Chapters 6.6
6.6 - Striga weed suppression using Desmodium intercropping
Jade Muileboom, University of Guelph, Canada
Suggested citation for this chapter.
Muileboom,J. (2022) Striga weed suppression using Desmodium intercropping. In Farmpedia, The Encyclopedia for Small Scale Farmers. Editor, M.N. Raizada, University of Guelph, Canada. http://www.farmpedia.org
The Problem: Striga
For over 100 million farmers in Africa, food insecurity and crop failure is sealed by the sudden appearance of a few brightly flowered plants nestled between their cereal crop (Hooper et al., 2010). These flowers while visually attractive are an indicator of a Striga infestation. Striga, an overarching term used to describe a Genus of parasitic weeds, is by far one of the most destructive weeds in Eastern Africa which causes annual economic losses of over 132 million dollars (Woomer and Savala, 2008). The most common variety S. hermonthica can be identified by its bright green stems and leaves and small purple, pink or white flowers, and grows to around 1m in height (CTA, 2007). Striga, as a parasitic weed, needs to exploit a host plant, stealing its nutrients in order to complete its life-cycle. Studies have found it can reduce crop yields well over 50% or in certain cases cause complete crop failure (Khan et al., 2016).
Striga can be difficult to recognize early as it penetrates the crop roots and feeds off a plant for several weeks before any stems appear above ground, and once it has emerged it is quick to produce flowers and seeds. Effected host plants may at first show signs of various nutrient deficiencies, appearing stunted or of below average biomass, ultimately leading to severely reduced grain yield (khan et al., 2016). Corn, sorghum, rice and sugarcane are generally the greatest victims of the weed, though other grains like millets are affected as well (Midega et al., 2010).
Rooting out striga is made more difficult by the nature of its seeds. Striga is a prolific producer of thousands of tiny black seeds, commonly called “black dust” by locals. These seeds germinate in response to root exudates (or signals) from nearby planted cereal crops, but can also persist dormant in the soil for over 15 years (Khan et al., 2016). Therefore, it is no surprise that striga currently infects more than 40% of arable land in Sub-Saharan Africa (Hooper et al., 2010).
The Intervention: Desmodium
Despite the concern of striga’s prevalence in countless cropping systems, there is great potential to eliminating not only the plants themselves, but also their seed bank in the soil through the use of desmodium. Desmodium, is a sprawling, perennial legume variety, indigenous to South America, that can be used as a forage, green manure and cover crop. Therefore, it can be used to add nitrogen to the soil, reduce erosion, and increase the health and milk production of livestock such as goats and cattle. Additionally, it has been observed to not cause bloat when fed to ruminants, even in large quantities (FAO, 2016). However, its arguably most important aspect is its impact on striga. Intercropping desmodium with striga-infested cereal crops, reduces the weed population dramatically. For instance, one study found striga counts were reduced up to 95% (Kifuko-Koech et al., 2012).
This feat can be explained by the release of various phytochemicals by the plant roots into the soil. Some of which cause the dormant striga seeds to germinate followed by others which then inhibit the striga root growth, preventing those seeds from attaching to a host plant and ultimately causing death. (Khan et al., 2008). This suicidal germination offers a proactive solution to the problem, targeting striga seeds before they can fully grow while clearing out the seed bank. Some researchers estimate that by incorporating desmodium into cropping systems one could eliminate striga seed entirely from a field within 6 years (Khan et al., 2008). As a cover crop, it reduces opportunities for other weed varieties to spread through the soil and complete with the main crop as well. It has also been found to repel stem borers which in combination with reduced striga prevalence has shown to increase cereal yields (Midega et al., 2010). Two main species of desmodium are used for intercropping and push-pull systems, D. uncinatum (Silverleaf) and D. intortum (Greenleaf).
Field Prep, Planting and Maintenance
Desmodium can be planted from seed or cuttings. If planted from seed, desmodium will grow best in finely textured soil, enriched with phosphorus fertilizer. However, if fertilizer is too expensive or unavailable mixing the seed a handful of seed with fine sand at a 1:2 ration is recommended (Khan et al., 2005). Around 1 kg of seed is needed for a 0.4 ha field, which should be drilled into 1-2 cm deep furrows between the maize rows, and then gently covered with a small amount of soil (Khan et al., 2005). If desmodium cuttings can be obtained from neighbouring farms or local agricultural extension services, they can be an easier and quicker method to establish desmodium. Provided the cuttings have at least two internodes and adequate soil moisture, desmodium cuttings can be an effective mode of propagation.
Maize and desmodium should be planted at the same time during the first season in alternating parallel rows. Also during the first season (around 4 months) it is recommended that desmodium be left to establish and that no cuttings of the plants foliage be done (Kifuko-Koech et al., 2012). In following seasons, the plants should be cut at the start of every season, again 4 weeks after the cereal crop was planted (to ensure proper crop establishment) and then a third time 18 weeks after planting for optimal striga reduction (though a range between 12-18 is acceptable) (Kifuko-Koech et al., 2012). The process of trimming desmodium is important in preventing it from outcompeting its companion crop.
Challenges
While Desmodium offers many opportunities for improving the livelihoods of subsistence farmers there remain some challenges in integrating this plant into current agricultural systems. Desmodium has shown to have varying results growing in soils that are either too dry or too acidic (below a pH of 5) (FAO, 2016). Desmodium can also be victim to other pests like the herbivorous bister beetles whom if left to feed on its flowers could negatively affect seed production (Lebesa et al., 2012). These pests are concerning as desmodium is not a prolific seed producer, which while adding value to the sale of its seeds, makes the loss of these potential profits a concern. Initial access to the desmodium seeds can also be a limiting factor as currently they are often not stocked in commercial seed enterprises.
Also, while desmodium has shown to increase the net profits of the farmers that intercrop it, there is a greater initial cost and labour requirement than mono-cropping systems. However, these costs (which include the initial purchase of desmodium seeds, and the labour required for planting and weeding) have been shown to significantly decrease after the first year, as it does not need to be replanted (Kifuko-Koech et al., 2012). Some further maintenance will be required to maintain the desmodium’s sprawl, taking cuttings to keep the spread contained, but the plant is not known to spread uncontrollably (rather one must be careful not to overcut or graze upon it). These cuttings can offer new economic prospects as well for farmers who choose to use them for fodder or sell to others for similar purposes. It should also be noted that some studies have found that during the first couple seasons the impacts of desmodium regarding striga suppression, increased yields and financial returns were not always pronounced, though in subsequent seasons they became much more evident and consistent (with striga being reduced by 76% and then 90%) (Kifuko-Koech et al., 2012).
Picture Based Lesson to Train Farmers
Further Practical Resources and Useful Reading
1). The CTA Practical Guide Series, No. 2: How to control Striga and stemborer in maize https://publications.cta.int/media/publications/downloads/1361_PDF.pdf
2). The Striga Technology Extension Project: Long Rains 2008 Report
3). Tropical Seeds LLC About: A potential source for desmodium seeds, while currently desmodium seeds are not in stock, they hope to have a supply for 2018. www.tropseeds.com
4). Dr. Samuel T. Ledermann Project Coordinator and Scientific Advisor for Biovision About: For more information regarding local organizations, providers or farmer co-ops with a desmodium material supply. Biovision has implemented previous projects focused on integrating desmodium into farming systems in Sub-Saharan Africa in partnership with ICIPE. Email: s.ledermann@biovision.ch Tel: +41 44 512 58 58 Website: http://www.biovision.ch/en/projects/sub-saharan-africa/push-pull/
References
1. Amudavi, D. M., Khan, Z. R., Wanyama, J. M., Midega, C. A. O., Pittchar, J., Nyangau, I. M., … Pickett, J. A. (2009). Assessment of technical efficiency of farmer teachers in the uptake and dissemination of push–pull technology in Western Kenya. Crop Protection, 28(11), 987–996. http://doi.org/10.1016/j.cropro.2009.04.010
2. FAO. (2017). Desmodium intortum. Retrieved January 26, 2017
3. FAO. (2017). Desmodium uncinatum. Retrieved January 26, 2017
4. Hooper, A. M., Tsanuo, M. K., Chamberlain, K., Tittcomb, K., Scholes, J., Hassanali, A., … Pickett, J. A. (2010). Isoschaftoside, a C-glycosylflavonoid from Desmodium uncinatum root exudate, is an allelochemical against the development of Striga. Phytochemistry, 71(8–9), 904–908. http://doi.org/10.1016/j.phytochem.2010.02.015
5. Khan, Z., Midega, C. A. O., Hooper, A., & Pickett, J. (2016). Push-Pull: Chemical Ecology-Based Integrated Pest Management Technology. Journal of Chemical Ecology, 42(7), 689–697. http://doi.org/10.1007/s10886-016-0730-y
6. Khan, Z. R., Midega, C. A. O., Njuguna, E. M., Amudavi, D. M., Wanyama, J. M., & Pickett, J. A. (2008). Economic performance of the “push–pull” technology for stemborer and Striga control in smallholder farming systems in western Kenya. Crop Protection, 27(7), 1084–1097. http://doi.org/10.1016/j.cropro.2008.01.005
7. Khan, Z. R., Muyekho, F. N., Njuguna, E., Pickett, J. A., Wadhams, L. J., Pittchar, J., … Lusweti, C. (2005). A Primer on Planting and Managing “Push-Pull” Fields for Stemborer and Striga Weed Control in Maize. Kenya.
8. Kifuko-Koech, M., Pypers, P., Okalebo, J. R., Othieno, C. O., Khan, Z. R., Pickett, J. A., … Vanlauwe, B. (2012). The impact of Desmodium spp. and cutting regimes on the agronomic and economic performance of Desmodium–maize intercropping system in western Kenya. Field Crops Research, 137, 97–107. http://doi.org/10.1016/j.fcr.2012.08.007
9. Lebesa, L. N., Khan, Z. R., Krüger, K., Bruce, T. J. A., Hassanali, A., & Pickett, J. A. (2012). Farmers’ knowledge and perceptions of blister beetles, Hycleus spp. (Coleoptera: Meloidae), as pest herbivores of Desmodium legumes in western Kenya. International Journal of Pest Management, 58(2), 165–174. http://doi.org/10.1080/09670874.2012.673032
10. Midega, C. A. O., Bruce, T. J. A., Pickett, J. A., Pittchar, J. O., Murage, A., & Khan, Z. R. (2015). Climate-adapted companion cropping increases agricultural productivity in East Africa. Field Crops Research, 180, 118–125. http://doi.org/10.1016/j.fcr.2015.05.022
11. Midega, C. A. O., Salifu, D., Bruce, T. J., Pittchar, J., Pickett, J. A., & Khan, Z. R. (2014). Cumulative effects and economic benefits of intercropping maize with food legumes on Striga hermonthica infestation. Field Crops Research, 155, 144–152. http://doi.org/10.1016/j.fcr.2013.09.012
12. Midega, C., Khan, Z., Amudavi, D., Pittchar, J., & Pickett, J. (2010). Integrated management of Striga hermonthica and cereal stemborers in finger millet (Eleusine coracana (L.) Gaertn.) through intercropping with Desmodium intortum. International Journal of Pest Management, 56(2), 145–151. http://doi.org/10.1080/09670870903248843