Gadenin日本語Get the appPrussic acid: the hazard to know before anything else
Sorghum and its close relatives contain dhurrin, a cyanogenic glycoside. When plant tissue is crushed or chewed, dhurrin breaks down and releases hydrogen cyanide, also called prussic acid. This is a real toxicity that kills livestock, and it is not a rare or theoretical event. Anyone growing sorghum where animals can reach it, or anyone thinking of cutting the crop to feed to goats, sheep, cattle or horses, needs to understand when the levels are high.
Concentration is not uniform across the plant or the season. It is highest in young leaves and in the growing point, lower in older leaves, and very low in the stem and essentially absent from the mature grain. This means the dangerous material is precisely the fresh green growth that looks most appealing as fodder. Short plants, under about two feet (60 cm), are the classic hazard.
Two events spike the levels sharply. The first is regrowth after cutting or grazing: those new shoots coming from the base are young tissue and can be more dangerous than the original crop. The second is frost. Frost ruptures cells and frees the enzyme that generates the cyanide, and a frosted sorghum field is genuinely lethal to stock for a period afterwards. Drought stress also raises levels, which is a cruel combination given that drought is exactly when a struggling pasture makes sorghum look attractive as an emergency feed.
For a home gardener growing sorghum for grain, syrup or brooms and eating none of the foliage, none of this affects the crop you harvest. Mature grain is safe, and syrup pressed from mature cane is safe. The hazard is entirely about green foliage and about animals. Keep fencing intact, do not throw thinnings or cut tops over the fence to livestock as a treat, and do not turn animals into a plot after frost.
| Plant condition | Prussic acid risk | Guidance |
|---|---|---|
| Seedlings under 24 in (60 cm) | High | Keep livestock off entirely |
| Regrowth after cutting | High | Wait until it is well over 24 in (60 cm) |
| Drought-stressed green growth | High | Do not graze during or just after stress |
| Frosted or frozen foliage | Very high | Keep stock off for at least a week after growth stops |
| Tall mature foliage, no stress | Lower | Still not zero. Introduce animals slowly and fed |
| Mature grain | Negligible | Safe as human or animal food |
| Pressed syrup from mature cane | Negligible | Safe |
Properly cured hay and well-fermented silage lose most of their prussic acid because the compound is volatile and dissipates during drying and fermentation. Fresh green chop does not. Never assume a cut-and-carry feed is safe because a hay from the same field was.
Aphids on the underside of the leaves
Sugarcane aphid is the pest that does real damage to garden sorghum. It colonises the undersides of the lower leaves and works upward, and the first thing most people notice is not the insect but the sticky honeydew coating the leaf surfaces below, often turning black with sooty mold. By that point the colony is already substantial. Turn leaves over and look every week from the time the crop is knee high, because catching this early is the difference between a nuisance and a lost crop.
A heavy infestation at heading prevents the head filling properly, and the honeydew gums up the head so badly that threshing becomes miserable. Strong jets of water knock colonies off effectively when the crop is still short enough to reach. Ladybirds and lacewings arrive on their own and often clear a moderate infestation, so avoid broad-spectrum sprays that remove them and leave the aphids to rebound.
Corn leaf aphid also turns up, usually clustered in the whorl before the head emerges. It is generally less damaging and the crop grows past it, but a whorl packed solid with aphids and honeydew can make the emerging head come out deformed.
| Symptom | Likely cause | Response |
|---|---|---|
| Sticky shine on upper leaf surfaces | Aphid honeydew from the leaf above | Check undersides immediately |
| Black sooty film on leaves | Mold growing on honeydew | Wash off, deal with the aphids |
| Purple or red blotches on leaves | Leaf spot diseases or anthracnose | Remove worst leaves, improve airflow |
| Head partly gray and fuzzy | Head mold in wet weather | Cut and dry immediately |
| Stalk snapped near the base | Borer or stalk rot | Cut and destroy, do not compost |
| Head stripped bare at the top | Birds | Net or bag the heads |
Birds, which will take the whole crop
Birds find sorghum at soft dough and stay until there is nothing left. Because the heads stand proud of the canopy on bare stalks, they are the most exposed and accessible grain in the garden, and a flock can strip a small block in two or three days. This is the loss that most often turns a promising home sorghum crop into no crop at all.
Netting the block is the only wholly reliable answer, and it needs to go on at soft dough rather than when you first see damage. For a handful of plants, slipping a paper or fine mesh bag over each head and tying it below works well and has the side benefit of catching any grain that shatters. Scaring devices help for a few days and then stop working; birds habituate quickly. If netting is not possible, the alternative is to cut early at black layer and finish the drying indoors, accepting slightly more work in exchange for keeping the harvest.
Head molds, lodging and volunteers
Wet weather during ripening brings head mold, showing as gray, pink or black fuzz between the seeds. A compact head holds moisture in its center and is more vulnerable than an open, loose head. There is no treatment once it appears; the response is to cut affected heads immediately and dry them hard under cover, and to discard any head where the mold has spread through the interior. Do not store moldy grain with sound grain, and do not feed it to animals.
Lodging after a storm is common in tall types and is dealt with by prevention rather than cure, but a flattened stand is not necessarily lost. Sorghum stems will bend upward at the nodes over the following week if the crop went over before the heads filled. Prop the outer rows with a stake and line, and leave the rest alone rather than trying to lift individual plants, which usually breaks them.
Finally, sorghum volunteers. Grain shattered onto the bed germinates readily the following season once the soil warms, and volunteer plants are unwelcome for a specific reason beyond the usual weediness: they are young, unstressed green growth of exactly the kind that carries the highest prussic acid, and they come up in beds where you no longer expect sorghum. Hoe them out while small. This matters most in gardens with poultry or grazing animals that might get into a bed you have moved on from.
What to record about problems
Note the date you first found aphids on the leaf undersides, not the date the damage became obvious. That first-sighting date, repeated over a few seasons, tells you when to start your weekly checks rather than checking blindly from emergence. Do the same for the date birds first showed interest, which is usually within a day or two of the same growth stage each year.
Record which blocks lodged and in which direction, and record any frost event against the state of the crop at the time. If you keep animals, keep a specific written note of any period during which sorghum ground was closed to them and why, because prussic acid risk is exactly the kind of thing that gets forgotten between one season and the next by whoever is doing the feeding.