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Article

Assessment of Alternative Warm-Season Annual Grasses for Forage Production in Water-Limited Environments

by
Diego F. Aviles
1,
Alondra Cruz
2,
Caitlyn E. Cooper
1,
Whitney L. Crossland
3,
S. V. Krishna Jagadish
2 and
Aaron B. Norris
1,*
1
Department of Natural Resources Management, Texas Tech University, 2903 15th Street, Lubbock, TX 79409, USA
2
Department of Plant and Soil Science, Texas Tech University, 2911 15th Street, Lubbock, TX 79409, USA
3
Department of Animal and Food Sciences, Texas Tech University, 1308 Indiana Ave, Lubbock, TX 79409, USA
*
Author to whom correspondence should be addressed.
Grasses 2025, 4(3), 36; https://doi.org/10.3390/grasses4030036
Submission received: 21 June 2025 / Revised: 29 August 2025 / Accepted: 2 September 2025 / Published: 10 September 2025
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Abstract

As traditional forage crops demand substantial water, exploring alternatives with lower water demands can mitigate the strain on water supplies. This pot study evaluated five annual warm-season forages (forage sorghum (FS) [Sorghum bicolor (L.) Moench], prussic acid-free forage sorghum (PF) [Sorghum bicolor subsp. Drummondii], sorghum x sudangrass hybrid (SS) [Sorghum bicolor x drummondii], sudangrass (SU) [Sorghum sudanense (Piper) Stapf], and pearl millet (PM) [Pennisetum glaucum (L.) R. Br.]) under two different irrigation treatments (40% and 80% ETo). Morphological (leaf area, leaf count, plant height), biomass yield, nutritional content (nitrogen (N), acid detergent fiber, and in vitro true digestibility (IVTD)), and water use efficiency (WUE) parameters were assessed at 35 and 49 days after planting (DAP). Irrigation effects varied with time, more strongly influencing nutritive value at 35 DAP and morphological traits at 49 DAP. WUE was significantly affected by irrigation at both timepoints. No single forage consistently outperformed across all metrics. PF and SU had the most biomass (p < 0.01), while PM had the greatest N content (p < 0.01). However, PF and SU had the highest WUE for biomass and digestible dry matter (p < 0.01). These findings suggest PF and SU may improve forage system sustainability under limited water availability.

1. Introduction

Climate change has reduced water security by altering the patterns, magnitude, and frequency of precipitation; increasing severe droughts and temperatures; and decreasing the supply and recharge rate of groundwater [1]. Having sustainable access to water is one of the main ways in which agricultural production systems can become resilient to, and mitigate, the effects of climate change. When it comes to forage–livestock systems, an opportunity for sustainable use of water resources is by exploring alternative forages that are more drought tolerant and can maintain production under limited water conditions. Traditional forage crops such as alfalfa (Medicago sativa L.) and maize (Zea mays L.) demand substantial amounts of water for growth, increasing the strain on already limited water supplies [2]. Embracing innovative water-efficient alternatives becomes crucial in regions like the semi-arid Texas High Plains, where water scarcity is a pressing issue. By transitioning to crops that require less water for growth and maintenance, such as certain sorghum (Sorghum spp.) varieties, farmers can reduce the strain on water supplies while still ensuring adequate forage for livestock production.
Current forage production systems in the Texas High Plains heavily rely on irrigation to produce high-quality and high-yielding forage [3]. This reliance on irrigation exacerbates the water scarcity problem in the region, as water resources are finite and increasingly strained due to factors like population growth and climate change. Additionally, the energy-intensive nature of irrigation further adds to the environmental footprint of traditional forage production methods. For example, Cardozo et al., 2016, revealed that greenhouse gases emissions are 71.2% higher in irrigated agriculture than in rainfed agriculture [4], with irrigation accounting for 28–50% of the total energy consumption in crop production [5]. Therefore, there is a pressing need to shift towards more sustainable forage alternatives that can thrive with minimal irrigation.
Sorghum spp. emerge as promising alternative warm-season annual forage crops due to their ability to thrive in arid and semi-arid climates with minimal irrigation while maintaining biomass production [6]. Sorghum’s drought tolerance can be attributed to reduced stomatal conductance and transpiration rate under high temperatures, delayed or accelerated maturity, and its ability to grow deep root systems to reach water in deeper soil profiles [7]. Sorghum also exhibits high water use efficiency (WUE), producing high yields per unit of irrigation water, with reported values ranging from 4.45 to 116 kg ha−1 mm−1 [7,8,9,10,11]. These characteristics make sorghum a well-suited forage for regions facing water scarcity challenges like the Texas High Plains.
Beyond its water-efficient characteristics, sorghum also offers significant nutritional value for livestock. On average, across growth phases, sorghum provides 9% crude protein and 58% total digestible nutrients, meeting nutritional requirements for most livestock production stages [12]. By incorporating Sorghum spp. into forage production systems, producers may be able to reduce water usage while maintaining livestock performance and promoting sustainable agricultural practices in water-stressed regions. However, there currently are many cultivars and hybrids of Sorghum spp. available and responses to drought will vary across these hybrids, especially since many of these responses are controlled by different genetic traits [7]. Additionally, the comparison of various alternative warm-season annual forages is limited in the literature, particularly at divergent soil moisture levels. Because of this, it is important to carry out comprehensive studies that compare available Sorghum spp. cultivars and their responses to divergent soil moisture conditions. This is particularly true for the novel prussic acid-free (PF) variety that has not been thoroughly assessed in semi-arid regions, where it has the greatest potential to influence production. This research uniquely incorporates the recently developed PF sorghum into a comprehensive water × species evaluation, filling a gap in understanding its performance compared to conventional forage sorghums. The aim of this study was to assess the production attributes, yield, and nutritive value of five Sorghum spp. cultivars under drought and irrigated conditions, providing guidance to producers for selecting the most suitable species for their production system.

2. Materials and Methods

This study was performed at the Texas Tech Native Rangeland located in Lubbock, TX, USA. The climate for the location consists of an average annual precipitation of 460 mm with most precipitation falling in the summer [13]. Temperature ranges from a high of 34 °C in the summer to a low of −3 °C in the winter [14]. Forages evaluated in this study included conventional, brown midrib (BMR) forage sorghum (FS) [Sorghum bicolor (L.) Moench], BMR prussic acid-free sorghum x sudangrass hybrid [Sorghum bicolor subsp. Drummondii], BMR sorghum x sudangrass hybrid (SS) [Sorghum bicolor x drummondii], sudangrass (SU) [Sorghum bicolor (Piper) Stapf], and BMR pearl millet (PM) [Pennisetum glaucum (L.) R. Br.]. Forages were seeded at a rate equivalent to 1,600,000 plants ha−1 in thirty 0.09 m3 pots (46 cm in height and 55 cm in top diameter) that were filled with a mixture of native topsoil (Pullman clay-loam) and vermiculite in a five-to-one ratio, with pea gravel placed in the bottom of the pots (~2.54 cm) to prevent soil from washing. Soil samples were collected and analyzed at a commercial laboratory (Table 1). Soil nutrient levels were representative of regional soils used for annual forage production and there was no attempt to alter nutrient composition prior to planting. Two irrigation treatments were evaluated: 40% and 80% evapotranspiration (ETo). The 40% treatment was selected to impose water restriction, while the 80% treatment represents the typical peak irrigation capacity for local producers. Irrigation was provided to each pot using one pressure compensating dripper and a 4-way assembly manifold connected to an angled arrow dripper. The water used for irrigation had a pH of 7.78, hardness of 200 ppm, turbidity of 0 NTU, and conductance of 1236 µS cm−1 [15]. A randomized complete block design was implemented with each irrigation and species combination being replicated in triplicate, with each row of pots serving as a block. This design was chosen to account for potential environmental effects (e.g., light and wind).
On 9 June 2023, seeds were planted by hand at a depth of 2.54 cm using 3 rows spaced 15.25 cm apart and a plant spacing of 5 cm apart. Fertilizer was applied at a rate of 33.7 kg of N ha−1 for all treatments using Scotts® Osmocote PRO (19-6-9) granulated. For irrigation, evapotranspiration (ETo) was calculated using the Penman-Monteith equation [16] and weather data from the West Texas Mesonet weather station located on the TTU Native Rangeland, ~300 m from the study site. The crop coefficients (Kc) were selected based on values reported for sorghum in the Texas North High Plains [17]. For reference, Lubbock’s average annual reference ETo is 1500.63 mm, while the average annual precipitation is 469.14 mm [18]. Two sampling/harvest dates were implemented, 35 and 49 days after planting (DAP) corresponding to the 14 and 28 July 2023. The clipping height applied was below the growing point for all species and ultimately precluded a second harvest due to poor regrowth across treatments. At 35 DAP, the calculated total irrigation requirements were 76.5 and 152.9 mm for the 40 and 80% ETo levels, respectively, with actual amounts of water applied being 79.1 and 156.3 mm. By 49 DAP the calculated requirements increased to 117.7 and 235.4 mm, while the applied irrigations were 121.4 and 239.7 mm for 40 and 80% ETo levels, respectively. The small differences between calculated and applied irrigation were mainly due to rainfall, as the pots were maintained outdoors and thus received natural precipitation in addition to scheduled irrigations.
Sampling in each pot involved randomly selecting three plants (one per row) to measure morphological parameters. Leaf area was measured using a LI-COR LI-3000c portable leaf area meter (LI-COR Environmental, Lincoln, NE, USA). Height was measured from ground level to the apex of the plant using a ruler. Afterward, selected plants were clipped at a height of 3 cm above the soil. The clipped samples were placed in a drying room at 55 °C until no further change in weight was measured. Once dried, each plant was weighed using a precision balance (accurate to 0.001 g) to determine dry matter biomass production per plant. Biomass per ha was calculated by multiplying individual plant biomass by planting population, with no correction for estimated plant mortality. The samples were then ground to a 2 mm particle size using a Wiley mill (Thomas Scientific, Swedesboro, NJ, USA) and stored until nutritive analyses were carried out.
Nitrogen (N) was obtained using the combustion method AOAC 990.03 [19]. Neutral detergent fiber (NDF) and acid detergent fiber (ADF) were determined by methods described originally by Van Soest & Robertson 1979 [20] using an ANKOM 200 fiber analyzer (ANKOM Technologies, Macedon, NY, USA). In vitro true digestibility (IVTD) and in vitro neutral detergent fiber digestibility (NDFD) were determined using 48 h batch in vitro fermentations [21,22] using a Daisy incubator (ANKOM Daisy II incubator, ANKOM Technologies) with rumen inoculum collected from fistulated steers maintained on a hay diet. Within a fermentation jar, all forage and irrigation combinations for an individual DAP were incubated simultaneously. A total of four jars per harvest date was used and served as laboratory replicates. Following 48 h of incubation, all bags were removed from the fermentation jars and individually rinsed with cold deionized water until the water ran clear. Post-incubation bags were washed with neutral detergent solution using the ANKOM 200 fiber analyzer and the same protocol for determining NDF followed by drying at 55 °C overnight. In vitro true digestibility and NDFD were calculated using the following equations [21].
I V T D ,   % = ( 1 − ( W 3 − W 1 × C 1 ) ) W 2 × 100
N D F D ,   % = ( 1 − ( W 3 − W 1 × C 1 ) ) W 4 × 100
where W3 is the dried bag weight containing sample residue following washing with neutral detergent, g; W1 is initial bag weight, g; C1 is the blank bag correction factor, g/g; W2 is initial dried sample weight, g; and W4 is initial NDF content of initial dried sample, g.
Water use efficiency (WUE) was calculated using total water input and different yield metrics. To determine water use, irrigation and precipitation were summed. However, for precipitation, only the millimeters of water required to reach field capacity were included, ensuring that excess water beyond soil retention capacity was excluded.
First, WUE was expressed as the ratio of total dry matter to seasonal water use [23]:
W U E   ( k g   m m − 1 ) = D r y   m a t t e r   y i e l d   k g   h a − 1 T o t a l   w a t e r   u s e d   m m   h a − 1
Additionally, digestible dry matter WUE (DDM–WUE)was assessed to account for forage quality:
D D M − W U E   ( k g   m m − 1 ) = D i g e s t i b l e   d r y   m a t t e r   y i e l d   ( k g   h a − 1 ) T o t a l   w a t e r   u s e d   ( m m   h a − 1 )
Lastly, nitrogen WUE (N–WUE) was evaluated by calculating the nitrogen yield per unit of water used:
N − W U E   ( k g   m m − 1 ) = N i t r o g e n   ( k g   h a − 1 ) T o t a l   w a t e r   u s e d   ( m m   h a − 1 )
Statistical analyses were performed using the GLIMMIX procedure in SAS version 9.4 (SAS Institute Inc., Cary, NC, USA). All response parameters were evaluated by DAP (harvest time) using a randomized complete block design to fit the full factorial of species and irrigation. The assessment of data by DAP was chosen due to being representative of production scenarios and the well-established relationship of plant maturity with plant morphology and nutritive value. Pots served as experimental units and row as a random factor. Normality and homogeneous variances were checked for all response variables. Significant interactions were assessed, if present, otherwise only the main effects were evaluated and discussed. Mean comparisons were performed using the LSMEANS statement with the adjustment proposed by Tukey–Kramer for significant effects. Significance was declared at p ≤ 0.05 and tendencies were assumed at p ≤ 0.10.

3. Results & Discussion

3.1. Plant Morphology—Leaf Area, Leaf Count, Plant Height, & Biomass

No interaction effects were observed for morphological and biomass parameters; however, the main effects of species and irrigation were present. At 35 DAP, the species with the highest leaf area were FS, PF, and SU (Table 2). By 49 DAP, PF had the greatest leaf area compared to the remaining species. There was no effect of irrigation on leaf area at 35 DAP, but at 49 DAP 80% ETo had the greatest leaf area. Leaf surface area plays a key role in the photosynthetic capacity of a forage, with a larger surface area generally representing a higher rate of total photosynthesis per plant [24]. A larger leaf surface area enhances light capture, which can contribute to increased carbohydrate accumulation and biomass, but its efficiency depends on structural costs and species-specific traits [25]. At 49 DAP, those receiving more irrigation had greater leaf area, agreeing with previous findings that observed a reduction in leaf area in grain sorghum when exposed to water stress [26]. Leaf surface area significantly influences carbon gain through photosynthesis but plays a pivotal role in water loss via transpiration [27]. Under limited water conditions, plants reduce leaf area as an adaptive response to conserve moisture, primarily by decreasing cell expansion and division to reduce transpiration, serving as an avoidance mechanism [28,29]. In this study, plants with the greatest leaf area were associated with higher biomass yields, aligning with previous findings that observed a positive correlation between biomass accumulation, leaf area, and light interception in forage sorghum [30]. Studies with forage sorghum have reported an increase in LA as irrigation levels increase [31,32], while drought limited LA [31]. Apart from water stress, LA also varies across harvest interval, with LA decreasing greatly after 110 DAP [31]. Although our study duration was much shorter and we did not compare harvest dates, a numerical reduction in leaf area at the later harvest date is apparent for most species.
Leaf count is another important variable that will impact a plant’s photosynthetic rate, biomass accumulation, and nutritional quality. In this experiment, PM had the highest leaf count observed at 35 DAP, while at 49 DAP, both PF and PM had the highest leaf count. Irrigation had no effect at 35 DAP, but at 49 DAP 80% ETo had greater leaf numbers. The leaf count values observed in this experiment are comparable to those reported in previous studies evaluating PM under different fertilization regimes, where the control group had an average of 5.2 leaves at 30 DAP [33]. In a field experiment, Ali et al., 2015, observed leaf counts ranging from 12.52 to 15.25 leaves per plant at 60 DAP in four different sorghum hybrids under rainfed conditions [34], while Saberi et al., 2011, reported 12.4 leaves per plant in their study on two different forage sorghums [35]. The varying leaf counts observed and the dynamic relationship with environmental factors across warm-season annual forages demonstrate the need for further research with forage hybrids under different growing conditions.
In the present study, SU exhibited the greatest plant height at both harvest dates. While irrigation had no effect on plant height at 35 DAP, plants receiving 80% ETo were the tallest by 49 DAP. Plant height was shown to decline under water stress in maize genotypes [36], and severe drought stress was found to reduce the height of sweet sorghum and sorghum x sudangrass by 41% and 30%, respectively [37]. Similarly, sudangrass under a short precipitation season (234 mm) was 20% shorter compared to sudangrass grown under a long precipitation season (306 mm), although no significant difference in height was found between harvests at 28 and 56 DAP during the short rain season [38].
Different yields can be expected across species, particularly as hybrids are bred for different purposes such as height [7], dry matter yield [39], leaf to stem ratios [39,40], fiber content [41], and photoperiod sensitivity [42]. In the present study, SU produced the most biomass at 35 DAP, whereas at 49 DAP the greatest biomass was observed in PF (Table 1). Regardless of harvest date, PM and SS had substantially less biomass production, approximating 50% less yield than the top producer at each harvest date. In field research, non-BMR SS (1120 kg DM ha−1) underperformed BMR lines of SS (1502 and 1695 kg DM ha−1) [43] which aligns with our observation of relatively low biomass production in SS (3460 kg ha−1). While our SS yields were higher than those reported in earlier field studies, they still lagged behind other species evaluated in our trial. Similarly, studies on forage sorghum have reported dry matter yields ranging from 8100 to 20,610 kg ha−1 [42,44,45]; our FS yields were within these values. In Brazil, sudangrass hybrids yielded from 6892 up to 23,913 kg ha−1 under three different soil and climatic regions [46], which is comparable to our SU yields.
At 35 DAP, irrigation did not significantly affect biomass; however, at 49 DAP, forages under 80% ETo had 20% greater biomass production. Similarly, Schittenhelm & Schroetter 2014 reported that average dry weights of sweet sorghum and sorghum x sudangrass were reduced by 37 and 35%, respectively, from well irrigated to drought stress treatments [37]. In maize, optimal irrigation yielded 7260 kg ha−1 in a semi-arid environment compared to 5050 kg ha−1 under 60% deficit irrigation [47]. In forage sorghum, optimal irrigation conditions with a N fertilizer application of 450 kg ha−1 were observed to yield up to 40,030 kg ha−1, whereas under water-limited conditions, sorghum yielded as low as 9780 kg ha−1 [48]. In the current study, the difference in biomass production between irrigation levels is lower relative to those observed in other works. The lesser degree of biomass reduction is likely a result of low nutrient status, as fertility was not adequate to support potential production at the highest irrigation level.
Additionally, harvest date had a larger influence on yield than did species, with numerically higher biomass observed at 49 DAP for all forage species. Similar trends were observed for sudangrass harvested at 28 and 56 DAP [38]. Komainda et al., 2018, observed that maize plants harvested at an earlier date had up to 13% lower yields than maize plants harvested at later dates [49]. Salama et al., 2020, also observed biomass accumulation in maize hybrids was greater when harvested at 55 DAP compared to 45 DAP, with yields of 30,890 and 22,920 kg ha−1, respectively [50]. Similarly for sorghum-sudangrass hybrids, average yields in Arkansas were 1120 kg ha−1 at 34 DAP and up to 7433 kg DM ha−1 at 63 DAP [42]. It is evident that harvest date plays an integral role when attempting to optimize forage yield, with species, harvest date, and irrigation all having influence to varying degrees.

3.2. Nutritive Value—Nitrogen and Fibrous Components

Nitrogen concentration at 35 DAP was impacted by species with the PM having the highest concentration (p ≤ 0.05), but by 49 DAP there were no differences among species (p > 0.05; Figure 1A). At 35 DAP, plants receiving 80% ETo had higher N levels compared to 40% ETo (p ≤ 0.05), but by 49 DAP there was no difference (p > 0.05; Figure 1B). Overall, PM harvested at 35 DAP was the only species to meet the assumed minimum N concentration requirement for ruminants (1.2% N) [51,52]. There was a species effect for NDF concentration, with the lowest NDF concentration at both 35 and 49 DAP observed in PF (p ≤ 0.05, Figure 2A). Irrigation had no effect on NDF at 49 DAP, but at 35 DAP, the 80% ETo treatment had the lowest concentration (p ≤ 0.05; Figure 2B). Whereas for ADF, there was an interaction effect of species and irrigation for both harvest dates (p ≤ 0.05). At 35 DAP, the lowest ADF content was found in PF at 80% ETo, whereas at 49 DAP it was PF at 40% ETo (Figure 3). Similarly, the greatest ADF content was seen in PM at 80% ETo for both harvest dates. Overall, ADF values observed were below the maximum allowable ADF concentrations for dry and lactating beef cows, 42 and 39%, respectively [53]. Noteworthy is the considerably lower NDF content for PF across both harvest times relative to the other species. As well, the NDF and ADF values for PF and SS were remarkably stable across harvest times. The NDF content for PF and SS was very similar at 35 and 49 DAP, with the same trend observed for ADF in the 40% ETo.
Plant N concentrations are known to decline as crops grow, largely due to a dilution effect as fibrous constituents increase. Nitrogen uptake is not only regulated by soil available N, but also by the plant growth rate and development [54]. Biomass production and N uptake are not always linearly related, particularly in older plants, where increased biomass does not necessarily correspond to higher N uptake [54]. At 35 DAP, the forages were between the five-leaf and growth point differentiation stage. During this developmental stage the forages were growing at a rapid rate and required greater amounts of N. Adequate N at this crucial growth period helps the plant maintain its chlorophyll content, which is necessary for photosynthetic activity, especially as it prepares to enter the reproductive phases [55]. Because of this, forages in the vegetative growth stages will have a greater concentration of N relative to older forages. Beck et al., 2007, reported N concentrations of 1.88 and 1.73 for two BMR-SS lines at 41 DAP when fertilized with 57 kg ha−1 (17-17-17) [43]. Similarly, levels of N ranging from 0.98% to 1.8% were observed in PM hybrids when fertilized at 120 kg N ha−1 [50]. The N values for SS in the present investigation are relatively lower in comparison, which is likely attributed to the lower fertility, given that soil N levels were low (Table 1) and pots were only provided the equivalent of 33 kg of N ha−1. The N content of PM aligns well with previous works but appears more volatile as concentrations exceeded the minimum requirements for cattle at 35 DAP but at 49 DAP, it decreased substantially and only approximated the minimum requirements.
Generally, N and fibrous constituents are inversely related due to the dilution effect whereby fibrous component accumulation exceeds N uptake. Previous research has observed negative correlations between cellulose and hemicellulose accumulation and N, with plant N concentration decreasing more than 50% when harvested 30 days apart [56]. Rinne & Nykänen et al., 2000, also observed a decrease in N content with physiological maturation during primary growth and regrowth [57]. In the current study PM and SS had the lowest biomass production at 35 and 49 DAP (Table 1), which could be a primary driver of their elevated N concentrations due to a lesser dilution effect. However, PM had substantially greater N content than SS at 35 DAP although similar biomass was observed. This could possibly be a result of differing morphology, as PM had increased leaf area per unit height. However, since leaf to stem ratio was not assessed in this study, we are only able to speculate on the innate dynamics. Ultimately, N concentration in plants is driven by a multitude of factors including species, hybrid, morpho-physiology, and soil available N.
In this study, increased NDF and ADF values were observed in all species as harvest time progressed, this was expected and agrees with previous works [57,58,59]. Environmental factors such as water availability and nutrient uptake significantly influence NDF content in forage crops. Water stress can alter plant growth patterns, leading to changes in fiber accumulation. For instance, drought conditions often delay plant maturation, which can result in increased NDF concentrations due to prolonged structural development. Additionally, water deficits can impair nutrient uptake, particularly N and phosphorus, further affecting cell wall composition and fiber content [60]. Generally, as plants mature, NDF content increases and results in a reduction in both the rate and extent of fiber digestibility. This trend has been widely observed across forage species and is a key factor influencing the decline in nutritive value over time [59,61]. Overall, the fiber values observed in this study align with those observed by Beck et al., 2007, who reported 65% NDF and 39% ADF at 41 DAP in BMR-SS lines [43].
Ferreira et al., 2021, reported that drought stress increased the concentration of NDF in grasses, which agrees with our results at 35 DAP but not at 49 DAP [62]. In contrast, other investigations have reported that plants under water stress have reduced cell wall contents [63]. At 35 DAP, differences in NDF and ADF content between irrigated and water-stressed forages were evident (Figure 2), likely reflecting early growth responses to moisture availability. Meanwhile, plants with less available moisture may have accelerated structural development or reduced leaf expansion, contributing to higher fiber concentrations. However, by 49 DAP, the effect of early water stress diminished as all forages approached more advanced maturity stages with increasing NDF and ADF levels. This suggests that maturity-driven fiber accumulation may overshadow earlier drought induced differences [64]. On the other hand, other research found no differences in fiber contents under different irrigation regimes for perennial grasses, suggesting that factors such as plant maturity and seasonal changes exert a stronger influence on fiber accumulation than irrigation alone [65]. Although only two irrigation regimes were tested, the results revealed minimal differences in fiber content across the irrigation treatments. The stability in fiber levels indicates the potential capacity to maintain forage quality under limited water conditions. Our results demonstrate the complex interplay of plant genetics, environmental factors, and growth dynamics, while highlighting the need for refining forage management strategies aimed at optimizing livestock nutrition and agricultural productivity.

3.3. Digestibility—IVTD & NDFD

All species had an IVTD exceeding 50% at 35 DAP, whereas at 49 DAP only PF surpassed 50% IVTD (Figure 4A). At 35 DAP, the species with the highest digestibility were FS, PF, and SU. By 49 DAP, there were no differences in IVTD between species. The same trend was observed for NDFD, with FS, PF, and SU showing the greatest digestibility at 35 DAP, but by 49 DAP there were no differences in NDFD among species (Figure 4B). The observed IVTD at both harvest dates tends to correspond with leaf area, but leaf to stem ratio would likely better describe this relationship.
Although not statistically assessed, a substantial numerical reduction in IVTD was observed for FS, PF, and SU between the two harvest periods which contrasts with other studies that observed minimal decreases in IVTD between 48 and 61 DAP for FS and SS [66]. Similarly, no difference in digestibility was observed over time when harvesting PM at boot stage (~49 DAP) vs. heading stage (~63 DAP) [67]. Of the species assessed, SU and FS had the greatest reduction in IVTD, decreasing 15.9% on average. Whereas SS and PM decreased 5.2%, on average, with PF acting as an intermediate with an 8.3% reduction in IVTD. However, across species, IVTD does not align well with fibrous components and demonstrated very weak relationships with NDF and ADF, r = −0.33 and −0.38 (p ≤ 0.01), respectively. Many studies, including the current one, generally rely on timed harvests; however, phenological maturity at harvest is considered the most important single factor determining forage quality [68]. In our investigation, plants were in vegetative growth stages during both harvest points, but at 49 DAP all plants species were approaching flag leaf stage. Because of the harvest stages, the corresponding reduction in digestibility is surprising.
In this study, no difference was observed between the irrigation regimes when evaluating IVTD. Similarly, under drought conditions, water supply had minimal effect on lignin concentration and did not impact the IVTD of maize for silage [62]. It has been suggested that drought stress generally increases cell wall concentration but enhances digestibility in maize due to reduced lignin production, leading farmers, nutrition consultants, and extension educators to believe that water stress enhances the digestibility of cell walls [62]. However, the current study suggests that more research is needed to understand whether digestibility is influenced by changes in the cell wall structure or composition under irrigated vs. drought stress conditions, and how these conditions can affect other plant metabolic functions.

3.4. Water Use Efficiency

At 35 DAP, the species with the highest dry matter WUE were PF and SU, whereas at 49 DAP, only PF had the highest dry matter WUE (Figure 5). Due to our study design, this aligns with the biomass values, where species with the lowest biomass values also had the lowest WUE. However, the 40% ETo treatment resulted in the highest dry matter WUE for both harvest times. We observed that although using less water results in lower biomass yield, it remains more efficient in terms of WUE. This agrees with previous work that observed increased WUE with increased moisture stress [69]. Between species, sorghum has generally outyielded pearl millet under moisture stress conditions, with sorghum having the highest WUE followed by pearl millet and maize [70,71]. Similarly, maize demonstrated lower WUE compared to forage sorghum and BMR forage sorghum for fresh silage yield [72]. Farhadi et al., 2022, observed sorghum and pearl millet as sustainable alternative forage options, achieving maximum green forage yield with optimal irrigation and the highest WUE under water-limited conditions [8]. Rostamza et al., 2011, reported that maintaining irrigation at 40% depletion of total available soil water increased WUE from 2.84 to 3.44 kg m−3 compared to 100% depletion, allowing for more biomass per unit of water [69]. For pearl millet cv. BRS-1501, the highest dry biomass yield was 12,100 kg ha−1 with 260 mm of irrigation but water productivity peaked at 80 mm, reaching 9.0 kg m−3 [73]. The WUE values obtained in this study fall within, or even exceed, the ranges reported in the literature. At 35 DAP, WUE ranged from 40.26 kg ha−1 mm−1 for PM to 67.55 kg ha−1 mm−1 SU, and increased at 49 DAP, reaching up to 100 kg ha−1 mm−1 for PF. These findings align with reported WUE ranges for sorghum cultivars (4.45–116 kg ha−1 mm−1) and pearl millet (8.4–92 kg ha−1 mm−1), with several values in this study clustering toward the higher end of those distributions [7].
A similar trend was observed for DDM–WUE. At 35 DAP, PF and SU had the highest values and at 49 DAP, PF remained the highest. Likewise, the 40% ETo treatment had the highest DDM–WUE at both time points (Figure 6). Our results align with previous studies, showing that increased irrigation in sorghum increases the individual plant water consumption and decreases the WUE, ultimately leading to a reduction in DDM–WUE [74,75]. Other studies identify leaf area development as a key factor influencing water use [76]; however, in our case, no relationship was found between the two. The DDM–WUE metric is particularly valuable, as it reflects the amount of digestible biomass available for livestock consumption per unit of water applied, offering a practical measure of forage quality under differing environmental conditions. Based on these results, PF harvested at 49 DAP emerged as the most efficient forage in terms of digestible biomass per unit of water used. PF showed a notable increase from 35 to 49 DAP, which is consistent with its unique performance among the forages, being the only one to maintain IVTD above 50% (Figure 4A) and achieving the highest biomass yield. Despite the additional 14 days of irrigation, the final harvest proved more efficient in converting water into usable forage. Although we did not assess economic feasibility of the various scenarios, for producers with the objective of maximizing digestible yield, the additional irrigation days may represent a worthy investment.
At 35 DAP, the species with the highest N–WUE were PF, PM, and SU, whereas at 49 DAP no differences were observed (Figure 7). The highest N–WUE was recorded at 40% ETo at both 35 and 49 DAP. Our results concur with previous research indicating that reduced irrigation levels can decrease biomass yield but enhance forage quality. Limiting irrigation reduced herbage yield but improved crude protein and digestibility per unit of water [77]. Similarly, other studies have demonstrated that increasing moisture stress, such as moving from full to deficit irrigation, reduced total dry matter, leaf area index, and N utilization efficiency, but increased crude protein content and water use efficiency [70]. N–WUE metric is important as it reflects the N output per unit of water applied, providing insight into the forage’s capacity to provide N efficiently in different production settings. It is important to note that crude protein is often the most critical and costly component in livestock rations, significantly influencing animal performance and overall feed efficiency [78]. After evaluating both N–WUE and DDM–WUE, PF and SU emerged as the top-performing forage species across both parameters, indicating their strong potential as the most efficient options under water-limited conditions.

4. Conclusions

This study’s results reveal intricate interaction effects between species and irrigation levels, and time across NDF and ADF. While N concentrations varied between species and time points, with 35 DAP generally having greater N concentrations in comparison to 49 DAP, fibrous constituents such as NDF and ADF increased as harvest time progressed. Remarkably, specific species such as PF and SS consistently exhibited superior fiber profiles within distinct DAP intervals. All species had IVTD greater than 50% at 35 DAP, but only PF maintained this level of digestibility at 49 DAP, with irrigation level having minimal effects. Biomass production was influenced by species and harvest time, with certain hybrids exhibiting higher yields at specific harvest dates, showcasing the importance of harvest time on biomass production. After evaluating both N–WUE and DDM–WUE, PF and SU emerged as the top-performing forage species across both parameters, indicating their strong potential as the most efficient options under water-limited conditions. Overall, these findings offer valuable insights into forage nutritional value and yield under contrasting water conditions, aiding producers in optimizing species selection and harvest timing for enhanced livestock nutrition and agricultural productivity.

Author Contributions

Conceptualization, D.F.A. and A.B.N.; methodology, D.F.A., C.E.C., A.C., S.V.K.J., W.L.C. and A.B.N.; formal analysis, D.F.A. and A.B.N.; investigation, D.F.A. and A.C.; resources, C.E.C. and A.B.N.; data curation, D.F.A.; writing—original draft preparation, D.F.A.; writing—review and editing, D.F.A., C.E.C., A.C., S.V.K.J., W.L.C. and A.B.N.; supervision, A.B.N.; project administration, A.B.N. All authors have read and agreed to the published version of the manuscript.

Funding

Texas Tech University, Davis College of Agricultural Sciences and Natural Resources.

Acknowledgments

We would like to express our sincere gratitude to Adrian Gonzales, TTU range barn manager, for his invaluable assistance. We also extend our heartfelt thanks to our lab peers for their collaboration and to the undergraduate research assistants for their efforts in processing samples. Seeds of sorghum hybrid carrying the PF TraitTM (Prussic Acid-Free; U.S. Patent No. 9,512,437) were supplied by S&W seed company (Longmont, CO, USA) under a material-transfer agreement. PF TraitTM eliminates dhurrin biosynthesis, thereby preventing prussic-acid accumulation in sorghum forage. The technology was invented by Mitchell R. Tuinstra, (Tuinstra et al., 2016, Purdue University) and is licensed exclusively to S&W seed company by the Agricultural Alumni Seed Improvement Association, Inc. The PF TraitTM and Prussic Acid FreeTM are trademarks of S&W seed company.

Conflicts of Interest

The authors declare no conflict of interest. S&W seed company had no role in the study design, data analysis, decision to publish, or preparation of the manuscript.

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Figure 1. (A) Effect of species on nitrogen concentration (%). (B) Effect of ETo on nitrogen concentration (%). Different lowercase letters (a, b) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass. The dashed line indicates the assumed minimum nitrogen required to meet ruminant requirements (1.2% N; 7% crude protein).
Figure 1. (A) Effect of species on nitrogen concentration (%). (B) Effect of ETo on nitrogen concentration (%). Different lowercase letters (a, b) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass. The dashed line indicates the assumed minimum nitrogen required to meet ruminant requirements (1.2% N; 7% crude protein).
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Figure 2. (A) Effect of species on NDF concentration (%). (B) Effect of ETo on NDF concentration (%). Different lowercase letters (a, b, c, d) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
Figure 2. (A) Effect of species on NDF concentration (%). (B) Effect of ETo on NDF concentration (%). Different lowercase letters (a, b, c, d) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
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Figure 3. (A) Effect of irrigation by species on ADF concentration (%) at 35 DAP. (B) Effect of irrigation by species on ADF concentration (%) at 49 DAP. Different lowercase letters (a, b, c, d, e) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
Figure 3. (A) Effect of irrigation by species on ADF concentration (%) at 35 DAP. (B) Effect of irrigation by species on ADF concentration (%) at 49 DAP. Different lowercase letters (a, b, c, d, e) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
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Figure 4. (A) Effect of species on in vitro true digestibility (%) at 35 and 49 DAP. (B) Effect of species on in vitro neutral fiber digestibility (%) at 35 and 49 DAP. Different lowercase letters (a, b, c) denote differences (p ≤ 0.05). The dashed line indicates assumed minimum digestibility necessary for production, 50% digestibility. FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
Figure 4. (A) Effect of species on in vitro true digestibility (%) at 35 and 49 DAP. (B) Effect of species on in vitro neutral fiber digestibility (%) at 35 and 49 DAP. Different lowercase letters (a, b, c) denote differences (p ≤ 0.05). The dashed line indicates assumed minimum digestibility necessary for production, 50% digestibility. FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
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Figure 5. (A) Effect of species on WUE (kg mm−1). (B) Effect of ETo on WUE (kg mm−1). Different lowercase letters (a, b, c) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
Figure 5. (A) Effect of species on WUE (kg mm−1). (B) Effect of ETo on WUE (kg mm−1). Different lowercase letters (a, b, c) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
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Figure 6. (A) Effect of species on DDM WUE (kg mm−1). (B) Effect of ETo on DDM WUE (kg mm−1). Different lowercase letters (a, b, c) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
Figure 6. (A) Effect of species on DDM WUE (kg mm−1). (B) Effect of ETo on DDM WUE (kg mm−1). Different lowercase letters (a, b, c) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
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Figure 7. (A) Effect of species on N WUE (kg mm−1). (B) Effect of ETo on N WUE (kg mm−1). Different lowercase letters (a, b) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
Figure 7. (A) Effect of species on N WUE (kg mm−1). (B) Effect of ETo on N WUE (kg mm−1). Different lowercase letters (a, b) denote differences (p ≤ 0.05). FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
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Table 1. Soil analysis results.
Table 1. Soil analysis results.
ResultsCL aInterpretation
pH85.8Mod. Alkaline
Conductivity (µS cm−1)277nanone
Nitrate-N (ppm)8naVery Low
Phosphorus (ppm)250Low
Potassium (ppm)6570Very High
Calcium (ppm)4941180High
Magnesium (ppm)61750Very High
Sulfur (ppm)6313Very High
Sodium (ppm)366naLow
a CL = critical level, the point at which no additional nutrient is recommended (excluding nitrate-N, sodium, and conductivity).
Table 2. Leaf area, leaf number, plant height, and biomass of five different warm-season annual species under 40 and 80% ETo irrigation regimes at 35 and 49 DAP. Values are estimated means and standard error of the means is presented.
Table 2. Leaf area, leaf number, plant height, and biomass of five different warm-season annual species under 40 and 80% ETo irrigation regimes at 35 and 49 DAP. Values are estimated means and standard error of the means is presented.
SpeciesIrrigationp-Value
ParameterFSPFPMSSSUSEM40%80%SEMSpeciesIrrigationS × I
35 DAP
Leaf area, cm24.12 a4.60 a3.23 b3.85 ab4.08 a66.274.093.8662.790.03140.36910.2444
Leaf number5.51 b5.43 b6.85 a5.35 b5.43 b0.215.665.760.18<0.00010.44090.1658
Plant height, cm83.47 b86.85 b54.68 d67.38 c106.33 a2.2130.7931.992.09<0.00010.16950.8042
Biomass, kg ha−14887.08 b5633.9 ab3301.07 c3460.03 c6529.92 a651.455043.724481.07567.8<0.00010.13110.4518
49 DAP
Leaf area, cm23.62 b5.85 a3.36 b2.95 b2.78 b64.293.17 y4.26 x58.2<0.00010.00250.3245
Leaf number3.78 b5.28 a5.20 a3.28 b3.87 b0.503.92 y4.65 x0.04<0.00010.00410.3266
Plant height, cm102.87 b100.75 b69.21 c77.68 c130.81 a1.5035.86 y39.93 x0.95<0.00010.00670.7347
Biomass, kg ha−19076.80 bc13,534 a7452.61 cd6674.70 d10,829 b1096.508512.73 y10,514 x966.08<0.00010.00130.6381
Different lowercase letters (a, b, c, d) denote differences (p ≤ 0.05) within species. Different lowercase letters (x, y) denote differences (p ≤ 0.05) within irrigation. S = species, I = irrigation, FS = forage sorghum, PF = prussic acid-free sorghum, PM = pearl millet, SS = sorghum x sudangrass hybrid, and SU = sudangrass.
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MDPI and ACS Style

Aviles, D.F.; Cruz, A.; Cooper, C.E.; Crossland, W.L.; Jagadish, S.V.K.; Norris, A.B. Assessment of Alternative Warm-Season Annual Grasses for Forage Production in Water-Limited Environments. Grasses 2025, 4, 36. https://doi.org/10.3390/grasses4030036

AMA Style

Aviles DF, Cruz A, Cooper CE, Crossland WL, Jagadish SVK, Norris AB. Assessment of Alternative Warm-Season Annual Grasses for Forage Production in Water-Limited Environments. Grasses. 2025; 4(3):36. https://doi.org/10.3390/grasses4030036

Chicago/Turabian Style

Aviles, Diego F., Alondra Cruz, Caitlyn E. Cooper, Whitney L. Crossland, S. V. Krishna Jagadish, and Aaron B. Norris. 2025. "Assessment of Alternative Warm-Season Annual Grasses for Forage Production in Water-Limited Environments" Grasses 4, no. 3: 36. https://doi.org/10.3390/grasses4030036

APA Style

Aviles, D. F., Cruz, A., Cooper, C. E., Crossland, W. L., Jagadish, S. V. K., & Norris, A. B. (2025). Assessment of Alternative Warm-Season Annual Grasses for Forage Production in Water-Limited Environments. Grasses, 4(3), 36. https://doi.org/10.3390/grasses4030036

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