Elsevier

Field Crops Research

Volume 198, November 2016, Pages 171-178
Field Crops Research

Early storage root bulking index and agronomic traits associated with early bulking in cassava

https://doi.org/10.1016/j.fcr.2016.09.004Get rights and content

Highlights

  • Harvest index and shoot mass are the key determinants of early storage root bulking varieties.

  • Study proposes that early harvesting of cassava in Malawi be 9 months after planting.

  • Yield loss due to early harvesting is lower than yield loss due to cassava brown streak disease infection.

Abstract

One of the attempts by farmers in counteracting the devastating effects of cassava brown streak disease (CBSD) on yield and quality of cassava is early harvesting. However, most varieties grown by farmers are often late bulking which increases the disease severity while on the other hand early harvesting results in significant yield losses. Farmers, therefore, need early storage root bulking cassava varieties in order to reduce the time to harvest leading to a faster rate of return to investment, while at the same time avoiding devastating effects of CBSD on yield and quality of cassava. The study was, therefore, conducted to identify high-yielding and early storage root bulking cassava genotypes as well as traits associated with early storage root bulking and estimate yield loss if any due to early harvesting. The overall aim was to generate information that would guide future improvement programmes for high-yielding and early-bulking cassava varieties in Malawi and other countries facing similar challenges. Trials were implemented using a square lattice design with three replications at two locations for two growing seasons with three harvest intervals (6, 9 and 12 months after planting, MAP). High yields were obtained of up to 9.5 t/ha at 6 and 17.8 t/ha at 9 MAP. Furthermore, the study revealed that yields obtained at 9 MAP were higher than those obtained at 12 MAP for some genotypes which suggests that such genotypes would be considered as early storage root bulking. Simple correlation analysis identified harvest index, storage root number, storage root diameter and storage root length as the selection criteria to achieve high fresh storage root yield (t/ha) and dry mass yield (t/ha). Path coefficient analysis allocated harvest index and shoot mass as the major selection criteria in improving fresh storage yield and dry mass yield. The study suggests that both source and sink capacities were important for determining early yield. Therefore, these two traits are the key determinants of early storage root bulking and should be used when selecting early-bulking cultivars and indirectly selecting for storage root number, storage root diameter and storage root length.

Introduction

Cassava (Manihot esculenta Crantz) plays an important role in traditional tropical cropping systems, more particularly on small farms in the subsistence farming sectors. It is often grown in mixed stands with other food or cash crops. Cassava’s importance is mainly derived from its wide range of adaptation, its tolerance to low soil fertility, drought, pests and diseases, a high dry matter yield per hectare, flexibility in planting and harvesting and diverse range of utilization (Ceballos et al., 2004, FAO, 2013, Leihner, 2002, MoAFS, 2007, Onwueme, 1978, Westby, 2002).

Cassava production is affected by numerous constraints that include pests and diseases, in particular cassava mosaic disease (CMD) and cassava brown streak disease (CBSD), low yield potential, long growth period/late storage root bulking, early postharvest deterioration, use of low-yielding varieties, and shortage of labour, land and capital for cassava production (Dahniya, 1994, IITA, 1990). The combination of late storage root bulking and CBSD infections makes the farmer vulnerable to food insecurity, as by the end of the season she/he realises yields that are well below the potential of the crop.

Cassava has no defined maturity period, which means that it can be harvested whenever economic yields can be obtained. Maximum dry matter (DM) accumulation in storage roots generally occurs between 300 and 360 days after planting (DAP), and is mainly influenced by changes in temperature (Alves, 2002). This is the period when most of the cassava is harvested (that is, 12 months after planting, MAP). However, the highest rates of DM accumulation in storage roots occur within 180–300 DAP (6–10 MAP), and varies according to genotype and environment. This infers that cassava harvesting can start as early as 6 MAP. According to FAO (2013), in cases where the storage root is used as food, the best time to harvest is between 8–10 MAP. Studies have reported high dry matter storage root yield (9.0–14.5 t/ha) at 7 MAP (Mtunda, 2009, Okogbenin and Fregene, 2002) and high fresh storage root yields (15.5–28.0 t/ha) at 6 and 7 MAP (Asante, 2010, Mtunda, 2009, Nair and Unnikrishnan, 2006, Okechukwu and Dixon, 2009, Tumuhimbise, 2013). A good measure of DM distribution in storage roots is its harvest index (HI, the ratio of storage root mass to the total plant mass), which represents the efficiency of storage roots production. Significant differences in HI have been reported among cultivars, indicating that it can be used as a selection criterion for higher yield potential in cassava (Alves, 2002, Kawano, 2003). HI values of 0.49–0.77 have been reported after 10–12 MAP (Alves, 2002), which means that varieties exhibiting HI within this range at 6–9 MAP could be early storage root bulking. Since early-bulking is partly due to a genotype’s ability to quickly accumulate assimilate reserves in its storage roots (Alves, 2002, Segnou, 2000), there is a need to exploit this variability in order to breed for early storage root bulking cassava varieties.

Development of early storage root bulking varieties has received much attention across the globe (Kamau et al., 2011, Nair and Unnikrishnan, 2006, Okechukwu and Dixon, 2009, Okogbenin and Fregene, 2002, Okogbenin et al., 2008, Olasanmi et al., 2014, Suja et al., 2010, Tumuhimbise, 2013, Tumuhimbise et al., 2014, Wholey and Cock, 1974), and more particularly in the wake of the CBSD epidemic which is threatening the cassava industry in east and southern Africa. Late harvesting of cassava (mainly due to late bulking) contributes to high CBSD incidence, which increases with plant age (Alvarez et al., 2012, Gondwe et al., 2003, Hillocks et al., 2001, Hillocks et al., 2002, Rwegasira and Rey, 2012). It is clearly documented that most farmers prefer early-bulking varieties that can also withstand pests and disease damage (Agwu and Anyaeche, 2007, Benesi et al., 2010, Chipeta et al., 2016, Dahniya, 1994, Munga, 2008, Okechukwu and Dixon, 2009, Tumuhimbise et al., 2012). A greater commitment, therefore, has to be made to develop early-bulking cultivars so that they reach full bulking before the disease (root necrosis) becomes severe. This in turn would effectively reduce the production period resulting in a faster rate of return to investment. In Malawi, due to scarcity of livestock feed during later months of the year (dry periods), most livestock fend for themselves which means keeping cassava in the field for longer time exposes the crop to the animals. This in turn increases cost of production as farmers resort into guarding their fields and if not, crop loss due to animal feeding. Therefore, productive early storage root bulking varieties would not only provide good storage root quality and productivity per unit area of land, but with early harvesting would also facilitate the release of land for other farming activities (for example, early land preparations for the following season, production of other short duration crops such as vegetables more especially in wetlands or areas close to water sources), and reduce exposure to biotic and abiotic stresses thereby increasing productivity. The objectives of this study were to: (1) identify high-yielding and early storage root bulking cassava genotype, (2) determine agronomic traits influencing early storage root bulking through path coefficient analysis, (3) estimate yield loss if any due to early harvesting.

Section snippets

Plant material

Planting material was sourced from national agricultural research stations and farmers’ fields. A total of 16 genotypes were evaluated (Table 1) and their selection was based on their popularity with farmers and their response to various diseases prevalent in Malawi.

Experimental sites

The trials were conducted in Malawi at two sites, Chitala Agricultural Research Station in Salima district (central Malawi) and Kasinthula Agricultural Research Station in Chikwawa district (southern Malawi) over two growing seasons

Variation due to genotype, harvest time, environment (location/season) and their interactions

Table 3 shows different sources of variation and their influence on the traits measured. Genotype had a highly significant (P < 0.001) effect on all the traits except for storage root diameter (cm), plant height (cm) and plant height at first branch (cm). Harvest time impacted highly significantly (P < 0.001) on all the traits except storage root number/plant, storage root diameter (cm) and plant height (cm). Environment (location/season) had a highly significant (P < 0.001) influence on a number of

Discussion

Malawi, like most of the east and southern African countries has been greatly affected by CBSD, a disease that increases with plant age. One of the attempts to reduce CBSD impact on yield and quality of cassava is early harvesting. However, most varieties in Malawi are late bulking (12–24 months) which entails that early harvesting on such varieties would lead to significant yield sacrifices among smallholder farmers. This study was therefore aimed at assessing the degree of variation in the

Conclusions

Results of this study show that cassava harvesting period can be reduced by the use of early storage root bulking varieties. The study identified Phoso, Mbundumali, Mulola and Maunjili as the early-bulking varieties. Growing such varieties would fetch high returns per unit time for the farmers both in terms of storage roots and leaf production. Therefore, productive early storage root bulking varieties would not only provide good storage root quality and high productivity per unit area of land,

Acknowledgements

Special thanks to the station managers of Chitala and Kasinthula Agricultural Research Stations and their technical staff for kindly hosting the trials. This study is part of a PhD study and the first author thanks the Alliance for a Green Revolution in Africa for funding the study.

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