Productivity of Grain Cowpea (Vigna unguiculata (L.) Walp.) as Influenced by Season, Genotype, Insect Pest Management and Cropping System in Southeastern Nigeria

Abstract

The first experiment involved nine improved cowpea genotypes and a local variety. The ten treatments were planted in two locations, namely the Research Farm of the College of Agriculture, Mgbakwu in Anambra State (060 17ʹN, 070 04ʹE; 83m asl) and the Experimental Farm of the Federal College of Agriculture, Ishiagu in Ebonyi State (050 58ʹN, 070 34ʹE; 197 m asl), over a period of two years and two seasons per year in each of the two locations. The experiment was spilt-plot arranged in randomized complete block design (RCBD) with three replications. The second experiment was conducted at the DEMACCO Integrated Farms Ltd., Ako, Nike in Enugu State (060 34ʹN, 070 35ʹE; 154 m asl). The experiment consisted of four promising genotypes selected from experiment one and a local variety used as check. An open pollinated maize variety (ACR9931) was intercropped with the five cowpea genotypes. The maize and cowpea genotypes were sown over a period of two years and two seasons in each year. The experiment was split-split plot arranged in RCBD with three replications. A total number of twenty nine parameters were sampled consisting of eleven growth, twelve grain yield and six insect pest damage components. Data were subjected to analysis of variance (ANOVA) using the GENSTAT, 2003 edition. Differences among treatment means were compared using F-LSD, while interaction of genotype by environment, genotype by traits and environment by traits were computed using GGE biplot analytical model. This study revealed the presence of genotype X season, genotype X insect protection and genotype X season X insect protection interaction for experiment one, while experiment two indicated the presence of genotype X season, genotype X cropping system, genotype X spray regime and genotype X season X cropping system X spray regime interaction. Growth, reproductive, grain yield and insect damage components were highly significant in all the environments. Yield and yield components were significantly higher in early season than in late season. Similarly, plant population and cowpea biomass were higher in early than late season. Pod length, number of seed per pod, number of branches and number of internodes were least influenced by the environments due to their high heritability. In all the environments, seed size was significantly higher in IT97K-277-2, IT97K-556-4 and IT93K-452-1, than the rest genotypes, while IT84S-2246-4 and IT90K-82-2 consistently expressed significantly lower seed size. The local variety produced significantly higher seed size than all the test genotypes when sprayed with insecticide in late season. The genotypes IT90K-277-2, IT97K-556-4 and local variety exhibited dual-purpose (grain and fodder) characteristics, while the rest genotypes were purely grain type. Most of the dual-purpose cowpeas are both indeterminate and long duration. The short growth duration and higher mean grain yield made IT93K-452-1 the best grain type cowpea because it combined these qualities with tolerance to most post flowering pests. The genotype IT93K-452-1 also produced reasonable grain yield in late season without chemical spray. IT98K-131-2 was an outstanding medium maturing genotype combining superior grain yield attribute with tolerance to both pre-and-post flowering pests in all the environments. Furthermore, this variety also produced satisfactory grain yield in late season without insecticide application. Genotype IT97K-556-4 on the other hand, harboured the highest population of most pests sampled in all the environments. This study further showed that thrips, Maruca, pod sucking bugs and bruchids were the most prevalent insect pests of cowpea in south eastern Nigeria, while aphids and Ootheca were the minor pests. Application of insecticides once each at flower bud initiation, full bloom and podding significantly reduced insect pest population and increased grain yield of cowpea significantly. Improved cowpea genotypes recorded significantly higher grain yield than the local check in all the environments. Medium to late maturing genotypes were better adapted to late season while early maturing genotypes performed well in both seasons. Bruchids, Maruca, pod sucking bugs and thrips were more abundant in late season than early season while aphids and Ootheca population were more widespread in early season than late season. Brown seeded cowpea genotype consistently harboured lower infestation by bruchids than white seeded types. This study also showed that insecticide treatment targeted at the critical growth stages especially at 50 percent podding and early sowing significantly reduced bruchids damage on stored cowpea seed. Grain yield loss assessment was negligible in early season for all the genotypes while in late season it was 100 percent for local variety, 34 percent for best yielding medium maturing genotype (IT98K-131-2) and 30 percent for best yielding early maturing genotype (IT93K-452-1). Percentage reduction in insect population when 3 sprays were applied relative to zero spray for aphids, bruchids, Maruca, Ootheca, pod sucking bugs and thrips are 121 percent, 240 percent, 174 percent, 45 percent, 38 percent and 270 percent respectively. Intercropping reduced dry fodder yield in early season by 22 percent and in late season by 41 percent. On the other hand, intercropping did not significantly reduce number of branches, internodes number, number of leaves, number of nodules, plant population, and root length. Meanwhile, peduncle length was significantly reduced by intercropping in both early and late season but varied widely among the genotypes tested, with local cowpea variety being most affected. Peduncle length in cowpea was obviously sensitive to stress imposed by intercropping particularly in late season and could be used as an index for determining cowpea cultivars adapted to intercropping environment. Intercropping in both seasons significantly reduced yield and yield components in cowpea but more in late than early season. Consequently, intercropping reduced grain yield in early season by 14 percent while in late season it reduced it by 25 percent. Also, intercropping in early season reduced days to maturity but did not affect 50 percent bloom and pod filling duration. However, in comparison with early season, all the genotypes in late season flowered and matured earlier, while on the contrary they took longer days to fill their pods. In both seasons, sole cropping generally produced higher grain yield than intercropping when sprayed with insecticide. Conversely, cowpea grain yield in intercropping were generally higher than yields from sole cropping when no insecticide was applied, suggesting less insect damage under intercropping. Early maturing genotypes produced significantly higher grain yield in early and late seasons and in both sole and intercropping, while medium and late maturing genotypes expressed their highest yield potentials in sole cropping in late season. Also, in late season, intercropping significantly reduced the population of bruchids, pod sucking bugs and thrips but did not affect the population of the rest insect pests. Highest grain yield components were realized in genotypes grown in intercropping with two sprays while in sole cropping early maturing genotypes required two sprays while medium and late maturing genotypes required three sprays to produce the highest grain yield. Late season planting reduced the population of aphids, Maruca and Ootheca by 122 percent, 183 percent, and 47 percent respectively, while early season sowing reduced the population levels of pod sucking bugs by 47 percent and thrips by 104 percent. Intercropping reduced the population of aphids, bruchids, pod sucking bugs and thrips by 40 percent, 9 percent, 8 percent, and 100 percent respectively. Meanwhile, intercropping increased the infestation of Maruca by 9 percent while Ootheca was unaffected by cropping system. Intercropping combination of ACR9931/IT98K-131-2 had positive effects on maize through the production of significantly higher yield and yield components of maize, while ACR9931/Local combination depressed components of maize yields. We found improved medium maturing, indeterminate cowpea cultivar with long peduncle length as most suitable for use in intercropping with maize in South-eastern Nigeria. Maize performed better under intercropping than sole cropping in early than in late season, in 2009 than 2010. The yield reduction in maize from cropping system, season and year effects was caused by decline in cob length, cob weight, number of cobs per plot, seed weight, 100 seed weight and harvest index, and not by number of plant stands. This revealed that maize productivity is more influenced by these traits.