Johnson and Sudan Grasses
Johnson grass (Sorghum halepense) and Sudan grass (S. sudanensis) and their hybrid varieties are annuals in northern states and perennials in southern states. Johnson grass is an indigenous coarse grass that spreads readily from its creeping rhizomes. The glabrous leaves are up to 1.5 ft (0.5 m) in length and form a sheath around the stem which may reach 3 to 8 ft (1 to 2 m) in height. Many individual small flowers are produced in a loose terminal panicle. Sudan grass and other grain sorghums such as milo are cultivated corn-like annuals that do not persist like Johnson grass.
Sorghums have primarily been associated with poisoning due to the accumulation of toxic levels of cyanide and, for ruminants, nitrates. Not all varieties of sorghum grasses are poisonous. It has been possible to select varieties that are safe to use for feeding animals. Those, such as Sudan and Johnson grasses, that are capable of accumulating high levels of cyanide and nitrates and can cause acute fatalities in animals consuming them are described later in this chapter in the section on sudden death. More commonly in horses, sorghum poisoning is due to very low concentrations of cyanide that result in chronic neurologic damage.
Horses, sheep, and cattle that consume cyanide-containing varieties of these grasses for prolonged periods develop a syndrome of hindleg incoordination, urinary bladder inflammation and incontinence, and weight loss. The disease results from lower spinal cord degeneration induced by the low levels of cyanide-producing substances in the plants that cause damage to the peripheral nerves.
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Incoordination resulting from consumption of toxic sorghum grasses is most noticeable when the horse is backed or turned, which may cause it to sit on its hindquarters or fall over. Paralysis of the urinary bladder results in the continual dribbling of urine, which causes skin scalding and hair loss of the lower hind legs in the male and of the perineal area in the mare. Affected mares may have paralysis of the perineum so that the lips of the vulva stay open, causing vaginal inflammation. Loss of tone to the rectum may occur, causing fecal impaction and constipation. Flaccid paralysis of the tail develops in some cases. Affected horses generally retain a good appetite, and physical parameters remain normal until bladder and kidney infections develop secondary to incontinence. As this occurs, the urine becomes thick and opaque and contains large amounts of amorphous sediment.
The clinical signs of incoordination and hindquarter flaccid paralysis are similar to those caused by rhinopneumonitis myeloenchepalitis, and sleeping sickness. However the history of eating sorghum grasses and the presence of urinary incontinence and bladder inflammation, which is not usually a feature of these other diseases, should confirm the diagnosis of chronic sorghum grass poisoning.
Animals slowly recover from the syndrome if they are removed from the toxic sorghum grasses before the bladder infection and incoordination become complicated by serious secondary problems, such as kidney infection. Complete recovery seldom occurs once horses have developed severe signs of incoordination and bladder infection. Improvement can be accomplished by not feeding toxic sorghum grasses and by treating urinary tract infections with appropriate antibiotics.
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