Exploring the Influence of Environmental and Crop Management Factors on Sorghum Nutrient Composition and Amino Acid Digestibility in Broilers

Abstract

This exploratory study expected crop management and climatic factors to significantly influence the nutrient composition and amino acid digestibility of tannin-free sorghum grain determined in broilers of 3 wks of age. Using data from six tannin-free sorghum samples harvested across the southeast USA, Pearson correlations were analyzed (r ≥ |0.8|; p < 0.05). Standardized ileal amino acid digestibility (SIAD) was determined in a previous study using eight replicate cages with 13 birds per sorghum sample. SIAD values were correlated with nitrogen fertilization, yield, seeding rate, and climatic data obtained by surveying the crop growers and from weather stations. Nitrogen fertilization positively correlated with dry matter and starch. Yield was positively associated with SIAD, while seeding rate was negatively correlated with dry matter and Lys. Fiber, particular neutral detergent fiber, showed an inverse relationship with SIAD. No significant correlations with climatic factors were found, which was likely due to the close proximity of growing locations (r ≤ |0.8|; p > 0.05). Despite the limitations of a small sample size (n = 6) and genetic variability within and between each sorghum sample, these findings provide preliminary insights into managing sorghum cultivation to enhance its nutritional value for poultry. Future research should explore larger datasets, from further locations apart, and standardized data collection measurements to be able develop predictive models for grain quality improvement.

1. Introduction

Sorghum is a resilient, drought-tolerant crop that thrives in poor soils, making it a practical and sustainable choice for growers in the southeastern United States, which is a region heavily invested in poultry production [1,2]. Its cultivation in this area has the potential to reduce reliance on corn and decrease transportation costs, which could benefit both growers and poultry producers [3]. Modern tannin-free sorghum varieties provide a nutritional profile comparable to corn without the negative effects associated with tannins, making it a promising feed ingredient for broilers [4,5].

In poultry nutrition, tannin-free sorghum has emerged as a promising alternative to corn, offering a comparable nutritional profile without adverse effects on broiler performance [5,6,7]. Historically, sorghum’s use in feed was limited due to its high tannin content, which reduced palatability and nutrient availability [8,9]. However, nearly all sorghum grown in the U.S. has been tannin-free, allowing researchers to explore its full potential as a feed ingredient [4].

Environmental and genetic factors are well known to influence grain nutrient composition, including in sorghum [10,11,12]. For instance, drought conditions have been linked to decreased starch content and altered starch properties [13]. As a result, it potentially leads to increased viscosity in grain extract and bird intestinal contents, which could be detrimental to digestibility [14,15].

Although previous studies demonstrated the influence of agronomic practices on grain AA content, little is known about their impact on its digestibility. Only one research study evaluated the impact of N fertilization of different triticale varieties (hybrids of wheat and rye) on AA digestibility using cecectomized laying hens. The findings showed that fertilization impacted the grain AA content and its digestibility in laying hens [16]. In addition to the study by Siegert et al. (2017) [16], a review of sorghum as feed ingredient for broilers investigated the AA digestibility of grain harvested in 2004 and 2005 [5]. Significant differences in digestibility were found between harvest years, which highlight the influence of genotype, agronomic practices, and environmental conditions on sorghum’s protein composition [17].

Previous research from the authors used 3-week-old broilers to evaluate the standardized ileal amino acid digestibility (SIAD) of eight tannin-free sorghum samples from southeastern US states (three from North Carolina, four from South Carolina and one from Georgia). Results indicated that genetic differences among samples likely influenced digestibility, but environmental factors and agronomic practices may have also played a role [18].

Based on this prior research, the current study investigates the influence of environmental factors and agronomic practices on sorghum digestibility and nutrient composition. By exploring these relationships, the study aims to provide insights that support the production of more nutritious grains for poultry and enhance sustainability in the poultry industry without compromising animal performance.

2. Materials and Methods

2.1. Tannin-Free Sorghum Samples

This short communication serves as a companion paper to a previous study conducted by the authors, which provides a comprehensive evaluation of the SIAD values of 8 tannin-free sorghum samples tested in 3-week-old male broilers [18]. The current work focuses solely on the correlation analysis of agronomic and environmental factors with the SIAD values. The prior manuscript thoroughly addresses details of the digestibility trials and associated data (SIAD values and nutrient composition of the samples). They are not repeated here to maintain focus on the new analyses [18].

Agronomic data were collected to assess the impact of tannin-free sorghum production variables on grain nutrient composition and digestibility. Only 6 out of the 8 sorghum samples tested for SIAD in the previous study were considered in this study, as it was not possible to collect agronomic data for all of them.

The 6 samples considered in the current study were obtained from different places across the southeastern USA during the 2023 harvest season (

Table 1. Information about the sorghum crops from where the grain samples were collected.

Crop Information	SC-Flo	SC-Pei	GA-Gf	NC-LW	NC-Pl	NC-T2
Country	USA	USA	USA	USA	USA	USA
State	SC	SC	GA	NC	NC	NC
Town	Florence	Pelion	Griffin	Windsor	Plymouth	Plymouth
Type of grower	Research Station	Private Grower	Research Station	Research Station	Research Station	Research Station
Soil type	Sandy loam	Sandy loam	Sandy loam	Sandy loam	Sandy loam	Sandy loam
Seeding rate (seeds/acre)	87,120	71,000	80,000	120,000	120,000	120,000
Total N applied (gal N/acre)	141	215	173	144	144	144
Days grown	111	113	122	103	145	103
Yield (bush/acre)	125.6	75	83.5	60.4	97.3	69.3

All grain samples were planted and harvested in 2023. Abbreviations: SC (South Carolina), GA (Georgia), NC (North Carolina).

). Three of these sorghum samples were obtained from North Carolina (NC-PL, NC-T2, NC-LW), while two samples were from South Carolina (SC-Flo, SC-Pei), and another one was from Georgia (GA-Gf). It should be noted that each sample consisted of different sorghum genetic varieties (identity preserved) reflected by different seed colors within the sample.

Each grower collaborated with this study by providing the harvested sorghum samples and was surveyed about the management of the crop during its cultivation. More precisely, growers shared the location of the crop, seeding rate, total N applied, planting and harvest date, and yield. All the available information is summarized in Table 1, where the details for each crop sample can be observed.

2.2. Weather Information

As shown in

Table 2. Crop periodization to assess the influence of the environment on sorghum digestibility.

Stage	GDD
Period 1: Vegetative (planting date—panicle initiation)	0–600
Period 2: Reproductive (panicle initiation—Flowering)	600–1100
Period 3: Grain Fill (flowering—harvest)	1100-Harvest

, each crop’s growth period was divided into three key developmental stages: vegetative, reproductive, and grain fill, in which the average temperature (ºC) and cumulative precipitation (mm) were measured.

This periodization was completed using cumulative growing degree days (GDDs), allowing for a more standardized and accurate comparison of environmental impacts on grain composition and digestibility across different sorghum samples and growing conditions. The vegetative stage was considered from the planting date until approximately 600 GDDs. The reproductive stage was defined from panicle initiation until flowering, spanning approximately 600 to 1100 GDDs. Finally, the grain fill period was considered from flowering to harvest.

To calculate the GDDs for the different sorghum samples, daily temperature and precipitation data were obtained from NASA’s POWER Data Access Viewer website using the geographical location of each crop [19] along with the planting and harvest dates. The GDDs were then calculated using a base temperature of 10 °C, following Equation (1) provided by Roozeboom and Prasad (2019) [20]:

$$GDD=\left[{\displaystyle \frac{(Max. Temp+Min Temp.) }{2}}\right]-Base Temp.$$

(1)

Information about developmental stages was required to match each stage to the corresponding GDD. Vegetative stage and anthesis date data were available only from the Florence, South Carolina (SC-Flo), and Georgia (GA-Gf) samples. To increase the reliability of the periodization for all crops, additional data from two weather stations in Florence, South Carolina, were collected for the years 2019–2023. This included planting dates and anthesis dates from sorghum crops grown at the research station during those years (one crop per year).

The GDDs for sorghum crops cultivated from 2019 to 2023 were calculated to estimate the cumulative GDD required to attain different growth stages. These estimates were then used as average references to divide each sorghum sample in the current study into the three developmental stages mentioned in the first paragraph. Once data were cleaned and organized, information about the precipitation and temperature for each crop categorized by developmental stage is shown in

Table 3. Precipitation and temperature data categorized by developmental stages for each sorghum sample.

Grain	Grow Stage	Age (Weeks)	Cum. GDDs 1 (°C)	Precipitation (mm)	Average Temp (°C)	Max. Temp (°C)	Min. Temp (°C)
SC-Flo
	1. Vegetative	7	595	195	23.8	33.4	12.4
	2. Reproductive stage	10	1094	123	28.5	35.6	20.2
	3. Grain fill	16	1831	226	28.0	37.8	16.2
GA-Gf
	1. Vegetative	7	598	188	22.5	33.8	9.7
	2. Reproductive stage	11	1090	80	27.6	38.1	17.2
	3. Grain fill	22	2409	180	26.9	39.9	12.8
NC-LW
	1. Vegetative	5	595	121	28.6	36.2	20.8
	2. Reproductive stage	9	1090	65	28.3	38.3	19.3
	3. Grain fill	15	1645	237	22.6	35.1	6.5
NC-PT
	1. Vegetative	7	581	115	22.9	34.4	12.1
	2. Reproductive stage	11	1087	120	28.7	36.2	21.3
	3. Grain fill	21	2184	303	25.0	38.3	6.5
NC-T2
	1. Vegetative	5	595	121	28.6	36.2	20.8
	2. Reproductive stage	9	1089	56	28.3	38.3	19.3
	3. Grain fill	15	1655	246	22.8	35.1	6.5
SC-Pei
	1. Vegetative	7	595	243	22.9	32.9	12.6
	2. Reproductive stage	11	1083	108	28.1	35.4	20.1
	3. Grain fill	17	1805	170	28.1	38.8	16.8

¹ GDDs = growing degree days.

.

2.3. Experimental Design and Statistical Analysis

This experiment was conducted in accordance with principles and specific guidelines approved by the Clemson University Institutional Animal Care and Use Committee (IACUC) and Southern Poultry Research and Feed, Inc., Animal Use Protocol (AUP) 2023-0191.

To obtain the digestibility values, the study was designed as a randomized, incomplete block with each cage considered an experimental unit. There were 8 cages used as replicates for each dietary treatment (8 × 6 sorghum samples = 48 cages total). A total of 624 broilers were randomly distributed into 24 thermostatically controlled poultry battery brooders (Model 0540, GQF Manufacturing Inc., Savannah, GA, USA). Each cage had 13 birds and measured 81 cm in width × 91 cm in length × 25 cm in height [18].

Once the data were obtained from surveying the different growers, Pearson’s correlation was used to assess the associations between production, nutrient composition and digestibility variables using the Multivariate and Correlation function of JMP Pro 16 [21]. Associations were considered important if r ≥ |0.8|; p < 0.05. Scatter plots were also used to confirm the important associations.

3. Results

3.1. Influence of Agronomic Factors on Grain Composition and Digestibility

• Fertilization: As shown in

  Table 4. Correlations of agronomic variables on nutrient composition and AA digestibility of sorghum grain.

  Variables	Correlation Coefficient	Significance
  Fertilization
  SIAD 1 Ser	−0.96	<0.0001
  SIAD Trp	−0.83	<0.0001
  SIAD Tyr	−0.87	<0.0001
  SIAD His	−0.82	<0.0001
  Dry matter	0.94	<0.0001
  Starch	0.89	<0.0001
  Yield
  Mean SIAD	0.88	<0.0001
  SIAD Cys	0.91	<0.0001
  SIAD Ile	0.94	<0.0001
  SIAD Met	0.95	<0.0001
  SIAD Phe	0.85	<0.0001
  SIAD Pro	0.95	<0.0001
  SIAD Val	0.86	<0.0001
  Seeding Rate
  Dry matter	−0.83	<0.0001
  Lys	−0.92	<0.0001
  Tyr	0.92	<0.0001

  SIAD ¹ = standardized ileal amino acid digestibility.

  , the amount of N applied to sorghum crops was positively correlated to dry matter and starch content (r ≥ +0.89; p < 0.0001). On the other hand, it was negatively correlated to the SIAD of Ser, Trp, Tyr and His (r ≤ −0.82; p < 0.0001).
• Yield: It was positively correlated with the overall AA digestibility of sorghum grain, particularly for Met, Cys, Pro, Ile, Val, and Phe (r ≥ 0.86; p < 0.0001; Table 4).
• Seeding rate: It was negatively correlated with dry matter and Lys content (r ≤ −0.83; p < 0.0001), while it was positively correlated with the amount of Tyr (r > +0.92; p < 0.0001; Table 4).

3.2. Influence of Climate Factors on Grain Composition and Digestibility

• Temperature and precipitation: There was not enough statistical evidence to report that temperature and rainfall were correlated significantly with the nutrient composition or SIAD of sorghum (−0.8 ≤ r ≥ +0.8; p > 0.05).

3.3. Influence of Sorghum Composition on Digestibility

Results are shown in

Table 5. Correlations of sorghum composition on AA digestibility of sorghum grain.

Variables	Correlation Coefficient	Significance
Dry Matter
Starch	0.9164	<0.0001
Ash	0.8276	<0.0001
Lys	0.9041	<0.0001
SIAD 1 Ser	−0.8451	<0.0001
Crude Protein
His	0.93	<0.0001
Ile	0.99	<0.0001
Leu	1.00	<0.0001
Phe	0.99	<0.0001
Val	0.98	<0.0001
Crude fiber
Crude protein	−0.87	<0.0001
His	−0.85	<0.0001
Ile	−0.86	<0.0001
Leu	−0.84	<0.0001
Phe	−0.86	<0.0001
Val	−0.90	<0.0001
Neutral detergent fiber
SIAD Ala	−0.88	<0.0001
SIAD Arg	−0.83	<0.0001
SIAD Glu	−0.88	<0.0001
SIAD Gly	−0.81	<0.0001
SIAD Leu	−0.86	<0.0001
SIAD Val	−0.81	<0.0001
Ash
Lys	0.88	<0.0001
SIAD Ala	0.91	<0.0001
SIAD Glu	0.91	<0.0001
SIAD Leu	0.90	<0.0001
Methionine
SIAD Gly	0.92	<0.0001
SIAD Thr	0.81	<0.0001

SIAD ¹ = standardized ileal amino acid digestibility.

.

• Dry matter: Grain dry matter was linked to starch, ash and Lys content (r ≥ +0.83; p < 0.0001) but negatively correlated with the SIAD of Ser (r ≤ −0.85; p < 0.0001).
• Crude protein: Sorghum total protein content was strongly associated with BCAA (Leu, Ile, Val), His and Phe levels (r ≥ +0.93; p < 0.0001).
• Crude fiber: There was an inverse relationship between sorghum crude fiber and crude protein as well for BCAA, His and Phe (r ≤ −0.84; p < 0.0001).
• Neutral fiber detergent: The content of NFD of grain sorghum was negatively correlated with the SIAD of Leu, Val, Arg, Glu, Gly, Ala (r ≤ −0.81; p < 0.0001).
• Ash content: It was positively correlated with Lys levels and with the SIAD of Leu, Ala, and Glu (r ≥ +0.88; p < 0.0001).
• Methionine: The amount of Met found in grain sorghum was positively linked to the SIAD of Gly and Thr (r ≥ +0.81; p < 0.0001).

4. Discussion

This study aimed to explore the influence of agronomic and environmental factors on the nutrient composition and digestibility of grain sorghum. The findings indicate significant correlations between specific agronomic practices, such as fertilization, yield, and the nutrient composition and digestibility of sorghum grain.

Even though, based on previous studies, fertilization impacted the protein content of grains [16,22,23], in our study, there was not a significant correlation between N fertilization and protein content. As the correlations indicate (Table 4), fertilization appears to be negatively correlated with the digestibility of certain AA. Specifically, N fertilization was positively correlated with dry matter and starch content but negatively correlated with the digestibility of Ser, Trp, Tyr, and His. This aligns with findings reported by the only in vivo experiment found assessing the impact of N fertilization on the cereal AA digestibility using cecectomized laying hens [16].

Siegert et al. (2017) found that N fertilization influenced the digestibility of AAs in different triticale varieties. While N fertilization increased the concentration of AA in the grain, leading to higher concentrations of digestible AA, it also reduced the digestibility of some AA, including Ala, Ile, Lys, Met, and Val, across all triticale varieties [16]. Considering the different cereals grains used between the latter and our study, their results are comparable to ours in sorghum, where fertilization reduced the digestibility of Ser, Trp, Tyr, and His.

The positive correlation between N fertilization and starch content in sorghum grain can be attributed to the role of N in starch synthesis. According to Yang et al. (2020), N is crucial for the synthesis of enzymes involved in various biochemical pathways, including starch synthesis [24]. Nitrogen fertilization enhances the activity of these enzymes, thereby facilitating greater starch accumulation in sorghum [25]. It is important to maintain a balanced N fertilization, as excessive N can negatively impact these pathways [24,25]. However, the effect of N on starch content remains inconsistent, as Kaufman et al. (2013) reported no significant effect of N fertilization on sorghum starch content [22].

Additionally, yield was positively correlated with overall SIAD of sorghum grain, particularly for Met, Cys, Pro, Ile, Val, and Phe. To date, the relationship between yield and in vivo sorghum digestibility has not been previously evaluated. The seeding rate also played a role in influencing nutrient composition and digestibility. It was negatively correlated with dry matter and Lys content while positively correlated with the amount of Tyr. These correlations highlight the complex interactions between agronomic practices and grain quality. This potentially suggests that higher-yielding sorghum crops are more digestible, albeit it should be noted that many confounding variables as well as other complex interactions may be influencing the variation in SIAD among samples.

Regarding sorghum composition, crude protein seems to strongly influence AA composition, particularly for Leu, Ile and Val (branched chain AAs, BCAAs), His and Phe. This association was expected, as AAs are the building blocks of proteins, and it is also consistent with the literature [26,27]. Therefore, a higher concentration of AA in a sample would indicate a higher protein content, leading to a positive correlation between crude protein and AA.

The amount of fiber in sorghum grain appeared to have a detrimental effect on SIAD. Crude fiber showed an inverse relationship with protein content, and NDF negatively correlated with SIAD. These findings were expected, as NDF includes non-starch polysaccharides (NSPs) such as hemicellulose and lignin, which are indigestible for birds. These NSPs reduce digestibility by increasing gut motility and the passage rate of nutrients through mechanical stimulation. This mechanical action burdens the interaction between enzymes and substrates, reducing the efficiency of nutrient absorption and utilization in the intestinal lumen [16,28].

Ash content was positively correlated with Lys and influenced digestibility, showing a significant positive relationship with the SIAD of Leu, Ala, and Glu. The positive impact of ash and the negative impact of fiber on SIAD align with the estimated coefficients for these variables in the multiple linear regression equation formulated by Ebadi et al. (2011) to predict sorghum SIAD. Thus, the ash and crude fiber content could serve as potential predictors of the nutritive potential of sorghum grain for poultry [29].

In the current investigation, our analyses did not reveal any evident relationships between environmental variables and the nutrient quality of grain sorghum. The existing literature indicates that the optimal temperature range for vegetative growth is 27–34 °C and that for reproductive growth is 21–35 °C [30,31]. Water requirements for sorghum vary between 450 and 650 mm, being most critical during flowering and gradually less during grain filling [32]. Therefore, it appears that the environmental conditions in our study (Table 3) were similar to the environment for the proper development of sorghum. However, the regional weather similarity of the cultivation sites in the southeast USA and the use of different hybrids within each crop may have minimized the environmental effects on nutrient composition and SIAD, making correlations difficult to detect.

We expected to find significant correlations as there is enough evidence showing that other abiotic stress, such as suboptimal temperatures, light stress, high humidity, and imbalanced water provision, affect nutrient composition and increase antinutritive components such as phytates and phenolic compounds in sorghum [31,33,34]. These changes can negatively impact the nutritional quality of sorghum when used as poultry feed [35].

In regard to temperature, heat stress has been reported to have a detrimental effect on sorghum growth development and grain quality [31]. The developmental stage and duration of heat stress vary in their effects, and susceptibility to this stress also depends on the sorghum variety [36]. Various studies agree that the most critical stage to avoid heat stress is the reproductive stage compared to the vegetative stage, which is due to a reduction in floret fertility [37,38,39]. Diurnal temperatures above 33 °C and nocturnal temperatures above 27 °C have been reported to cause reproductive failure, including floret and embryo abortion [40]. Short periods of heat stress exposure have been associated with a lower number of seeds [41], while longer periods affect grain filling, leading to a negative effect on seed weight [38].

Regarding grain quality, heat stress has been reported to negatively affect starch content [33,42,43]. Another study did not show an influence of high temperatures on starch content but did observe a decrease in protein digestibility and an increase in grain hardness [43]. High temperatures can affect the optimal functioning range of enzymes, impacting biochemical processes such as starch synthesis, thereby affecting their proportion in the grain and the amylose/amylopectin ratio [31]. Conversely, low temperatures can also stress sorghum, affecting grain composition and quality. A reduction in starch and protein content has been reported in crops subjected to low temperatures, although some hybrids are more tolerant to these climates [44,45].

Although sorghum is known to be the most drought-tolerant among cereals, there is still a limit beyond which its quality and yield are compromised [31]. Drought negatively impacts grain quality for several reasons. Water deficiency decreases nutrient uptake and the transport of nutrients, ultimately threatening grain viability [31]. Various studies have reported changes in the nutritional composition concerning starch, protein, and fat content [46,47,48]. Excess water and waterlogging also negatively affect sorghum grain [31]. Waterlogging creates anaerobic conditions in the soil, impeding proper energy metabolism, enzyme functioning, and photosynthesis. It also harms root tissue, burdening nutrients and phytohormone transport that regulate grain development and nutrient accumulation [49,50].

5. Conclusions

While this companion paper’s limited sample size constrains the statistical power and generalizability of the findings, the observed correlations underscore the potential influence of agronomic and environmental variables on grain quality. Notably, N fertilization positively correlated with dry matter and starch content, while yield was positively associated with SIAD. In contrast, seeding rate showed a negative correlation with dry matter and Lys content. Fiber, particularly NDF, was inversely related to SIAD.

These preliminary results highlight intriguing patterns but must be interpreted cautiously, as correlation does not imply causation. The sample size, combined with the genetic and relatively close geographical locations, might have comprised the result of this study.

Therefore, this study serves as a foundation for future research, emphasizing the need for larger, more comprehensive datasets and robust experimental designs to validate and expand upon these findings. By advancing understanding in this area, further studies can contribute to optimizing sorghum’s potential as a sustainable and nutritionally valuable feed grain for the poultry industry.