Prussic acid is another term for cyanide.  Cyanogenetic plants, those known for producing prussic acid,  produce cyanide molecules while growing.  Under optimal conditions the cyanide molecules are chemically bonded to sugars known as glucosides and are not toxic.  But problems begin when the glycosides are broken down, thus
releasing the cyanide molecules through a process called hydrolysis (the addition of water).  Freed prussic acid molecules are quickly absorbed into the blood stream and are fatal if the concentration is high enough.

SYMPTOMS  

Death from prussic acid poisoning can occur quickly - within 15 to 20 minutes and seldom takes longer than 30 to 45 minutes.  The prussic acid enters the blood stream and combines with hemoglobin to form cyanoglobin which inhibits the ability of cells to take up oxygen from the blood.   An effected animal's blood and normally pink tissues
(like gums) will appear bright red.  Symptoms include labored breathing, staggering, foaming at the mouth, falling to the ground, spasms, convulsions, and in the worst case scenario, death which results from respiratory paralysis.  Animals that survive longer than 2 hours usually recover fully.

TREATMENT

Treatment is available but death occurs so quickly that unless you are right there with treatment in hand it is unlikely that you will have enough time to save dying livestock - especially if very many animals are stricken at the same time.  This is one time that an
ounce of prevention is definitely worth more than a pound of cure, so if there is any question in your mind as to the safety of a forage, I suggest that you let one animal graze for awhile before risking  your entire herd.  Just don't use your prized herd sire as the guinea pig!    Treatment involves administering  sodium nitrite and sodium thiosulfate.  Talk with your veterinarian about dosages.

HIGH RISK PLANTS

There are approximately 1000 plant species in 250 genera that are known to be cyanogentic.   Perhaps the most commonly known ones here in Texas are the sorghums, sudangrass, and Johnsongrass.  Most sorghums are known for having high levels of prussic acid and therefore are generally not recommended for grazing, although there are some "grazing" varieties that have been developed that are reasonably safe.  Sudangrass contains about 40% less prussic acid than most sorghums.  There are crosses between the two, and even some 3- and 4- way crosses, that contain very low levels.  You are encouraged to contact your extension agent to find out which ones do well in your area.   Other commonly known plants include plants of the Prunus genus, such as wild black cherry trees, chokecherry, and pincherry.

CONDITIONS AFFECTING PRUSSIC ACID LEVELS

The riskiest times to graze Cyanogenetic plants in when they are young and growing and when they are undergoing environmental stress.
Growth stages.    Leaf blades tend to contain anywhere from 2 to 25 times more prussic acid than stems.  Upper leaves generally contain more than lower leaves.  Heads are low in prussic acid and seeds don't have any.  As a result, young growing plants contain higher levels than mature plants mainly because the leaf to stem ratio is higher.  As the
plant matures, the stem grows larger so the total prussic acid content is thereby lowered.  It is generally recommended to not graze suspect plants until they are at least 18 inches tall.
 Fertilizer.  Too little phosphorus, especially if combined with high amounts of nitrogen, is thought to increase the concentration of prussic acid, mainly by slowing the maturation process and thus preventing the plant from leaving it's immature (read: highly concentrated) stage.  Therefore it is recommend that soil samples be taken  to determine the correct amounts of fertilizer for your particular field.
 Freezing.   As temperatures near freezing, the glycosides breakdown and release prussic acid molecules.  Very high levels of free prussic acid can result, but will begin dissipating after the plants thaw and wilt.  It is generally recommended to wait 5 to 8 days before grazing again, longer if conditions cause a lot of new growth.
 Drought.  This is probably the most common cause of prussic acid poisoning.  Drought stricken plants are prevented from growing out of the high leaf-to-stem ratio and therefore prussic acid levels remain high.  Then as plants begin wilting, hydrolysis begins and the level of freed prussic acid increases even further.  It is recommended to avoid grazing drought stressed Cyanogenetic plants.  But don't graze immediately following a drought breaking rain because the plants will be putting forth new growth which is high in prussic acid.  It is recommended to wait  a few days before grazing.

MANAGING HIGH RISK FORAGES 

Once you understand when the levels of prussic acid poisoning are likely to be dangerous, managing the risk becomes fairly straightforward.
 Pasture.  Don't allow livestock to graze until plants are at least 18 inches tall.  Once plants are eaten down, move your livestock to a different pasture until the resulting new shoots are at least 18 inches tall.  Avoid drought stressed plants.  Avoid rapidly growing plants following a drought breaking rain.  Following a freeze, wait at least 5 to 8 days after plants thaw and wilt before grazing.   When possible, sow varieties known to have low levels of prussic acid - your extension agent can help you find varieties most suitable for your area.
 Hay.  Prussic acid isn't a problem in properly cured hay because up to 75% of the prussic acid dissipates while curing.  However, if you are concerned you can take a sample of the hay to your extension agent who can test it for prussic acid levels.
 Green Chop.  Green chop is generally safe as it is not possible for the animal to selectively eat only the plant parts that are higher in prussic acid such as the leafs.  The stems offset some of the concentration in the leaves.  Just be careful to avoid using plants known to be at peak concentrations of prussic acid.  Again, if there is
a question your Extension Agent will be happy to test it for you.  
 Silage.  Silage is generally safe as the prussic acid escapes as a gas during fermentation.  It is recommended to not feed new silage for at least 3 weeks after harvesting and storing.