Maize-sesame intercropping in Southeast Tanzania : Farmers' practices and perceptions, and intercrop performance

Keywords: Intercropping,farm household objectives, marginal factor returns, cost-benefit analysis, inter-seeding time, Land Equivalent Ratio, maize, sesame, niche differentiation, spatial arrangement.InSoutheast Tanzania, the major food crop maize is often inter-seeded with the cash crop sesame using an additive design. Farmers consider maize an essential crop for securing their basic food requirements, whereas sesame is added to generate cash. In this research, farmers' motives for adopting maize-sesame intercropping systems were studied. Concurrently, experiments were conducted to evaluate the performance of the maize-sesame intercropping systems and to explore options for improvement.A household survey revealed that in the study area arable cropping is the main occupation, responsible for 75% of the mean annual household income. Cashew, maize and sesame were found to be the most important crops contributing to the household income. About 90% of the farmers intercropped maize and sesame to diversify their cash income. All farmers consider maize the more important crop, as it should secure the basic food requirements of the household. The risk of crop failure associated to growing sesame in pure stand is an important reason for adding sesame to maize. Growing the intercrop also puts less demand to labour and fertile land, both of which are limited in supply. Furthermore, maize and sesame are regarded as good companion crops, which additionally contribute to restoration of soil fertility and weed suppression.Simultaneous sowing of maize and sesame caused reductions in maize grain yield, of on average 27%. These reductions decreased with delayed inter-seeding times. Conversely, delayed seeding led to significant reductions in sesame yield, caused by a direct effect of sowing time and an increased competitiveness of maize. Based on a long-term averagemaize :sesame price ratio of 1:3.5, simultaneous sowing turned out to be the option with the highest gross financial returns. At the same time, the study demonstrated that there are associated risks with simultaneous sowing such as seedling mortality of sesame due to water logging and severe reductions in maize grain yield (up to 60%). Farmers generally introduce sesame about two weeks after maize, to reduce those risks. This study showed however that the recently developed improved sesame varieties are not very well suited for late introduction, due to their poor competitiveness. As sesame is mainly grown in intercrop, future breeding efforts should not only consider characteristics as yield, seed colour and seed oil content, but should also take into account characteristics as competitive ability and growth duration, that determine the suitability of sesame in intercropping systems.Density experiments revealed that maize was more competitive than sesame, but more importantly the experiments also demonstrated that maize and sesame are partially complementary in resource acquisition. Niche differentiation forms the basis for a yield advantage in intercropping. This observation confirms the notion of farmers that maize and sesame are good companion crops. At both the relatively high fertile site and the poorly fertile site, P/N ratios of shoot tissue of maize and sesame were high (between 1/1.5 and 1/6.4), indicating that nitrogen was a major limiting factor in the study area. At the low soil fertility site (Mkumba), both maize and sesame in pure stand responded significantly to N and NP-fertilization. At this site, the application of nitrogen fertilizer resulted in 2.5 and 3.6 fold increases in pure stand yield of maize and sesame, respectively. In intercrop, N fertilization increased the dominant position of maize in the intercrop. Consequently, only maize profited from N fertilization, as for sesame the advantage of additional N was counterbalanced by the presence of a more competitive maize crop. Nitrogen recovery was highest in the intercrop. The results question the general fertilizer recommendations, which advice to supply N and P in nearly equal amounts.Studies on spatial arrangement showed that, both in pure stand and intercrop, sesame seed yield was independent of sowing method (row or broadcast). In the intercrop, grain yield of maize was affected by the method of sowing sesame. When broadcast sown, sesame caused reductions in maize grain yield ranging from 53 to 69%. These reductions were only 19 to 55% with row sowing. Two to three maize plants per station were found optimal in pure stand as well as in intercrop. This last result indicates that farmers' practice of growing two-three plants per station is superior to the institutional recommendation of growing one plant per stationThe results of this study clearly indicate that recommendations for intercropping should be based on intercropping research and cannot simply be extrapolated from results obtained with pure stands of the respective component crops. Efforts for breeding improved varieties should also consider the use of the crops in intercropping. Furthermore, a proper analysis of experimental results requires a thorough understanding of farmers' objectives and production constraints. At the same time, an inventory of farmers' objectives and production constraints without exploring the options for improvement seem ineffective. For these reasons it was recommended that future projects should put emphasis on participatory research in teams of social scientists, technical scientists and farmers in the process of co-innovation to improve the well being of farmers and rural households

Genotype x Environment Interaction and Stability Analysis for Yield and its Components in Selected Cassava (Manihot Esculenta Crantz) Genotypes in Southern Tanzania

The present investigation was carried out to study stability performance over three environments for root yield and its components in twelve genetically diverse genotypes of cassava using a Randomized Complete Block Design. The partitioning of (environment + genotype x environment) mean squares showed that environments (linear) differed significantly and were quite diverse with regards to their effects on the performance of genotypes for root  yield and majority of yield components. Stable genotypes were identified for wider environments and specific environments with high per se performance (over general mean) for root yield per plant. The investigation revealed that the genotypes Kiroba     (21.72 t ha-1) and NDL 2006/487 (19.5 t ha-1) were desirable and relatively stable across the environments. Other genotypes NDL 2006/850 was suitable for favourable situations, while genotypes NDL 2006/104 and NDL 2006/283 were suited to poor environments for root yield. Keywords: G X E Interaction, Stability Analysis, Cassava, Root Yield, Yield Component

Assessment and Selection of Superior Genotypes among Elite Cassava Genotypes by Farmers and Scientists in Southern Tanzania.

A participatory genotype/variety selection involving farmers in three agro ecological zones in Southern Tanzania and scientists from Naliendele Agricultural Research Institute (NARI) and Sokoine University of Agriculture (SUA) to select superior cassava genotypes is described. Initial situation analyses of the communities indicated that cassava is increasing in importance both as a food and a cash crop. Most farmers utilized landraces of cassava; improved varieties were scarcely mentioned. Nine newly improved cassava genotypes obtained from advanced yield trial at NARI were planted in each agro ecological in a randomized complete block design. During cropping season (2011/2012) the genotypes selected by farmers were almost the same as those selected by scientists after analysis.  This selection process has identified four superior genotypes from amongst nine elite genotypes; namely Kiroba, NDL 2006/487, NDL 2006/438 and Naliendele.  Farmers were relatively consistent in their selection from different stages of crop growth, and their selections corresponded with their stated selection criteria. Official variety release requires additional multilocational and inspection trials and postharvest assays but otherwise seems harmonious with a participatory breeding approach; involvement of farmers in this stage may facilitate early variety release, an important factor in cost-effectiveness. Adoption of a participatory approach, with farmers and scientists taking on new roles and decentralization of activities, implies a concomitant transfer of influence and resources. Keywords: Participatory Variety Selection, Farmer selection criteria, Variety release

Correlation and Path Analysis between Yield and Yield Components in Cassava (Manihot esculenta Crantz) in Southern Tanzania

An experiment to study the relations among root yield and some traits of cassava using correlation and path coefficient analysis was conducted in the Sothern Tanzania during the 2011 – 2012 cropping season. The design of the experiment was a Randomized Complete Block with three replications using twelve genotypes. Highly significant and positive correlations were found between plant height and stem girth (r = 0.5900***), plant height and roots per plant (r = 0.4463***), plant height and harvest index (r = 0.3005***), branches per plant and roots per plant (0.2441***), stem girth and roots per plant (0.5046***) and roots per plant and harvest index (0.2647***).  Also, highly significant and positive correlations were found between root yield and plant height (r =0.5436***), stem girth (r = 0.3874***), roots per plant (r = 0.7053***) and harvest index (r = 0.3025***). Compared to the simple correlation analysis, path analysis of roots per plant, plant height and root size evolved the highest direct influence 0.619, 0.290 and 0.153, respectively. Therefore, indirect selection for higher root yield may be effective for improving these characters. This study suggests the relative higher value of residual effect (0.51) indicated more yield components should be considered in the future to account for the variation in cassava root yield. Key words: Correlation, Path analysis, Cassava, Yield, Yield components

Genetic Variability among Six Traits in Twelve Cassava (Manihot esculenta Crantz) Genotypes in Southern Tanzania

Twelve cassava genotypes were evaluated to assess genetic variability for root yield and its components at three locations (Naliendele, Mtopwa and Nachingwea) in Southern Tanzania during 2011/2012 cropping season. Data exploited for this purpose were, number of branches per plant, plant height, root number, root weight, shoot weight and root dry matter. Genotype x location interaction was significant for all the characters studied indicating considerable influence of the environment on the expression of the traits. High heritability and relatively high genetic gain were observed in plant height (72.90% and 36.67%), stem girth (69.41% and 33.63%) and roots per plant (44.88 and 37.05%) respectively, suggesting that the traits are primarily under genetic control and that reliable selection with simple recurrent phenotypic selection would be rewarding. Most of genotypic correlations were significant and positive. Though genotypes differed significantly at (p > 0.05) for all the traits studied, two genotypes (NDL 2006/487 and NDL 2006/438) and two varieties (Kiroba and Naliendele), were found to constitute a pool of germplasm with adequate genetic variability from which selection will bring about significant progress in cassava improvement programs. Keywords: Cassava genotypes, Root yield, Genetic variability, Heritability, Genetic gain, Genetic correlations

QTL associated with resistance to cassava brown streak and cassava mosaic diseases in a bi-parental cross of two Tanzanian farmer varieties, Namikonga and Albert

Article purchasedCassava production in Africa is compromised by cassava brown streak disease (CBSD) and cassava mosaic disease (CMD). To reduce costs and increase the precision of resistance breeding, a QTL study was conducted to identify molecular markers linked to resistance against these diseases. A bi-parental F1 mapping population was developed from a cross between the Tanzanian farmer varieties, Namikonga and Albert. A one-step genetic linkage map comprising 943 SNP markers and 18 linkage groups spanning 1776.2 cM was generated. Phenotypic data from 240 F1 progeny were obtained from two disease hotspots in Tanzania, over two successive seasons, 2013 and 2014. Two consistent QTLs linked to resistance to CBSD-induced root necrosis were identified in Namikonga on chromosomes II (qCBSDRNFc2Nm) and XI (qCBSDRNc11Nm) and a putative QTL on chromosome XVIII (qCBSDRNc18Nm). qCBSDRNFc2Nm was identified at Naliendele in both seasons. The same QTL was also associated with CBSD foliar resistance. qCBSDRNc11Nm was identified at Chambezi in both seasons, and was characterized by three peaks, spanning a distance of 253 kb. Twenty-seven genes were identified within this region including two LRR proteins and a signal recognition particle. In addition, two highly significant CMD resistance QTL (qCMDc12.1A and qCMDc12.2A) were detected in Albert, on chromosome 12. Both qCMDc12.1A and qCMDc12.2A lay within the range of markers reported earlier, defining the CMD2 locus. This is the first time that two loci have been identified within the CMD2 QTL, and in germplasm of apparent East African origin. Additional QTLs with minor effects on CBSD and CMD resistance were also identified

Sequencing wild and cultivated cassava and related species reveals extensive interspecific hybridization and genetic diversity

Cassava (Manihot esculenta) provides calories and nutrition for more than half a billion people. It was domesticated by native Amazonian peoples through cultivation of the wild progenitor M. esculenta ssp. flabellifolia and is now grown in tropical regions worldwide. Here we provide a high-quality genome assembly for cassava with improved contiguity, linkage, and completeness; almost 97% of genes are anchored to chromosomes. We find that paleotetraploidy in cassava is shared with the related rubber tree Hevea, providing a resource for comparative studies. We also sequence a global collection of 58 Manihot accessions, including cultivated and wild cassava accessions and related species such as Ceará or India rubber (M. glaziovii), and genotype 268 African cassava varieties. We find widespread interspecific admixture, and detect the genetic signature of past cassava breeding programs. As a clonally propagated crop, cassava is especially vulnerable to pathogens and abiotic stresses. This genomic resource will inform future genome-enabled breeding efforts to improve this staple crop