Storage Root Yield and Related Traits of Ten Cassava Varieties at Different Harvesting Age

Cassava (Manihot esculenta Crantz) is an important staple and industrial crop in Nigeria (the highest producer globally) as well as other countries in Africa, Asia and Latin America. It is an essential staple crop and affordable source of carbohydrates in Sub-Saharan Africa where the crop has been projected to have a major role in future food security [1,2]. In 2023, Africa accounted for about 64% of the 333.7 million metric tons while Nigeria alone accounted for 18.8% (62.7 million metric tons) of the total global production [3]. Other major producers are Democratic Republic of Congo, Thailand, Ghana, Brazil and Indonesia. Cassava leaves and young shoots are also eaten as vegetables in many parts of Africa [4]. Late root bulking has been identified as a major factor leading to rejection of improved cassava genotypes in sub-Saharan Africa [5]. Rate and duration of bulking determine the Storage Root Yield (SRY) of cassava; hence, high-yielding varieties will exhibit high bulking rate for long period [6]. Meanwhile, it has been established that some cassava varieties bulk early and slow down; some start early and continue bulking at high rate for a long time while some start bulking late [6-9]. According to Olasanmi et al. [9], cassava varieties should be evaluated for SRY at different plant ages to ascertain the best harvesting age for each. Also, the time of harvest has been identified as one of the factors affecting yield of cassava in addition to others such as temperature, weed management, spacing, soil properties, partitioning of assimilates, planting method, pests and diseases [2, 10-14].

Cassava plants left on the field during dry season are exposed to pests which have few options at this period because almost all other arable crops such as sweet potatoes, yam, maize, rice and cocoyam are harvested at the end of the rainy season or shortly into the dry season. Such destruction by pests during dry season alone can lead to considerable loss in root yield as other animals such as bush fowls and rabbits which were not known to feed on cassava before joined the host of other pests in destruction of cassava roots in the last few years. From about a month into dry season, cassava plants start to shed leaves to reduce evapotranspiration. This is accompanied by reduced or no photosynthesis, hence, the cassava plant can hardly add photo assimilates to the storage roots during the dry season. Knowledge of SRY of elite cassava varieties at different harvesting ages will therefore help identify early bulking varieties that can be cultivated in areas with short rainy season and harvested at the onset of dry season to maximize productivity of cassava. This will increase profitability of cassava production and identification of the early bulking varieties will enable use of vast arable land in some parts of the savannah that is hitherto not well utilized for cassava production. Cassava is grown in areas where rain lasts for about 6-10 months of the year and in Nigeria for example, the major cassava growing belt has rainfall for about 7-9 months. This means cassava will grow effectively by synthesizing assimilates for about 6-8 months before onset of dry season, but harvesting is delayed till around 12 MAP. It has been reported that cassava varieties responded differently to the planting dates [15].

For cassava cultivated between March and April, assessment for yield at 12 MAP falls at the onset of rainfall the following season. At this period, cassava plants are just recovering from the effects of dry season, hence, some of the photosynthates (dry matter) in the storage roots are mobilized to produce new leaves. Cassava harvested and processed at this period are therefore very low in dry matter resulting in low gari, fufu, flour or starch productivity. Harvest index is the proportion of assimilates partitioned from the source to the sink after meeting all the growth needs of the plant [16,17]. The sink in most cases in cassava is the storage root which is processed into different products. Ologunde et al. [15] reported significant increase in fresh root yield as the cassava age increased and therefore recommended harvesting at 13 MAP rather than 9 and 11 MAP if there is no pressure on the land. To maximize dry matter yield of cassava, harvesting should be carried out between the end of rainy season and shortly before the end of dry season when high harvest index is more achievable. This therefore calls for adequate information on storage root yield of existing cassava varieties at different ages to enable farmers to plan adequately. The objective of this study was therefore to determine the storage root yield and related traits of selected cassava varieties at different harvesting ages.

Ten cassava varieties cultivated by Nigerian farmers were evaluated for Storage Root Yield (SRY), Number of Storage Roots per plant (NSR), number of rotten roots and harvest index and at different harvesting age at the Research field of the Department of Crop and Horticultural Sciences at Parry Road, University of Ibadan, Nigeria with coordinates N 07°27.134’ and E 003°53.425’. It was a factorial experiment involving ten varieties and three harvesting ages (8, 10 and 12 MAP) laid out using randomized complete block design with three replications resulting in 90 experimental plots. The land used for the experiment was cleared of bush, ploughed twice and ridged manually in each season. Each plot size was 20m 2 and 20 stem cuttings of each variety were planted at a spacing of 1mx1m to give a population of 10,000 plants/ha. Weeding of the plot was carried out as required and the field was fenced to prevent pest infestation. Using three plots for each variety at each harvesting age, plants in the net of each plot were harvested at 8, 10 and 12 MAP to determine SRY and yield-related traits of the cassava varieties. Data were collected on fresh weight of leaves and non-plantable stems (L+NPS, kg), plantable stem weight (PSW, kg), weight of stump (SW, kg), Number of Storage Roots (NSR) number or rotten roots and storage root weight (SRW, kg). The data was used to estimate the Total Biomass (TB), Storage Root Yield (SRY) (t/ha) and Harvest Index (HI) for the varieties at each harvesting age as shown below:

Data on number of roots per plant, number of rotten storage roots, SRY and HI were subjected to descriptive analysis and analysis of variance using Statistical Analysis Software (Version 9.0). Significantly different means were separated at 0.05 level of probability.

Harvesting Age (HA) significantly influenced Storage Root Yield (SRY), Number of Storage Root per plant (NSR) Harvest Index (HI) and number of rotten roots/plants among the cassava varieties evaluated at Ibadan in two seasons (Table 1). There was also significant varietal influence on each of the three parameters. Significant difference across the years was observed for SRY, NSR and HI. Interaction between HA and year was significant for NSR and HI while year by variety interaction was significant for SRY and HI. However, HA by variety interaction was not significant for any of the three parameters. Also, interaction between HA, variety and year was not significant for each of the parameters. Lower NSR was recorded for all varieties at 12 MAP than 8 and 10 MAP except for varieties IITA-TMS-I011371 and IITA-TMS-I972205 (Figure 1). Also, NSR was higher at 8 MAP than 12 MAP for all varieties except for varieties IITA-TMS-I011368 and IITA-TMS-I920057. The level of rotting of storage roots at each harvesting age is shown in Figure 2. No rotting of storage roots was observed at 7 MAP for all varieties except TMEB419. The level of rotting was higher at 12 MAP than 10MAP for all varieties except for IITA-TMS-I011412 and IITA-TMS-I30572. The lowest level of storage root rotting was observed in varieties IITA-TMS-I972205 and IITA-TMS-I30572 at 10 and 12 MAP, respectively. However, the highest level of rotting was observed in varieties IITA-TMS-I980505 and TMEB419 at 10 and 12 MAP, respectively. The harvest index was lower at 10 MAP than other harvesting ages for all the varieties except for varieties IITA-TMS-I070593 with almost the same values at the three HAs and TMEB419 with lower HI at only at 10 MAP than 7 MAP (Figure 3).

Table 1:Mean squares of yield and related traits of ten cassava varieties evaluated in Ibadan in two seasons.

Figure 1:Number of storage roots/plant of ten cassava varieties at different harvesting age over two seasons.

Figure 2:Average number of rotten roots/plants of ten cassava varieties at different harvesting age over two seasons.

Figure 3:Harvest index of cassava varieties at different harvesting ages over two seasons.

The SRY among the cassava varieties ranged from 12.21 to 20.09 with an average of 17.72 (+2.81) t/ha at 8 MAP; 24.60 to 41.43 with an average of 31.54 (+5.95) t/ha; and 25.91 to 49.44 with an average of 37.55 (+6.34) t/ha at 12 MAP (Table 2). The average SRY among the varieties was significantly higher at 12 MAP (37.55t/ha) than at 10 MAP (31.54t/ha) while the SRY at 10 MAP was significantly higher than what was obtained at 8 MAP (17.72t/ha). The SRY of all the ten varieties at 8 MAP was significantly lower than the respective yield at 10 and 12 MAP. Varieties IITA-TMS-I011371, IITA-TMS-I070539, IITA-TMS-I920057, IITA-TMS-I972205, IITATMS- I980505 had significantly higher SRY at 12 MAP than 10MAP (Figure 4). Meanwhile, there was no significant difference between SRY of each of varieties IITA-TMS-I011368, IITA-TMS-I011412, IITATMS- I070593, IITA-TMS-I30572 and TMEB419 at 10 MAP and 12 MAP. However, varieties IITA-TMS-I011368, IITA-TMS-I011412 and TMEB419 had consistently higher SRY at 10 MAP than 12 MAP over the two seasons while higher SRY was recorded for varieties IITATMS- I070593 and IITA-TMS-I30572 at 12 MAP. Among the cassava varieties, the increase in SRY from 8 to 10 MAP ranged between 36.1 and 176.9% with an average of 81.2% while the increase from 10 to 12 MAP ranged between -6.3 and 65.6% with an average of 21.4% (Table 3). The largest yield gains ranging from 32.4 to 178.4% were observed from 8 to 12 MAP with an average of 116.3%. No decrease in yield was observed from 8 to 10 MAP for any variety while from 10 to 12 MAP, yield losses of 2.7%, 6.3% and 6.0% were observed for varieties IITA-TMS-I011368, IITA-TMS-I011412 and TMEB419, respectively.

Table 2:Storage root yield of ten cassava varieties evaluated in Ibadan at different harvesting age over two seasons.

Figure 4:Average fresh root yield at different age over two seasons.

Table 3:Change in fresh root yield of ten cassava varieties across harvesting ages.

The significant effect of harvesting age on Storage Root Yield (SRY), Number of Storage root per plant (NSR), Harvest Index (HI) and number of rotten roots/plants among the cassava varieties evaluated in this study justifies the need to understand performance of existing cassava varieties at different growth stages. The observed seasonal variation also corroborates previous submissions on effect of GxE interaction of performance of cassava varieties [18-20]. It was also reported earlier that genotypes and the environment have significant influence on HI [21,22]. Cassava varieties should therefore be evaluated across different environments (locations and years) to ascertain their stability. This will enable farmers to choose varieties more adapted to each environment to maximize productivity of each variety [23]. The lower Number of Storage Roots (NSR) observed among all the varieties at 12 MAP than 8 and 10 MAP except for two varieties at each early harvesting age reflects higher number of rotten roots observed at 12 MAP for most of the varieties. However, the rotting observed for variety TMEB419 at 8 MAP, though very little, indicates that the variety has poor inground storability and should therefore be harvested early. This is further corroborated by the reduction in storage root yield of the variety from 10 to 12 MAP. Varieties IITA-TMS-I011412 and IITATMS- I30572 with lower rate of rotting at 12 MAP than 10 MAP indicates that the two varieties have good in-ground storability and are therefore better cultivated by farmers who do not have adequate capacity to harvest all the plants at the same time. Such varieties will fit better into a system where piecemeal harvesting is inevitable, hence, they offer the advantage of flexible harvesting date allowing farmers to keep the roots in the ground until needed [24].

The relatively similar Harvest Index (HI) observed for variety IITA-TMS-I070593 across different harvesting ages is an indication that partitioning of photo-assimilates by the variety is relatively similar over the growing stages. The observed higher HI for other varieties at 8 and 12 MAP than 10 MAP indicates that less photo-assimilates are partitioned to the storage roots around 10 MAP than other parts of the plants. This implies that the cassava varieties had more vigorous vegetative growth at 10 MAP resulting in lower harvest index because they had limited capacity to allocate assimilation to the storage roots [25] at such stage. While high plant vigor is a genetic characteristic, it can also be influenced by environmental factors [25,26,7]. However, the decision on when to harvest cassava depends more on storage root yield rather than harvest index. The harvest index values observed in this study are within the ranges of 0.41 to 0.63 [27]; 0.51 to 0.55 [28], 0.39 to 0.59 [29]. 0.32 to 0.72 [22] reported in previous studies. One of the advantages cassavas has over other root and tuber crops, if not over all crops, is that it can be harvested all year round because of in-ground storability of many varieties, ability to keep adding assimilates to storage roots for more than 12 months, flexible planting windows and ability to survive long dry season of more than three months. To explore these and other attributes that make cassava one of the crops natures has put in place to fight hunger, there is need to understand the storage root yield dynamic of varieties cultivated by farmers. This will enhance ongoing transformation of cassava from subsistence to commercial crops, most especially in Africa.

Varieties IITA-TMS-I011371, IITA-TMS-I070539, IITATMS- I920057, IITA-TMS-I972205, IITA-TMS-I980505 with significantly higher yield at 12 MAP than their respective yield at 10 MAP is better harvested late at 12 MAP to maximize farmers’ profit. Varieties IITA-TMS-I011368, IITA-TMS-I011412, IITATMS- I070593, IITA-TMS-I30572 and TMEB419 with statistically similar SRY at 10 MAP and 12 MAP are better harvested early at 10 MAP to maximize profit and make the land available for dry season vegetable production or short fallow period before the land is cultivated the following rainy season. The two varieties (IITATMS- I011412 and TMEB419) with higher yield at 10 MAP than 12 MAP should be harvested early to maximize their potential, hence, they will be better adapted to areas where there is enough moisture to support good growth of cassava for about 8-9 months. Also, the seven varieties (IITA-TMS-I011368, IITA-TMS-I070593, IITA-TMS-I30572, IITA-TMS-I920057, IITA-TMS-I972205, IITATMS- I980505, TMEB419) with SRY of 18.54 t/ha (more than average yield across all the varieties) and above at 7 MAP can be cultivated in areas with short rainy season of about 6-7 months.

Cassava can grow and continue to photo-assimilate to the storage roots for some months during the dry season depending on soil characteristics, hence, early bulking varieties can mature into dry season when farmers may have enough time to harvest and process [9]. According to Santanoo et al. [30] & Janket et al. [31], one of the varieties cultivated under rainfed and exposed to drought during dry season in their experiments, had statistically similar storage root yield and dry weight but higher starch concentration at 12 MAP as compared to the well-irrigated plants. Janket et al [31] reported further that plants exposed to drought during the dry season (November to January) at active canopy development and early root bulking stages produced significantly higher root biomass and higher starch content at 12 MAP than those receiving continuous irrigation.

This shows that varieties with relatively high yield at 8 MAP identified in this study will most likely fit in more into regions with about 6 months of rainfall since they will be able to continue to grow and add more assimilates to the storage roots for some months into dry season. Also, early harvesting will minimize destruction of cassava storage roots by pests during dry season and allow the land to rest for about 3 months before it is prepared for another planting. Variation in storage root yield has also been reported among cassava varieties due to different planting dates [15], hence, the need to evaluate these cassava varieties at different planting dates in addition to the information on their yield at different plant ages. The average percentage increase in storage root yield of 81.2% from 8 to 10 MAP suggests that these varieties are better harvested at 10 MAP in areas where cassava can grow for at least 8 months effectively when soil moisture is not limited. Also, the average storage root yield increase of 21.4% from 10 to 12 MAP suggests that harvesting at 12 MAP may be more profitable though higher yield increase was observed between 8 and 10 MAP on average. This is in tandem with reports by Enesi et al. [32] who reported a yield increase of 51.6%, 46.2%, and 38.9% from crop age of 9 to 11 MAP than 10.9%, 14.4% and -0.6% between crop age of 11 and 13 MAP, respectively at three locations. However, each variety should be managed based on its storage root yield at each harvesting age because some varieties had more root yield at 10 MAP than 12 MAP.

In this study, ten cassava varieties were evaluated for storage root yield at different harvesting ages. The varieties identified to have higher storage root yield at 10 MAP than other harvesting ages will most likely fit well into current cassava growing belt of Nigeria where rainfall lasts for about 8 to 9 months in a year. This is since cassava can continue to grow effectively for more than a month without rainfall once the plants are well established before onset of dry season. However, where there is no market pressure, pest attacks, pressure on land and other constraints, varieties with higher yield at 12 MAP (IITA-TMS-I011371, IITA-TMS-I070539, IITA-TMS-I920057, IITA-TMS-I972205, IITA-TMS-I980505) should be harvested as such a later stage. Varieties IITA-TMS-I011368, IITA-TMS-I070593, IITA-TMS-I30572, IITA-TMS-I920057, IITAIITATMS- I972205, IITA-TMS-I980505, TMEB419 with reasonable storage root yield at 8 MAP can be cultivated in areas with shorter rainy season of about 6 months to expand cassava growing belt of Nigeria. These varieties should be evaluated across different agro-ecologies in Nigeria to understand the effect of genotype by environment interaction on their yield at different harvesting age.