Crotalaria juncea Genotype Biomass Accumulation in Northern Semi-Arid and Humid-Continental Climates

Abstract

Crotalaria juncea (sunn hemp) is a tropical forage legume used as a cover, forage, and fiber crop. Sunn hemp seed production occurs primarily in India because it requires short days to flower and set seed. Seeds available for production are typically non-specific genotypes instead of true breeding varieties. As sunn hemp is grown in more locations, understanding not only its performance in different growing conditions but also variations in genotype performance is critical for production management. We evaluated the growth and biomass accumulation of four genotypes (KMB1, KMB2, Thailand Original Sunn, and ‘Tropic Sunn’) of sunn hemp grown in northern semi-arid and humid-continental environments, Wyoming (Adams ‘22 and ‘23 (irrigated), Wyarno ‘23 (rainfed)) and Minnesota (Morris ‘22 and ‘23), USA. Thailand Original Sunn had the fastest growth rate (height over time) but the slowest canopy closure (NDVI over time), while KMB1 had the slowest growth rate but the fastest canopy closure. While growth rates varied among sunn hemp germplasm, there were no marked differences in biomass accumulation when harvested at 60 and 90 days after planting. Although the genotype did not have a significant effect on biomass accumulation, the environment affected not only growth but also biomass accumulation. At 60 DAP, the sunn hemp biomass averaged 1836, 489, 2459, 3334, and 731 kg ha⁻¹ in the Adams ‘22, Adams ‘23, Morris ‘22, Morris ‘23, and Wyarno ‘23 environments, respectively. At 90 DAP, the sunn hemp biomass averaged 6459, 4573, 7979, 7403, and 2220 kg ha⁻¹ in the Adams ‘22, Adams ‘23, Morris ‘22, Morris ‘23, and Wyarno ‘23 environments, respectively. The growth rate, canopy closure, and biomass accumulation differed when compared between the semi-arid environments and the humid-continental environment, with the humid-continental environment producing faster growth and higher biomass. These findings support the hypothesis that genotypes are likely to perform as predicted within growing regions, but there may be room to improve performance in different environments through selective breeding.

1. Introduction

Sunn hemp (Crotalaria juncea) is used as both a cover crop and a forage crop, expanding from southern areas into northern areas of the United States [1,2]. Sunn hemp is an annual, warm-season legume that originated in India (Figure 1) [3]. Its rapid growth, deep tap roots, ability to fix atmospheric nitrogen, and drought tolerance make it a desirable cover crop for use in short-growing windows during the heat of summer [4,5].

Sunn hemp is currently being distributed by seed dealers throughout the United States and marketed for cover cropping, livestock grazing, and wildlife food plots. Seeds available for purchase are often ‘common’ seeds, meaning the distributors know the seed production company and often the genotype, but a variety has not been tested and released. Sunn hemp seed is produced in the tropics, primarily in India and Thailand, because sunn hemp is typically a short-day plant that does not reliably flower and set seed north of 28° latitude [6]. Depending on the nursery location and seed company providing the sunn hemp seed, the genotype used for seed production may vary [7,8]. Variations in genotypes being distributed by seed dealers could lead to differences in sunn hemp performance, including biomass accumulation and forage quality.

There are varieties of sunn hemp that have been released by various breeding programs: Auburn University released ‘AU Golden’ and ‘AU Darbin’; the United States Department of Agriculture and the University of Hawaii College of Tropical Agriculture and Human Resources jointly released ‘Tropic Sunn’; Ubon Ratchathani (Thailand) released ‘Crescent Sunn’ and ‘Ubon’; ‘Blue Leaf’ and others are marketed from India; Sunn Global Biologics LLC released ‘Global Sunn^®’ [9,10,11,12]. ‘Tropic Sunn’ was released in 1983. Due to its flowering response to short days, it does not reliably produce seeds in the continental United States, even in the southernmost parts of the country, but will set seed in Hawaii, USA [13]. The ability to reliably produce seeds within the continental USA would allow for a more local seed source to help meet demands, lower seed costs, and potentially allow for the development of regionally adapted varieties. Breeding programs, such as at Auburn University, aim to develop sunn hemp varieties that could produce seed in the southern part of the continental USA. ‘AU Golden’ has been shown to produce seed in Alabama and north-central Florida, which is most likely because the variety was developed from day-neutral germplasm obtained from the USDA germplasm collection (PI 322377) [10,14,15]. However, seeds from these true varieties are not available for commercial sale in most areas.

As sunn hemp utilization expands into northern US latitudes, research targeting the varietal selection, production potential, and crop management is needed to aid producers and seed suppliers in making sound management decisions. Multiple studies have reported on the biomass accumulation and forage quality of sunn hemp across geographic regions, but seed sources in the current body of research vary and may contribute to differences in sunn hemp performance. Sunn hemp dry matter accumulation has been shown to vary based on the growing environment, e.g., when harvested between 55 and 60 days after planting, biomass dry matter accumulation was 0.6–2.2 Mg ha⁻¹ in Lingle, Wyoming (Figure 1, #5) [16], 1–3.3 Mg ha⁻¹ in 60 days in Ona, Florida (Figure 1, #7) [12], 1.6 Mg ha⁻¹ in Columbia, Missouri (Figure 1, #6) [17], 3.5 Mg ha⁻¹ in Yogyakarta, Indonesia (Figure 1, #8) [18], and 5 Mg ha⁻¹ in Nakhone Ratchasima, Thailand (Figure 1, #9) [19]. Of these studies, only Garzon and Lepcha [12] used a named sunn hemp variety; the other studies used common seeds, making it difficult to know what biomass accumulation differences between studies are due to growing conditions and which are due to the genotype. Growth and biomass accumulation of sunn hemp are determining factors of its suitability as a forage or cover crop in different growing regions. To best understand sunn hemp production potential, research is needed not just on sunn hemp growth across regions but also on how genotypes perform relative to one another across regions.

In this study, we evaluated the performance of four sunn hemp genotypes produced in distinct growing regions of India, Thailand, and the United States. The unique seed production regions (Figure 1) were selected based on their diverse climate conditions and the possibility that the genotypes would have unique phenotypes. The objective of this study was to evaluate these four genotypes of sunn hemp for differences in growth rate and biomass accumulation across harvest times and growing environments and test the following hypotheses:

Hypothesis 1:

Sunn hemp genotypes will have different growth rates and biomass accumulation relative to each other.

Hypothesis 2:

Sunn hemp genotypes will differ in their performance across environments.

2. Materials and Methods

2.1. Study Site

Experiments were conducted across three locations (Wyoming: Wyarno and Adams, and Minnesota: Morris) in the summers of 2022 and 2023 (

Table 1. Information for five study environments in Morris, Minnesota and Sheridan, Wyoming in 2022 and 2023. Wyoming environments include the Adams irrigated farm and Wyarno dryland farm. Fertilizer rates, planting dates, and harvest dates are given for each environment and harvest time (days after planting).

Environment	Year	Irrigation	Fertility N-P-K kg ha−1	Planting Date	45 DAP 1 Harvest	60 DAP Harvest	75 DAP Harvest	90 DAP Harvest
Morris ‘22	2022	Rainfed	3.7-17.7-0.0	14 June	25 July	11 Aug	N/A	12 Sep
Morris ‘23	2023	Rainfed	0.0-17.6-13.7	1 June	17 July	31 July	N/A	29 Aug
Adams ‘22	2022	Irrigated	N/A	3 June	18 July	08 Aug	17 Aug	01 Sep
Adams ‘23	2023	Irrigated	N/A	24 May	N/D	25 July	14 Aug	25 Aug
Wyarno ‘23	2023	Rainfed	N/A	22 May	10 July	21 July	11 Aug	21 Aug

¹ DAP = days after planting; N/A = Not Applicable.

). Each unique site year is referred to as an environment. Plots were established in 2022 and 2023 at the University of Wyoming Sheridan Research and Extension Center Adams Ranch in Sheridan, WY, under pivot irrigation (Adam’s ’22 and Adam’s ’23), in 2023 at the University of Wyoming Sheridan Research and Extension Center Dryland Research Farm (Wyarno) east of Sheridan, WY (Wyarno ’23), and in 2022 and 2023 at the USDA-ARS Swan Lake Research Farm near Morris, MN (Morris ’22 and Morris ’23) (Figure 1). The University of Wyoming’s Adams Ranch (irrigated) and Wyarno Dryland Research Farms have Warno clay loam soil with a 0–15% slope. The Swan Lake Research Farm has Barnes loam soil with a 2–5% slope. Sheridan, WY, has a northern semi-arid climate with average summer high and low temperatures of 29 and 11 °C and annual precipitation of 409 mm (averages from 2006 to 2020) (ncei.noaa.gov/access/us-climate-normals/ (accessed on 5 August 2024)). Morris, MN, has a humid-continental climate with average summer high and low temperatures of 26 and 14 °C and annual precipitation of 660 mm (averages from 2006 to 2020) (ncei.noaa.gov/access/us-climate-normals/ (accessed on 5 August 2024)). Cumulative growing degree days (GDD base 10 °C) and precipitation are given for the duration of the study for each environment (Figure 2). Adams ‘22 plots received weekly supplemental irrigation at a rate of 25.4 mm week⁻¹. A total of 254 mm was applied during the growing season. Limited supplemental irrigation was applied to the Adams ‘23 plots due to above-average precipitation received during the growing season. Plots received supplemental irrigation at a rate of 17 mm week⁻¹ for six weeks starting 1 July 2023 for a total of 102 mm of applied irrigation in 2023. Cumulative precipitation plus irrigation is shown for Adams ‘22 and ‘23 in Figure 2.

2.2. Field Preparation, Design, and Planting

Sunn hemp was seeded across all environments when minimum air and soil temperatures were above 14 °C. Planting dates for each environment are presented in Table 1. Seeds were provided by GreenCover Seed (GreenCover, Bladen, NE, USA) and KMB Seeds (KMP Group LLC, East Brunswick, NJ, USA); KMB1 (Mirzapur, Agra, and Dudhinagar, Uttar Pradesh, India), KMB2 (Purnia and Saharsa, Bihar, India), Thailand Original Sunn (Thailand Sunn; Buriram, Chamni, & Pakhon Chai, Buri Ram Province, Thailand), and ‘Tropic Sunn’ (Kauai, HI; USDA-ARS varietal release). Genotypes were sourced from different seed production regions (Figure 1) but had not undergone any breeding selection, with the exception of Tropic Sunn. A seeding rate of 28 kg ha⁻¹ pure live seed (PLS) was used for all genotypes. Plots were arranged in a randomized complete block design with four replicates of each treatment.

Plots in the Wyoming locations were 1.5 m wide and 6.1 m long in 2022. In 2023, the plot size was increased to 6.1 m wide by 6.1 m long. Seeds were drilled on 17.9 cm rows at a depth of 2.5 cm. Fields were disked, cultivated, and packed prior to planting. Weeds were manually removed from plots as needed. Plots in the Minnesota location were 1.8 m wide and 7.6 m long. Seeds were drilled on 25 cm rows at a depth of 5 cm. Prior to planting, fields were prepared with light tillage using a field cultivator. Weeds were manually removed from plots as needed to maintain a nearly weed-free stand.

2.3. Data Collection

2.3.1. Crop Growth

Plant height was measured weekly starting one week after emergence (emergence occurs between 2 and 12 DAP depending on conditions), with the exception of the Morris ’22 environment, where data collection started two weeks after emergence. Three plants were measured per plot from the soil surface to the top of the apical meristem, and the average of the three was used for each plot-level observation. On the same day the heights were taken, a crop-normalized difference vegetation index (NDVI) was measured as a proxy for canopy closure using a Holland Scientific handheld CS-45 Rapid Scan (Holland Scientific Inc, Lincoln, NE, USA). Measures were taken 60 cm above the canopy in the second crop row from the edge, and the average NDVI per plot was recorded. Optical bands of 670 nm (red light; RED), 730 nm (far red light; RE), and 780 nm (near-infrared light; NIR) were measured, and the NDVI was calculated as (NIR − RED)/(NIR + RED).

2.3.2. Biomass Collection

Biomass was collected at three harvest times, 45, 60, and 90 days after planting (DAP; Table 1), across all environments and additionally at 75 DAP at the Wyarno and Adams environments. At the Wyoming locations, the biomass was hand sampled in two 0.25 m² plots in 2022 and used by a forage harvester in 2023. At the Minnesota location, the biomass was hand-sampled from three one-meter rows from each plot. The biomass was not sampled from the outside rows of the plots to avoid edge effects. The hand-sampled biomass was dried at 60 °C to a constant weight and weighed to determine the dry biomass accumulation for the area harvested to provide the kg ha⁻¹. The biomass harvested with the forage harvester was subsampled; subsamples were dried to determine the moisture content, and the moisture content was used for a dry weight conversion of the total biomass sample.

2.3.3. Seed Viability

Germination tests were run for the Thailand Sunn genotype from seed collected at Adams ‘22, Morris ‘22, and Morris ‘23. In 2022, pods were harvested at the end of the season in each of the Thailand Sunn plots at each environment, and two samples of 25 seeds/plot were placed into Petri dishes on germination paper with 4 mL of distilled water and placed inside a growth chamber at 25 °C for one week in the dark. In 2023, the pods were bulk-harvested from all four Thailand Sunn plots in the Morris ’23 environment, and three samples of 30 seeds/plot were germinated, as in 2022. Seeds were considered germinated when hypocotyl emergence was visibly evident.

2.4. Data Analysis

Effect of environment, genotype, DAP, and the environment by genotype interaction on height and NDVI were analyzed using PROC GLM in SAS v9.4 (Copyright© 2023 by SAS Institute Inc., Cary, NC, USA). The environment and genotype were treated as fixed discrete variables, and DAP was treated as a fixed continuous variable. A partial model was run to determine the categorical relationships, and the environment by the genotype interaction term was significant for the height; therefore, the height was analyzed within each environment. The environment by the genotype interaction term was not significant for NDVI, but to keep the analysis consistent between the two response variables, NDVI was also analyzed within each environment. The height and NDVI were modeled by the DAP for each environment using PROC GLM, where the genotype was a fixed, discrete variable and DAP was a fixed, continuous numeric variable. Best-fit regression models were selected in PROC GLM based on the significance of polynomial terms in the respective models. All models were first run as a third-order polynomial, and the highest-order non-significant terms were removed from each model until the least-complex model remained. When the genotype was significant for an environment, models for each genotype were developed; if the genotype was not significant for an environment, the modeled data included all genotypes together.

The biomass was modeled for each harvest time to determine the effect of the environment, genotype, and their interaction. The environment, genotype, and harvest time were all treated as fixed discrete variables. A Tukey adjustment was used to compare treatment means, and significance was based on a p-value < 0.05.

3. Results

3.1. Growth

3.1.1. Genotype Height over Time

The environment, genotype, days after planting, and the interaction of the environment and genotype significantly affected the sunn hemp height (

Table 2. ANOVA for the effects of growing environment, sunn hemp genotype, environment by genotype interaction, and days after planting for height and NDVI.

	Height	NDVI
Environment	<0.0001	<0.0001
Genotype	<0.0001	0.0463
Environment × Genotype	0.0029	0.8680
Days After Planting	<0.0001	<0.0001

). The genotype and DAP had a significant effect on height for all environments, except Wyarno in 2023, where the genotype was not significant (p-value = 0.083; Figure 3). Generally, sunn hemp growth over time followed a third-order polynomial regression, showing slow initial growth, faster growth from 50 to 75 DAP, and a decrease in the growth rate after 80 DAP (Figure 3). The exception was the Morris ’22 environment, where all genotypes except Thailand Sunn had a linear growth rate, and Adams ’23, where KMB2 had a second-order growth rate (Figure 3). For all environments where the genotype was significant, Thailand Sunn had the fastest growth rate, reaching 100 cm height first, followed by Tropic Sunn, KMB1, and KMB2, respectively (Figure 3). The Morris environments had faster growth rates, reaching 100 cm between 50 and 60 DAP, than the Adams and Wyarno environments, which reached 100 cm between 70 and 90 DAP (Figure 3).

3.1.2. Genotype NDVI over Time

The environment, genotype, and DAP all had a significant effect on the NDVI (canopy closure; Table 2). When evaluated by the environment, the genotype was only significant in the Adams ’23, Morris ’22, and Morris ’23 environments (Figure 4). The NDVI response over time had a third-order polynomial regression in all five environments, with the exception of certain genotypes in the Morris environments (Figure 4). Where the genotype was significant, KMB1 had a higher NDVI than the other varieties, while Thailand Sunn was the lowest, indicating that KMB1 has a faster canopy closure (Figure 4). This is the opposite of what was measured for the growth rate, where KMB1 was the shortest of the genotypes.

3.2. Biomass Accumulation

The environment affected biomass accumulation within each harvest time (

Table 3. ANOVA for the effects of environment, ecotype, and their interaction on biomass accumulation at each harvest time (DAP).

	45 DAP	60 DAP	75 DAP	90 DAP
Environment	<0.0001	<0.0001	<0.0001	<0.0001
Genotype	0.0025	0.6577	0.0233	0.8692
Environment × Genotype	0.7738	0.2811	0.0224	0.3743

). The two Minnesota environments (Morris ‘22 and ‘23) had a higher biomass accumulation than all of the Wyoming environments (Adams ‘22 and ‘23 and Wyarno ‘23) at the 45, 60, and 90 DAP harvest times (

Table 4. Mean biomass accumulation (kg ha⁻¹) by harvest time for each genotype by environment. Mean biomass for all genotypes within environment (DAP Average) and for genotypes across all environments (Genotype Average) are given for each harvest time. Different lowercase letters indicate significant difference (p < 0.05) between genotype average and between DAP average. For the 75 DAP harvest time different uppercase letters indicate significant difference (p < 0.05) between all environment by genotype combinations.

45 DAP Harvest Time
	Wyarno ‘23	Adams ‘22	Adams ‘23	Morris ‘22	Morris ‘23	Genotype Average
KMB1	234	490	ND	1567	2033	1121	ab
KMB2	120	275	ND	1215	1688	861	b
Thailand Sunn	434	465	ND	1764	2138	1249	a
Tropic Sunn	185	353	ND	1053	1465	791	b
45 DAP Average	243 c	396 c		1400 b	1831 a
60 DAP Harvest Time
	Wyarno ‘23	Adams ‘22	Adams ‘23	Morris ‘22	Morris ‘23	Genotype Average
KMB1	719	2452	544	2633	2833	1837	NS
KMB2	584	1327	417	2565	3281	1635	NS
Thailand Sunn	879	1818	458	2150	3735	1808	NS
Tropic Sunn	741	1748	538	2489	3485	1800	NS
60 DAP Average	731 d	1836 c	489 d	2459 b	3334 a
75 DAP Harvest Time
	Wyarno ‘23	Adams ‘22	Adams ‘23	Morris ‘22	Morris ‘23	Genotype Average
KMB1	1798 B	4673 A	2685 B	ND	ND	3052	a
KMB2	1899 B	4264 A	1980 B	ND	ND	2714	ab
Thailand Sunn	2190 B	4827 A	2250 B	ND	ND	3089	a
Tropic Sunn	1883 B	2520 B	2283 B	ND	ND	2229	b
75 DAP Average	1942 b	4071 a	2299 b
90 DAP Harvest Time
	Wyarno ‘23	Adams ‘22	Adams ‘23	Morris ‘22	Morris ‘23	Genotype Average
KMB1	2301	6824	4942	7858	6457	5676	NS
KMB2	2245	5841	4573	7560	8033	5650	NS
Thailand Sunn	2199	7231	3846	7558	7473	5661	NS
Tropic Sunn	2133	5941	4931	8939	7651	5919	NS
90 DAP Average	2220 d	6459 b	4573 c	7979 a	7403 ab

). The Adams ’23 environment had no harvestable biomass at the 45 DAP harvest and the lowest biomass (489 kg ha⁻¹) at the 60 DAP harvest. However, by the 75 DAP harvest, Adams ’23 surpassed the Wyarno ’23 biomass accumulation, 2299 and 1942 kg ha⁻¹, respectively (Table 4).

The average biomass varied by the genotype for all locations at the 45 (791–1249 kg ha⁻¹) and 75 (2229–3089 kg ha⁻¹) DAP harvests but not the 60 (1635–1837 kg ha⁻¹) or 90 (5650–5919 kg ha⁻¹) harvest (Table 3). The genotype effect on biomass accumulation was most notable at the 45 DAP harvest, where Thailand Sunn had the highest biomass accumulation (1249 kg ha⁻¹) and Tropic Sunn (791 kg ha⁻¹) and KMB2 (861 kg ha⁻¹) had the lowest (Table 4). The 75 DAP harvest was only performed in the Wyoming environments (Table 4). There was an interaction between the environment and genotype at the 75 DAP harvest. The Adams ‘22 environment had a higher biomass accumulation for all varieties (4264–4827 kg ha⁻¹) than the Wyarno ‘23 and Adams ‘23 environment (1798–2684 kg ha⁻¹), except for Tropic Sunn, which had the same biomass accumulation across all three environments (1883–2520 kg ha⁻¹) (Table 3 and Table 4).

3.3. Seed Viability of Thailand Sunn

Interestingly, Thailand Sunn consistently and uniformly flowered at 60 DAP across all environments (Figure 5). To test seed viability, pods were harvested in Morris ’22, Adams ’22, and Morris ‘23 and tested for germinability. None of the seeds harvested from Adams ‘22 germinated. However, field-harvested Thailand Sunn seeds had an average of 57% and 76% germination for Morris ‘22 and Morris ‘23, respectively.

4. Discussion

4.1. Sunn Hemp Genotypes Had Variable Growth

Sunn hemp growth patterns were comparable across environments, with all genotypes showing a slow initial growth (0–30 DAP) and more rapid growth after 30 DAP (Figure 3 and Figure 4). This is in agreement with previous growths reported for sunn hemp [16,20]. The exception to this trend was the Morris ’22 growth, which was linear for three of the four genotypes. This inconsistency in the growth rate can be attributed to starting the height data collection at 30 DAP when sunn hemp begins its more rapid growth. We expect that if data from 0 to 30 DAP were available to use in the model, the growth would have followed the conventional growth pattern. Interestingly, genotypes having faster growth rates (height over time) were not the same as those whose canopies closed fastest (NDVI over time). Thailand Sunn had the fastest growth rate but the slowest canopy closure (Figure 3 and Figure 4). Inversely, KMB1 had the slowest growth rate but the fastest canopy closure (Figure 3 and Figure 4). Shorter plants tended to branch more, while the taller, more rapidly growing genotypes produced fewer branches. Similar observations were reported by Morris and Antonious [21], who evaluated branching patterns and heights across 16 sunn hemp accessions. Stand density is a factor that can affect growth. The four genotypes used had thousand seed weights (TSW) of 41.5, 34.2, 29.2, and 40.8 g for KMB1, KMB2, Thailand Sunn, and Tropic Sunn, respectively. In-field populations were not directly measured, but the uniform seeding rate of 28 kg ha⁻¹ PLS would result in estimated seed populations of 67.5, 82.0, 96.0, and 68.6 seeds m⁻², respectively, for KMB1, KMB2, Thailand Sunn, and Tropic Sunn. Thailand Sunn, having the lowest TSW and highest estimated seed population, would be expected to have the highest population of plants. Higher plant populations can result in competition for light, which results in tall, thin plants. This would align with the height and NDVI data observed for Thailand Sunn. Likewise, KMB1 had the highest TSW and lowest estimated seed population, resulting in lower plant populations, reducing competition for light, and resulting in shorter, branched plants. This also aligns with the height and NDVI data observed for KMB1. Additional work on the genotype by an in-field population would be needed to determine if the observed growth differences between genotypes were a result of genotypic differences or plant density.

While the genotype had a significant effect on both the growth rate and canopy closure, the same response was not observed for biomass accumulation. Where height was the most similar between genotypes at 45 DAP, biomass was the most different between genotypes. At 60 and 90 DAP, biomass accumulation between genotypes was not different, while a separation in height and NDVI was more apparent. This may indicate that the variation in growth (height and NDVI) across genotypes helps to maintain the yield. Garzon et al. [12] reported no genotype effect on biomass accumulation between the sunn hemp varieties Ubon, Blue Leaf, and Tropic Sunn. However, a number of studies assessing sunn hemp genotypes have reported differences in biomass accumulation between varieties and genotypes, though there is some overlap between different genotypes [8,12,21,22]. These results support the hypothesis that the genotype may be an important variable in sunn hemp growth, and closer attention to genotypes used in research and for production may be warranted.

4.2. Sunn Hemp Growth Varied Across Environments

The growing environment had the biggest effect on sunn hemp performance (Table 2 and Table 3). The Minnesota environments supported the highest growth rates, canopy closure, and biomass accumulation. Morris had higher GDD accumulation and precipitation during the growing season than the three Wyoming environments (Figure 2). Additionally, Sheridan, WY, is at 44.7972° N, while Morris, MN, is at 45.5919° N, meaning that the Minnesota environments had a longer day length during the growing season. Furthermore, the environment in Wyoming is at an elevation of approximately 1160 m above sea level, has an average growing season (frost-free days) of 120 days, and has an average annual precipitation of 40.9 cm. Comparatively, Morris, MN, is at 345 m above sea level, has an average growing season of 145 days, and average annual precipitation of 66 cm. Sunn hemp growth responds to the day length, temperature, and moisture [22,23,24]. Kasirajan et al. [22] reported that the fiber yield of sunn hemp was positively associated with GDD and longer photoperiods. Within the Wyoming environments, Adams ‘22 had higher biomass accumulation than Adams ‘23 and Wyarno ‘23 (Table 4). The hot and dry conditions of the 2022 growing season resulted in crop failure under rainfed conditions but an optimal performance with supplemental irrigation in the Adams ‘22 environment (Figure 2, Table 4). Conversely, the high precipitation and low cumulative GDD of the Adams ‘23 environment resulted in biomass accumulation equivalent to that of the rainfed Wyarno ‘23 environment (Figure 2, Table 4). Irrigation alone was not enough to ensure a successful crop in Wyoming, but it can be a valuable tool to enhance sunn hemp growth during dry years.

The differences in environmental conditions and the growth response of sunn hemp indicate that the genotypes evaluated in this study are better suited to the humid-continental environment rather than the northern semi-arid environment. At 60 DAP, for the Morris ‘22 and Morris ‘23 environments, the range of biomass accumulation across genotypes was 2150 to 2633 kg ha⁻¹ and 2833 to 3735 kg ha⁻¹, respectively. In comparison, Garzan et al. [12] reported that across five genotypes evaluated in Ona, Florida, the range of biomass accumulation at 60 DAP was 1010 to 3360 kg ha⁻¹, indicating that sunn hemp biomass accumulation in Morris, Minnesota, at 60 DAP can reach that of a more southern, subtropical region of the US. However, further studies are needed to determine if there are other sunn hemp germplasm that could perform more consistently and with higher biomass yields in semi-arid environments, such as those in Wyoming, or if the tropical origin of sunn hemp will limit the environmental conditions where it will thrive. Of the four genotypes evaluated here, none were better suited to one environment over another.

Harvest time is an important component of forage production. A later harvest often results in higher biomass accumulation but lower forage quality [12,16]. As expected, we saw biomass accumulation increase with later harvest dates. This response was universal across the different genotypes and environments. Biomass accumulation after 60 days is often reported, as this is when the crop is optimized for yield and quality. However, these studies are often from more southern climates, and forage quality may not respond the same in northern climates. Where the Minnesota climate had similar biomass accumulation to that reported for sunn hemp grown in Florida after 60 days, the Wyoming environments reached these amounts at 75–90 DAP, indicating later harvest times may be recommended for this region (Table 3). Future research on forage quality should be assessed across the environment, harvest time, and genotype to determine the optimal harvest time for each environment and if the genotype affects the forage quality.

4.3. Seed Production Is Feasible in More Northern Environments

Currently, seed production is primarily performed in the tropics. The expansion of seed production into the mid-western United States could help meet market demands further north in the continental US and lower seed costs for those producers while also creating a more local seed source that could help to eliminate the potential for seed-borne pathogens from seeds produced in more tropical areas of the world. Our results demonstrate that viable seeds can be produced (with germination at 76% in the Morris ‘23 environment) as far north as 45.5919° N using the genotype Thailand Sunn. An additional germplasm evaluation is needed to see if there are other genotypes that respond similarly. However, these results are encouraging and show the potential for breeding sunn hemp adapted to more northern climates of the United States, with the potential for seed production in these areas as well. Further studies are needed on Thailand Sunn to determine if seed production from this common genotype could be carried out successfully in northern production regions. Based on the data we have presented here, Thailand Sunn is a viable option for use as a parent in US sunn hemp breeding programs. Furthermore, selections could be made from within Thailand Sunn that would increase seed viability even further in the northern regions.

5. Conclusions

Genotypes varied in their growth, but this did not translate into a difference in biomass accumulation during most harvest times. The only indication that individual genotypes may be better adapted to different environments was for the growth rate, where the environment by genotype interaction term was significant. However, the same interaction was not observed for biomass accumulation. Growth rate, canopy closure, and biomass accumulation were significantly lower in our semi-arid environments (Wyoming) than in our humid-continental environments (Minnesota). The average biomass accumulation peaked at 6.4 and 8.0 Mg ha⁻¹ in Wyoming and Minnesota, respectively. Further germplasm screening is needed to determine if there are genotypes that will respond better in more semi-arid conditions. However, the response of the genotype to the environment was not significant, indicating that the seed source should not affect performance in different environments and that common seeds will perform as expected for specific growing regions. Until breeding programs are able to produce enough seeds to be commercially available, it appears that the common sunn hemp available through seed companies should not be a cause of concern for growers.