Breeding cowpea (Vigna unguiculata [L.] Walp) for improved yield and related traits using gamma irradiation.
Horn, Lydia Ndinelao.
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Cowpea is an important grain legume widely grown in sub-Saharan Africa for food and feed. In Namibia cowpea productivity is considerably low due to a wide array of abiotic and biotic stresses and socio-economic constrains. The overall objective of this study was to develop farmers’ preferred cowpea varieties with enhanced grain yield and agronomic traits through mutation breeding. The specific objectives of the study were to: (1) assess farmers’- perceived production constraints, preferred traits and the farming system of cowpea, and its implication for breeding in northern Namibia, (2) determine an ideal dose of gamma radiation to induce genetic variation in selected cowpea genotypes, (3) identify desirable cowpea genotypes after gamma irradiation of three IITA acquired cowpea varieties widely grown in Namibia including Nakare (IT81D-985), Shindimba (IT89KD-245-1) and Bira (IT87D-453-2) through continuous selections from M2 through M6 generations, (4) determine G x E interaction and yield stability of elite mutant cowpea selections and to identify promising genotypes and representative test and production environments, and (5) select elite cowpea varieties that meet farmers’ needs and preferences through farmers’ participation and indigenous knowledge. Participatory rural appraisal (PRA) study was conducted across four selected regions of northern Namibia including Kavango East, Kavango West, Oshikoto and Omusati where cowpea is predominantly cultivated involving 171 households. The majority of respondent farmers (70.2%) grow local unimproved cowpea varieties. About 62.6% of interviewed farmers reported low yields of cowpea varying from 100-599 kg/ha, while 6% of respondents achieved good grain harvests of 1500-1999 kg/ha. Farmers who grow local unimproved avarieties also indicated that the local varieties were not readly available and most have lost them to prolonged droughts and poor rainfall. Most farmers (59.1%) produced cowpea for home consumption, while 23.4% indicated its food and market value. Field pests such as aphids (reported by 77.8% respondents), leaf beetles (53.2%) and pod borers (60%) and bruchids (100%) were the major constraints. Striga gesnerioides and Alectra Vogelii (Benth) were the principal parasitic weeds reported by 79.5% respondent farmers. Soil fertility levels were reported to be very low across regions and all farmers did not apply any fertilizers on cowpea. Farmers-preferred traits of cowpea included a straight pod shape (reported by 61.4% respondents), a long pod size bearing at least 10 seeds (68.4%), white grain colour (22.2%) and high above ground biomass (42.1%). Inter-cropping of cowpea with sorghum or pearl millet was the dominant cowpea farming system in northern Namibia. About 68.4% of farmers used a relatively smaller proportion of their land (<1 ha) for cowpea production, while only 9.9% allocated more than 5 ha-1. Before a large scale mutagenesis an appropriate dose of radiation should be established on target genotypes. Therefore, seeds of the following three cowpea genotypes widely grown in Namibia: Nakare (IT81D-985), Shindimba (IT89KD-245-1) and Bira (IT87D-453) were gamma irradiated using seven doses (0, 100, 200, 300, 400, 500 and 600 Gy) at the International Atomic Energy Agency, Austria. The optimum doses at LD50 for the genotypes Nakare and Shindimba were 150 and 200 Gy, respectively while genotype Bira tolerated high dose of 600 Gy. Using linear regression model, the LD50 for genotypes Nakare, Shindimba and Bira were established to be 165.24, 198.69 and 689 Gy, respectively. Large scale mutagenesis were undertaken through gamma irradiation using seeds of the three varieties (Nakare, Shindimba and Bira). Field experiments were conducted in order to identify agronomically desirable cowpea genotypes. Substantial genetic variability was detected among cowpea genotypes after mutagenesis across generations including flowering ability, maturity, flower and seed colours and grain yields. Overall 34 elite cowpea mutants were selected from 37 genotypes including 3 parental lines showing phenotypic and agronomic stability. The selected 34 promising mutant lines along with the 3 parents were recommended for adaptability and stability tests across representative agro-ecologies for large-scale production or breeding in Namibia. The lines were subjected to G x E study conducted at three selected sites (Bagani, Mannheim and Omahenene) and two cropping seasons (2014/2015 and 2015/2016) providing six environments. The following four promising mutant genotypes: G9 (ShL3P74), G10 (ShR3P4), G12 (ShR9P5) and G4 (ShL2P4) were identified with better grain yields of 2.83, 2.06, 1.99 and 1.95, t.ha-1, in that order. The parental lines designated as G14 (Shindimba), G26 (Nakare) and G37 (Bira) provided mean grain yields of 1.87, 1.48 and 1.30 t.ha-1, respectively. The best environments in discriminating the test genotypes were Bagani during 2014/15 and Omahenene during 2014/15. Participatory cowpea varietal selection was undertaken in the northern Namibia using a set of newly developed 34 elite cowpea varieties. Genotypes were evaluated along with the three parents. Field evaluations were conducted across three selected villages in Omusati Region of northern Namibia where the crop is predominantly cultivated. Test varieties were independently assessed and scored using nine agronomic traits involving 114 participating farmers. Overall, the following 10 farmers-preferred cowpea varieties were selected: R9P5 (Sh200), R3P4 (Sh100), R4P1 (Sh100), L3P74 (Sh100), R1P12 (Nk100), R8P9 (Nk150), R5P1 (Nk150), R2P9 (Nk150), R10P5 (Nk150) and R11P2 (Bi600) for their larger seed size, white grain colour, high pod setting ability, insect pest tolerance, early maturity, longer pod size, drought tolerance, high biomass and pod yields. Generally, the study identified valuable cowpea mutants derived from three local varieties Shindimba, Bira and Nakare using gamma irradiation. The identified genotypes are phenotypically and agronomically stable and recommended to distinct, uniformity and stability (DUS) trials for varietal registration and release in northern Namibia.