Hydrogen cyanide in flax forage
General considerations
The leaves, stems and seeds of fresh flax forage may contain toxic amounts of hydrogen cyanide (HCN). As chewing and rumen bacteria activities increase cyanide production, cyanide poisoning is more likely to happen in ruminants (EFSA, 2007). In livestock, a HCN content of more than 750 mg/kg DM is considered hazardous, 500-750 mg/kg DM is doubtful, and less than 500 mg/kg DM is considered safe (Kahn, 2005). In particular, regrowth after harvest or after frost can be unsafe as new leaves and stems contain higher levels of HCN than mature leaves, making mature flax forage safer than younger flax, even though its nutritive value is lower. The danger is greater if the immature flax is frozen. It may be possible to mitigate the risk through generous applications of fertilizer: in sorghum, also a HCN accumulator, moderate N fertilizer and good P and K levels can help to limit the concentration of HCN in forage (Government of Saskatchewan, 2008b).
Pasture
If livestock is allowed to graze the flax swath, care should be taken that no hungry animal enters the field if the flax has been recently cut. Feeding hay or grain will reduce the HCN concentration in the digestive tract. Standing flax straw should not be grazed as the act of trampling and chewing releases the cyanogenic glycoside and the ß-gycosidase enzymes (Government of Saskatchewan, 2008b).
Processing
After harvest, hay making and ensiling can reduce HCN content by more than 50% as it is released as cyanide gas. Storage also helps to reduce the HCN content (Government of Saskatchewan, 2008b).
Nitrate poisoning
The flax plant tends to accumulate nitrates after frost. If the plant is alive and allowed to regrow for a week, it can metabolize the nitrates, thus reducing the risk of nitrate poisoning. If the plant has been killed by the frost, the crop should be harvested as soon as possible and then tested for nitrates. If the nitrate content is higher than the safe level, the flax forage should be mixed with other feeds to reduce the level of nitrates in the diet (Government of Saskatchewan, 2008b).
Rumen impaction
In Canada, official recommendations regard flax straw as a cause of rumen impaction and suggest providing adequate protein and energy to prevent this undesirable effect (Government of Saskatchewan, 2008a). However, some reports consider that the fibres are digested like other fibrous materials and do not form indigestible balls in the stomach (Duke, 1983).