [7].

Alike miscanthus and giant reed, switchgrass is considered a very attractive perennial energy crop in a very short term [8]. Basically, the wide public approval of switchgrass may be explained by two main potential advantages characterizing this crop: (i) it produces vital seeds, which allow to enormously reduce plantation costs compared to crops propagated by rhizomes; (ii) it seems well adapted to common forage machines, an attitude which is especially appreciated by farmers. Nonetheless, nearly all studies reporting switchgrass yields are mostly based on small hand-harvested plots which could provide discrepancies and misassumptions in upscaling to actual farm situation with mechanized systems [9], an uncertainness that makes farmers sometime reluctant in cultivating this crop. According to this, two recent studies on switchgrass addressing the effects of nitrogen dose [10] and energy balances [9] clearly demonstrated the need of commercial size data encompassing a diversity of spatial conditions (soil type, microclimates, slopes etc.) to validate small plot results and economic analysis [11], [12].

Therefore, this study focused on quantifying the biomass loss at field scale using conventional harvest and baler machines. Biomass production deriving from hand-harvested plots within the same field was taken as referencing potential biomass yield. The discrepancy between plot and field yields was calculated taking separate all the possible causes of biomass loss (e.g. not recovered feedstock, irregular cut height, tiller density etc.). Biomass loss during handling and transport were disregarded accounting for only 0.4% of bale weight [5].