In looking for a green manure crop, the best plants to try are those especially well adapted to local climate and soil conditions. In the search for the ideal crop, it is easy to overlook a potentially useful class of plants–local weeds. Scientists at the International Centre for Research in Agroforestry (ICRAF) in Kenya have recently determined that the weedy shrub Tithonia diversifolia has potential as a green manure crop.
Farmers collaborating with the ICRAF research project chopped stems and leaves of shrubs commonly found growing on the margins of fields and carried them into the corn fields as a green manure. Three separate experiments were done.
Experiment 1: Of six species, two of which were leguminous, Tithonia was found to increase maize yields the most (43-80%) over a control treatment which received no fertilizer. The common tropical weed Lantana camara produced the second-highest yield increases (24-38%), though many of the farmers “did not apply the prescribed quantities” of Lantana, possibly due to its thorns. Besides tithonia and lantana, the six shrub species used as green manure in the ICRAF trial were Cassia (or Senna) spectabilis, Calliandra calothyrsus, Grevillea robusta, and Psidium guajava (guava). The rate of green manure application in the experiment was 5 t/ha (2 t/acre).
Experiment 2: Green manures of Tithonia and Lantana were applied at three different rates (5, 10 and 20 t/ha or 2, 4, 8 t/acre) to corn and compared to three rates of phosphorus fertilizer (12.5, 25, and 50 kg/ha or 11, 22, 44.5 lbs./acre) applied to similar plots. At 5 t/ha, the lowest application rate, the two green manures both produced higher yields than the phosphorus-only fertilizer at the highest rate of application (50 kg/ha). At 5 t/ha, Tithonia mulch produced significantly higher yields of maize than Lantana mulch. Higher application rates produced higher yields of maize for both green manures, but at these rates there was no significant difference between yields produced by one versus the other.
Experiment 3: Kenyan farmers in two regions used Tithonia and Lantana mulches on their farms at the rate of a little less than 5 t/ha (100 kg/5x5 m plot or 2 t/acre). Both mulches increased maize yields, but over 95% of the farmers preferred the Tithonia mulch.
Besides its lack of popularity with farmers, Lantana is toxic to most grazing animals. In horses it is responsible for “pinknose,” a sensitivity of the skin to the sun caused by ingesting Lantana. In researching this article, I found an entire, well-written website on Lantana camara [no longer available] The author of the website, Amanda Rinehart, cites several studies which indicate that Lantana camara is allelopathic (suppresses growth) to both crops and other weeds.
Tithonia diversifolia is a shrub native to Central America but which has become naturalized in countries throughout the tropics, being found in Kenya, India, Ceylon, Cuba, and Colombia [Katto and Salazar, see below]. We grow it at ECHO. I personally have seen it growing in Haiti and Ecuador. A relative of the sunflower (see photo), this tender shrub grows from 1.5 m to 4.0 m (5 to 13 ft.) high and nearly as wide if left uncut. Its bright yellow flower heads are so ornamental that they are in the background of a postcard sent to me from Haiti.
At ECHO we occasionally feed our rabbits the leaves of Tithonia. A paper by Katto and Salazar of the Center for Investigation in Sustainable Systems of Production Agriculture (CIPAV) (published in Spanish in Livestock Research for Rural Development. 6(3), 1995.) reports that it is used as forage for sheep and guinea pigs as well, with protein levels of up to 28.5% in the leaves (dry weight). (I was surprised to read in “ICRAF Updates” on the internet that Tithonia has “no fodder value.”)
ECHO does not carry seed of Tithonia or Lantana, because of their known weed potential (though relatives of both are sold as ornamentals in the southern US). However, most tropical countries likely already have populations of these plants. A word of caution when evaluating such plants as green manures: ICRAF has not yet determined if the use of weeds as green manures would increase local weed populations in and around farms.