Impact of Weeds on Groundnut Yield

Weeds can compete with groundnuts for light, nutrients in soil, and water causing major yield losses if they are not controlled. Weeds can also interfere with digging and lifting plants at harvest. Broadleaf weeds, annual and perennial grasses, and sedges are present in many fields and if left uncontrolled, can result in significant yield reduction. When and how often a farmer controls weeds will influence the final yield of groundnuts. To ensure the highest yields, growers need to begin the first weeding (usually by hand, using hoes) no later than 3 weeks after planting. Additional weeding at 6 and 9 weeks after planting will also protect yield from weeds that emerge later. The first 30 days is the most important time to weed and is often referred to as the critical period of weed interference. Once flowering and pegging begins, weeds should be pulled by hand to avoid destroying flowers and developing pods or disturbing the soil.

Planting in rows rather than broadcast seeding or intercropping increases efficiency of hand-weeding. Intercropping limits the use of herbicides to control weeds because of the potential for damage to crops other than groundnut. A thick and uniform stand of groundnuts can improve weed control by allowing groundnut plants to shade the soil and minimize growth of weeds. By establishing a groundnut population of 222,000 plants per hectare and using an appropriate herbicide at planting or hand-weeding within 30 days of planting, a farmer can avoid an additional weeding later in the season and preserve yield.

Herbicide Guidelines for Suppression of Weeds

Some farmers may choose to buy herbicides. Use of herbicides, especially immediately after planting, can reduce the time required for hand removal and protect groundnuts from weed interference. For example, research in Ghana indicated that 40 or more hours were required to hand weed a single hectare of groundnuts (Figure 5.4). Using herbicides at planting decreased this time commitment dramatically, and even though the herbicides were expensive, the weed control they provided was cost effective and brought documented financial benefits. When labor to remove weeds by hand is limited, herbicides can be an effective tool for farmers to minimize weed interference during the first month of the season.

Trials conducted in Malawi in 2020 and 2021 at two locations show a benefit from the use of herbicides in terms of reduced labor and increased yield. This was true for both the use of preemergence and postemergence herbicides.

It is important to make sure any herbicides used in groundnuts are approved by regulatory agencies in Malawi and that they are used in accordance with the manufacturer's product label.

Several herbicides can be applied to groundnut fields before sowing or just after sowing but before groundnuts and weeds emerge. Glyphosate and paraquat and other herbicides containing the active ingredient acetochlor, alachlor, diclosulam, pendimethalin, S-metolachlor, and trifluralin are available in Malawi and provide good suppression of weeds during the first 30 days of the season. These herbicides control weeds that might emerge for several weeks after application, but do not kill weeds that already are growing when the chemical is applied.

Glyphosate is a relatively safe herbicide to handle and apply. However, paraquat is highly toxic to humans and animals and should be applied only when people use all safety precautions. If adequate personal protection equipment is not available, paraquat should not be used.

Consistent control requires application several weeks before sowing if a disc is not used and then a second application immediately after sowing. If a disc is used to prepare field for sowing, these herbicides can control weeds that begin to emerge after disking, especially if sowing is delayed because of labor issues or excessive rain. Do not apply these herbicides if soil begins to crack due to groundnut emergence. These herbicides are considered non-selective and will kill not only groundnuts but also weeds once they have emerged.

Once groundnuts have emerged, herbicides such as bentazon and fluazifop-p-butyl can be used. Bentazon controls broadleaf weeds and sedges while fluazifop-p-butyl will control grasses. Make sure the correct adjuvant is added with the herbicide. These herbicides control only the weeds that have emerged, and the herbicide comes in contact with. There is no residual weed control in the soil from these two post-emergence herbicides.