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.
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.
