Sustainable Methods of Managing Weeds in Dryland Farming – Part 2

Article: Matshediso Motshabi

The parasitic witchweed, Striga species, is one of Botswana’s most notorious weeds to eradicate. When left unchecked, this weed can result in significant financial losses from our harvest, and chemical control alone is insufficient to manage it. Striga and other parasitic weeds cause significant harm to the host crop before emerging above ground, making it difficult to effectively control them with traditional hand or mechanical weeding methods. Crop scientists have created sustainable management strategies for the Striga species that combine biological, mechanical, chemical, and cultural controls. Few of these methods, however, can completely eradicate Striga, so it is typically necessary to combine several of them (integrated control), depending on the farming system.

Hand weeding is one of the most dominant cultural and mechanical practices, and Batswana farmers generally loathe it because it must be done frequently and is time-consuming. To avoid seeding, it is advised to start hand-pulling the parasitic weed when it begins to flower at 3 weeks old. The most cost-effective hand-pulling method is continued for an additional three to four years on the least infested fields. Another solution is to alternate non-susceptible crops on infested land. Striga seed germination is further interrupted by rotation with non-host crops, which lowers the amount of seeds present in the soil. However, this method needs to be used continuously for at least three years to be effective.

This parasitic weed can be distracted from your valuable crop by using trap crops, which is a practical management strategy that ensures no harvest loss. Striga incidences on your farm are significantly reduced when hosts are interplanted with non-host crops. The most popular intercrop method, which involves planting cowpeas between rows of maize, significantly lowers the amount of Striga species that attack both of these crops. Similar to this, an integrated pest management push and pull strategy has demonstrated that combining fodder legumes like Brachiaria grass and Desmodium species with a maize or sorghum crop deters both Striga species and defoliator insect pests. Intercropping with trap crops cause the weed to germinate but are not themselves parasitized when trying to control Striga asiatica, S. alectra, or S. hermonthica in a maize setting. Among other crops, these include cotton, lablab, lucerne, sunflower, cowpea, chickpea, and cotton. The reason for this unusual mechanism is that, as opposed to the aforementioned Striga species, other Striga species parasitize the trap crops. Some genotypes of cereals, particularly those for maize and sorghum, can tolerate Striga species. It has been demonstrated that planting these tolerant cultivars at the correct time will reduce infestation rates and harvest losses.

There are numerous agrochemicals available on the market under different trade names that are used to control this parasitic weed, but you should pay attention to their active ingredient. These chemicals are either herbicides or germination stimulants. In the absence of a suitable host, germination stimulants encourage the germination of Strigaseeds, which lowers the amount of seed reserves in the soil. They work best when used during your field’s resting period. Ethylene, ethephon, strigol, and strigol analogues are the active ingredients in germination stimulants; please use these substances in accordance with the recommended dosage. Among the well-known broad-spectrum herbicides used to control Striga, Dicamba is most effective when applied early in the summer’s ploughing season, preferably in the first few weeks of September. Another broad-spectrum herbicide that controls early season Striga is Imazapyr, but it only performs well with selective varieties of maize. Additionally, Imazapyr and Pyritobac applied in combination in a seed dressing provide effective Striga control. In sub-Saharan Africa, a weevil insect from the Smicronyx species feeds on witchweed seeds and is a significant economic player in the region. Fusarium nygamaiand F. semitectum var. majus, are two fungal pathogens that have been isolated from emerging Striga plants, have been used to significantly reduce germination and/or kill Striga species.

In other African nations, various integrated weed management techniques are used to manage Striga species in commercial cereals and pulses. In order to lessen the prevalence of witchweed in farms, perhaps it is time for our local agricultural scientists to create cultivars resistant to the weed, investigate potential ethno-botanicals that control the weed, and definitely create an integrated weed management plant suitable for our local cultivars. FMB