The plant widely known as coffeeweed (Senna occidentalis) is a common concern in agricultural fields and disturbed habitats across tropical and subtropical regions. This species is classified as a troublesome weed due to its aggressive growth and ability to compete with cultivated crops. However, its toxicity is the most significant issue for landowners and livestock owners. Coffeeweed is not a source of commercial coffee; it contains compounds that can be harmful if consumed, making its proper identification and control a priority.
Identifying Features of Coffeeweed
Coffeeweed is typically an upright, slender shrub that can grow between 0.5 to 2.5 meters tall and often emits an unpleasant odor when its foliage is damaged. Its leaves are compound and pinnate, consisting of three to seven pairs of broadly elliptic to egg-shaped leaflets with pointed tips. A dark-colored gland near the base of the main leaf stalk helps distinguish it from similar species.
The plant produces bright yellow flowers, each with five petals, usually found in small clusters in the upper leaf axils. The fruit develops into a straight or slightly sickle-shaped pod, measuring up to 13 centimeters long. These pods contain multiple seeds that are flat, brown, and obovoid, and their resemblance to coffee beans gives the plant its common name.
Toxicity and Dangers to Animals
The toxic compounds in coffeeweed are concentrated most heavily in the seeds, though the entire plant poses a risk. The primary toxins are anthraquinone derivatives, including a compound known as a dianthrone, which are known to interfere with cellular function. These compounds act as mitochondrial poisons, damaging the mitochondria and impairing energy production in cells.
This mitochondrial damage leads to a specific type of muscle degeneration called myopathy, which is the most common symptom of poisoning in livestock, especially cattle, poultry, and swine. Animals that consume the plant may exhibit weakness, incoordination, and myoclonus (involuntary muscle spasms of large muscle masses). In cattle, the muscle tissue in the hind and forelimbs is often the most affected, and severe cases can result in the animal being unable to stand.
Poultry are also susceptible, with low-level contamination in feed leading to a marked reduction in egg production and lower feed efficiency. The anthraquinone-derived toxins can target the ovaries in birds, causing damage such as yolk leaking. Human consumption of the seeds, often as a coffee substitute, is also a risk due to the severe laxative and gastrointestinal distress caused by the anthraquinones. The toxic effects can be cumulative, with animals becoming ill several days after initial ingestion.
Control and Removal Methods
Management of coffeeweed requires an integrated approach that combines mechanical, cultural, and chemical controls. These methods are necessary to prevent the plant from reaching maturity and producing toxic seeds.
Mechanical Control
Mechanical removal, such as hand-pulling or cultivation, is highly effective. This must be done when the plants are small seedlings before they develop deep taproots or set seed. Repeated cultivation can deplete the seed bank in the soil and is a useful strategy in row crop systems.
Chemical Control
Coffeeweed is challenging to manage chemically, as it does not respond effectively to many common herbicides. Pre-emergence herbicides, such as fluometuron and norflurazon, provide effective control of germinating seeds in crops like cotton. Post-emergence applications of specific herbicides, like methazole, are also effective when the plants are still in the seedling stage.
Cultural Control
Cultural controls focus on creating an environment where the weed cannot thrive, with a healthy, dense ground cover being the most important factor. In pastures, maintaining a good stand of desired forage species through proper grazing management and fertilization prevents the bare ground coffeeweed needs to germinate and establish. Consistent monitoring and follow-up work are necessary for sustainable management, as the plant reproduces entirely by seed and can quickly re-establish in disturbed areas.