Correlation analysis for grain yield and other agronomic parameters for 90 single crosses hybrid maize evaluated in three agrological zones in Ghana

A trial was conducted to determine the correlation between grain yield and agronomic parameters of 90 F1 early maturing maize hybrids in 2012 in Fumesua, Ejura and Kpeve; representing the Forest, ForestSavannah Transition and CoastalSavannah Transition zones of Ghana, respectively. The objective of the work was to determine the correlation between grain yield and other agronomic parameters of maize across three locations. Randomized Complete Block Design (RCBD) with two replicates was used for each location. Results from the correlation analysis revealed that grain yield was significantly and positively correlated with plant height (r = 0.633), cob length (r =0.610) ear height (r =0.410), and cob diameter (r = 0.401). However, there were nonsignificant correlation between grain yield and days to silking. Nevertheless, among agronomic traits, ear height, plant height seed length, seed diameter, cob length and cob diameter were positively and significantly correlated, indicating that increase in any one of these traits could lead to increase in the other. It was recommended that hybrids that showed the highest correlation with grain can be selected to improve grain yield.


INTRODUCTION
Maize (Zea mays L.) is the most widely grown cereal in the world, and it is the third most important cereal crop after wheat and rice which serves as a primary staple food in most developing countries (Khalil et al., 2011;Badu-Apraku et al., 2010;Obeng-Bio, 2010).The maize plant has wider adaptability hence can be cultivated in different growing conditions from latitude 58°N to 40°S, below sea level and at altitude higher than 3000 m, and in areas with rain-fall of about 250 to 5000 mm per annum and with growing season ranging from three months to about 13 months (Golam et al., 2011).
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License of cultivation, processing, storage and transportation has increased its potential for combating food security challenges posed by population increase and changes in climatic conditions due to global warming in West and Central Africa (Badu-Apraku et al., 2010).
Maize is grown in approximately 25 million hectares in Sub-Saharan Africa, largely by subsistence farmers that produced 38 million tons in 2005 to 2008, primarily for food (Smale et al., 2011).From 2005 to 2008, maize represented an average of 27% of cereal area, 34% of cereal production and 8% of the value of all primary crop production (FAO STAT, 2014;Smale et al., 2011).
In Ghana, maize is the highest ranking cereal in terms of production and consumption followed by rice (Twumasi-Afriyie et al., 1992).The domestic demand is growing because it serves as a major source of daily calories and dietary protein for most people who are under privileged, since poverty makes it difficult for such people to afford meat (MiDA, 2006).
According to a MOFA (2006) annual report, maize accounted for 50 to 60% of total production area of cereals with average yield approximately at 1.6 metric tons per hectare, but yields as high as 4.5 to 5.0 metric tons per hectare can be realized by farmers using improved seeds and good management practices.

Research location
The experiment was conducted in three agro-ecological zones of Ghana.The locations where the experiment was conducted were different in mean seasonal rainfall (Table 1).Fumesua lies in the Forest ecology zone of Ghana.Ejura lies in the Forest -Savannah Transition ecology and Kpeve lies in the Coastal-Savanna Transition.The three experimental sites experience a bimodal rainfall pattern.The major season stretches from April through July, and the minor season from August to November (Table 2).

Experimental design
The ninety F1s were constituted into a hybrid trial and planted in a random complete block design at each of the sites.Prior to planting, the site was thoroughly prepared with plough and harrow using tractor.Each entry was planted in a one row of 5 m, spacing between hills of 0.45 m and spacing between rows of 0.75 m with two replications at each of the three evaluation sites.Each plot in the trial contained 11 hills, and each row contained 22 plants to obtain a target plant density of approximately 60,000 plants ha-1 .

Crop husbandry
Pre-emergence and post emergence chemical weed control was done with an application of Gramoxone and Atrazine respectively.Hand weeding was also done when necessary to control weeds during the growing period.NPK 15-15-15 fertilizer was applied at the rate of 30 kg N ha -1 and 60 kg P2O5 ha -1 as basal fertilizer at two weeks after planting and top-dressed with additional N at 60 kg N ha-1 at 4 weeks after planting.The trials were conducted under rainfed condition, and other management practices were done according to the recommendations of the specific areas.

Data collection
Data were collected on the following parameters prior to harvesting:

Days to 50% tasseling
This was calculated as the number of days from the date of sowing to the day at which 50% of plants in a plot showed full tassel emergence.

Days to 50% silking
The number of days from the date of sowing to the day on which 50% of the plants in a plot showed complete silk emergence.

Plant height
The height of five randomly selected plants in a plot were measured in centimeter with a graduated measuring stick from the ground level to the node bearing the flag leaf and averaged.

Ear height
The ear heights of the five previously selected plants in each plot were measured in centimeters from ground level to the node bearing the uppermost ear and averaged.

Cob length
The length of the cob was measured in centimeters using Vernier caliper (from the base of the ear to the tip).Five cobs were chosen at random from each plot and averaged.

Cob width
The widths of five randomly selected cobs were measured in centimeters as the thickness of the ear using Vernier caliper.Ten cobs were chosen at random from each plot and averaged.

Seed length
The length of ten randomly selected seeds were measured in centimeters using Vernier caliper and averaged.

Seed diameter
The widths of ten randomly selected seeds were measured in centimeters as the thickness of the seed using Vernier caliper and averaged.

Data analysis
The Analysis of Variance (ANOVA) according to Steel and Torrie (1980) for grain yield and agronomic parameters was conducted using Statistical Analysis System version 9.2 (SAS, 2003).Least significance difference test (p≤0.05)was used to determine the level of significance among measured parameters.Pearson coefficients of correlation were calculated using the hybrids' least square means for all parameters to determine associations among these parameters.
Correlation coefficients ranged in values between -1 and +1; a perfect negative relationship and a perfect positive relationship respectively
The result for days to silking showed that there were highly significant differences among genotypes (P<0.01).Days to silking ranged from 38 to 54 days.The mean days to silking (Table 4) was at 50 days.TZEI-45 × TZEI-47 was the latest to reach mid-silk, while TZEI-9 × TZEI-12 was the earliest to reach mid-silk.
From the result (Table 3), differences in days to tasseling among genotypes were highly significant (p<0.01).Mean days to tasseling (Table 4) was 47.6 days.Days to mid-tasseling ranged from 43 to 51.2 days.TZEI-12 × TZEI-13 recorded the highest number of days to tasseling while TZEI-9 × TZEI-12 recorded the lowest days to tasseling.The mean square (Table 3) for seed diameter showed highly significant differences among genotypes (p<0.01).

Identification of superior hybrids
The primary trait, grain yield, is a quantitatively inherited trait with low heritability.Several studies have indicated that highly significant phenotypic correlations between yield and many secondary traits can be found (Nzuve et al., 2014).The use of secondary traits in breeding can increase breeding progress as compared to selection for yield alone (Edmeades et al., 1997).
A superior maize hybrid must be high yielding and also must possess desirable agronomic characters.The correlation studies revealed that plant height was strongly associated with grain yield, ear height, days to tasseling, cob length and diameter, seed length and seed diameter.This indicates that any one of these traits could be used to select for the other.
The significant differences recorded for the different traits among the genotypes studied (Table 3) implied that the maize genotypes included in this study had diverse genetic backgrounds (Vashistha et al., 2013;Reddy et al., 2012).
Thus, the genetic variability recorded in this study could be exploited by plant breeders to develop hybrids adapted to the diverse environments in sub Saharan Africa to improve food security status (Feuillet et al., 2012).The significant genotype by environment interaction showed a wide variability with regard to the tested genotypes and the environments involved in this study (Alake et al., 2008).
Results on correlation among parameters are presented in Table 5.The correlation studies among traits showed that grain yield was positively correlated to days to tasseling, ear height, plant height, cob length, cob   diameter, seed length and seed diameter with the highest effect on plant height (r = 0.633) and cob length (r = 0.610) and ear height (r = 0.410).The associations were highly significant (p < 0.001).This indicates that improvement on any of these characters could help improve grain yield.Similar results were reported by Bocanski et al. (2009) and Malik et al. (2005).They observed high and positive correlation between grain yield and plant height (r = 0.953), ear height (r = 0.867), and cob length (r = 0.959).Meanwhile, days to silking has no significant correlation with grain yield, plant height, seed length, seed diameter, cob length and cob diameter.This result is in agreement with Golam et al. (2011), who reported that grain yield and plant height did not correlate with days to silking.However, among agronomic traits, ear height, plant height, seed length, seed diameter, cob length and cob diameter were positively and significantly correlated, indicating that increase in any one of these traits could lead to increase in the other.
Ear height significantly correlated with plant height seed length, seed diameter, cob length, and cob diameter.The strong correlation between ear height and plant height with grain yield suggested that tall plants with high ear placement gave better yields compared to the shorter plants with lower ear placement.This could be attributed to the high dry matter accumulation function carried out by the high number of leaves possessed in the case of tall plants (Al-Tabbal et al., 2012).
There were negative correlations between ear height and days to silking and days to tasseling (r = -0.369and r = -0.332).This result is in agreement with that of Malik et al. (2005).The negative correlation indicates that increase in days to silking and tasseling could indirectly reduce yield through stalk and root lodging (Malik et al., 2005).Plant height had low correlation with days to silking, but highly correlated with ear height (r = 0.668), days to tasseling (r = 0.10692), seed diameter (r = 0.2204), cob length (r = 0.4568), cob diameter (r = 0.3989) and seed length (r = 0.3396).
This result indicates that increase in plant height could lead to increase in these characters which could result in yield increase, since plant height has been observed to be controlled by the expression of many genes and the interactions between these genes (Bello et al., 2012).Therefore, traits that positively contribute the highest correlation with grain yield such as plant height, cob length, and ear height and cob diameter can be chosen as superior characteristics to help improve maize grain yield (Table 5).

Conclusion
The correlation studies among traits showed that grain yield was highly correlated with plant height, ear height, days to tasseling, cob length, cob diameter, seed length, and seed diameter with plant height contributing the highest effect (r = 0.633) followed by cob length (r = 0.610) and cob diameter (r = 0.402).This genetic diversity and the strong genetic association between grain yield and the agronomic traits would help in indirect selection thereby aiding breeders in the development of better hybrids for resource poor farmers.

Table 1 .
Description of the test environment used in the trial.

Table 2 .
List of 90 single cross hybrid maize evaluated at three locations in southern during 2012 growing season.

Table 3 .
Mean squares values of combined ANOVA for agronomic traits of early maturing maize single cross hybrids evaluated during 2012 growing season.

Table 4 .
Combined mean grain yield (kgha -1 ) and agronomic performance of maize hybrids evaluated across three locations in Ghana during 2012 growing season.

Table 5 .
Correlations among nine parameters of maize single cross hybrids evaluated across three locations in Ghana during 2012 growing season.