«Understanding growth of East Africa highland banana: experiments and simulation Kenneth Nyombi Thesis committee Thesis supervisor Prof. Dr. K.E. ...»
Understanding growth of East Africa highland banana:
experiments and simulation
Prof. Dr. K.E. Giller
Professor of Plant Production Systems
Dr. Ir. P.A. Leffelaar
Associate Professor at Plant Production Systems Group
Dr. P.J.A. van Asten
International Institute for Tropical Agriculture, Kampala, Uganda
Other members Prof. Dr. Ir. L.F.M. Marcelis, Wageningen University Prof. Dr. Ir. H.J. Bouwmeester, Wageningen University Dr. E. Hoffland, Wageningen University Dr. E. Malézieux, CIRAD, Montpellier, France This research was conducted under the auspices of the C.T. de Wit Graduate School of Production Ecology and Resource Conservation.
Understanding growth of East Africa highland banana:
experiments and simulation Kenneth Nyombi Thesis submitted in fulfilment of the requirements for the degree of doctor at Wageningen University by the authority of the Rector Magnificus Prof. Dr. M.J. Kropff, in presence of the Thesis Committee appointed by the Academic Board to be defended in public on Tuesday 2 March 2010 at 4 p.m. in the Aula.
Kenneth Nyombi Understanding growth of East Africa highland banana: experiments and simulation, 198 pages.
Thesis, Wageningen University, Wageningen, NL (2010) With references, with summaries in English and Dutch ISBN: 978-90-8585-550-7 Dedication To my dear family, living and deceased, with deep gratitude for their support. Most especially to my mother, wife and sons for their love, encouragement, patience and sacrifices during the study period.
Prayer Glory to him who is able to give you the strength to live according to the Good news which I preach, and in which I proclaim Jesus Christ, the revelation of a mystery, kept secret for endless ages, but now so clear that it must be broadcast to the peoples everywhere, bringing them to the obedience of faith. This is what scripture predicted and it is all part of the way the eternal God wants things to be. He alone is wisdom. Give glory therefore to him through Jesus Christ for ever and ever. Amen Romans 16:25−27.
Abstract East Africa Highland banana yields on smallholder farms in the Great Lakes region are small (11−26 Mg ha−1 cycle−1 in Uganda, 21−43 Mg ha−1 cycle−1 in Burundi and 25−53 Mg ha−1 cycle−1 in Rwanda). The major causes of poor yields are declining soil fertility and soil moisture stress. In order to improve production, knowledge on highland banana physiology, growth patterns and response to fertilization is important, to establish the potential yield of the crop, to quantify the yield gaps between potential and actual yield, and to explore options for closing the yield gaps.
Measurements of plant morphological characteristics, radiation interception and biomass (by destructive harvesting) were taken in experimental fields in central and southwest Uganda. Results showed that total leaf area can be estimated by using height and girth (used to estimate middle leaf area) and number of functional leaves. The light extinction coefficient, k determined from photosynthetically active radiation (PAR) measurements over the entire day was 0.7. Banana plants partitioned more dry matter (DM) to the leaves during first phase of vegetative growth, with the pseudostem becoming the dominant sink later with 58% of total DM at flowering, and the bunch at harvest with 53% of the total DM. Changes in dry matter partitioning influenced the allometric relationships between above-ground biomass (AGB in kg DM) and girth (cm), the relationship following a power function during the vegetative phase (AGB = 0.0001 (girth)2.35), and exponential functions at flowering (AGB = 0.325 e0.036 (girth)) and at harvest (AGB = 0.069 e0.068 (girth)). This thesis shows that allometric relationships can be derived and used to estimate biomass and bunch weights.
In fertilizer trials, yield increases above the control (13.0 Mg ha−1 yr−1) ranged from 2.2−11.2 Mg ha−1 yr−1 at Kawanda, to more than double at Ntungamo, 7.0−29.5 Mg ha−1 yr−1 (control 7.9 Mg ha−1 yr−1). The limiting nutrients at both sites were in the order KPN. Differences in soil moisture availability and texture resulted in higher yields and total nutrient uptakes (KNP) at Ntungamo, compared with Kawanda. Per unit dry matter yield, highland bananas take up a similar amount of N (49.2 kg finger DM kg−1 N), half the amount of P (587 kg finger DM kg−1 P), and five times the amount of K (10.8 kg finger DM kg−1 K), when compared with cereal grain. Calibration results of the static nutrient response model QUEFTS using data from Ntungamo were fair (R2 = 0.57, RMSE = 648 kg ha−1). The calibrated QUEFTS model predicted yields well using data from Mbarara southwest Uganda (R2 = 0.68, RMSE = 562 kg ha−1).
A new dynamic radiation and temperature-driven growth model, LINTUL BANANA 1 was developed to the compute potential yields of East Africa highland banana. The model considers (i) the physiology of the highland banana crop; (ii) the plant dynamics (i.e. three plant generations, Plant 1, 2 and 3 at different stages of growth constituting a mat); and (iii) three canopy levels formed by the leaves of the three plants.
Average computed potential bunch dry and fresh matter were slightly higher at Ntungamo (20 Mg ha−1 DW; 111 Mg ha−1 FW), compared with Kawanda (18.25 Mg ha−1 DW; 100 Mg ha−1 FW), and values compared well with banana yields under optimal situations at comparable leaf area index values (20.3 Mg ha−1 DW; 113 Mg ha−1 FW). Sensitivity analysis was done to assess the effects of changes in parameters (light use efficiency, LUE;
the light extinction coefficient, k; specific leaf area, SLA; the relative death rate of leaves, rd; relative growth rate of leaf area, RGRL; and the initial dry matter values) on bunch dry matter, leaf dry matter and leaf area index (L) at flowering. Sensitivity results for Kawanda and Ntungamo showed that changes in LUE1 resulted in more than proportional increase in bunch DM (1.30 and 1.36), a higher leaf DM (0.60 and 0.67) and L at flowering (0.60 and 0.67). Changes in rd1 values reduced bunch dry matter, leaf dry matter and L at flowering. Changes in SLA1 reduced only leaf DM, whereas both leaf DM and L at flowering were reduced by changes in k1 at both sites. Initial dry matter values had a small effect (sensitivity 0.0263) for bunch DM, leaf DM and L at flowering.
Based on the model results, it is clear that the potential yield of East Africa highland bananas is more than 18 Mg ha−1 DW. Management options that increase LUE and reduce the relative death rate of leaves, and improvements in parameters related to light interception (SLA and k) are important to increase yield.
General introduction Chapter 1
1.1. Background Bananas and Plantains (family Musaceae) are rhizomatous, monocotyledonous herbs grown in the tropics and sub-tropics, where they are adapted to a wide range of temperature and water supply regimes (Stover and Simmonds, 1987). Their fruits are important staple and cash crops for about 100 million people in the Great Lakes region (i.e. Uganda, Tanzania, Rwanda, Burundi, parts of eastern Democratic Republic of Congo, and Bukoba and Kilimanjaro areas of Tanzania). Globally, bananas are the fourth most important food commodity after rice, wheat and maize in terms of gross value of production. In Uganda, the East Africa highland cooking bananas (Musa spp., AAAEAHB) locally known as ‘Matooke’ is the most abundantly cultivated crop and rank highest among the food crops (NARO, 2000). The Ganda and Soga people to the north of Lake Victoria, the Konjo and Bamba in the Rwenzori region and the Gishu people around Mount Elgon have traditionally exclusively relied on bananas (Allan, 1965). Amongst the Ganda, it is often stated that a meal without bananas is no meal.
Per capita banana consumption is estimated at about 0.7 kg per person per day.
Cooking banana fingers are peeled to obtain the pulp, which is steamed in banana leaves, and then smashed to form a mash that is consumed with a sauce (e.g. vegetable, fish or beef). The nutritional value of cooking highland banana is 4.04 g crude protein, 0.64 g crude fat and 367.62 kcal per 100 g of dry weight (Muranga et al., 2007). It is estimated that Uganda produces over 10% of the world’s bananas and plantains, with only 0.02% exported. Cultivation is mainly on plots less than 0.5 ha around the homestead, characterized by high cultivar diversity (4-22 per farm), but medium scale plantations ( 1 ha) are common in western Uganda (Gold et al., 1998; Gold et al., 1999; Bagamba, 2007).
Bananas are believed to have originated in Indochina and South East Asia, where the earliest domestication is considered to have occurred, and the greatest diversity of wild Musa species (Musa acuminata – AA and Musa balbisiana - BB) is found today (Simmonds, 1962). Hybridizations between the various sub-species of the polymorphic species of Musa acuminata gave rise to a variety of AA diploid cultivars. Through chromosome restitution during meosis, diploid AA gave rise to triploid AAA types. First introductions of bananas to sub-Saharan Africa are thought to have occurred after the birth of Jesus Christ, through North and Eastern Africa by Arab traders (Stover and Simmonds, 1987; Vansina, 1990; Price, 1995). East Africa highland bananas are diverse, unique to this region and are thought to have arisen as a result of somatic mutations (Simmonds, 1966). Using plant morphological characteristics, Karamura (1998)
characterized and classified over 130 highland banana cultivars into five major clone sets;
Musakala, Nakabululu, Nakitembe, Nfuuka and Mbidde (brewing type). The Musakala clone set includes cultivars like Kisansa, Musakala, Mudwale and Mpologoma which are popular due to their large bunches with loosely packed clusters. World wide, the vast majority of bananas grown today are triploids – AAAs, with the AABs and ABB being starchy and tetraploids (e.g. AAAA) for dessert purposes.
1.2. Morphology and growth requirements
A banana plant consists of the corm (true stem) from which fibrous roots (primary, secondary and tertiary), the pseudostem and the leaves grow. Plant height varies with the cultivar, ranging from 2 m for dwarf cultivars to 6 m for tall cultivars. The corm is differentiated into the central cylinder (vascular bundles) from which roots develop and an outer cortex (parenchyma cells). The terminal growing point of the corm is a flattened dome, with leaves formed around it in spiral succession (Purseglove, 1988). The enveloping leaf sheaths form the pseudostem that supports the leaves. The midribs and petioles support the lamina, which intercepts photosynthetically active radiation (PAR).
The centre of the meristem (flattened dome) is transformed into the inflorescence (bunch) after production of 25−40 leaves. During leaf production, a vegetative bud is produced 180° opposite each leaf, on the outer surface of the cortex, a few of which later develop into suckers. Sword suckers have a strong attachment to the mother plant and develop a thick rhizome of their own, which can be used for propagation. In the early stage of development, their leaves are small, thin and bract like structures, later developing into broad sword leaves and eventually large functional leaves (Robinson, 1996). The mother plant and the suckers form a ‘mat’. The mat thus consists of plants at different stages of growth developing at their own rate. At bunch maturity, the shoot is cut away and the selected lateral buds or suckers form the next crop cycle.
Bananas require a deep, well drained retentive loam soil with high humus content and good water holding capacity for satisfactory growth (Purseglove, 1988). However, bananas also grow well on lighter sandy soils with good soil organic matter content (Delvaux, 1995; Stover and Simmonds, 1987). In Uganda, bananas are grown on diverse soil types from the relatively heavy Ferralsols, Nitisols and Acrisols mainly in the Lake Victoria basin (Zake et al., 2000) to Fluvisols and Plinthosols in mainly Tororo and Pallisa (Bekunda et al., 2002). Generally, bananas require an abundant supply of nutrients, compared with other crops (Turner, 1985; Robinson, 1996; Purseglove, 1988),
particularly potassium and nitrogen. Simmonds (1962) reported that most banana species grow best in open sun provided moisture is not a limiting factor. As a rule of thumb, soil water should not be allowed to fall below 20−30% of the total available water (TAW) if optimum growth is to be maintained (Robinson and Bower, 1987). Uniformly warm (optimum air temperature of about 25−27 °C) conditions are important for optimal growth (Turner and Lahav, 1983).
1.3. Production problems in banana based farming systems of Uganda
Reduced productivity and loss of sustainability (i.e. low yields and reduced mat life) have been widely reported among highland banana farmers especially in the central part of Uganda (Bekunda and Woomer, 1996; Bekunda, 1999). For example, banana yields are reported to have declined from 8.4 Mg ha−1 yr−1 in 1970 to 5.6 Mg ha−1 yr−1 in the late 1980s (Gold et al., 1999). Irrespective of the accuracy of the yield estimations and the extent of yield decline, it is clear that the widely reported actual banana yields on smallholder farms (5−30 Mg ha−1 yr−1) are far below the estimated potential yields ( 70 Mg ha−1 yr−1). Low yields are attributed to poor soil fertility, moisture stress, pest pressure (banana weevil - Cosmopolites sordidus; nematodes - Radopholus similis, Helicocotylenchus multicinctus and Pratylechus goodeyi), diseases (black sigatoka Mycosphaerella fijiensis, banana wilt, banana streak virus, banana bacterial wilt) and poor crop husbandry (Gold et al., 1999; NARO, 2000).
Soils under bananas are generally old and highly weathered (Zake et al., 2000).