Section 3: Variety Selection, Seed Quality and Plant Populations
Assets to consider when selecting a variety include:
• pest resistance, which is essential to optimize yield and quality
• predictability and durability of yield across environments for risk management
• drought and disease resistance, such as resistance to leaf spot disease and rosette
• short maturity times, which allow a farmer to adapt to early or unpredictable dry seasons
Commercially available varieties and key characteristics in Malawi are listed in Table 3.1. Several improved groundnut varieties (both Spanish and Virginia types) have been released by DARS through research with CGIAR and other partners in Malawi. The latest varieties combine high yield, disease resistance and other preferred market traits. Understanding how these varieties interact with planting date, plant populations and other available inputs can help optimize yield of these varieties.
Agro-Ecologies for Groundnut Cultivation
Malawi has four main agro-ecologies based on climatic conditions and differences in altitude: the high altitude areas, the mid-altitude or plateau areas (900-1200 m), the lakeshore, and the Shire Valley. The lakeshore and the Shire Valley are often regarded as a lowland agro-ecology (up to 899 m). Groundnut is grown from near sea level to >1500 m, but over 70% is produced in the mid altitude and plateau areas, covering Lilongwe and Kasungu in central Malawi, and Mzimba district in northern Malawi. Groundnuts are mainly grown as a rainfed crop; however, off-season production is possible in some parts of Nkhatabay and Karonga districts. The crop is either grown as the sole crop or intercropped with maize, sorghum, millets, soybean, pigeon peas or other plants. The Virginia-type varieties are promoted in the mid-altitude agroecology while the Spanish-type varieties are promoted in the lowland agroecology.
Importance of Seed Quality
Regardless of the variety, a farmer should use quality seed with high germination and seedling vigor. Seed that is produced under poor growing conditions, dried too quickly or to a low moisture level, has pest damage or is stored under warm and humid conditions often will have low germination and seedling vigor. Seed grown in soil lacking adequate nutrients, especially calcium and boron, are less likely to germinate and may produce plants with less vigor. Drought stress during the previous year also limits kernel development and subsequent germination and growth of seedlings. Growers should know the source of their seed and the quality of seed before purchase. Seed damaged through improper mechanical shelling can reduce germination. Kernels that are split or have damage to the seed coat will not germinate.
Source of Seed
To realize high yields, farmers should use good quality seed from known and reliable seed sources such as International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Department of Agricultural Research Services (DARS) or a registered body of certified seed traders. A farmer who obtains seed of an improved variety may opt to save some of the crop to use as seed in subsequent seasons, but groundnut, like other legumes, should be harvested and saved as seed only three times. After three years of recycling, fresh seed should be obtained from known and registered seed sources for optimum production. Saved seed should be kept in shells until shortly before planting.
Conducting a Seed Germination Test
Prior to planting seed in the field, growers should determine the germination rate so that they can adjust seeding rates, if necessary, to achieve an optimum plant population. Germination and plant density can have important implications for a farmer trying to reach a certain yield per hectare or needing a total amount of production from the farm. To test germination, shell approximately 20-25 pods, sow 40 seeds in soil and water well enough to germinate. If 30 plants emerge, the seed is considered high quality and no adjustment at planting is necessary. If 20-30 plants emerge, you should plant a bit closer or put 2 seeds together in the row. If fewer than 20 plants emerge, the seed is poor quality and you will need to plant 3 seeds together in the row or purchase new seed. The proper plant density is important to get the best yield, so it is better to sow seed to achieve the proper plant density than to spread the crop over a larger land area. If you have additional area available, either purchase more seed to sow for the proper density or plant another crop on the additional land. A farmer should keep some seed in order to plant where seeds failed to emerge. This will establish greater uniformity of groundnut stands and can reduce weed interference and rosette disease.
Seed Preparation and Treatment
Groundnut to be used as seed should be selected carefully and graded before sowing. Before sowing, pods should be hand-shelled and sorted to eliminate skinned, immature, moldy, and small or disfigured seeds. Farmers may choose to treat seeds with an insecticide/fungicide mixture to minimize the impact of insects and pathogens that cause seedling disease and stand loss. The most common fungicide is Thiram which can be sourced from agricultural chemical companies such as Chemicals and Marketing, Agricultural Trading Company, Farmers’ Organization, Crop Serve and other registered agro-dealers. Farmers should use caution when using Thiram to treat seed and in handling the treated seed.
Establish Adequate Groundnut Population
In order to get the best yield, it is critical to have an adequate plant population. A final plant stand of 220,000 plants per hectare is optimal for the greatest yield. Historical recommendations in Malawi indicated that optimum yields could be obtained if groundnut is planted on ridges spaced at 60 cm for Spanish (133,300 plants/ha) and 75 cm for Virginia varieties (89,000 plants/ha). However, more recent research indicates growers should sow seed closer. A higher seeding rate and denser plant population can result in less groundnut rosette, suppress weeds, bring a higher yield, and result in less aflatoxin contamination at harvest, in some instances.
Recommended ridge and plant spacing for varieties differs. Table 3.2 gives maximum plant stand at different seed and row spacings. Regardless of row spacing, growers should plant seed no more than 10 cm apart for Spanish (5 cm along the twin/double row) and 15 am apart for Virginia varieties (7.5 cm along the twin row).
Though farmers sometimes choose to plant seed farther apart, it is important to follow spacing guidelines in order to establish a canopy and optimize photosynthesis early in the growth cycle. The recommended spacing may be different for different types of groundnut. For example, Virginia varieties often have a more prostrate or running growth habit and will produce pods further away from the crown of the plant, while Spanish varieties often grow more upright requiring plants to be established closer together.
The use of double rows, especially for Spanish types, is recommended. Seed should be planted in two lines on top of the bed following a zig-zag pattern, so that each seed in a line is 10 cm apart for Spanish varieties and 15 13 cm apart for Virginia types. This will result in spacing of 5 cm and 7.5 cm along the double row, and result in optimal plant densities and good ground cover. Double rows have been shown to increase yield, but it does increase the amount of seed sown per unit area. However, the goal is to optimize yield.
The actual seeding rate (kg per ha) depends on seed size (100 seed weight) and should reflect the germination percentage. After the groundnut plants emerge, growers should fill in gaps in plant stands as quickly as possible. Establishing plants no more than 10 or 15 cm apart not only increases yield potential but also minimizes the impact of rosette disease. The guidelines for plant spacing apply to all planting dates and for all commercially-grown varieties in Malawi. When seedling rate and plant density are optimum, the groundnut canopy will close more quickly, decreasing soil temperatures in the pegging zone which can increase the likelihood that pegs will survive in soil. Weed control is often enhanced when the groundnut canopy is closed more quickly.
Planting Configurations
Some farmers may choose to use broadcast planting because they have limited time and labor when the rains begin, but this approach can lead to erratic stands and poor stand establishment. Weed management is also more challenging when groundnut seeds are broadcast rather than planted in rows. Flat-sowing in rows or on raised beds is recommended in areas where the soils are loose to prevent losing pods to the ground (pod retention) during harvesting. When groundnut is grown on flat, the spacing between rows should be reduced to 45 cm. Replanting in areas that have not germinated should be done within one week of seedling emergence.
Intercropping versus Monocropping
Many farmers choose to use intercropping, an approach that allows maize to benefit from BNF and can increase overall diversity of crops. Intercropping can also mitigate risk and can allow farmers with limited land to expand production in a different dimension, upward rather than laterally across landscapes. However, intercropping results in competition of the taller crop in the canopy (maize and cassava, for example) with lower-growing crops like groundnuts.
Intercropping rather than monocropping limits mechanization and presents challenges in synchronization of the crops in this system. If crops are not synchronized, one crop can be damaged when the other crop is harvested. Incorporating pesticides into an intercropped system is also difficult, especially with herbicides that may damage sensitive crops growing close to tolerant crops.
Land Preparation
This should start early enough (July or August) to ensure that residues from the previous crop are decomposed before the rains come and the growing season begins. This will also give ample time for the farmer to plant when the first effective rains begin. Ensure that all residues are buried, plowing is deep enough and soils are loose and friable which is important for rooting, water penetration, and pegging.
Sowing
In Malawi, planting should be initiated when the first effective rains begin. There has been some success with dry planting when it occurs within 2 weeks prior to the first rains, but this can be risky, since seeds can deteriorate if they remain in the ground for an extended period prior to germination. If seed is treated with inoculant for BNF, the bacteria in the inoculant does not survive if exposed to hot and dry conditions for a prolonged period of time. The suggested dry planting dates are based upon rainfall patterns for that area.
Groundnuts should not be planted after January 15 because of relatively high populations of aphids that transmit groundnut rosette virus. Planting this late will result in dry conditions during the latter part of the growing cycle. Prolonged drought during pod fill and near harvest causes lower yields and greater potential for aflatoxin contamination compared with early plantings that generally have more abundant rain throughout the growing cycle.
The first step in planting is to make a one or two (if planting double rows) grooves that are 5-7 cm deep along the top of the ridge. The amount of seed dropped in dependent upon germination of seed. In most cases, germination of seed will not exceed 75%. If the germination rate is 75% or higher, plant one seed per hole. If the rate is between 50% and 75%, plant two seeds per hole. If the rate is less than 50%, plant three seeds in each hole (or obtain better seed). The farmer should place the seed, then cover the groove with soil and press firmly to ensure adequate soil-seed contact and uniform seedling emergence. It is also important to replant in areas where seeds do not germinate within the first week after seedling emergence.
Plant as Soon as Rains Begin
The optimum planting date in Malawi for groundnuts coincides with the first predictable and sustained rains. This can occur in late November or early to late December, but can vary from year to year and across geographies in the country. Planting on time allows good germination and quick groundcover to suppress weeds and reduce rosette disease. Planting as early as possible also reduces the risk the crop will face drought later in the season (February/March) which can affect pod filling and increase the risk of infection by molds that cause aflatoxin contamination. Dry planting, within 2 weeks of the onset of rains, can also lower rosette pressure, increase yields, and reduce aflatoxin, but it comes with the risk that if the rains are delayed, the farmer will have poor germination and fewer plants. Finally, it is important not to wait until all maize is planted to start planting groundnut as it likely will be too late and result in a poor crop.
