1. Introduction
Every year a significant amount of surface of crop lands remain idle because of low return margins, high water pumping costs, and limited and saline water supplies. In recent years, there has been a great interest in the use of agricultural crops to produce biofuel. Among the most important reasons are to reduce our dependence on fossil fuels, to reduce air contamination and mitigate greenhouse gases, and to develop markets for existing and new crops. An important challenge to grow bioenergy crops is the water availability, which is one of the critical factors to produce biofuels or any agricultural crops. In the past several years some authors have suggested to produce biofuel using marginal water (saline water) and marginal lands to increase its sustainability, avoid competition with food crops, or even to produce these biofuels in arid environments. Most of the arid environments are affected with saline and sodic soils, which represent 23% and 37% of the cultivated lands, respectively. Main causes of soils with high salinity are the high temperatures, which tend to increase evaporation demand and bring up salts to the root zone, and scarce precipitation, which prevents removal of salts from the soil by leaching. Salt-affected and sodic soils as well as saline water supplies represent a major challenge for irrigated crop production in many parts of the world since they are a threat to sustain productivity in irrigated agriculture [
1].
Sorghum is promising as a bioethanol crop because it efficiently uses water, and it is well-adapted to semiarid regions where soil salinity is too high to grow most common crops economically and where groundwater with high salinity is the most important water source [
2]. Energy sorghum hybrids were genetically modified from forage sorghums to prolong their vegetative growth stage, and to produce more biomass, and they can reach up to six meters in height under optimal growth conditions [
3]. Also, energy sorghum hybrids have demonstrated to produce high biomass yields in a lifecycle of 60 to 90 days [
4]. They contain significant quantities of lignin, cellulose, and hemicellulose in their leaves, and their stems can be converted into ethanol [
5]. Therefore, plant species such as the lignocellulosic species can produce more biomass, and, consequently, they can also produce more bioenergy. The challenge is to find plant feedstocks that produce more biomass with fewer inputs (mainly fertilizer and water). Concentrations of soluble salts in the soil root zone beyond the threshold levels can reduce crop growth and affect yield of most agricultural crops. According to Leland and Eugene [
6], sorghum has a salt tolerance threshold of 6.8 dS m
−1, and its yield is reduced approximately 16% per dS m
−1 of soil salinity increase; therefore, sorghum is generally classified as a moderately tolerant crop. This threshold value and yield can vary depending on climate, soil conditions, sorghum varieties, and cultural practices [
1]. There are several situations associated with crop production in relation to saline-sodic soils and saline water. For example, some locations may present both soil and water saline and/or sodic conditions, and other locations may present only saline water but have healthy soils.
Competition for land use between biofuel production and food/feed crops could be prevented if biofuel crops were grown on marginal lands (saline or sodic soils) and aquifers with limited water quality (high salinity). In arid and semiarid areas irrigated with groundwater with high salinity, sorghum could offer the advantage of being a biomass crop that can be used for bioenergy production. The objective of this field study was to evaluate the dry biomass productivity, water use efficiency, and expected net returns of two energy sorghum cultivars grown in two different climatic environments with different saline and sodic soils and different irrigation water quality. Results of this study will help producers to identify opportunities and make better decisions to improve their crop productions.
3. Results
3.1. Environmental Conditions
The daily mean air temperature and total precipitation during the growing seasons in Pecos were 26.2 °C and 104 mm in 2014 and 27.9 °C and 46 mm in 2015. While in the experiments located in Weslaco, they were 27.7 °C and 148 mm in 2014 and 26.8 °C and 238 mm in 2015. The GDUs slightly varied through years and locations during the study period. In Pecos, the cumulative GDUs were 2063 °D and 1924 °D in 2014 and 2015, respectively, and in Weslaco they were 1897 °D and 1986 °D in 2014 and 2015, respectively. The lower daily mean air temperature and the low GDU observed in Weslaco in 2014 probably were due to the early sowing.
3.2. Sorghum Emergence
Sorghum emergence was affected by soil salinity in both cultivars. There was a numerical between locations in both years. However, there was not a significant difference between cultivars in each location. The average plant emergence in Pecos (irrigated with water of 6.4 dS m−1) was 41% lower than Weslaco (irrigated with water of 1.42 dS m−1). The overall plant emergences in Pecos were 63,906 and 65,006 plants ha−1 in 2014 and 2015, respectively, While the plant emergences in Weslaco were 108,516 and 110,765 plants ha−1 in 2014 and 2015, respectively.
3.3. Dry Biomass
The final dry biomass productivity at each growing season was obtained for each cultivar in both locations. Separate analysis of variance results for dry biomass productivity in Pecos and Weslaco showed no significant effect between cultivars (
P > 0.05) in both growing seasons (
Table 5). However, the combined analysis of variance showed that there was a significant difference in dry biomass between cultivars (
P < 0.05), whereas the interactions between cultivar and year were not significantly different (
P > 0.05). The averages of final dry biomass are reported in
Figure 3. Dry biomass productivity response was statistically different between cultivars. Although, there was not an analysis of variance to compare both locations because the dry biomass values were numerically different.
The sorghum cultivars Blade ES 5140 and Blade ES 5200 reached maximum productivities of 25.2 and 28.7 Mg h
−1, respectively. These final dry biomasses performed as expected and were comparable with yields obtained in similar conditions in previous studies [
16].
The observed average dry biomass productivities in Weslaco for cultivar Blade ES 5140 were 24.7 and 25.2 Mg ha
−1 in 2014 and 2015, respectively, whereas for cultivar Blade ES 5200 they were 28.0 and 28.7 Mg ha
−1 in 2014 and 2015, respectively. Differences in dry biomass in each cultivar were not significantly different between years despite the limited rainfall observed in growing season 2014 (which was 40% less than that observed in the 2015 growing season), the fewer cumulative GDUs recorded in 2014, and the fewer days in the 2014 growing season with day lengths larger than 12:20 h. A faster accumulation of stem biomass over leaf biomass in both cultivars was observed during the study period. Thus, at the end of growing seasons, stem biomass comprised about 82% of the total biomass of the cultivar Blade ES 5140 and 84% of the cultivar Blade ES 5200. These results were similar to those reported by Olson and Ritter [
17] that observed a stem biomass partition of 83%, whereas Meki and Ogoshi [
18] reported a stem biomass partition of 73% of the total biomass. As expected for both cultivars, flowering was not observed in any of the growing seasons.
In contrast to the results obtained in Weslaco, average dry biomass productivity of both Blade ES 51400 and Blade ES 5200 were around 50% lower in Pecos, TX. In Pecos, the two cultivars did not perform as expected because of the saline environment. A higher salinity was observed in Pecos (4076 ppm of total dissolved salts) compared to those reported in Weslaco (913 ppm of total dissolved salts). Additionally, the SAR was also higher in Pecos (19.2) compared to that in Weslaco (3.6). The dry biomass variability observed (
Figure 3) may be attributed to differences in soil properties and salinity among plots considering that we only sampled for dry biomass in area of 4 m
2 at harvest. Cultivars Blade ES 5140 and Blade ES 5200 reached an average dry biomass productivity of 12.0 and 14.1 Mg ha
−1, respectively.
3.4. Stem Diameter and Height of the Plant
Averages in stem diameter and height at harvest time at both experimental sites are presented in
Figure 4 and
Figure 5. Significant differences (
P < 0.05) were observed in both stem diameter and plant height between Blade ES 5140 and Blade ES 5200 across harvest dates in both growing seasons, 2014 and 2015 (
Table 5), and in both locations.
At the Weslaco site, stem diameters ranged from 40 to 54 mm in both cultivars at harvest. The stem diameters were 46 mm for the Blade ES 5140 and 49 mm for the Blade ES 5200 in 2014. Similar diameter thicknesses were observed in 2015, with diameter thicknesses of 47 and 50 mm for the Blade ES 5140 and Blade ES 5200, respectively. In contrast to Weslaco, stem diameters observed in Pecos ranged from 26 to 45 mm in both cultivars at harvest. Average stem diameters were 34 mm for Blade ES 5140 and 36 mm for Blade ES 5200 in 2014, whereas in 2015 Blade ES 5140 was 34 mm and Blade ES 5200 was 38 mm.
Plant heights at the end of both growing seasons were influenced by both sowing date and soil conditions in both experimental locations. The average heights observed in Weslaco in 2014 were 2.34 m for the Blade ES 5140 and 2.45 m for Blade ES 5200, whereas in 2015 they were 2.36 m for Blade ES 5140 and 2.49 m for Blade ES 5200. On the other hand, Pecos presented lower plant heights than those observed at Weslaco. Blade ES 5140 showed average plant heights of 1.31 and 1.29 m in 2014 and 2015, respectively, whereas Blade ES 5200 showed 1.41 and 1.40 m in 2014 and 2015, respectively.
3.5. Water Use Efficiency
The dry biomass productivity at the end of each growing season and the sum of the daily crop evapotranspiration were used to determine the water use efficiency (WUE) for each cultivar in both locations. A separate analysis of variance for WUE showed no significant effect (
P > 0.05) between cultivars, neither Pecos nor Weslaco, in both growing seasons (
Table 5). However, a combined analysis of variance (cultivar
year) showed that there was a significant difference in WUE between cultivars (
P = 0.014) in Pecos, and the interaction (cultivar
year) was not significant (
P > 0.05), whereas in Weslaco the sources of variations were not significantly different (
P > 0.05). The average WUE is reported in
Figure 6. WUE response was statistically equal between cultivars but different between locations.
At the Weslaco site, WUE ranged from 3.49 to 5.87 kg m
−3 in both cultivars (
Figure 6). The cultivar Blade ES 5200 obtained the highest average WUE values, 4.42 and 5.50 kg m
−3 in 2014 and 2015, respectively, whereas Blade ES 5140 obtained 3.90 and 4.83 kg m
−3 in 2014 and 2015, respectively. In contrast to Weslaco, Pecos observed lower values of WUE. At Pecos, values of WUE ranged from 0.81 to 2.07 kg m
−3 in both cultivars. The cultivar Blade ES 5200 obtained the highest average values of WUE, 1.63 and 1.53 kg m
−3 in 2014 and 2015, respectively, whereas Blade ES 5140 obtained 1.39 and 1.30 kg m
−3 in 2014 and 2015, respectively.
3.6. Soil and Water Chemistry
At the Weslaco site, the sorghum evapotranspiration (ETc) were 633 mm in 2014 and 545 mm in 2015, whereas at Pecos, the total ETs for the sorghum crop were 871 and 910 mm in 2014 and 2015, respectively. At Weslaco, a higher ET was observed in 2014 probably because the sorghum was grown eight days longer, resulting in more water use by the crop. Additionally, 2014 was a warmer year than 2015.
The total water applied for the Pecos Experiment was 1067 mm in seven irrigation applications. More irrigation water was applied at Pecos following local recommendations to leach salts and avoid crusting of the soil, which could impede germination. At the Weslaco Research station, the irrigation applied was 248 mm in 2014 and 304 mm in 2015 in three irrigations events. Less irrigation was applied in Weslaco because more precipitation was received during the growing season. Pecos has a dryer environment than Weslaco, and less rainfall was received.
The soil properties at the beginning and end of the experiment for both locations are presented in
Table 6. The soil properties did not change in Weslaco. The soil presented very low salinity and sodicity in Weslaco. However, the Pecos station presented high salinity of the soil and sodicity at the beginning of the experiment. The salinity and sodicity were reduced probably because there were very heavy rains before planting in 2015. The heavy rains could have leached some salts, and this may be the reason for the lower salinity at the end of the experiment. The Pecos station also has a high calcium content, which may help to reduce some sodicity. Although, the sodicity in Pecos was high at the beginning and end of the experiments. Additionally, seven irrigation events were applied during the season to keep a lower balance of salts in the soil.
3.7. Economic Analysis Results
Empirical results suggest that, under arid and semiarid environments in South Texas and current ethanol prices, it is not economically feasible to produce ethanol from neither sorghum cultivar Blade ES 5140 nor Blade ES 5200. Expected net returns are shown in
Table 7. Namely, average net returns of −1058 USD ha
−1 for Blade ES 5140 and −1131 USD ha
−1 for Blade ES 5200 were estimated under the arid conditions of Pecos. These compared to net returns of −1103 USD ha
−1 and −1156 USD ha
−1 for the Blade ES 5140 and Blade ES 5200 cultivars in the semiarid environment of Weslaco, respectively. In addition to yields, different cost structures were observed in both sites. Particularly, higher variable production costs were estimated in Weslaco mainly due to additional harvesting and hauling expenses compared to Pecos. On the other hand, higher irrigation costs were incurred in Pecos, where an average of seven irrigations were conducted compared to four irrigations executed in Weslaco. Moreover, in Pecos the irrigation cost was influenced by the pumping cost, which depended on the electricity cost (i.e., 0.094 USD kw-hr
−1), and the pumping depth, which was approximately 85.3 m deep. Also, using ground water generated additional repair and maintenance costs, machinery depreciation, and equipment investment. In Weslaco, surface water was used, and the water price scheme consisted of an initial flat rate of 34.6 USD ha
−1 and 29.6 USD ha
−1 per irrigation.
A break-even analysis was conducted to identify the minimum ethanol price needed to cover all feedstock and production processes (
Table 7). Under observed biomass productivity and considered biofuel production technologies, break-even results indicated that considerably higher ethanol prices were needed in Pecos compared to Weslaco. Specifically, ethanol prices ranging between 0.67 USD L
−1 and 0.70 USD L
−1 are required to generate non-negative net returns in the Pecos site compared to the counterpart prices of 0.53 USD L
−1 and 0.56 USD L
−1 needed in Weslaco. The break-even prices suggested for Pecos and Weslaco are higher and lower than the 2014 ethanol price reported by Enciso and Jifon [
14], respectively. Thus, favorable energy market conditions have existed in the recent past that support the production of ethanol from energy sorghum under semiarid environments.
4. Discussion
The difference of dry biomass productivity between locations was the main cause for the combination of soil conditions and the quality of irrigation water in addition to the distribution of precipitation, which was more evenly distributed in Weslaco than Pecos. In Pecos, during 2014, most of the rainfall was received by the end of the growing season during the months of September and October, while in Weslaco the rainfall pattern allowed to supplement irrigation and relief crop water stress. Similarly, in 2015, most of the rainfall was received at the end of the season in Pecos, while in Weslaco almost twice as much rainfall was received, and it was also evenly distributed during the growing season.
The dry biomass productivities obtained at the Weslaco site are comparable to those reported by Chavez and Enciso [
16], in which the energy sorghum was grown under similar conditions. They obtained productivities that ranged from 26.57 to 28.05 Mg ha
−1, which is similar to the ones observed by Rocateli and Raper [
19], who reported dry biomass yields of 26.0 to 31.6 Mg ha
−1 in a study conducted in the southeastern US, and the dry biomass reported by Palumbo and Vonella [
20], who reported 20.9 to 26.4 Mg ha
−1 in a Mediterranean environment. However, the dry biomass productivities obtained at the Pecos location were lower than most of the productivities reported in literature. In a similar arid environment, Ganjegunte and Ulery [
21] conducted a study to evaluate the dry biomass productivity of an energy sorghum cultivar irrigated with both urban wastewater with an electrical conductivity of the irrigation water (ECw) of 2.5 dS m
−1 and fresh water with an ECw of 1.3 dS m
−1. They reported productivities ranging from 18.24 to 33.19 Mg ha
−1. They concluded that bioenergy sorghum is highly tolerant to elevated salinity. In a similar study, Almodares and Hadi [
22] evaluated the stem biomass productivities of sweet sorghum under different levels of saline water. They observed yields of 54.07 Mg ha
−1 (wet basis) at 2 dS m
−1 and 50.65 Mg ha
−1 at 12 dS m
−1.
In
Figure 3, it can be observed that the dry biomass productivity in Pecos was lower than the one in Weslaco. This point is explained by the adverse soil conditions and the high saline concentration of the irrigated water, since the salt in the soil solution reduces leaf growth and, to a lesser extent, root growth, and it decreases stomatal conductance and, thereby, photosynthesis [
23]. The average yield in Pecos was 13.05 Mg ha
−1 and in Weslaco 26.63 Mg ha
−1 for the two cultivars during the two years of the study. The average dry biomass yield in Pecos was approximately 51% lower than the Weslaco biomass. Biomass accumulation varies among growing seasons and locations basically because of changes in weather conditions. The energy sorghum is very sensitive to the photoperiod, and it should be sown on dates with optimal temperatures and also that exceed the photoperiod trigger of 12 h 20 min, otherwise, energy sorghum cultivars behave like photoperiod-insensitive, short-day sorghums [
18]. According to Ritchie and Singh [
24], the rate of biomass accumulation is principally influenced by the amount of light intercepted by plants over an optimum temperature range.
Differences in water use efficiency among cultivars over seasons or years are caused, most of the time, because of the climatic conditions presented during the growing seasons, crop management, soil characteristics, and the capacity of crops to both extract water from the soil and produce biomass [
25]. The WUE values of the Weslaco sorghum cultivars were higher than those observed in Pecos (
Figure 6). Seasonal water use varied in both locations. More irrigation water was applied at the Pecos experiments to meet the water needs of the crop (
Table 2) because of the higher water demand caused mainly by the higher temperatures and the low relative humidity recorded during the growing seasons.
The experimental results of WUE obtained in the Weslaco experiments were in agreement with those reported by Rooney and Blumenthal [
4] who obtained WUE values that ranged from 3.0 to 4.7 kg m
−3 in a study carried out in the High Plains of Texas. Narayanan and Aiken [
26] reported that WUE values for sorghum ranged between 3.39 and 7.63 kg m
−3. All of these results and the results from Weslaco are within the commonly reported range of 2.8 to 12.6 kg m
−3 for sorghum [
27,
28]. However, the opposite case was observed in the results obtained in the Pecos experiments, which ranged from 0.81 to 2.07 kg m
−3 and are considerably below the range previously mentioned.
The salinity at Pecos was higher, and the soils presented more sodic conditions. More water was necessary to apply the leaching fractions and maintain a water balance in the soil. The cultivars Blade ES 5140 and Blade ES 5200 reached an average dry biomass productivity of 12.0 and 14.1 Mg ha
−1, respectively. These sorghum dry biomass productivities were similar to those reported for similar saline environments in Iran by Ranjbar and Ghadiri [
29] who used irrigation with saline water of 6 dS m
−1, and they obtained 10.15 Mg ha
−1 of dry biomass. In another experiment conducted in Northern Greece, Vasilakoglou and Dhima [
30] tested six different sorghum cultivars under a soil salinity of 6.9 dS m
−1 and obtained dry biomass productivities ranging from 8.1 to 20.9 Mg ha
−1.
The reduced biomass yield and higher water consumption, particularly observed in Pecos, combined with lower ethanol prices made economically not feasible the production of ethanol from energy sorghum under Texas arid and semiarid conditions. Substantial gains in productivity observed under semi-arid growing conditions in recent years [
14] suggest that low energy prices are one the main limiting factors to cellulosic-based ethanol production.
5. Conclusions
The results obtained in this study demonstrated that dry biomass productivity, plant height, stem diameter, and WUE of energy sorghum decrease significantly when it is grown under severe saline and sodic conditions commonly encountered in arid environments. The dry biomass yields of Pecos (arid environment) were 50% lower, due to poor germination, than those observed in Weslaco (semi-arid environment).
The lower values of WUE for Pecos resulted because of the lower productivity produced by higher salinity and sodic conditions, and it was necessary to apply more water to leach salts and to meet the higher evaporative demand. Furthermore, economic analysis indicated that, under current energy prices, it is not economically viable to produce ethanol using energy sorghum in Pecos and Weslaco, respectively. However, a favorable ethanol price outlook could make the semiarid cropland of Texas a feasible feedstock production alternative.