Nitrate poisoning
Nitrates are generally non toxic to ruminants but their transformation into ammonia in the rumen produces toxic nitrites that bind with haemoglobin and prevent blood from binding with oxygen, resulting in oxygen starvation of the tissues and animal death in the most severe cases (Marais, 2001). Nitrate levels ranging from 0.5% DM to 1% in the plants are considered potentially toxic to ruminants while levels higher than 1% DM are considered dangerous (Yaremcio, 1991).
Levels of nitrate ranging from 0.24 to 0.98% have been reported in pearl millet (Krejsa et al., 1984). Grazing pearl millet can potentially cause nitrate poisoning, and above 0.6% nitrate reduced milk yield in dairy cattle (Newman et al., 2006). High nitrate levels can happen under drought (Sedivec et al., 1991) or after heavy applications of nitrogen fertilizer (FAO, 2009). A period of four to six weeks of growth is recommended before grazing since young pearl millet plants may have nitrate levels toxic to livestock (Sedivec et al., 1991). Recommended grazing height above 30-40 cm normally guarantees low levels of nitrate since nitrates are mainly concentrated in the lower parts of the stems (below 15 cm) (Lang, 2001). Making hay does not reduce nitrate levels but ensiling can result in a 40 to 60% nitrate reduction (Newman et al., 2006).
Alkaloids
Some pearl millet varieties contain alkaloids at levels ranging from 17 to 101 mg kg-1 (Krejsa et al., 1984). Water stress as well as added N fertilizer appear to increase nitrate and alkaloid contents. Non-drought-stressed forage was found to contain 1% nitrate and 10 to 20 mg/kg alkaloids, while drought-stressed forage had 3% nitrate and 180 to 460 mg/kg alkaloids (Krejsa et al., 1984; Rouquette et al., 1980). Supplying water to pearl millet during dry periods decreases its alkaloid content. Under extreme drought stress conditions, palatability and total consumption were negatively altered as total alkaloids and nitrate content increased (Rouquette et al., 1980).
Oxalic acid
Oxalic acid acts as an antimetabolite, interfering with protein metabolism and thereby reducing the quality of pearl millet forage (Rachie et al., 1980). High levels of oxalic acid were found in twigs, and weight losses were observed when they were offered as sole forage (Verma et al., 2004). Oxalic acid, combined with high K, Mn and Na content is suspected to have a deleterious effect on the milk fat content of cows’ milk (Andrews et al., 1992). Limiting K fertilizer and correcting Ca and Mg deficiencies appear to reduce oxalate content while increasing milk butterfat (Schneider et al., 1970 cited by Andrews et al., 1992). Soaking pearl millet forage before feeding dairy cows also decreases oxalate toxicity (Parveen et al., 1988 cited by Andrews et al., 1992).
Other feeding risks
Pearl millet forage contains HCN but not enough to be hazardous to stock, including horses, and does not contain tannins (Newman et al., 2006). Rust diseases in pearl millet result in decreased nutritive value for livestock. Rust-resistant varieties have been developed to solve this problem (Andrews et al., 1992).