Sorghum contains hydrogen cyanide and nitrates/nitrites, which may be toxic for animals if they are ingested at a high level. There are varietal differences between sorghum strains (Luthra et al., 1976).
Hydrogen cyanide
Dhurrin, a cyanoglycoside, is present in the aerial parts of most sorghum varieties. It is particularly concentrated in the young leaves and tillers and in plants that are suffering from drought. Dhurrin is hydrolysed into hydrogen cyanide in disrupted plant tissues or in the rumen of animals that consume the forage. HCN is highly toxic and can kill grazing animals. Forage sorghum can potentially contain between 100 and 800 mg HCN/kg plant DM. It is lethal to ruminants if consumed at 2 mg/kg body weight. Symptoms of acute HCN poisoning in ruminants are excitement, difficult breathing, muscle tremors, staggering, convulsions, collapse and death, which can occur in less than 15 minutes but generally within 1 or 2 hours. A chronic form of HCN poisoning also exists that results in sulphur deficiencies in ruminants (Pedersen et al., 2000).
In the plant, HCN levels usually decline with age, reaching non-toxic levels 45-50 days after planting (Hanna et al., 2001). The HCN is partly destroyed when the fodder is made into hay or silage, where it degrades in 2 or 3 weeks after ensiling (Undersander et al., 2003). Low HCN potential varieties of sorghum have been developed (Hanna et al., 2001).
Nitrates and nitrites
Sorghum is a nitrate accumulator and, while nitrates are generally non-toxic to ruminants, their transformation into ammonia, within the rumen, produces toxic nitrites. Nitrites bind with haemoglobin to form methemoglobin, preventing blood from binding with oxygen, resulting in oxygen starvation of the tissues, accompanied by increased pulse and respiration rates, and animal death in the most severe cases (Marais, 2001; Fahmy et al., 2010).
Nitrate levels ranging from 0.5 to 1% DM in the plant are considered potentially toxic to ruminants. Nitrate levels higher than 1% DM are considered dangerous (Yaremcio, 1991). Nitrates are more concentrated in the stems than in the leaves (Aii, 1975; Harada et al., 2000).
Several environmental conditions result in nitrate accumulation, notably severe drought, high applications of N fertilizer and killing frost (Pedersen et al., 2000). A well-managed programme of N fertilizer application can reduce nitrate accumulation (Wilmoth et al., 2000).
To prevent nitrate poisoning, livestock should not be given full access to forage sorghum: in strip grazing at high stocking rates, livestock only choose the leaves that have less nitrates than the stem. The sward should not be grazed immediately after plant stress and should be allowed to mature, as this decreases its nitrate content. Ensiling may also be a way of reducing nitrate levels (by 40 to 60%). If nitrate accumulation is suspected, the herd should be fed with other feeds prior to entering the stand so that nitrates are diluted in the ration (Pedersen et al., 2000).