Evaluation of Cassava (Manihot esculenta Crantz) Productivity in Relation to Termite Attacks in Rural Area in Tivaouane (Senegal)

In Senegal, the cassava harvest, produced mainly in the department of Tivaouane (Thies), is 7.5 t/ha on average for a potential of 40 t/ha. The main variety produced in Tivaouane is Soya. The objective of this study is to evaluate the productivity of the soya variety farmed in Tivaouane in relation to termite damage. Specifically, it is intended to i) evaluate losses caused by termites and missing plants ii) evaluate the average number of tubers per plant, the average weight of a tuber and iii) calculate the yield of cassava production of the Soya variety farmed in Tivaouane. The methodology is based on sampling in order to evaluate parameters such as losses due to termites and missing plants, evaluation of the number of tubers per plant, the average weight of a tuber and the productivity of this variety of cassava. The average loss due to termite attack on dead feet is 1.2 t/ha and the loss due to missing feet is estimated at 3.4 t/ha. The average number of tubers per stand is 2.8 with an average weight of 1.1 kg per tuber. The theoretical yield obtained is 11 t/ha. The low average number of tubers is related to the variety, the quality of the soil and the crop conditions. However, the productivity of this variety of cassava cropped in Tivaouane is still low considering the potential of Senegal in terms of annual cassava production. An improvement in crop conditions, such as the way cuttings are planted, would be an advantage in increasing the productivity of the Soya variety. Keyswords: agricultural productivity, Soya variety, cassava, losses, termites, Tivaouane

grown in Tivaouane (7.5 t/ha) are low compared to Nioro (22.5 t/ha) according to Agriculture Statistics Analysis and Forecasting Directorate (DAPSA) data (2017). These weaknesses would be partly related to abiotic, anthropic, etc. factors that cause some missing plants during production and biotic causes, such as pest pressure, including termites that cause a lot of damage. However, there are five species of termites distributed among fungis growers (Odontotermes erraticus, Microtermes lepidus and Macrotermes subhyalinus) and lignivores (Psammotermes hybostoma and Microcerotermes sp.) have been identified on cassava (Sonko et al., 2019) to be a pest of the cassava plantations in Tivaouane Indeed, termite attacks are made from the outside under earth veneers or from the inside with or without earth filling of the gnawed parts. They lead to reduced growth or death of infested cassava cuttings and young plants.
These species are responsible of attacks and damage on cassava, whose severity and incidence are respectively of the order of 37.5% and 48% (Sonko et al., 2019). Thus, would be responsible for the losses on cuttings, plants and yield reductions recorded in Tivaouane. They can cause very serious damage and negatively affect the profitability and performance of the varieties grown in Tivaouane. The objective of this study is to analyze the productivity of this Soya variety farmed in Tivaouane in relation to termite damage. It specifically aims to i) evaluate losses due to termites and missing plants; ii) evaluate the average number of tubers per plant, the average weight of a tuber; and iii) calculate the yield of cassava production of the Soya variety produced in Tivaouane.

Study Environment
The study was conducted in Tivaouane department (Thies-Senegal). Located 92 km from Dakar the Senegalese capital, the department of Tivaouane covers an area of 3221 km 2 , it is larger than the department of Thies, 1873 km 2 and the department of Mbour, 1607 km 2 . According to ANSD/SRSD (2011), at the Tivaouane department level, 70% of the soils are sandy dune (dior dune soil), 15% clay-sandy (dior deck), 10% clay-humic (deck) and 5% hydromorphic.
The climate of Tivaouane is characterized by the existence of two seasons: the dry season and the rainy season. The long dry season runs from October to July. The rainy season runs from July to October. The region enjoys a mild climate with an average annual temperature of 32 °C (ANSD/SRSD, 2017). The lowest temperatures are recorded between January and February and the highest (35 °C) between March and October (ANSD/SRSD, 2017).
The study has been conducted in Tivaouane in cassava farms under 5 months of age (figure 1), regarding the impact of termites on young cassava plants and on harvest ready plots (over 12 months) for the evaluation of productivity and yield. Software R version 3.6.3 (2020-02-29) was used to compare the weight of tubers between the different farms visited.
The NMT and PMT are then used to evaluate the theoretical performance (RT).

Yield of Cassava Tuber Production
Considering all the cuttings planted at the beginning with a spacing of 1 m, there should be 10000 cassava plants in 1 hectare. Thus, the theoretical yield of cassava production is obtained with the following formula: where, The number 10,000 represents the total number of cassava root planted on 1 ha with a 1 m spacing.
The actual production yield is defined here as the difference between the theoretical yield and losses (due to termites and missing plants).

Assessment of Losses Related to Missing and Dead Feet Attacked by Termites
Losses related to missing feet and attacked dead feet are evaluated and presented in Table 1. For all sites combined, losses from missing feet observed during sampling are estimated at 3.406 t/ha and dead feet attacked by termites at 1.208 t/ha.
At the Koul site, the losses (t/ha) related to missing feet are 3.534 t/ha and those related to dead feet attacked by termites are 1.657 t/ha.
At Taïba Ndiaye, the losses (t/ha) related to missing feet totaled 2.469 t/ha and those related to dead feet attacked by termites are 1.014 t/ha.
In Tivaouane, the losses in cassava production due to missing and dead plants attacked by termites are respectively estimated at 4.216 t/ha and 0.953 t/ha.

Average Number of Tubers per Plant
The average number of tubers per stand of cassava (NMT) varies from 1.4 (Pire station) to 3.7 (Ndeukou station 1). The average of all stations combined is 2.8 tubers per stand ( Table 2). The p-value is significant (p 2.2-16), so there are significant differences in the average number of tubers according to the sites with the Kruskal-Wallis test.

Average Weight of Tubers per Plant
The average weight of cassava tubers per plant (PMT) also varies from one station to another. The lowest PMT (0.5 kg) is observed at the Pire station and the highest (1.7 kg) at Mborine station 1 (Table 3) with p-value = 4.149-9. The PMT, all stations combined, is 1.1 Kg/tuber. The p-value is significant (p = 4.1-9), so there are significant differences in average tuber weight between sites with Kruskal-Wallis test.

Yield of Cassava Production in Tivaouane
The average theoretical yield is 11 t/ha (Table 4). From one station to another, it varies from 5 (Pire station) to 17 t/ha (Mborine station 1). Considering the losses recorded, the actual yield obtained is 5.8 t/ha.

Discussion
Information obtained on cassava productivity in Tivaouane revealed that it is low. The planting density of cassava plants is 10,000 plants/ha and the average number of tubers produced per plant is 2.8. The significant difference observed at the site level in terms of average number of tubers would be related to the cropping practices and the degree of termite attack. Indeed, according to Raffaillac and Second (1997), the number of roots that develop into tubers varies according to varieties, environmental conditions but also cultivation techniques. According to these authors, in a total of 24 roots, 18 will be transformed into tubers for a planting density of 6000 plants/ha and only 6 will give tubers if the density is 15000 plants/ha. The average weight of a tuber obtained in this study is 1.1 kg and corresponds to the interval of the average weight given by Favier (1977) in which the weight of a tuber root is between 200 g and 3 kg. This difference in tuber weight would be closely related to the consequences of termite attacks but also to the number of tuber roots. The losses on the Soya variety obtained in this study, related to termites (1.2 t/ha) and missing plants (3.4 t/ha), could explain the low national cassava yields. These results confirm those of Raffaillac and Second (1997) on parameters that could influence root tuberization. Apart from these parameters, the actions of termites on cassava also contribute strongly to the reduction in the number of tuberized roots. In West Africa, termites have been reported as a pest of cassava in northern Ghana (Maayiem et al., 2012). In Malawi, southern Africa, Munthali et al. (1999) reported termite attacks on cassava plantations. Some termite species usually attack the stems from the inside starting from the roots and/or crown (Pearce, 1997) causing severe damage. These losses may be increasing as other termite damage parameters such as the impact of attacks on the number of tuberized roots had not been considered in this study. Losses related to the missing feet would be closely linked to termites. These plants disappear following termite attacks that transform the implanted cutting into sand, creating an empty space from which the missing plants disappear. Losses due to these missing feet not only impact production and yield, but also cause economic losses. Considering the theoretical yield obtained, which is 11 t/ha, the actual yield obtained at Tivaouane is estimated at 5.8 t/ha. The yield obtained is low because in classical conditions, the yield per hectare varies between 8 and 15 t/ha (Kouakou et al., 2015). This yield is quite similar to that obtained by DAPSA in 2017 during the evaluation of the 2014-2015 crop year in the department of Tivaouane. During this crop year, national production in terms of tubers was 254,983 tons and a national yield of 8.2 t/ha compared to a potential of 1,000,000 tons per year (DAPSA, 2017).

Conclusion
At the end of this study, on the evaluation of yield losses and productivity of the variety Soya of cassava cultivated in Tivaouane, it was retained that the yield losses related to missing plants amounted to 4.216 t/ha and that of termite losses 0.953 t/ha. In addition, some of the losses related to missing plants are caused by termites. The productivity of a cassava plant of this Soya variety is very low, averaging 2.8 tubers with an average weight of 1.1 kg per tuber. With a theoretical yield of 11 t/ha and an effective yield of 5.8 t/ha taking into account losses, the Soya variety has a low productivity from an agronomic point of view. These results show that the productivity of this cassava variety produced in Tivaouane remains low, however, considering Senegal's potential in terms of annual cassava production.