How Toxic is Eastern Black Nightshade?

Comfort Ateh* and Jerry Doll

During the mid 1970s, there was much concern about black nightshade among soybean growers. This resulted in an extensive taxonomic study which found that true black nightshade (Solanum nigrum L.) is only found in the western region of the USA. The correct nomenclature of Wisconsin's nightshade species is eastern black nightshade (Solanum ptycanthum Dunn) and it is only found east of the Rocky Mountains. The purple-red color usually present on the lower surface of the seedling leaves of eastern black nightshade helps distinguish it from other nightshade species.

Eastern black nightshade is a highly branched annual plant that grows 1 to 3 feet tall. Flowers have five reflexed petals and fruits are green berries that turn black at maturity. The plant can be found in cultivated fields, gardens, waste places and overgrazed pastures. Seeds germinate in the spring and early summer and plants flower from July to October with seed maturing in August through October.

In recent years, eastern black nightshade has caused serious economic losses to soybean producers, and it may well pose hazards to forage livestock producers. In certain cases, eastern black nightshade contaminates the harvested crop resulting in either a lower market value or perhaps rejection. For example, nightshade fruits harvested with peas, snap beans and soybeans are impossible to remove mechanically from the harvested crop. Ripe fruits are often crushed during harvest which stain the crop seed. Green and ripe fruits contain a sticky, saponin-rich juice that adheres to harvest equipment and increases the likelihood of plugging the machine as dust and plant debris coat the internal parts. This juice also coats soybean seed, allowing foreign matter to stick to it, thereby increasing the grain moisture content

The spread of the eastern black nightshade has been favored by the following:

its tolerance to the commonly used dinitroaniline herbicides (Treflan, Prowl, Surflan) as well as metribuzin (Sencor, Lexone) which generally give good control of other broadleaf weeds.

reduced tillage which leaves nightshade seeds near the surface for easy germination and emergence.

solid-seeding and reduced row cultivation in soybeans.

using contaminated harvest equipment.

Poisonous plants are among the important causes of economic loss to the livestock industry. They affect animals in many ways: death, chronic illness and debilitation, decreased weight gain, abortion, birth defects, increased parturition interval, and photosensitization. In addition to these obvious losses, poisonous plants can also result in loss of forage, increased labor and management costs, and frequently, interference with proper harvesting of forage. Poor management and/or pasture conditions are more often the cause of livestock poisoning than simply the presence of the poisonous plants.

Situations conducive to poisonous plant ingestion include:

overgrazing of pastures (most significant).

lack of suitable forage in periods of drought.

incorporation of toxic forbs in hay or greenchop.

unpalatable plants becoming palatable and acceptable when frosted or sprayed with herbicide.

livestock introduced to new pasture often eat anything within immediate reach, including toxic plants.

Not all plants often cited as "poisonous" are unpalatable, and they do not always kill or otherwise harm animals when consumed. In most cases, animals are affected only when they consume too much of the poisonous plant. It is important to note that definitive diagnosis of suspect plant poisonings is difficult. In general, diagnosis of plant poisoning is based on availability, grazing evidence, presence in the hay and existence of plant parts in the rumen contents. Diagnostic lesions are usually lacking and analytic methods to identify toxic components are severely limited.

All Solanum spp. plants should be considered poisonous, but there is little firm evidence about the toxicity of most species. A variety of alkaloids have been extracted from Solanum species including glycoalkaloids and an alkamine fraction. The toxic principle, a glycoalkaloid called solanine, is found in leaves, shoots and unripe berries. Possible symptoms after animals ingest nightshade include acute hemorrhage, gastroenteritis, weakness, excess salivation, dyspnea, trembling, progressive paralysis, prostration, and death.

Solanine, a highly toxic alkaloid, has been extracted from eastern black nightshade. Similar to the influence of light on the development of solanine in potatoes, factors like soil, climate and season have an enormous effect on the alkaloid content of eastern black nightshade. Hence, in some places, the plant is harmless while it can be toxic in others. Nevertheless, the commonest source of solanine poisoning in farm animals has been with potato tubers, and most cases of solanine poisoning have occurred in European countries.

Solanine is a glycoalkaloid which contains three sugar residues (galactose, glucose and rhamnose) attached to an aglycone moiety called solanidine.

Solanine is poorly absorbed from the gastrointestinal tract, has a rapid fecal and urinary excretion, and hydrolyses to the less toxic and poorly absorbed solanidine. Animal species differ considerably in reaction to solanine toxicity and little is known about the chronic (long-term) toxicity of solanine.

Immature eastern black nightshade plants and berries are considered potentially toxic. Rapid ingestion of large quantities of highly toxic fruits of this plants can result in coma and rapid death. The concentration of toxic principle decreases in berries to safe or negligible levels as the plant matures. This alkaloid, solanidine, has been reported to cause nausea, vomiting, bloat, paralysis and even death if the plant is consumed before it has reached maturity. Even though livestock poisonings and deaths have been attributed to nightshade ingestion, species identification in such poisonings is not exact. There is still need for more research into eastern black nightshade poisoning to more precisely determine the toxicity and hazard in various growth stages and in dry hay and silage.

If pastures are well managed, eastern black nightshade is not a serious threat. It may pose a threat in hay as the poisonous alkaloids remain toxic in dry hay. Grazing and haying are not advised for severe infestations of eastern black nightshade in legumes or legume-grass mixtures. Such areas should be clipped and dried and the forage destroyed by burning. Though no serious problems of eastern black nightshade poisoning have been reported in Wisconsin, farmers should be on the alert and control small infestations which could be a potential source for livestock poisoning.

*  Graduate Assistant, Dept of Agronomy, Univ of Wisconsin
  October, 1993