African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 4, Num. 4, 1996, pp. 393-397

Effects of planting date on yield components and seed viability in soybean in Southern Guinea Savanna, Nigeria

L.L. BELLO, A.A. OJO, M.O. ADEYEMO and Y.M. OMOJOR^1

Department of Crop Production, University of Agriculture P.M.B. 2373 Makurdi, Nigeria

^1 Benue State Agricultural and Rural Development Authority, Makurdi, Nigeria

(Received 16 September, 1994, accepted 28 October, 1996)

Code Number: CS96080
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ABSTRACT

A field study was conducted during two growing seasons in the Southern Guinea Savanna of Nigeria, to assess the effect of early and late plantings on three soybean (Glycine max (L.) Merrill) cultivars. Planting was done two weeks before, at, and two and four weeks after the recommended planting time. Seed yield and yield components (pods per plant and branches per plant) were measured. Planting date x cultivar interaction was significant for seed yield and yield components. There were significant differences in average seed yield among planting dates and a progressive decline in average seed yield with delay in planting after the recommended date in both years. Pods per plant and branches per plant, in early planting, were more than or comparable with those planted within the recommended periods. However, there was reduction in these parameters with late plantings. There were no significant differences in seed viability after eight to ten months of ambient storage of the cultivars. Thus, flexibility of planting certain soybean cultivars earlier than the recommended planting period in the Southern Guinea Savanna is feasible without detrimental effect on quality and quantity of the seeds produced.

Key Words: Glycine max, yield performance

RESUME

Une etude sur le terrain a ete menee pendant deux saisons de croissance dans la savane Guineenne au Nigeria, pour evaluer l'effet de planter tot sur trois cultures de soja (Glycine max (L.) merrill). On a plante dans une periode de deux semaines en avance, et deux et quatre semaines plustard apres la periode recommandee pour planter. La date x d'interaction pour planter emit considerable pour la production de semence et les composants de semence. Il y a eu de differences considerables en moyenne de production de semence avec retard de planter apres les dates recommandees pendant les deux annees. Les gousses par plante et les branches par plante, pendant la periode de planter tot, ont plus pousse en comparaison avec celles durant les periodes recommandees. Cependant, ii y avait une reduction dans ces parametres par rapport a la periode de planter tot. I1 n' y avait pas de differences significatives dans la viabilite de semence apres huit a dix mois de stockage de cultures. Ainsi, le flexibilite de planter certaines grains de soja avant la periode recommandee dans la savance au sud de la Guinee est faisable sans effet detrimental sur la qualite et la quantite de produit de semence.

Mots Cles: Glycine max, la performance du rendement

INTRODUCTION

Soybean (Glycine max (L.) Merrill) is generally planted in the Southern Guinea Savanna of Nigeria between mid-June to mid-July (Fisher, 1980). Planting during this period ensures adequate rainfall during seedling establishment, flowering and pod filling stages for maximum seed yield. It also ensures bright and sunny weather at maturity. If soybean matures during the rains, both seed quality and viability are greatly affected (Nangju, 1977). Since the establishment of this planting period, genes for seed viability have been incorporated into improved varieties to prolong the shelf life of the seeds at ambient storage (Wien and Kueneman, 1981 ).

Soybean planted after recommended period (June 15th - July 15th) frequently yields less as a result of insufficient moisture during flowering and pod filling. Large yield reductions attributable to late planting have been reported (Pal et al., 1983; Olufajo et al., 1984).

The occurrence of rains towards the end of May in Southern Guinea Savanna, and the interest among farmers to grow improved soybean varieties, in addition to the genetic improvement in seed viability expressed in improved cultivars, suggest the possibility of production advantage from planting soybean earlier than presently recommended. A wider range of planting dates may be helpful to growers during the planting season, when many competing activities demand for attention. Earlier reports on planting dates had been silent on seed longevity (Pal et al., 1983; Olufajo et al., 1984). Soybean production could be increased substantially in Benue State by introduction of improved varieties that are stable in yield over a range of planting dates. Extensive testing is required to identify the genotypes that show the least interaction with environments, or possess the greatest stability of performance. The cheaper alternative is the unilocational test under different micro-environmental conditions created by varying planting dates.

Therefore, the objective of this study was to determine if flexibility in planting date exists in the Southern Guinea Savanna and can be exploited without compromising yield, yield components and seed longevity.

MATERIALS AND METHODS

Three varieties of soybean (TGX 536-02D, Samsoy 2 and TGX 923-2E) were grown in 1991 and 1993 growing seasons at the University of Agriculture, Makurdi Research Farm on a sandy loam soil. These varieties are of different maturity groupings. TGX 536-02D is an early variety which matures within 100 days, while Samsoy 2 and TGX 923-2E mature within and more than 120 days after planting (DAP), respectively.

Two weeks prior to, within, and two and four weeks after the recommended period were evaluated. The experiment was of a split-plot design, with planting dates as main plots and cultivars as the subplots. Each subplot consisted of four rows of 6 m length, with 75 cm between and 5 cm within row spacing. The two centre rows of each plot were end-trimmed at maturity to a uniform length of 4 m prior to harvesting. The trial had three replicates. Planting, which was done fortnightly, started on 1st June and ended on 10th August in both years.

The two centre rows of each plot were harvested and weighed for seed yield. Fifteen plants were taken at random from each plot to determine number of pods per plant and number of branches per plant. In Benue State, Farmers keep soybean seeds for next planting at ambient condition for 78 months. Therefore, in this study, threshed seeds for each plot at each planting date were stored at ambient temperature for eight to ten months for seed viability testing, depending on the maturity grouping of the variety and the time of planting. Twenty-five seeds were used to determine the viability for each planting date in a germination trial using three replicates. The numbers of seedlings that emerged after ten days were counted and converted to percentages for analysis.

Analysis of variance was done on each variable and, where differences existed, means were compared using Fisher's protected LSD. Both planting date and cultivar were considered fixed factors in the model. In order to increase the sensitivity of the main and interaction effects, main plot and subplot error terms were pooled for those traits found to have error terms homogeneous at 0.50 level of significance (Carmer et al., 1969).

RESULTS AND DISCUSSION

Planting date x year interaction and year effects were highly significant for yield and its components. This might probably be due to unequal amount of rainfall observed during the growing seasons (Table 1) in the different years. For this reason, soybean performance for the two years is discussed separately.

TABLE 1. Mean monthly units and total monthly units for the 1991 and 1993 growing seasons and a 10-year average in Makurdi, Benue State

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Months      Mean monthly temperature (C)    Total monthly rainfall (mm)
           -----------------------------    -------------------------------
           1991    1993  10 year average     1991    1993   10 year average
---------------------------------------------------------------------------
May        28.1    28.6       28.6          183.2     52.9      135.7 
June       27.8    27.5       27.3           82.7    268.1      137.7
July       27.0    26.5       26.5           96.6    341.3      213.3
August     26.7    26.3       26.5          264.3    174.6      236.4
September  26.8    26.6       26.6          185.0    137.2      214.6
October    26.8    27.1       27.3          161.5    186.9      109.1

Seed yield and yield components showed significant differences among cultivars and planting dates (Table 2). Cultivar x planting date interactions were significant for these parameters in both years. However, there were no significant differences in seed-viability for planting dates, cultivars, or planting date x cultivar interactions (Table 2). In 1991, early planted soybean yielded more than that planted on the recommended and later dates (Table 3). Delayed planting decreased yields, with greater reductions after the recommended period similar to those reported in other studies (Pal et al., 1983; Olufajo et al. 1984). A similar trend occurred in the 1993 growing season, but the highest yields were observed within the recommended period, probably because of differing climatic conditions for the two years.

 TABLE 2. Analysis of variance for seed yield, branches per plant, pods per plant and seed viability in different planting date study of soybean in 1991 and 1993

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Characteristic                     Source of variation
                 ----------------------------------------------------------
                  Planting date      Cultivar         Planting date 
                                                    x Cultivar interaction
                 ---------------    ------------    ----------------------- 
                   1991    1993     1991    1993       1991    1993
---------------------------------------------------------------------------
Seed Yield          **      **        *      **          *      **
Branches per plant  **      **       **      **         **      **
Pods per plant       *       *        *       *          *       * 
Seed viability      NS      NS       NS      NS         NS      NS

TABLE 3. Mean yield and other characters of three soybean cultivars planted on different dates in 1991 and 1993

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Variable        Early planting   Recommended planting   Late planting  CV%  
                    date                  date              date    
              -----------------   ----------------   ------------------ ---
              June 1st  June 15   June 29  July 13   July 27  August 10
---------------------------------------------------------------------------
                                         1991

Yield(Kg ha^-1) 4138.9a  3888.9a  3314.8b   2453.7c  1866.6d  1745.4d  20.0
Branches per plant  17a    20a      19a       11c       9c       7c    25.0
Pods per plant     211a   175b     191ab     115c     100cd     80d    23.0

                                         1993

Yield(Kg ha^-1) 1662.0bc 1814.7bc 2407.3a   2055.5ab  1453.6c  953.7d  35.5
Branches per plant  10a     8b       8b        8b        7b      4c    13.0
Pods per plant     112a     88ab   108a      112a       80b     34c    22

However, the combined 2-year averages indicated a significant and progressive decline in yield with delayed planting (Table 4). Generally, the three planting dates in June were statistically comparable, while delayed planting beyond June resulted in drastic reductions in seed yield.

TABLE 4. Mean seed yield for three soybean cuitivars in different planting date studies in 1991 and 1993 at Makurdi, Benue State, Nigeria

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Cultivar      Early planting      Recommended planting     Late planting 
                  date                     date                date
             ------------------   --------------------   ------------------
            June 1st    June 15    June 29    July 13    July 27  August 10
---------------------------------------------------------------------------
TGX536-O2D   3035a+     2889a       3111a      2125b      1475b    1445a 
Samsoy2      3077a      3028a       3195a      2833a      2061a    1514a 
TGX923-2E    2590b      2639a       2278b      1806b      1444b    1090b 
2 year mean  2901++     2852a       2861a      2255b      1660c    1350d

Interaction between planting date and weather patterns are known to affect soybean development (Fisher, 1980). Hanks and Thorp (1957) found that soil ,crust strength and soil moisture stress affect the emergence of soybean seedlings. The decrease in yield in early planting during the 1993 growing season may be related to crusting because of erratic rainfall that occurred at the onset of the season which made it difficult for the hypocotyl to push the cotyledons above the soil hence there was delayed emergence. However, in this study, soil crust strength per se was not measured.

Pods per plant and branches per plant in early planting were more than or comparable with those planted within the recommended planting periods. Branches per plant and pods per plant decreased with later planting. Delayed planting has been reported to decrease number of branches and number of pods per plant (Pal et al., 1983; Olufajo et al., 1984).

Table 4 shows the responses of the different soybean cultivars to planting date. Generally, Samsoy 2 and TGX 536-02D were the highest yielding cultivars and were not significantly different, except for July 13 and 27 plantings when Samsoy 2 produced significantly higher seed yield. TGX 923-2E performed consistently the poorest, although its yield was statistically similar to those of Samsoy 2 and TGX 536-02D on June 15 and that of TGX 536-02D on July 27.

Individual cultivar data indicated that certain genotypes were more sensitive than others with respect to planting date in this location. Samsoy 2 was the most stable cultivar whereas TGX 53602D and TGX 923-2E were particularly sensitive to delayed planting at this location.

The lack of significant effect of planting date on seed viability at ambient storage temperature suggested that the three cultivars used maintained their viability for at least 8 months, irrespective of planting date. One of the main constraints to increased production of soybean in Nigeria is the rapid loss in seed viability. Excellent germination and resistance to deterioration at ambient storage was associated with small seeded varieties (Dassou and Kueneman, 1984). Dassou and Kueneneman (1984) gave a general rule of thumb that "large-seeded genotypes are highly susceptible to incubator weathering and have poor seed longevity". This rule has been a guiding principle in genetic improvement programmes of soybean in the country. All large-seeded genotypes from 15g and above per 100 seeds are always discarded either at selection for single plants in the F4 or in later generations or from progeny rows.

The three varieties used (TGX 536-02D, Samsoy-2 and TGX 923-2,E) on the average weighed 10, 12.6 and 11.1 g per 100 seeds, respectively. The small seed size of these cultivars might have been responsible for their abilities to maintain seed viability.

CONCLUSIONS

Similar trends in average yield performance across planting dates occurred in both years. There was a progressive decline in yield with planting delayed beyond the recommended period. However, since planting date effects differed considerably between years, additional planting date research is necessary before planting date guidelines are changed.

Individual cultivars were variable in yield performance on different planting dates. Samsoy 2 was the least sensitive to planting date and was consistently the highest yielding cultivar at early, recommended and late plantings.

There were no significant differences in seed viability after eight months of ambient storage of the cultivars used. As new cultivars are developed, additional testing may be advisable to determine consistency of performance on altered dates of planting.

Although there was considerable environmental variation in crop performances, wider flexibility in planting date with selected cultivars is possible in the Southern Guinea Savanna of Nigeria.

REFERENCES

Carmer, S.G., Walker, W.M. and Self, R.D. 1969. Practical suggestions on pooling variances for F tests of treatment effects. Agronomy Journal 61:334-336.

Dassou, S. and Kueneman, E.A. 1984. Screening methodology for identification of soybean varieties resistant to field weathering of seed. Crop Science 24:774-779.

Fisher, N.M. 1980. A note on agronomic research with soybean at the Institute for Agricultural Research, Ahmadu Bello University, Zaria. Proceedings, First National Meeting of Nigerian Soybean Scientists held at the Institute of Agric. Research and Training, Ibadan, pp. 33-40.

Hanks, R.J. and Thorp, F.C. 1957. Seedling emergence of wheat, corn, grain sorghum and soybeans as influenced by soil crust strength and moisture content. Soil Science Society American Proceeding 21:357.

NanKlu, D. 1977. Effect of date of harvest on seed quality and viability ofsoybeans. Journal of Agricultural Science 89: 107-112.

Olufajo, O.O., Pal, U.R., Nnadi, L.A., Adu, J.K. and Asenime, I.O.R. 1984. Influence of cultural practices on soybean yield in Nigeria Savanna. Proceedings of the fourth National Meeting of Nigerian Soybean Scientists 4:112-131.

Pal, U.R., Olufajo, O.O. and Nnadi, L.A. 1983. Response of soybean to sowing date, planting arrangement and fertilizer application in Nigerian Savanna. Proceedings of the third National Meeting of Nigerian Soybean Scientists 3: 108-124.

Wien, H.C. and Kueneman, E.A. 1981. Soybean and deterioration in the tropics. Varietal differences and techniques for screening. Field Crops Research 4:123-132.

Copyright 1996 The African Crop Science Society