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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 7, Num. 1, 1999, pp. 21-33
African Crop Science Journal,

African Crop Science Journal,
Vol. 7 No. 1 1999 pp. 21-33

Effect Of Planting Methods And Soil Moisture On Cassava Performance In The Semi-Arid Sudan Savanna Belt Of Nigeria

E. OKOGBENIN, I. J. EKANAYAKE and M.C.M. PORTO1

TRIP, International Institute of Tropical Agriculture, PMB 5320, Ibadan, Nigeria
1 FAO, Viale delle, Terme, di, Caracalla-00100 Rome, Italy

(Received 16 June, 1998; accepted 25 January, 1999)

Code Number: CS99003

ABSTRACT

Climatic and edaphic factors are important determinants of the growth and yield potential of an ecological environment. Among other cultural practices, planting methods play a very vital role in the performance of a crop. The effects of planting methods and soil moisture on cassava (Manihot esculenta) performance in the Sudan savanna region of Nigeria were assessed under field conditions at the International Institute of Tropical Agriculture (IITA) in Minjibir, Kano State. Six planting methods in monoculture were evaluated in two crop seasons. These were horizontal planting on furrow or ridge, inclined planting on flat or ridge, and vertical planting on flat or ridge.Two genotypes were compared: TMS 91934, an improved IITA clone; and Dakata Uwariya, a land race. Dakata Uwariya was significantly better (P < 0.05) than TMS 91934 in plant height and root dry matter content; TMS 91934 was better in leaf formation and leaf retention. Ridge-based methods positively influenced root yield production and leaf formation, while flat or furrow methods were advantageous in number of plants at harvest. Horizontal and inclined planting were the best methods in general. Results showed that cassava performance in the Sudan savanna of Nigeria was influenced by genotype, planting methods and soil moisture. Sustainable development of cassava in the semi-arid agroecology essentially depends on the use of clones with good drought adaptation, combined with efficient cultural practices for good growth and yield.

Key Words: Leaf formation, leaf retention, Manihot esculenta, planting methods, root dry matter, root yield

RÉSUMÉ

Les facteurs climatiques et édaphiques sont des composantes importantes déterminant le potentiel de croissance et de rendement dans un environnement écologique donné. Entre autres pratiques culturales, les méthodes de plantation jouent un rôle, on ne peut plus vital, dans la performance d=une culture. Les effets des méthodes de plantation et de l=humidité du sol sur la performance du manioc (Manihot esculenta) ont été évalués en plein champ dans la ferme de l=Institut international d=agriculture tropicale (IITA) à Minjibir dans l=État de Kano, en zone de savane soudanienne. Six méthodes de plantation, en condition de monoculture, ont été évaluées pendant deux campagnes culturales: plantation horizontale en sillons ou en billons, plantation inclinée sur le plat ou en billons et plantation verticale sur le plat ou en billons. Deux génotypes ont été comparés: TMS 91934, un clone amélioré de l=IITA et Dakata Uwariya, un cultivar local. En ce qui concerne la hauteur du plant et la teneur en matières sèches des tubercules, Dakata Uwariya s=est avéré significativement meilleur (P<0,05) que TMS 91934; par contre, TMS 91934 a présenté une meilleure formation et rétention foliaires. Les méthodes de plantation en billons ont positivement influencé le rendement en tubercules et la formation foliaire, tandis que les méthodes de plantation sur le plat ou en sillons se sont révélées avantageuses en termes de nombre de plants à la récolte. En général, les méthodes de plantation inclinée se sont avérées les meilleures. Les résultats ont indiqué que la performance du manioc dans la zone de savane soudanienne du Nigéria a été influencée par le génotype, les méthodes de plantation et l=humidité du sol. Un développement durable du manioc dans la zone agroécologique semi-aride repose essentiellement à la fois sur l=utilisation de clones dotés d=une bonne adaptation à la sécheresse et sur le recours aux pratiques culturales efficaces en vue de l=obtention d=une bonne croissance et de rendements élevés.

Mots Clés: Formation foliaire, rétention foliaire, Manihot esculenta, méthodes de plantation, matière sèche des tubercules, rendement en tubercules

INTRODUCTION

Cassava (Manihot esculenta L.) is known for its wide adaptation to different edapho-climatic conditions. It is a major source of calories in tropical Africa and is particularly important in those areas where food supply is constantly threatened by environmental constraints (Porto et al., 1994). Cassava is highly productive in hot humid climates (El-Sharkawy et al.,1990; Ramanujam, 1990). It can grow in areas with as little as 500 mm rainfall per year and survives in areas with dry periods of 5-6 months, since it has a conservative pattern of water use (Cock, 1985). As such, cassava is widely distributed in the tropical and sub-tropical ecosystems of Africa. Most crop improvement efforts have been concentrated in the humid and sub-humid regions, due to their relatively greater importance in production. Less studied is the potential of cassava in environments outside the humid and sub-humid regions.

Nigeria contributes significantly to world production of cassava, and emerged as the largest producer since the beginning of the 1990s, with an estimated output of 31.4 million metric tonnes in 1995 (FAO, 1996). Cassava, a staple food in Nigeria, is rapidly expanding from the humid rainforest in the south to the marginal lands of the Sudan savanna zone in the north. As a drought tolerant crop, it offers farmers a new opportunity for cropping their lands throughout the lengthy dry season, typical of northern Nigeria, and therefore provides small-scale farmers with more food, feed and income.

Cassava planting materials (cuttings) are planted horizontally, vertically, or inclined, with or without tillage. Cuttings are planted on flat or slightly undulating land, ridges, mounds or heaps (IITA, 1990). Appropriate planting position varies with cassava variety, soil characteristics, and climate. In regions of medium to heavy soils with adequate rainfall (1000-2000 mm), any of the planting positions is suitable since the moisture will be adequate for bud sprouting. In areas of sandy soils or erratic rainfall, however, vertical planting provides better sprouting for cuttings than horizontal planting where buds may rot because of greater heat in the soil than surrounding air (Toro and Atlee, 1985).

The physiological basis of cassava tolerance to sporadic and extended drought has been studied (Connor et al., 1981; Connor and Cock, 1981; Connor and Palta, 1981; El-Sharkawy, 1993), including its highly responsive stomata, deep root system, and effective carbon fixation system for enhanced photosynthesis under water stress.

Given the future role of this crop in Nigeria in alleviating poverty and hunger in farm families, various planting methods of cassava were evaluated in the dry savanna. The primary objective was to identify appropriate cultural practices suitable for improved crop performance in this agroecology, both in vegetative growth and underground storage root production, given the long dry season characteristic of this zone.

MATERIALS AND METHODS

The research farm of the International Institute of Tropical Agriculture (IITA) at Minjibir located near Kano in the semi-arid Sudan savanna belt of northern Nigeria was used as the test site (5° 12' N and 7° 23' E, 490 m a.s.l.). Mean annual pan evaporation is about 1,701 mm. Rainfall (unimodal) starts in June and stops in October, with mean annual precipitation of 831.6 mm. Average monthly temperatures ranges from a minimum of 12-23° C to a maximum of 29-37° C. The soils are classified as eutric Regosols.

Trials were carried out in the 1994/95 and 1995/96 planting seasons using an improved cassava clone (TMS 91934) and a local variety (Dakata Uwariya). Six planting methods (treatment combinations of land preparation and cutting orientation) commonly used for cassava cultivation were evaluated. These planting methods were horizontal planting in furrow or ridge, vertical planting on flat or ridge, and inclined planting on flat or ridge. The land was ploughed and harrowed for all planting methods.

Using a split-plot experimental design, TMS 91934 and Dakata Uwariya were planted in main plots (i.e., flat or ridge), with the planting methods as sub-plots, replicated three times. Planting was done in August of both years. Rainfall amounts in 1994/95 and 1995/96 were 669 and 591 mm, respectively. Planting was done at a spacing of 1 m x 0.8 m in both seasons. In the first season, the site was 200 m from a lake. In the second season the trial was duplicated with the first site at 50 m and the other at 200 m from the lake (i.e., with a distance of 150 m in between the two sites). The depth of the water table was expected to interact with the planting methods. The plot closer to the artificial lake was denoted as the high water table site (HWTS), while the plot farther away was denoted as the low water table site (LWTS). Soil physico-chemical data were taken and the experimental sites were weeded and kept clean throughout the cropping period.

Starting from four weeks after planting (WAP), plant height, leaf formation, shedding, and retention, as well as neutron probe measurements of soil moisture at 30 cm intervals to 180cm depth were collected every three weeks. Life span of leaves was recorded for those tagged in the period between 9 and 26 WAP during the dry season (since leaves tagged had not all fallen, so that their life span could not be calculated at later stages). Leaf retention (longevity) was measured in weeks. Fresh root and shoot yields, root dry matter and shoot dry weight, number of plant stands and plantable cuttings produced at harvest were measured. Shoot yield was determined on both fresh weight and dry matter basis. The plant population per unit area at harvest was taken to assess plant establishment and survival under the various planting methods for each genotype. Data were analysed (by ANOVA) using SAS.

RESULTS

Plant growth. Distinct genotypic differences were observed in the plant growth of the elite clone TMS 91934 and Dakata Uwariya (Fig. 1). Dakata Uwariya was significantly taller (P < 0.05) than TMS 91934 in both trials, with mean plant heights of 139 and 79 cm, respectively, at 12 months after planting (MAP).

Plant growth was more rapid in the HWTS than the LWTS (Fig. 1). Soil moisture contents were only significantly different between the two sites at both 150 cm and 180 cm soil depths (Table 1). Soil moisture contents of the HWTS were 19% for the 150 cm and 25% at the180 cm depths, while in the LWTS, soil moisture was 16% at both the 150 and 180 cm depths. Significant variation were observed in soil moisture content at all depth levels between TMS 91934 and Dakata Uwariya, with moisture values being less for TMS 91934 (Table 1).

Dakata Uwariya was not appreciably affected in plant height at the two sites, for any specific period in growth, although plants were slightly taller at the HWTS than in the LWTS (Fig. 1a). However, plant height differed significantly across the two sites at 13, 21, 26 and 31 weeks after planting (WAP) for TMS 91934 (Fig. 1b).

Figure 1: Plant height at each stage of plant growth in the cropping season: (a) Dakata Uwariya at low and high water table sites; (b) TMS 91934 at low and high water table sites. (*indicates significance at the 5% level probability).

TABLE 1. Soil moisture content at different depths in low and high water-table sites for Dakata Uwariya and TMS 91934

Depth (cm)

Soil moisture (% by vol.)

Soil moisture (% by vol.)

LWTS

HWTS

Difference

D.Uwariya

TMS 91934

Difference

30

6.13

5.91

0.22

6.53

5.52

1.01**

60

9.70

9.24

0.46

9.79

9.15

0.64*

90

11.97

11.27

0.70

12.16

11.08

1.08**

120

14.48

14.45

0.03

15.28

13.65

1.63**

150

16.33

19.08

2.75**

18.73

16.68

2.05**

180

15.79

25.49

9.70**

21.39

19.89

1.50*

*, ** Significant at the 0.05 and 0.01 levels of probability, respectively
LWTS = Low water table site
HWTS = High water table site

Plant height responses were not significant for either TMS 91934 or Dakata Uwariya between the planting methods. However, combined analysis over a two year period across varieties showed significant differences in plant height between the planting methods with inclined planting on ridges as best (Table 2). The ridge based planting methods resulted in taller plants than the flat/furrow based methods. Overall mean plant height was, therefore, significantly greater on ridge (116 cm) than on the flat/furrow surface (106 cm). With respect to cutting orientations, plant height was only significantly better (P < 0.05) on ridge (114 cm) than in the flat/furrow (101 cm) when cuttings were planted horizontally (Table 3). No significant interaction was observed for year x clone x planting method on height.

Plant vigour measured in terms of stem production (i.e, the number of commercial cuttings obtained) was similar for the two genotypes used in this study. Stem production was not influenced by planting methods, but more cuttings were produced at the HWTS than at LWTS. The interaction between clone and planting method was also significant for stake production.

TMS 91934 produced more leaves than Dakata Uwariya in two seasons (Fig. 2a and 2b). TMS 91934 had an average cumulative leaf number of 633 leaves per plant in 49 weeks as compared with 497 leaves for Dakata Uwariya. Differences between the sites in leaf formation at any specific period of time for Dakata Uwariya was not significant. TMS 91934 was significantly better in leaf formation between 9 and 13 WAP at HWTS as compared with LWTS (Fig. 2a and 2b). Cumulative leaf formation was not affected by site differences. However, significant responses in cumulative leaf formation were observed between planting methods; inclined planting on ridge and horizontal planting on ridge were the best (Table 2). Ridges tended to increase cumulative leaf formation when cuttings were planted horizontally or in inclined orientation. It was only under vertical cutting orientation that plants on furrow/flat surface recorded increased leaf production. There was no significant interaction between clone and planting method for cumulative leaf formation for the two seasons.

Figure 2: Leaf formation at growth intervals throughout the cropping season: (a) Dakata Uwariya at low and high water table sites; (b) TMS 91934 at low and high water table sites. (*indicates significance at the 5% level probability).

Leaf shedding and retention. Leaf shedding measured as number of leaf scars, increased during the dry season of both years. Leaf shedding expressed as a percentage of leaf formed for TMS 91934 and Dakata Uwariya were 20 and 26%, respectively. Although Dakata Uwariya recorded more leaf shedding, it did not differ in cumulative leaf scars from TMS 91934. Leaf scars per plant were not different between the high and low water table sites, except at 21 WAP when the two varieties significantly (P < 0.01) dropped more leaves at the low than at the high water table site. Varieties were not different in leaf scars taken as an average for the two water table sites at each specific period except at 49 WAP (P < 0.05).

Cumulative leaf scars were more with horizontal planting on ridge and inclined planting on ridge in each of the seasons (Table 2). These two planting methods had the highest average cumulative leaf scars for two seasons. Cumulative leaf scars were not affected by the planting methods for either TMS 91934 or Dakata Uwariya. Based on data obtained from the HWTS and LWTS, it was noted that the highest leaf shed was observed between 26 and 36 WAP and that leaf scars obtained at 26, 31 and 35 WAP were significantly different (P < 0.05) from that obtained at any other time of the experiment. Overall mean leaf shed per plant for the entire growth cycle was significantly higher in the HWTS than the LWTS. Cutting orientation (horizontal, inclined and vertical cutting placement or position) were similar in leaf scars and percentage leaf drop either on ridge or in flat/furrow (Table 3). These traits were not influenced by the form of land preparation types (ridge vs. flat/furrow) for each cutting position (Table 3). Dakata Uwariya had significantly more leaf drop than TMS 91934. In general, more leaves were dropped on ridge (146) than flat/furrow (115), while percentage leaf drop was more on flat/furrow surface (24%) than on ridge (23%).

TABLE 2. Effect of planting methods on mean performance of two varieties (Dakata Uwariya and TMS 91934) in plant height, plant stands, leaf formation, leaf shed, root dry matter and dry shoot yield in two seasons (1994-1996)

Planting method

Plant height (cm)

Cumulative leaf formed (no.)

Cumulative leaf scars (no.)

Leaf drop (%)

Plant stand (no)

Dry shoot yield
(t ha-1)

Dry root yield
(t ha-1)

Horizontal/furrow

101

498

114.57

23.07

10677

3.42

1.52

Horizontal/ridge

114

643

151.86

25.14

9419

3.94

1.58

Inclined/flat

109

466

120.35

24.84

11285

3.49

1.49

Inclined/ridge

122

716

158.41

22.67

8898

3.83

2.10

Vertical/flat

109

533

126.2

23.45

9158

3.22

1.23

Vertical/ridge

112

532

127.86

23.77

10330

3.13

1.86

LSD (0.05)

11.8

134

25.43

2.46

NS

NS

0.59

TABLE 3. Orthogonal contrast of land preparation and cutting orientation for plant height, leaf formation, leaf scars, plant stands, root dry matter and dry shoot yield

Group

Type

 

Mean squares

Dry shoot
(t ha-1)

Root dry matter yield
(t ha-1)

Comparison

Plant height (cm)

Cumulative leaf (no.)

Cumulative leaf scars (no.)

Leaf drop (%)

Plant stands (no.)

Land preparation

Flat/furrow

H vs. I + V

245.44

7.38

0.58

2.82

67773.44

0.01

0.11

I vs. V

0.08

13662.00

351.11

8.70

2734165.33

0.19

0.21

(101.65)

(37081.14)

(3345.35)

(13.15)

(3200567.17)

(1.24)

(0.29)

Ridge

H vs. I+ V

26.69

869.27

304.5

3.26

610742.25

0.90

0.67

I vs. V

280.33

102508.57

2800.52

10.79

20306.08

1.40

0.14

(151.56)

(43326.04)

(3214.21)

(10.36)

(8509579.63)

(0.96)

(0.58)

Cutting orientation

Horizontal

R vs. .F

560.33*

59347.27

4005.15

0.53

4752725.33

0.91

0.01

(73.79)

(30971.50)

(1985.60)

(7.26)

(309039.77)

(1.42)

(0.25)

Inclined

R vs. F

520.08

188576.54

4345.69

29.27

17093307

0.37

1.08

(197.72)

(65337.26)

(4896.52)

(18.13)

(7345802.69)

(1.23)

(0.88)

Vertical

R vs. F

40.33

7.84

1007.42

0.68

4121924.08

0.02

1.27*

(88.63)

(32591.74)

(3365.92)

(12.84)

(8357004.21)

(0.63)

(0.14)

* Significant at the 0.05 level of probability; mean square error in parentheses; H = horizontal; I = inclined; V = vertical; R = ridge; F = flat/furrow

Leaf retention is a measure of leaf longevity or leaf age at senescence. It tended to decrease initially but increased later in the dry season in both genotypes. Mean leaf longevity for TMS 91934 was 102 days as compared with 81 days for Dakata Uwariya for leaves tagged between 9 to 26 WAP. Combined analysis of data for the two cropping seasons showed that there was no significant difference between the planting methods in leaf life span for each variety (Table 4).

Mean leaf longevity for the high and low water table sites was significantly longer for TMS 91934 than Dakata Uwariya for leaves tagged between 9 WAP and 26 WAP. Dakata Uwariya leaf longevity did not vary significantly between the HWTS and LWTS. Neither did TMS 91934 exhibit significant differences in leaf longevity between the sites with the exception of the leaves tagged at 21 WAP, which were better retained at HWTS.

TABLE 4. Mean leaf retention (weeks) of Dakata Uwariya and TMS 91934 in two seasons

Planting method

Dakata Uwariya

TMS 91934

Horizontal/furrow

11.50

14.75

Horizontal/ridge

11.70

14.10

Inclined/flat

11.80

14.80

Inclined/ridge

11.45

14.25

Vertical/flat

11.35

14.70

Vertical/ridge

11.65

14.65

P (0.05)

N.S

N.S

N.S. = Nonsignificant

Plant stands, root and shoot yields at harvest. An average of 10,084 plant stands ha-1 was harvested for TMS 91934; this was not different from the mean plant stands of 9,838 ha-1 for Dakata Uwariya. Planting methods significantly influenced total plant count, with inclined planting on flat showing the highest plant stands (Table 2). Plant stands were unaffected by stake positioning in either ridge or flat/furrow based planting methods (Table 3). For each of the positionings, plant stands did not differ between flat/furrow and ridge. Overall, however, the difference between plant stands on flat/furrow (10764) and ridge (9158) across all cutting orientations were significant. Horizontal planting on furrow and on ridge were the best planting methods for plant stands at the two water-table sites. While there was no significant interaction between water-table site and planting method, significant interaction (P < 0.01) was observed between water-table site and clone.

Dry shoot yield showed no significant difference between cutting orientations either on furrow/flat or on ridge (Table 3). Similarly, dry shoot yield did not differ significantly between ridge and flat/furrow for each cutting orientation analysed (Table 3). The two cassava varieties were not significantly different in dry shoot yield, although it was higher in Dakata Uwariya in the two seasons. With the exception of horizontal planting on ridge, which had the highest dry shoot yield, and which was significantly better than vertical planting on ridge, no other comparison between any two planting methods showed significant difference (Table 2). Dry shoot yield was also unaffected by the two locations, i.e., HWTS and LWTS (Table 5). However, fresh root yield was significantly higher in the HWTS than at the LWTS (Table 6). And, mean fresh root yield on ridge (7.53 t ha-1) was generally better than those with flat or furrow planting (5.30 t ha-1).

TABLE 5. Combined analysis of variance for dry root and shoot yield (t ha-1) of Dakata Uwariya and TMS 91934 for the low and high water-table sites

Source

df

Mean squares

Dry root yield

Dry shoot yield

Location (Loc.)

1

25.657**

0.308

Rep. within location

4

5.109

7.320*

Variety (Var.)

1

1.712

0.417

Loc. x Var.

1

0.428

0.768

Rep (location x variety)

4

0.423

1.277

Planting method

5

1.638

1.389

Loc. x planting method

5

2.622*

1.745

Var. x planting method

5

1.758

3.534

Loc. x variety x planting method

5

2.095

2.752

*, ** Significant at the 0.05 and 0.01 levels of probability, respectively

TABLE 6. Fresh root yield (t ha-1) of Dakata Uwariya and TMS 91934 at low and and high water-table sites

Planting method

Low water table site

High water table site

Dakata Uwariya

TMS91934

Dakata Uwariya

TMS91934

Horizontal/furrow

3.2

3.1

12.4

6.2

Horizontal/ridge

3.8

5.1

6.5

13.1

Inclined/flat

5.0

2.9

9.0

7.9

Inclined ridge

3.1

4.4

11.2

9.4

Vertical/flat

5.6

3.0

3.6

2.6

Vertical/ridge

5.1

3.1

4.2

14.0

Mean

4.3

3.6

7.8

8.9

LSD (0.05)

N.S

1.9

8.3

9.7

N.S.= Not significant

Root dry matter (DM) yields differed significantly between the varieties used in this study, with mean values of 1.87 t ha-1 (28.4% DM) and 1.36 t ha-1 (22.5% DM) for Dakata Uwariya and TMS 91934, respectively (Table 2). Root DM yield did not vary significantly between the cutting positions, either on the ridge, on the flat or furrow (Table 3). No significant variation in DM yield of tuberous root was observed between ridge and flat/furrow in both horizontal and inclined cutting orientation. However, ridging produced significantly better root DM yield than furrow/flat surface planting (Table 3). All ridge-based planting methods produced better root DM yield than those on furrow and flat surface (Table 3), while root DM yield was twice as high in the HWTS than the LWTS.

DISCUSSION

Genotype, soil water availability and planting methods affected the overall performance of cassava in the semi-arid savanna belt of Nigeria. The two contrasting water-table sites provided a good assessment of soil moisture influence on the growth pattern and yield of the two cassava varieties evaluated. Soil moisture readings at various depths in this study indicated that cassava absorbed moisture mostly within the 0 to 120 cm depth; in this soil zone, moisture levels did not differ between the two sites. TMS 91934 revealed higher water extraction than Dakata Uwariya at all soil depths, implying that TMS 91934 has a higher water requirement or use. Cassava roots can extract water down to a 2.5m depth (El-Sharkawy et al., 1992; El-Sharkawy, 1993).

Plant height was influenced by planting method, with horizontal and inclined planting on ridge emerging superior to vertical planting. That planting methods did not lead to appreciable difference in plant height within each variety suggests a strong genetic influence on this trait and explains the significant difference in plant height between the two genotypes. By manipulating the environment through appropriate cultural practices, such as planting method, one could achieve the maximum benefit of the inherent genetic potential of cassava for plant height. Dakata Uwariya was relatively stable in plant height, indicating that this variety is fairly adapted and tolerant to moisture stress. On the contrary, TMS 91934 showed remarkable response to moisture gradient. TMS 91934 is, therefore, sensitive to moisture stress in terms of plant height.

The growth pattern of Dakata Uwariya differed from TMS 91934, with few or no lateral branches as compared with the latter, which has a multiple shoot system due to higher lateral branching. There is a considerable difference among genotypes in their ability to form lateral branches (Hunt et al., 1977), and by implication, for stem production (in terms of commercial cuttings produced) and leaf formation. TMS 91934 compensated for its low plant height by producing a lot of branches, making stem production in this genotype comparable to that of Dakata Uwariya which has higher plants and therefore good production of commecial cuttings. TMS 91934, with its high branching characteristic, thus produced more leaves than Dakata Uwariya. Within the 12-month growth cycle in each season, more leaves were formed with increased rains but declined during the dry season. Earlier work has shown that leaf appearance is markedly decreased by water stress (Hunt et al., 1977; El-Sharkawy et al., 1992; El-Sharkawy, 1993). Higher soil moisture at the HWTS stimulated leaf production, thus cumulative leaf formed was more in the HWTS than in LWTS. Consequently, the effect of soil water deficits on crop growth was demonstrated by greater plant growth in HWTS than LWTS. This implies that the crop effectively utilized underground water for growth as soil moisture was more at the deeper depth levels beyond 150 cm in the HWTS, compared with the LWTS. Leaves were produced throughout the growth period. Cassava plants are known to produce new leaves continously (Veltkamp, 1986). Horizontal and inclined plantings on ridge, as in plant height, were very effective in leaf formation, and were the best planting methods for plant vegetative growth.

TMS 91934 produced more leaves and had less leaf shedding than Dakata Uwariya, an indication that TMS 91934 was more tolerant to stress than Dakata Uwariya. This means that Dakata Uwariya utilises leaf shedding as an effective drought avoidance mechanism. Among the drought avoidance mechanisms of cassava is its ability to shed leaves to reduce evapotranspirational losses under water stress conditions. Drought can cause considerable leaf drop, and thus shorten leaf life (IITA, 1990; El-Sharkawy et al., 1992; El-Sharkawy, 1993). Planting methods that tend to favour leaf production profusely shed more leaves than planting methods with less leaf formation. Hence, horizontal and inclined planting on ridges recorded the highest leaf shedding in terms of cumulative leaf scars. Another possible reason for higher leaf shed in high leaf producing planting methods is that under higher leaf production, there is tendency for shading. Mutual shading of leaves greatly limits leaf life and this accelerates leaf sheding (Rosas et al.,1976; Veltkamp,1986). Although leaf shedding may imply a relative decrease in photosynthesis and, therefore, reduction in assimilate production, the planting methods that favoured leaf shedding (and high leaf formation) still had relatively less proportion of leaf drop. Such planting methods promoted higher net leaf number which in turn favoured physiological growth processes.

In cassava leaf longevity varies with clone and environmental conditions, and the maximum leaf life is 210 days (Irikura et al., 1979), while the minimum leaf life is approximately 40 days (CIAT, 1978). As dry season progresses, plants adapt by reducing leaf number as well as leaf size to meet available moisture during drought stress at the peak of the dry season. This adjustment allows for longer leaf life as found out in this study. Cock (1985) reported that in dry periods, total leaf production is smaller and that leaf life is longer when number of active apices is reduced. Leaf longevity was not influenced by planting methods as no significant variation was observed in each of the variety. On average, TMS 91934 was better in leaf longevity than Dakata Uwariya. The active leaf life in TMS 91934 is thus longer, meaning higher duration of photosynthetic activity for this clone. Since TMS 91934 had more leaf production and had higher net leaf number per plant, this should favour higher photosynthate production.

Plant stand provides a good assessment of the establishment of cassava under the various planting methods. That plant stand was effectively favoured by flat or furrow surface for all cutting orientations combined showed that minimum tillage helps stimulate plant establishment. This is very important in semi-arid climatic conditions where high solar radiation and evaporation may accelerate loss of sub-soil moisture which accompanies tillage. Minimum tillage conserves soil moisture for sprouting of cuttings which mitigates the effect of short dry spell that often occurs at the onset of the rainy season. Rapid cutting dessication that occurs due to a long interval between rains after planting under the characteristic high temperature of the Sudan savanna could cause eventual death of cuttings. Where cuttings sprout, such plants may lack the vigour and development to sustain it through the prolonged dry season leading to the eventual senescence of such plants before attaining maturity.

TMS 91934, with shorter internodes, had more nodes for bud development leading to better sprouting, compared to the local variety (Dakata Uwariya). With fewer nodes, horizontal planting was likely to aid more sprouting for Dakata Uwariya. Plant stand at harvest depends, among other factors, on the growth and development of the plants at the initial stages of the growing season.

Fresh shoot yield, which refers to the top biomass, was a direct reflection of plant growth. It was very high on horizontal and inclined planting on ridge. Since plant height and leaf formation were favoured by these planting methods, expectedly, these planting methods emerged as the highest yielding for this parameter. High moisture stimulated higher fresh shoot yield in the HWTS compared with LWTS. For dry shoot yield, TMS 91934 was relatively stable at both LWTS and HWTS compared with Dakata Uwariya.

Fresh root yield was highly influenced by planting methods. Since root yield was often better on ridges than on the flat or furrow surface, it indicates that ridging is quite essential for cassava production in the Sudan savanna. The effect of water-table and by extention soil moisture was markedly pronounced as shown by the yield result at HWTS and LWTS. This revealed moisture as a limiting constraint to cassava production in the semi-arid zones. Fresh root yield were generally lower than the 11.9 t ha-1 reported by Nweke et al. (1994) for leading traditional cassava growing countries in sub-Saharan Africa. This can be attributed to the problem of moisture constraint in the Sudan savanna, in addition to the fact that the area used for this study is highly sandy, with very low organic matter and nitrogen, and given that the land had been continually cropped over the years.

TMS 91934, with a high leaf fomation and a probable potential for high photosynthesis in the dry savannas, did not translate this advantage into positive effect on yield. TMS 91934 recorded lower soil moisture content than Dakata Uwariya. This suggests that TMS 91934 had higher water use for each unit yield, compared to Dakata Uwariya. And, apparently, dry matter partitioning favoured vegetative growth in TMS 91934 a cultiviar developed for the sub-humid and humid agroecological zones as opposed to partitioning for root grown in Dakata Uwariya.

In most of the growth and yield parameters evaluated, performances were often better on the ridges compared with furrow or flat surface planting, suggesting that this type of land preparation is beneficial. Although this study showed that the different planting orientation of cuttings was not apreciably different in performance, the treatment combination of cutting orientation and land preparation (denoted as planting methods in this study) was effective, leading to significant differences in performance for most of the growth and yield parameters considered. Ridges tend to aid storage root development. Since they provide the right physical environment for good tuberisation. They also facilitate the aggregation of top soil around the growing plants, thus increasing the amount of nutrients available for plant growth and development. Kang and Wilson (1982) pointed out the obvious advantage of growing crops in 30 cm high ridges over growing them on the flat, stressing that yield reduction with planting on the flat may in part be related to physical soil impedence. Adequate loosening of the soil during preparation generally improve drainage and soil aeration, reduce root rot, and increase yields (Howeler et al., 1993).

Variations may occur in performance of planting methods from one season to the other as well as from one site to the other, such that no one particular planting method may always emerge as the best at all times, as they may be influenced by soil characteristics, climatic factors and variety. However, horizontal and inclined planting on ridge were relatively fairly stable as high performing planting methods in this study in terms of plant growth and yield.

CONCLUSIONS

In this study, the influence of planting methods and moisture on plant growth and yield were established. Genotypic differences were found to affect plant response in adaptation and tolerance to water stress conditions. Combination of land preparation and cutting orientation (planting methods) was effective, leading to significant differences between the planting methods for growth parameters and yield. From this study, ridging is a favourable land preparation method for cassava production in the Sudan savanna. The two most promising planting methods are horizontal and inclined planting on ridge, which produced best growth and yields. Since root yield is the most desired economic part of cassava, and given that vertical planting on ridge compared favourably well with inclined and horizontal planting on ridge, vertical planting on ridge may also be considered as a suitable planting method for cassava cultivation in the Sudan savanna zone of Nigeria.

ACKNOWLEDGEMENTS

This study was funded by the Centro Internacional de Agricultura Tropical (CIAT), as part of the CIAT-IITA collaborative project in Africa and by the core budget of IITA. We express our gratitude to all CIAT-IITA project staff for their invaluable assistance in data collection and preparation of the manuscript. The input of all reviewers of this paper are highly appreciated.

REFERENCES

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