Evaluation of Grain Yield of Heat Stress Resilient Maize Hybrids in Nepal

Midhills, foot hills and river basin areas of are generally under spring maize cultivation. These areas along with the areas from Terai and Inner Terai where spring and early summer maize are grown are the most affected from heat stress. Identification and selection of suitable varieties and traits for high temperature tolerance is vital to produce heat resilient genotypes.With a view to identify high yielding heat stress resilient maize hybrids, genotypes received from International Maize and Wheat Improvement Centre (CIMMYT) were evaluated in alpha lattice design with two to three replications at Rampur, Nepalgunj and Surkhet in 2013/014, 2014/015 and 2015/016. Total 57 trials consisting of 7764 maize hyrbids were evaluated under heat stress conditions. At Nepalgunj and Surkhet, trials were planted in March/April. Out of the tested genotypes, 24 hybrids were found promising based on grain yield, plant and ear height. These 24 hybrids along with four National Maize Research Program (NMRP) developed and two multi-national companies’ hybrids as checks were tested in multilocation trials at Dumarwana, Nijgadh, Nawalparasi and Rampur in 2014/015. Out of these 24 hybrids, eight were selected and tested during 2015/016 in Dumarwana, Nijgadh, Nawalparasi, Anandapur and Rampur. Based on results combined over years and locations CAH-151 (8629 kg ha) and CAH-153 (8955 kg ha) were registered for general cultivation as Rampur Hybrid-8 and Rampur Hybrid10, respectively. Other promising hybrids were CAH-1511 (8800 kg ha) followed by CAH-1515 (8678 kg ha), RML95/RML-96 (8486 kg ha), CAH-1513 (8258 kg ha) and RML-86/RML-96 (7544 kg ha), respectively. Stability analysis revealed that CAH-151, CAH-153, CAH-1515, CAH-1511 and RML-95/RML-96 are stable hybrids having good performance.


Introduction
Maize is the second most important crop in terms of area (891583 ha) and production (2231517 t) with productivity of 2503 kg ha -1 (MoAD, 2017).Out of the total maize area; 8.76, 72.29 and 18.95% belong to mountain, hills and Terai with productivity of 2075, 2461 and 2725 kg ha -1 , respectively.The average productivity of improved maize is 2547 kg ha -1 compared to local (1754 kg ha -1 ).Seed replacement rate (SRR) of maize is quite low (9.5%).As envisioned in National Seed Vision 2013-2025(SQCC, 2013) by 2025, projected SRR for maize is about 31.57% with average productivity of 3.33 t ha -1 .Heat stress reduces grain yield due to a decline in harvest index (Craufurd et al., 2002;Ferris et al., 1998).
The reasons for low maize yield in Nepal are high temperature, drought, stalk rot infestation, maize borer and shoot fly infestation, poor crop management, high input rates and use of low quality, substandard seed.Heat and drought stress have emerged as a common problem worldwide which can reduce maize crop productivity (Ali et al.,2015 ).Heat stress in the flowering and grain filling periods due to elevated temperatures drastically affect crop productivity.A record drop in maize production was reported in many maize-growing areas of the world (Van der Velde et al., 2010).It is predicted that maize yield might be reduced up 70 % due to increasing temperatures (Khodarahmpour et al.,2011) Rise in temperature (above 38 0 C) reduces the pollen viability and silk receptivity resulting in poor seeds set and reduced yield.Spring maize planted in maize-rice system is estimated to be about 15.5% of the total maize area are mostly affected by heat stress, and yield losses may reach up to 75% (Koirala et al., 2013) in Nepal.Mainly anthesis and silking of these maize coincide with high temperatures which cause leaf firing and tassel blast resulting poor pollination.
Among the different measures to increase maize production under adverse climatic conditions, development of high temperature tolerant maize varieties is good option to increase and maize production under heat stress condition (Shrestha et al., 2014).Therefore, this study was carried out to evaluate and identify heat stress resilient maize hybrids.

Genetic Materials and Experimental Sites
Heat stress resilient maize hybrids received from Heat Tolerant Maize for Asia (HTMA) -CIMMYT were experimented at National Maize Research Program (NMRP), Rampur;  2).Likewise, in 2015/016 at NMRP Rampur and RARS Nepalgunj nine and seven experiments with 812 and 820 genotypes, respectively were evaluated (Table 3).Thus, total 57 experiments consisting of 7764 genotypes were tested under this project in Nepal.Rampur was considered as normal (favorable), and Nepalgunj and Surkhet as heat stress environments.At RARS Nepalgunj and Surkhet planting was managed in such a way that maximum temperature coincided with anthesis and silking stages of the crop based on previous years meteorological data.Out of the tested 7764 genotypes from 2013/014 to 2015/016, 24 hybrids were found promising based on grain yield, plant and ear heights.These 24 hybrids along with four NMRP developed and two multi-national companies' hybrids as checks were experimented in multilocation trials at Dumarwana, Nijgadh, Nawalparasi (Keureni) and Rampur in 2014/015 (Table 4).Further more, among these 24 hybrids, eight were selected and tested in multilocation trials again in 2015/016 at Dumarwana, Nijgadh, Nawalparasi, (Gaindakot-5, Sitalnagar), and Rampur with one additional site Anandapur in Chitwan (Table 5).
NMRP Rampur has humid and subtropical climate with cool winter (2-3 o C) and hot summer (43 o C).The annual rainfall is over 1500 mm with a distinct monsoon period (>75% of annual rainfall) from mid-June to mid-September.The soil texture of NMRP Rampur is sandy loam having 5.43 pH value with 3.95% organic matter and 0.15% nitrogen content.Similarly, available phosphorus, potassiun, boron and zinc in the soil is 33.76, 145.29, 0.17 and 1.58 (ppm), respectively.At RARS Nepalgunj, average annual rainfall is 1000-1500 mm.However, delayed onset and early termination of monsoon is a regular feature, causing occasional failure of crops.The maximum and minimum temperatures are 46 o C and 5.4 o C respectively, with relative humidity ranging from 27 to 94%.Humidity remains low in most parts of the year.Soils of the station have sandy to silty loam, poor in organic carbon and available N but medium in available P2O5 and K2O; pH varies from 7.2-7.5.Bara district falls under subtropical region of Nepal.The average maximum temperature ranges from 22.7 o C to 34.52 o C and minimum temperature ranges from 8.54 o C to 25.9 o C with an average annual rainfall of 1550 mm.The rainfall distribution is not uniform across the year, 84% of the total rainfall is received from June to September.Likewise, ARS Surkhet has subtropical climate with average annual rainfall of 747 mm, of which 80% occurs between June and September.Temperature ranges from 7.9 0 C to 36.9 0 C in the month of January and June, respectively.The soil of the station is sandy loam in texture and acidic in reaction with pH value from 5.3-5.8.

Experimental Design and Cultural Practices
Experiments were conducted in alpha lattice design with 2-3 replications depending upon trials in 2013/014 and 2014/015.Plot size was 4m long single row and replicated twice except in HTMAHT-86 and CCAFS-HT-14 which were replicated thrice.Row-to-row and plant-to-plant spacing was maintained at 75 cm and 25 cm, respectively.Two seeds per hill were planted and thinned to single plant during first weeding.Fertilizer was applied @120:60:40 N:P:K kg ha -1 .In multilocation trials during 2014/015 and 2015/016, plot size was 10 rows of five metre long.One location was considered as a replication.Row-to-row and plant-to-plant distance was maintained at 60 cm and 25 cm, respectively.Two seeds per hill were planted and thinned to single plant during first weeding.Fertilizer was applied @180:60:40 N:P:K kg ha -1 .In all the years of experimentation, half of the N and full dose of P2O5 and K2O were applied as basal dose.The remaining half dose of N was applied in two splits at knee-high and pretasseling/silking stages.Other agronomic practices were carried out as per recommended.

Data Recording and Measurements
Grain yield, plant and ear heights were recorded.Grain yield was estimated using formula adopted by Shrestha et al. (2015) by adjusting the grain moisture at 15% and converted to the grain yield kg per hectare.
Plant height is the distance from the base of the plant to the top of the plant from where tassel starts branching.It was measured as an average of the randomly taken five plants.This was recorded after 2-3 weeks of flowering until just prior to harvesting.Ear height was measured at the same time from those plants which were used for plant height.It is the distance from base of the plant to the uppermost ear bearing node.

Statistical Analysis
The statistical analysis was done using computer software MSTATC version 1.2 (Freed, 1990) applying 5% level of significance for various trials from 2013/014 to 2015/016.Stability analysis was carried out using GEA_R version 2.0.
Generally, different genotypes behave differently because of differences in gene responses or in their potential performance in different environments (Brandiej, 1994).
A lot of information on agronomic and physiological traits for high temperature tolerance in maize is available (Steven et al., 2002;Sinsawat et al., 2004).The final plant height reflects the growth behavior of a crop, besides genetic characteristics, availability of essential nutrients, space, water and environmental condition under which it is grown.Increase in temperature affects the plant growth which ultimately influences the plant height.In 2014/015, Plant height of the tested genotypes were found highly significant and ranged from 125 to 191cm.The lowest plant height was recorded in Z478-9 (125 cm) followed by Z478-10 (139 cm), Z478-8 (145 cm), Z480-2 (151 cm) and Z480-1 (153 cm) (Table 15).In 2015/016, plant height among the tested hybrids differed significantly.The lowest plant height was recorded in CAH-1513 (158 cm) followed by RML-86/RML-96 (165 cm), CAH-1515 (170 cm), RML-95/RML-96 (171 cm) and CAH-153 (173 cm).It might be due to genetic difference among different maize hybrids and efficient utilization of high temperature to increase the plant growth (Bakker and Van Uffelen, 1998).Hybrid CAH-158 was the tallest (194 cm) among the tested hybrids (Table 16).The ranking of varieties based on plant height was more or less similar in both the years and location.Genotypic variation was evident for plant height when combined over years and locations.Similarly, highly significant differences were observed for locations.Genotype × Environment interactions were found non-significant.Plant height of the genotypes ranged from 157 cm (CAH-1513) to 194 cm (CAH-158).Results have been summarized in Table 17.

Table 3 :
Experimental details of heat stress resilient maize genotypes evaluated at NMRP Rampur and RARS Nepalgunj during 2015/016

Table 4 :
On-farm multilocation testing of selected hybrids at various locations during 2014/015

Table 5 :
On-farm multilocation testing of selected hybrids at various locations during 2015/016.

Table 6 :
CIMMYT code and pedigree of genotypes tested in multilocation trials during 2014/2015.

Table 6 :
CIMMYT code and pedigree of genotypes tested in multilocation trials during 2014/2015.

Table 7 :
Geographical description of experimental locations.

Table 8 :
Weather data of NMRP Rampur during crop growing season 2013/014

Table 9 :
Weather data of NMRP Rampur during crop growing season 2014/015 Month

Table 10 :
Weather data of NMRP Rampur during crop growing season 2015/016