Effect of Variety Selection on Growth, Development and Yield of White Yam in Southwestern Nigeria

: An on-farm yam experiment was conducted to confirm the agro-climatic potential of Abeokuta, South-western Nigeria for three white yam varieties ( Dioscorea rotundata ). Three varieties of yam (Efuru, Ise-osi and Oniyere) was selected and related to crop growth and yield. The experiment was laid out in randomized complete block design in three replicate. The result showed that all yam varieties evaluated were suitable for planting in the area. However, Efuru and Ise-osi synchronized perfectly with the pattern of Actual Water Availability and produced good vegetative growth with Leaf Area Index LAI, of 1.08 and 0.91 thereby leading to high tuber yield of 12 t ha G and 11.64 t ha G , respectively. Oniyere had LAI of 0.44 resulting in a 1 1 lower tuber yield of 11.53 t ha G . 1


INTRODUCTION
growth cycle with the period of effective water availability Crop production in the tropics as in other parts of by a n unpredictable distribution, variability and the world is sensitive to environmental factors [1].
seasonality. The selection of cultivar (variety) has been However, according to Verrnier et al., [2], climatic factor noted to be among the factor that contributes to the appeared to be more marked both in terms of its variation realization of a successful cropping [11][12][13]. This study over space and time than the other requirements. I n intend therefore to confirm the agro-climatic potential of particular, climate has both direct and indirect effect on the study area for three white yam varieties in order to crop production. The direct effect is manifested by the determine the appropriate variety that will synchronize the effect of climatic factor on the other components of the crop growth cycle with period of effective water plant environment [3][4][5]. Climate determines the nature of availability. the soils, the crops that can be grown and the type of farming that can be practiced in any region among others MATERIALS AND METHODS [1]. Generally, the major climatic parameters involved in crop production are rainfall, temperature, light and Description of Study Area: This study was conducted photoperiod [6,7]. Although soil and thermal factors are at the Teaching and Research farm of University of not constraints in the study area and the duration of rain Agriculture along Alabata road, Abeokuta (7° 15'N, is within the range of optimum annual rainfall (1000-3°25'E) in Odeda Local Government Area of Ogun State, 1500mm) reported for yam growth [8][9][10]. It is not enough South Western Nigeria (Fig. 1)  respectively. The region is characterized by relatively high Hence, for successful cropping, it is pertinent to identify temperature with mean annual air temperature being about the characteristic of variety and then synchronize the crop 30°C.
particularly because the rainfall in the area is characterized The soil at the experimental site was categorized as a cultivars (Efuru, 'A ; Ise-osi 'A ; and Oniyere 'A ) were well-drained tropical ferruginous soil. The A horizon of used. The choice of selection was due to the fact that the the soil is an Oxic Paleudulf of the Iwo series with 83% cultivars were the most important edible yams widely sand, 5% silt and 12 % clay with a pH of 6 considered grown by farmers in the University's extension villages tolerable by yam cultivation [13].
around the study area. Grass mulch was applied after Experimental Design and Field Measurement: intensity was nil (in otherwords before sunrises, when the The experimental site, comprised of a piece of land Cambell Stokes Sunshine recorder was unable to receive (30 x 60m ) had previously carried beans and groundnut enough radiation to burn the Sunshine recording card). 2 intercrop but had been fallowed for over 3 years (from After sprouting, the yams were staked to about 3m high 2004-2006). The site was cleared manually using cutlass in and the vines were trained regularly. No fertilizer and November 2006, in preparation for the 2007 cropping insecticide were applied and all plots were regularly hand following the popular practice by the farmers in the weeded. Bush rat was controlled by regular clearing of the study area. This period marks the preparatory period for surroundings of the project site. During each of the the cultivation of early yam planting in the study area.
phonological stages, daily observation of air and rainfall Yam mounds were made manually using African hoe (mm) were made at meteorological enclosure adjacent to during the two experimental years. The mounds were of the experimental field. Phonological crop growth height 60cm and spaced 1.5 x 1.5m a walk way of 1m parameter and yield characters were also measured. Data 2 between adjacent row. The mound tillage system was collected were subjected to analysis of variance selected for the study not only because it is the most (ANOVA) using GenStat Release 7.2 statistical software widely use method in the study area, but also because it (Discovery Edition 3) to evaluate the effects of variety on improves the soil aeration and hydrothermal conditions yam growth and yield. The significant difference o f for crops emergence, root development, crop growth and treatment means were determined using least significance yield [14]. Three local white yam, Dioscorea rotundata difference (LSD) 5% level of probability [15].

RESULTS DISCUSSION
The effect of variety selection on the growth and The selection of variety is among the factors that development of yam for 2007 and 2008 experimental years contributes to the realization of a successful cropping is shown in Tables 1 and 2. It was observed that variety [11][12][13]. Hence, the fact that the total rainfall (1177.2 does not significantly influenced emergence rate, vine and 1201.6mm for 2007 and 2008 experimental years, length, stem branches, branch length, number of roots respectively) recorded at the study site fell within the and root length of yam for both trials. However, variety range of optimum annual rainfall (1000-1500mm) significantly (P< 0.05) influenced vine diameter, number of reported for yam growth is not enough criteria for leaves of yam and leaf area index, LAI for 2007 and 2008 suitable crop variety selection in the study area. For experimental year. It was also observed that Efuru has the instance if we were to base our assessment only on the largest number of leaves and LAI followed by Ise-osi, total annual rainfall, the present study area would fall then Oniyere in both experimental years.
within the optimal range for yams cultivars and as such The effect of variety selection on the yield and yield we might assume that no further investigation was components of yam for 2007 and 2008 experimental years meaningful. However, recalling the fact that rainfall in the is shown on table 3 and 4. There was no significant area is characterized by an unpredictable distribution, difference (P> 0.05) in tuber length, tuber weight, number variability and seasonality, then if a high total rainfall of tubers and tuber yield of selected yam varieties planted received is as a result of one or two sporadic in the study area. However, variety selection significantly downpours, widely separated by periods of dry spells, as influenced tuber diameter of yam influenced tuber experienced in the 2007 experimental year, such diameter of yam particularly in 2008 cropping season. rainfall might not contribute meaningfully to crop growth.   Usually such downpours often generate flash REFERENCES floods while the little amount of rainfall happens to evaporation rather than effectively recharging the soil for subsequent use by plant. It follows therefore that adequacy for good plant growth does not depend solely on total rainfall but a combination of rainfall and evaporation.
Furthermore, temperature varied between 21°C and 30.5°C in both the 2007 and the 2008 cropping season. This was similar to the temperature range suggested for a good crop growth and yield in Nigeria [8,9]. Orkwor [8] reported a similar range that resulted in a good crop growth and yield in Umudike, East of Nigeria.
Since soil and thermal factors are not constraints though the duration of rains is appreciably longer and more reliable in Southwest than elsewhere in Nigeria. It might be possible to develop a cropping pattern that would involve planting early maturity variety in the study area. A selection of yam variety with appropriate phenologies that synchronize the crop growth cycle with the period of effective water availability is required. For instance, it was observed from study that though there were no significant difference in most of the yam growth parameters measured like the emergence rate, vine length, number of stem branches, number of stem roots, branch length, tuber length, tuber diameter, tuber weight, number of tuber and the yield for the different yam variety planted during the 2007 and 2008, there were still some significant difference in some major parameter like the number of leaves, vine diameter and the LAI. It was observed that the Efuru and Ise-osi has the higher number of leaves and LAI followed by the Oniyere. This implies that though all selected yam are suitable for planting in the study area. There are still some early maturing and moisture tolerant varieties that could have a larger canopy (LAI) and produce more yield even if the difference.

CONCLUSION AND RECOMMENDATIONS
From this study, it is obvious that knowledge of climatic conditions can allow us to develop a seasonal management strategy for yam production i n Southwest Nigeria. The study revealed that judging from the duration of the period of effective water availability, rainfall during the moist period, extreme lateness of onset of rainfall and water requirement by each of selected yam varieties, the production potential of some variety such as Efuru and Ise-osi can be encouraged in the study area.