Data sheets: Sorghum

Introduction

Sorghum (Sorghum bicolor) is a monocotyledonous, herbaceous plant belonging to the Gramineae family. It is cultivated in the summer in temperate zones between 25-30ºC (77-86ºF) in both hemispheres and is drought resistant. It is the fifth most cultivated cereal in the world and the fourth most used for animal feed (at the EU level), mainly as a energy source.

The variety with yellow-brown grains, known as white sorghum, is the most commonly used variety in animal feed. It has been selected for its low tannin content and low phenolic compounds. By contrast, brown and dark brown sorghum varieties have high tannin contents, which are condensed in the pericarp to protect the seed (a unique characteristic with respect to other cereals). These varieties are more resistant to drought and are grown in arid areas. The presence of condensed tannins interferes with the digestive processes, reducing the energy and protein digestibility. Therefore, dark grain varieties are not as widely used for animal feed.

The sorghum grain is comprised of approximately 6% bran or pericarp, 10% germ, and 84% endosperm and aleurone. More than 50% of the endosperm is waxy, which, unlike corn, has smaller starch grains intertwined in a network of hydrolysis-resistant prolamin and glutein-based proteins, which hinders starch availability. It has a high starch content, which can even exceed that of corn (± 65%), with an average fat content of around 3%. The proteins that surround the starch grains, together with the germ, provide a protein value higher than the corn grain, around 9% considering all varieties but a greater value if only white varieties are considered. However, despite lower starch availability, the reduced fiber content (2.5%) and higher lipid content (±3%) means that sorghum has an energy value more similar to corn than that of other cereals (EN/kg). The average ratio of amylose to amylopectin is 20:80 but in waxy varieties the ratio of amylopectin can be almost 100%.

The presence of condensed tannins in dark varieties and phenolic and flavone compounds, which are present in all varieties (dark and white), contribute to a reduction in the amino acid availability, compromising their digestibility. Specifically, condensed tannins are the result of the polymerization of flavan-3-ol units such as catechin, epicatechin or leucocyanidine linked by C-C bond and lacking a glycidic nucleus. Because of their inability to be hydrolyzed, they are involved in various antinutritional activities (e.g. micronutrient sequestration) and, after being absorbed cannot be metabolized, making them responsible for an increased need for certain nutrients to be excreted (e.g. sulphur containing amino acids). However, there is a great lack of knowledge about the antioxidant and astringent properties of these compounds, which could be of interest to the pig industry.

Comparative Study of Nutritional Values

Systems used in the comparison are: FEDNA (Spain), CVB (the Netherlands), INRA (France), NRC (United States) and Brasil (Brazil).

BRASIL is the only evaluation system that clearly differentiates between two different grades of sorghum according to their tannin content (high or low) while FEDNA, CVB, INRA and NRC consider a single grade. It is assumed that in these systems only the varieties selected for low tannin content for use in animal feed (called white sorghum) are considered.

The range in moisture content is very consistent for the different evaluation systems and does not vary by more than 3 percentage points with a minimum of 10.6% for NRC, which finds a higher starch content. There is a positive correlation between the percent dry matter and the starch content (r² = 0.66), but this number is strongly influenced by the high starch content considered by NRC. If we do not consider this more extreme value in the calculation, the correlation remains positive, although it decreases (r² = 0.43).

Unlike other cereals such as wheat, which has a negative correlation between protein content and starch (r² = -0.66, where 72% of the protein is concentrated in the endosperm, 20% in the pericarp and aleurone layer and 3% in the germ) or corn that has a positive correlation (r² = 0,55, where 31% of the protein is concentrated in the germ, 26% in the endosperm and 12% in the pericarp and aleurone layer), when looking at sorghum, dispersion of both variables is maximum and no consistent relationship is observed.

These relationships have a direct impact on the estimated net energy value. Starch has the greatest contribution to the energy value, with a wide variability (10 percentage points, and >13 if we consider the extreme value from NRC). This is determinant (r² = 0.68) in explaining the over 360 kcal/kg difference in NE observed between the most extreme values (2780 kcal/kg from NRC vs 2417 kcal/kg from BRAZIL high tannins). We observe a positive correlation (r² =0.48) between the NE and the fat content which, although determinant, is less variable than starch.

Compared to the other evaluation systems, BRAZIL (for high tannin varieties) and NRC give the lowest and highest values respectively for dry matter (-1.5% and +2.5%), starch (-13.3% and +17.1%), protein (-4.3% and +3.5%), fat (-17.4% and +20.2%) and NE (-5.3% and +8.4%) with respect to the average of the other systems. The evaluation of white sorghum varieties (without tannins) results in very similar values with the exception of NRC, which clearly finds a upward deviation in protein, starch and fat, and lower fiber than can be explained by the deviation observed in NE for this cereal.

In terms of total amino acids, using lysine as a reference and the same protein content (9.0±0.30%), it can be observed that CVB shows a value 5.5% above the average, with the NRC value being almost 6% below the average. Values for BRAZIL (low in tannins), INRA and FEDNA, are all very similar and near the average value (2.3%). This suggests that the evaluation of the varieties supposedly considered by NRC with respect to lysine are more similar to the evaluation offered by BRAZIL for varieties with a high tannin content. The remaining amino acids are quite proportional to lysine, with the exception of threonine and branched chain amino acids (valine and isoleucine) that present a lower variability between the systems. However, NRC stands out with a tryptophan value that is almost 30% below the average. The digestibility coefficients of protein and lysine present a range between 73.7-81.3% with a mean value of 77% which is can be attributed to the >80% value presented by BRAZIL for varieties with low tannin content.

Recent findings

1. Is the kafirin profile capable of modulating the ileal digestibility of amino acids in a soybean meal-sorghum diet fed to pigs.
The effects of kafirins from sorghum on protein and amino acid ileal digestibility has been evaluated en vivo in pigs. Two sorghum-soybean meal (SBM) diets were formulated: 1) low kafirin (LK) content (32.2 g/kg) and 2) high kafirin (HK) content (48.1 g/kg). A control diet (maize-SBM) and a reference SBM-diet were also prepared. The maize-SBM diet was more digestible than the sorghum-SBM diets, only with respect to valine. The AID of valine in the maize-SBM diet was higher than that in sorghum-SBM diets. The changes in kafirin profile between the diets only affected the AID of threonine, which decreased by 9.5 percentage units in LK diet compared with HK diet. Regarding the AID of cereals, maize exhibited greater AID than sorghum, with respect to valine and serine. The higher content of γ-kafirins in sorghum negatively affectes threonine and serine digestibility, implying that the AID of amino acids is affected more by the profile than the content of kafirins.

2. Low-tannin white sorghum contains more digestible and metabolisable energy than high-tannin red sorghum if fed to growing pigs.
The present study was conducted to determine and compare the energy values in yellow-dent corn, three low-tannin white sorghum cultivars and three high-tannin red sorghum cultivars when fed to growing pigs. The DE and ME were lower for red sorghum than for corn while the values for corn were lower than those obtained for white sorghum. The ATTD of GE was higher than for red sorghum but was lower than white sorghum. Tannin had a high negative correlation with DE and ME (both, r = -0.99; P < 0.01) and the ATTD of GE (r = -0.92; P < 0.01). Tannin was negatively correlated with CP (r = -0.85; P < 0.05), or positively with kafirin/CP (r = 0.88; P < 0.01) and phenols (r = 0.77; P < 0.05). Therefore, tannin content in sorghum may be the main anti-nutritional factor.

3. The effect of microbial phytase supplementation of sorghum-canola meal diets with no added inorganic phosphorus on growth performance, apparent total-tract phosphorus, calcium, nitrogen and energy utilization, bone measurements, and serum variables of growing and finishing swine.
The effect of phytase supplementation of sorghum-canola meal diets without added inorganic phosphorus was evaluated. The supplementation of 400 U of phytase/kg of diet was adequate for growth performance. For both experiments, the phytase treatments did not increase Ca, N, or energy absorption (amount/d or % of intake), and had no effect on serum Ca and P concentrations, or the serum activity of alkaline phosphatase, total acid phosphatase, or tartrate-resistant acid phosphatase in sorghum-canola diets.

4. Digestible energy of sorghum grain for pigs could be predicted using a computer-controlled simulated digestion system.
Prediction equations for digestible energy (DE) of sorghum grains fed to pigs have been generated based on chemical composition. Differences between determined and predicted values are smaller than 0.02 in 25 or 24 out of the 28 samples for ATTD of GE or DM. Tannin content negatively affects the in vitro enzyme digestion of energy content in sorghum grain. The 3-step enzymatic method using computer-controlled simulated digestion system could be a promising in vitro digestibility technique to predict DE and digestibility of GE in sorghum gain for pigs

5. Gastrointestinal morphophysiology and presence of kafirins in ileal digesta in growing pigs fed sorghum-based diets.
The effects of three sorghum-based diets with different levels of tannins and kafirins on some gastrointestinal morphophysiological characteristics was evaluated in growing pigs. Four different cereal-soybean meal diets were used: a corn-based (control diet; C), a low tannin and low kafirin sorghum-based (LTLK), a low tannin and high kafirin sorghum-based (LTHK), and a high tannin and high kafirin sorghum-based (HTHK). The different sorghum diets did not affect the performance of the pigs or the majority of the evaluated morphophysiological parameters. The total trypsin activity was higher in pigs fed the HTHK diet. The highest intensity/area of the kafirin fractions was noted in ileal digesta from pigs fed LTHK and HTHK diets. The simultaneous presence of high levels of tannins and kafirins could affect the digestion of sorghum proteins.

References

FEDNA: http://www.fundacionfedna.org/
FND. CVB Feed Table 2016. http://www.cvbdiervoeding.nl
INRA. Sauvant D, Perez, J, y Tran G, 2004, Tables de composition et de valeur nutritive des matières premières destinées aux animaux d'élevage.
NRC 1982. United States-Canadian Tables of Feed Composition: Nutritional Data for United States and Canadian Feeds, Third Revision.
Rostagno, H,S, 2017, Tablas Brasileñas para aves y cerdos, Composición de Alimentos y Requerimientos Nutricionales, 4° Ed.