1. Introduction
South Africa is considered to be a food self-sufficient country in Africa. However, 20% of its rural, resource-poor households routinely experience nutritional food insecurity and poverty [
1]. Furthermore, South Africa is a water-scarce country and only 12% of its land is suitable for agriculture [
2]. About 1.3 million hectares of its crops are under irrigation, which consume more than 60% of the available water [
3]. For the agricultural sector to play an important role in the economy, it needs to be transformed from relatively few dominant crops produced by commercial farmers into a more diverse, climate-resilient production system that cater to both commercial and small-scale farmers. Broadening the food base through the introduction, selection, and breeding for more climate-resilient crops will contribute to food and nutritional security. Cassava, being a tropical crop adaptable to diverse environments and climatic conditions, could provide South Africa with a novel food and industrial crop that could be grown widely.
Cassava is a source of dietary energy to over 800 million people in tropical and subtropical areas of the world [
4], and is also used for industrial starch. It is grown primarily for its enlarged storage roots and can be harvested from 8–24 months after planting [
5]. More than 40% of the African population consumes cassava as a staple food, and it is the second most important crop on the continent after maize. Globally, the mean fresh root yield of cassava is 12.8 t ha
−1, with the highest and the lowest yields of 13.3 t ha
−1 and 8.9 t ha
−1 being observed in Asia and Africa, respectively [
6].
Farmers can grow and harvest cassava on marginal soils with minimal inputs, on a sustainable basis, which is important to rural, resource-poor farmers [
7]. Cassava can be grown on marginal lands, in low-fertility, acidic soils, and under variable rain-fed conditions ranging from less than 500 mm per year in the semi-arid tropics to more than 1000 mm per year in the sub-humid and humid tropics [
8]. Due to cassava’s climate resilient nature, it is a critical crop for many farming communities and rural poor in Africa [
9]. However, relatively little cassava is grown in South Africa, despite a growing community of both commercial and small-scale farmers who wish to grow the crop. Cassava can be converted into a variety of food products, and can be used as a livestock feed, and factory byproducts can be used as poultry feed [
9] Furthermore, the natural high storability of cassava in the soil in comparison to other root crops allows farmers to access optimum market opportunities and earn the best possible market prices [
5]. The high cost of fossil fuels, combined with the need to reduce greenhouse gas emissions, necessitates a search for renewable fuel sources. Cassava, which has a high percentage of easily fermentable sugars and a relatively high level of drought tolerance, can also be used as an alternative source of biofuel. It is also grown globally as a source of industrial starch.
It is estimated that, in South Africa, maize accounts for approximately 95% of the country’s starch production, 37% of the crop being used for food, 40% for feed, 18% for export, and 5% for industrial starch. Due to changing climatic conditions, and competition between industries utilizing maize products, the local producers of industrial starch do not meet local demands. Hence, South Africa is importing large quantities of starch annually [
10]. Cassava starch is preferred in South Africa and fetches a higher price on the market than maize, potato, or wheat starch [
11]. This study was initiated with the aims to evaluate the performance and adaptability of exotic cassava cultivars across different environments in South Africa and to recommend genotypes for cultivation.
3. Discussion
In plant breeding programs, multi-environment trials play an important role in cultivar evaluation and selection for primary production and commercialization. In multi-environment trials, it is commonly assumed that the residual error variance is homogenous across all considered environments. However, heterogeneity of error variances across environments generally exists in multi-environment trials [
12]. Ignoring error variance differences across environments often limits the accuracy of genotype evaluations and the reliability of varietal selection [
13]. Hence, an error variance homogeneity test is recommended if the research is conducted in more than one environment. In this study, the error variance was varied across environments, and logarithmic data transformation was therefore performed to standardize the error variance.
Assessment of variability is a prerequisite for crop improvement to assess the potential of the genotypes as a base for genetic improvement. Significant variability has been observed among cassava cultivars and selection of desirable characters will lead to progress in plant genetic improvement. The significant genotypic effect observed in this study signifies that the tested genotypes showed appreciable levels of variations across environments. The wide range of variability observed on the mean performance of the tested genotypes across environments suggested that there were differences in their adaptability to the different environments. Egesi [
14] observed similar results for cassava FRY in multi-environmental trials. Importantly, for all the studied traits, the environmental mean square was higher than the genotypic MS. The highly significant environmental variation observed indicates that the environments of the six test sites differed greatly in temporal and spatial environmental conditions [
15]. It would have been ideal if the results were supported with soil and weather information to further elucidate the cause of the significant variation in genotypic performance and environment variation. The results demonstrated the importance of the evaluation of genotypes in multiple environments with variable agro-ecological conditions before recommending a crop, and especially specific cultivars, for production and commercialization [
16].
The broad sense heritability estimates for yield and yield component traits were a bit lower than the findings of Ntawuruhunga and Dixon [
17] and Adjebeng-Danquah [
18]. Heritability estimates influence the amount of genetic gain that can be achieved in selection for a trait of interest [
19]. However, broad sense heritability does not necessarily give a full indication of genetic gain that can be achieved through selection since it includes both additive and non-additive components of the variation [
20].
The different environments favoured the expression of traits differently, where some environments favoured certain genotypic performance over others. The results showed that Nseleni was a less favourable environment, and the genotypic performance of all cultivars was poor. This could be partly attributed to poor soil quality at Nseleni, which has a sandy soil with low levels of soil organic matter, suggesting that the level of N availability for optimal cassava production was too low [
21]. It could also be attributed to the heavy infestation of various weeds observed at Nseleni. In cassava, weed species composition, level of infestation, and exposure time directly affect cassava yield. Maur [
22] reported that weeds that were not removed for longer than 70 days could cause cassava yield losses of up to 51%.
The performance of cultivars in Mutale was better for most of the traits evaluated, compared to the rest of the environments because the cultivars were kept for longer than 12 months. Prior research has shown that the harvesting period has a strong positive association with dry matter accumulation and fresh root yield [
23]. In this environment, large and longer roots were harvested. The size and shape of storage roots are also dependent on genotypic and environment factors [
24].
To enhance cassava root yield and quality, it is important to understand storage root initiation and development [
25]. This can be done by assessing root number, root width and length, and carbohydrate accumulation. Aina [
26] reported that root traits such as NR, root size, and HI directly correlated with storage root yield. This was also confirmed in our PCA analysis, in which more than 59% the variation was contributed by root traits such as NCR, RL, RWd, and RWt. This signifies that when selecting for high FRY in cassava, these traits should be taken into consideration. A cassava plant can form up to 14 storage roots per plant, depending on the genotype [
25]. In this study, a wide range of total NR (13.8) and NCR (7.8) was observed per plant, suggesting considerable scope for improvement by selection. It has also been reported that HI values higher than 0.5 can be achieved, and typically 6–12 storage roots are produced per plant at a planting density of 10,000 plants/ha [
8].
The DMC of cassava roots is an important trait for the selection of cassava for industrial processing. In other studies, dry matter in cassava roots have ranged between 20 and 47%, with values above 30% considered to be high [
27]. Mehouenou [
28] reported an exceptionally high DMC of 55.2% for the cultivar Oueminnou. In this study, DMC ranged from 30.6% for UKF5 to 49.5% for UKF9, with a mean of 43.5%. The values observed in this study were a bit higher than the previous studies [
27,
29,
30]. However, this could be attributed to differences in harvesting dates between the various trials. Cultivar UKF9 (49.5%) and P1/19 (48.5%) were the top performing genotypes in terms of DM yield. The high dry matter cultivars identified in this study could be grown as a feedstock for industrial starch. They would also serve as good parents in a breeding program to improve cassava DMC.
In cassava, plant height is one of the criteria used for selection of genotypes at the early stages of breeding [
31]. PH is an important trait, because cassava plants are conventionally propagated using stem cuttings, hence tall plants are preferred. UKF8 was the tallest, at 2.07 m, followed by P4/10 and MSAF2. In the current study, multipurpose genotypes such as MSAF2 and UKF4 were identified with both high BIO and FRY, signifying the potential of these cultivars for food, feed, and biofuel feedstock. The advantage of using cassava as biofuel feedstock over many other crops is that cassava can thrive in degraded [
32] and relatively low fertility soils, where the cultivation of other crops would be uneconomical [
33]. Hence, cassava is being evaluated for the production of bioenergy, China being the leading producer [
34].
In the present study, FRY was significantly and positively correlated with most of the root related traits, implying that selection for a given root trait will not have a detrimental effect on yield and other traits [
35,
36,
37]. In cassava, storage root yield is the function of the number of storage roots per plant, mean fresh root weight, and dry matter content [
38,
39]. Tumuhimbise [
40] also reported that FRY was positively and significantly correlated with RWd, HI, BIO, and NR. Storage root phenotyping for important yield and starch traits is often done very late in the breeding cycle, and may takes 12 to 18 months, depending on the genotypes and the environmental conditions. Hence, direct selection for root yield can be slow. However, the process can be accelerated by measuring positively correlated traits at the early growth stages of the crop [
41]. Okogbenin [
42] also suggested the possibility of using HI at seven months after planting as an indirect selection trait for FRY.
In this study, no significant trait association were observed between root DMC and NR, NCR, and FRY. A lack of association between DMC and FRY was also reported by Okogbenin [
42], Ekanayake [
43], and Rao [
44]. Low correlation observed between some traits may also be beneficial in permitting independent manipulation of the traits [
36,
45]. Based on the result of the correlations in this study, root parameters had profound effects on final root yield [
39].
5. Conclusions
The identification of superior genotypes and suitable testing environments based on genotypic performance is key for any breeding program. This study was conducted in six locations for one season and the effect of the seasonal variation was not considered. In most of the literature, in cassava seasonal variation was overlooked because cassava is a biannual crop that can grow from 8–24 months after planting. Ssemakula and Dixon [
50] reported that location effects were more pertinent for cassava than year effects. However, Dixon and Nukenine [
51] suggested that testing at 3–5 locations for 2–3 years with 3–4 replications per location is the optimum combination to get precision in cassava yield trials. It is ideal to test genotypes across locations and years to maximize genetic gain through selection.
The analysis of variance revealed the presence of highly significant variation among genotypes, environments, and their interaction. However, the magnitude of the environmental and the G × E interaction effects were significantly higher than the genetic effect for all the studied traits, indicating the need for testing cassava genotypes in multiple environments before effective selection and recommendation can be made. MSAF2 and UKF4 showed the highest mean performance for BIO, FRY, and other root traits, whilst P1/19 and UKF9 were the top in terms of DM yield. UKF4 was found to be the highest yielder, and relatively stable for root yield across environments. In contrast, MSAF2 did not perform consistently across environments because it is highly susceptible to the local strains of cassava mosaic disease (CMD). However, MSAF2 could be used as a parent to breed for novel clones that combine large number of marketable root and high fresh root yield. UKF4 can be recommended for immediate use as a food, feed, and biofuel feedstock, while P1/19 and UKF9 could be grown for industrial applications.
Understanding the relationship between environmental and demographic parameters is an important first step in predicting the quality of the testing environments. The basis for genetic improvement is to identify representative environments where the traits of interest are consistently expressed at the levels appropriate for selection [
52]. Based on genotypic and environmental means, Mandlakazi and Mutale were found to be good environments for storage fresh yield, whereas Shatale and Mabuyeni were intermediate and Masibekela and Nseleni were low environments due to high CMD prevalence and weed infestation, respectively. Therefore, Mabuyeni (KwaZulu-Natal), Shatale (Mpumalanga) and Mandlakazi (Limpopo) could serve as varietal testing sites in the future cassava-breeding program. This study is a pioneering study on cassava production in South Africa, and provides baseline information on the performance of currently available cassava cultivars and the identification of suitable screening environments for future breeding and agronomic studies.