Sorghum is the only cereal that contains tannins. Tannins are associated with enhanced agronomic qualities such as reduced pre-harvest molding, enhanced resistance to pathogens and pests, lower bird depredation (in "bird-resistant" sorghums, see below) and lower pre-harvest germination. However, tannins are antinutritional factors as they bind with proteins, precipitate them and make them unavailable during digestion. Therefore, the nutritive value of feeds containing tannins is consequently reduced. There are roughly two types of sorghum (Taylor, 2001):
- High-tannin or “bird-resistant” sorghums. Those sorghums can be recognized because of a pigmented testa situated below the pericarp. Samples of sorghum grain are assessed by the “Bleach test” which removes the pericarp and make the pigmented testa turn black while the unpigmented testa remains light coloured.
- Low-tannin sorghums.
Due to breeding efforts aiming at eliminating sorghum tannins, the majority of sorghums currently produced are tannin-free in the USA (99% of the production), Europe (Vignau-Loustau et al., 2008), Australia, India and Thailand (Awika et al., 2004; Subramanian et al., 2000; Walker, 1999). However, in countries where bird predation is an important issue, the use of low-tannin sorghums may not be economically advantagious and the use of high-tannin cultivars is likely to remain important (Kyarisiima et al., 2004; Taylor, 2003). In Eastern and Southern Africa, traditional sorghum varieties of moderate tannin content are widely grown for staple food and alcoholic beverages (Awika et al., 2004), though there are local variations. For example, varieties containing tannins are common in Niger and Senegal, but only rarely grown in Mali and Burkina Faso (Abdoulaye et al., 2006). In Southern Africa, small-scale farmers intercrop tannin and tannin-free sorghums in areas prone to high bird predation in order to reduce grain losses (Awika et al., 2004). In Argentina, varieties with tannins are still cultivated (Massigoge et al., 2002).
In poultry, tannins are known to reduce growth rate, egg production and protein utilization, and to damage the mucosal lining of the digestive tract. Increasing protein content or amino-acid levels in the diet may alleviate the deleterious effects of tannins.
In pigs, feed intake and growth rates are also reduced by high-tannin sorghums, with feed efficiency being reduced by between 5% and 10% compared to that with low-tannin sorghum.
In ruminants, the effects are less negative since condensed tannins are complexed and precipitated by the rumen microflora (Reed, 1995). The remaining active tannins may have beneficial effects, such as bloat prevention and increasing the amount of by-pass protein (Waghorn, 2008; Reed, 1995). High-tannin sorghums have been found to have antioxidative activity within the beef cattle muscles and are able to enhance meat quality (Reed, 2008).
The adverse effects of high-tannin sorghum can be alleviated by various methods:
- Reconstitution followed by anaerobic storage.
- Formaldehyde treatment, acid treatment, NH4OH, NaOH, K2CO3, CaO, urea (Russell et al., 1989).
- Providing greater amounts of amino-acid in the diet: 0.15% methionine (Armstrong cited by King et al., 2000), or a mixture of choline and methionine (Daghir, 2008).
Sorghum ergot is a fungal infection caused by Claviceps sorghi, which produces alkaloids. Several observations in Australian piggeries have reported feed refusal, low milk production in sows and subsequent loss of litters when diets were based on grains containing 1% to 20% sclerotia. In cows, 20% sclerotia caused a drop in milk production. In poultry, 5% sclerotia induced respiratory difficulties, diarrhoea and death. No effect were noticed at 1.25% sclerotia. Sorghum ergot problems can be prevented through an appropriate visual inspection of the sorghum grain (Bandyopadhyay et al., 1998).
Like maize, sorghum is susceptible to various Fusarium spp. Mycotoxins such as aflatoxin, ochratoxin, zearalenone, deoxynivalenol or fumonisins may thus occur (their effects on livestock are fully described in the Maize grain datasheet). However, fungal attacks in sorghum are less frequent than in maize because sorghum grows in warmer and drier climates than maize. Moreover, when Fusarium is present in sorghum it produces much less fumonisin than in maize (Visconti et al., 1994).