It is believed that the previous two years of drought contributed to Tifton 85 grass producing cyanide gas that killed some cattle near Austin, Texas. Tifton 85 is a 1992 hybrid of Bermuda grass (Tifton 68 from Tifton, Georgia, US) and a South African grass, grown for its cold tolerance, high protein and digestibility.
The cattle died of prussic acid ( cyanide poisoning )
Cyanogenic glycosides in plants yield free hydrocyanic acid (HCN) aka prussic acid when plants are damaged.
Young plants and leaves of older plants contain dhurrin which can break down to release cyanide gas. This tends to be highest in young rapidly growing plants, especially those stunted by drought or damaged frost or other mechanical means. Heavy fertilizing with nitrogen in areas low in phosphorus is more likely to produce the gas. Treatment with 2,4-Dichlorophenoxyacetic Acid ( a broad leaf herbicide and pesticide ) also increases the risk.
It does decrease as plants die, decreasing slowest in drought stricken plants. The remaining acid may be concentrated in new shoots when regrowth begins. The darker the leaves, the higher the concentration.
Plants that can produce cyanide include: Apple, Apricot, Cherry, Peach, Elderberry, Flax, various sorghums, various grasses, hydrangea, lima bean, and others.