Breeding and testing single-cross maize hybrid QT55 in provinces in the North, South Central and Central Highlands of Vietnam

In many places around the world, population growth and climate change have been posing huge challenges to agriculture. There are increasing demands for food products in both quantity and quality. As a result, it is essential to develop new hybrid maize varieties with high yield, quality and resistance. In this study, single-cross maize hybrid QT55 was created from a combination of two maize hybrids (D4 x D54). This hybrid maize was tested in some provinces in the North, South Central and Central Highlands of Vietnam. Study results show that in comparison with control maize cultivars, QT55 was a medium early single-cross maize hybrid (medium growth time) with the yields from 69.44 to 75.38 quintals ha -1 and ranged from 60 to 100.2 quintals ha -1 during intensive farming. Additionally, QT55 demonstrated higher resistance to stalk borer and other diseases such as the banded leaf, sheath blight and stalk rot. It is less likely to fall, cold tolerant and drought tolerant are better. Single-cross maize hybrid QT55 was best planted in Spring and Autumn-Winter crops (in Northern provinces) and Winter-Spring, SummerAutumn crops (in South Central and Central Highlands). Keywords— single-cross maize hybrid QT55, breeding, testing, medium early, high yield, drought tolerance.

INTRODUCTION Maize (Zea mays L.) is one of the important cereal crops, ranking second after wheat based on production (Hallauer and Carena, 2009;Dowswell, 2019). Maize plays a significant role in providing human foods, animal feed, the raw material for the processing industry and biofuels (Shiferaw et al., 2011;Hochman and Zilberman, 2018; Loy and Lundy, 2019). The crop contains a huge amount of cellulose, vitamins, minerals and antioxidants (Ai and Jane, 2016). It does not only provide essential nutrients for human health but also contributes to preventing cancers (Jayaram et al., 2015). In addition, maize is easily cultivated under various climatic conditions, offering great financial support to farmer households (Becerril and Abdulai, 2010;Mathenge et al., 2014). As a result, maize has been widely cultivated in many countries around the world.
In Vietnam, maize has been cultivated in seven agroecological regions. In 2018, the cultivated area of maize in Vietnam was recorded to reach 1039 thousand hectares with the mean yield of 47.2 quintals ha -1 and the output of 4905.9 thousand tons (General Statistics Office of Vietnam, 2018). However, maize cultivation in Vietnam has still failed to meet the domestic consumption demand, leading to annual import of millions of tons of maize for animal feed processing. Statistics show that 10.18 million tons of maize were imported into Vietnam in 2018 (Ministry of Industry and Trade of the Socialist Republic of Vietnam, 2018).
Provinces in the North, South Central and Central Highlands of Vietnam are agriculture-oriented, in which maize is an important crop. The constraints and challenges in maize cultivation faced by farmer households in these regions are mainly the cultivation in riparian zones, narrow areas, medium-qualified and poorly fertile soil, mostly drought soil with water deficiency happening in 70% of the total cultivated area. The majority of currently cultivated maize varieties are imported hybrid maize cultivars (accounting for over 60%) at high prices. Farmer households can hardly take control of the crop seeds. There is a lack of high-qualified medium early hybrid maize cultivars through domestic seeding selection as well as suitable cultivation techniques for each cultivar, leading to the fact that some maize cultivars are suffering from pests and diseases at the serious rate and tending to have degenerated. This is also one of the main reasons why Vietnamese commercial maize production is less competitive than that of other countries in the world. Under the pressure of rapid population growth and urbanization, the cultivation area has been decreasing (Wang, 2019). Therefore, the demand for maize can be met when crop yield improves. Additionally, climate change also adversely affects crop growth and development (Calzadilla et al., 2013). Development of new hybrid maize varieties with high yield, high quality and good resistance is a common and inevitable trend of the world (Dass et al., 2009;Schroeder et al., 2013;Abate et al., 2015). This study, therefore, focuses on breeding a new hybrid maize with medium length of growing time, high and stable yield, high resistance to pests and diseases, less likelihood to fall, good drought stress tolerance, which can be suitably planted in major crops of provinces in the North, South Central and Central Highlands of Vietnam.

II. MATERIALS AND METHODS 2.1 Research materials
Single-cross maize hybrid QT55 was created from the hybrid combination D4 (III115144) x D54 (BOD22). The maternal line D4 (III115144) is originated from tropical region and selected as pour line by self-pollination method. Paternal line D54 (BOD22) is originated from tropical region and selected as pour line by selfpollination method.

Research venue and time
Tests of self-pollination, maintenance and evaluation of combining abilities among maize lines were conducted at the Center for plant evaluation and seed testing and plant products in

Conducting self-pollination, maintenance and evaluating combining abilities among maize lines
Self-pollinated parental lines of QT55 were selected as pure lines by traditional self-fertilization method, together with self-pollination by full-sib, half-sib selection methods.
Evaluation of combining abilities covered general combining ability (GCA) and specific combining ability (SCA) among 8 pure-line maize cultivars in diallel crossing as mentioned in Experimental method 4 by Griffing (1956).

Testing new hybrid maize cultivar in different ecological regions
Breeder testing and basic testing of Single-cross maize hybrid QT55 were conducted in accordance with "National technical regulation on testing for Value of Cultivation and Use of Maize varieties" -QCVN 01-56:2011/BNNPTNT by Ministry of Agriculture and Rural Development (2011).
Production testing of single-cross maize hybrid QT55 in experimental ecological regions was carried out based on the process of local hybrid maize cultivation in those localities. The experiments were arranged in a sequential manner, not repeated with control cultivars included. The testing area was 1000 m 2 /cultivar/location/crop with a plant population density of 57-64 thousand plants ha -1 and fertilizer rate of (10 tons of completely decomposed manure or 2 tons of micro-organic fertilizer + 160 kg N + 90 kg P 2 O 5 + 110 kg K 2 O) ha -1 .

Evaluating drought tolerance
Drought tolerance of maternal lines (D4), paternal lines (D54) and single-cross maize hybrid QT55 were evaluated at the stage of seedlings with 5 -6 leaves in covered net houses by CIMMYT (1985) method.

Statistical analysis
Combining ability (including GCA and SCA) of maize cultivars was analyzed based on the dry grain yield of hybrid combination thanks to IRRISTAT 5.0/ Linetest/ Dialen 2 software. Yield data gained from breeder testing and basic testing was statistically processed with IRRISTAT 5.0 and Excel 3.2. In production testing, maize was harvested in randomized-block design by statistical method, and the mean yield value was calculated with Excel 3.2 software.  Table  1.

Breeder testing on single-cross maize hybrid QT55
In two Spring crops and one Autumn-Winter crop, the yield of QT55 was recorded at the range from 62.4 to 85.1 quintals ha -1 , significantly higher than that of the control cultivars DK9901 and CP.333 at the significance level of 95%. The highest yield in Spring crops was in the range from 69.5 to 85.1 quintals ha -1 . The mean yield was 75.38 quintals ha -1 , surpassing that of the control varieties (DK9901, CP.333) with the difference of 9.66 quintals ha -1 (Table 3).

Agronomic characteristics of single-cross maize hybrid QT55
QT55 is a medium early cultivar, similar to DK9901 line. They took 118-120 days to reach harvest in Spring The cob length was recorded at the range from 17.6 to 18.9 cm, and the mean cob length, which was 1.6 cm longer than that of cultivar DK9901, reached 18.2 cm. QT55 line had from 12 to 18 rows per cob with the mean number of 15 rows per cob, higher than those of control cultivar DK9901 (12-16 rows per cob with the mean number of 13.6 rows per cob). The average 1000-grain weight of QT55 was 306.5 grams and higher than that of the control cultivar DK9901 (270.8 grams). The grain percentage per cob in QT55 was from 54.3% to 81.3% with the mean value of 67.8%, roughly equivalent to that of cultivar DK9901 (67.5%) ( Table 4).   Table 5 shows that QT55 was rarely infected with the stalk borer (rating scale 1-2), corn earworm (rating scale 1-2) and corn leaf aphid (1 point). The same results were found in testing on DK9901. QT55 was seldom infected with banded leaf (rating scale 0-2) as well as sheath blight (9.5%); stalk rot was not detected in QT55 (0 points). The corn stalk was strong and good at anti-falling (8.8% of the number of roots collapsed plants and 1% of the number of stem broken plants); fairly drought tolerant and which was cold-resistant, equivalent to DK9901.

Yield of single-cross maize hybrid QT55
The yield of single-cross maize hybrid after basic testing conducted in the Red River Delta and Northern midland and Mountainous region was presented in Table  6. In 2014 Winter crop, the yield of QT55 in 3 testing locations ranged from 48.55 quintals ha -1 to 84.22 quintals ha -1 . The average number was 63.55 quintals ha -1 , which was higher than the one of DK9901 at 2.77 quintals ha -1 (4.6%). In 2015 Spring crop, the yield of QT55 in 4 testing locations ranged from 58.50 to 82.73 quintals ha -1 , the average number (68.71 quintals ha -1 ) was higher than the figure of DK9901, which reached only 1.56 quintals ha -1 (2.3%). In 2015 Winter crop, the yield of QT55 recorded in 5 testing locations ranged from 50.42 to 67.27 quintals ha -1 with an average of 59.72 quintals ha -1 , which was similar to the ones found in DK9901 (60.23 quintals ha -1 ). After 3 basic testing cases in the Red River Delta and Northern midland and mountainous region, QT55 had average yield at 63.67 quintals ha -1 , which was 1.6% higher than the results recorded in the control variety, DK9901.
The yield of single-cross maize hybrid after basic testing conducted in the North Central of Vietnam was showed in Table 6. In 2014 Winter crop, the yield of QT55 in 2 testing locations ranged from 59.87 to 63.72 quintals ha -1 . The average number was 61.79 quintals ha -1 , which was higher than the one of DK9901 at 0.91 quintals ha -1 (1.5%). In 2015 Spring crop, the yield of QT55 in 2 testing locations ranged from 60.95 to 67.87 quintals ha -1 , the average number (64.91 quintals ha -1 ) was 1.85 quintals ha -1 higher than the figure of DK9901 (2.9% higher). In 2017 Winter crop, the yield of QT55 recorded in 2 testing locations ranged from 56.82 to 58.80 quintals ha -1 with an average of 57.81 quintals ha -1 , which was 2.48 quintals ha -1 higher than the ones found in DK9901 (4.5%). In 2018 Spring crop, the yield of QT55 in 2 testing locations was from 62.83 to 71.29 quintals ha -1 with the average of 67.06 quintals ha -1 , which was similar to the ones of DK6919 (68.13 quintals ha -1 ). After 4 basic testing cases in the North Central of Vietnam, QT55 achieved average yield at 62.89 quintals ha -1 , which was 1.7% higher than the results recorded in the control varieties (~1.04 quintals ha -1 ).  Table 6 depicts the yield of QT55 after basic testing in South Central of Vietnam. In 2014-2015 Winter-Spring crop, the yield of QT55 in 3 testing locations ranged from 68.40 quintals ha -1 to 82.60 quintals ha -1 . The average number was 76.40 quintals ha -1 , which was higher than the one of CP.333 at 2.60 quintals ha -1 (1.5%). In 2015 Summer-Autumn crop, the yield of QT55 in 3 testing locations ranged from 54.4 to 80.30 quintals ha -1 , the average number (65.3 quintals ha -1 ) was lower than the figure of CP.333 at 4.66 quintals ha -1 . In 2017 Winter-Spring crop, the yield of QT55 recorded in 3 testing locations ranged from 60.6 to 84.5 quintals ha -1 with an average of 70.06 quintals ha -1 , which was similar to the ones of CP.333 (70.36 quintals ha -1 ). After 3 basic testing cases in the South Central of Vietnam, QT55 achieved an average yield at 70.58 quintals ha -1 , which was nearly the same as the results recorded in CP.333 (71.38 quintals ha -1 ).
The yield of QT55 after basic testing in Central Highlands is also described in Table 6. In 2015 Summer-Autumn crop, the yield of QT55 in 3 testing locations ranged from 74.97 to 84.01 quintals ha -1 , the average number (79.91 quintals ha -1 ) was 2.14 quintals ha -1 higher than the figure of CP888 (2.8%). In 2015 Autumn-Winter crop, the yield of QT55 reached 80.32 quintals ha -1 , which was similar to the ones of CP888 (80.14 quintals ha -1 ). In 2015-2016 Winter-Spring crop, the yield of QT55 was 82.03 quintals ha -1 . This yield was 7.14 quintals ha -1 higher than the yield of CP888 (9.4%). After 3 basic testing cases in Central Highlands, QT55 achieved an average yield at 80.62 quintals ha -1 , which was higher than the results recorded in CP888. The higher amount was 3.16 quintals ha -1 (3.5%).

Production testing on single-cross maize hybrid QT55
The results of testing on QT55's production in the Red River Delta and Northern midland and mountainous region is shown in Table 7. QT55 had medium length of growing period (which was longer than DK9901 from 2 to 5 days and similar to LVN99). This variety was a healthy plant that was less infected with the stalk borer and corn earworm as well as other diseases such as the banded leaf, sheath blight and stalk rot. The corn stalk was strong and good at anti-falling; fairly drought tolerant and cold resistant. This plant could be grown in various conditions. The yield of QT55 ranged from 60.9 to 72.4 quintals ha -1 with an average of 65.46 quintals ha -1 . This productivity was higher than control varieties including DK9901, LVN99 and LVN10 from 0.8 to 18.9%. Table 7 also describes the production testing of QT55 in North Central of Vietnam. It can be seen that QT55 had medium length of growing period (which was longer than DK9901 from 1 to 4 days). The plant grew well, achieved medium height and was able to create earn corn at medium speed. The ears were long and beautiful with yellow and orange and half tough kennels, which were popular among consumers. QT55 was less infected with the stalk borer and corn earworm as well as other diseases such as the banded leaf, sheath blight and stalk rot. The corn stalk was strong and good at anti-falling; fairly drought tolerant and cold resistant. This variety could be grown in various conditions and was the most suitable for main crop during the year such as Spring crop and Autumn-Winter crop as well as different types of soil in North Central of Vietnam. The average yield of QT55 was 76 quintals ha -1 (ranging from 60.0 to 100.2 quintals ha -1 ), which was higher than the control variety (DK9901) from 10 to 15.4%.
The production testing of QT55 in South Central of Vietnam is presented in Table 7. This variety had medium length of growing period (which was longer than DK9901 from 2 to 7 days). The plant grew well, achieved medium height and was able to create earn corn at medium speed. The ears appeared to belong and beautiful with yellow and orange and half tough kennels. The yield of QT55 ranged from 80 to 85.1 quintals ha -1 (82.55 quintals ha -1 on average), which was higher than the control variety (DK9901) from 10.8 to 18.19%. It was less infected with the stalk borer and corn earworm as well as other diseases such as the banded leaf, sheath blight and stalk rot. The corn stalk was strong and good at anti-falling; fairly drought tolerant and cold resistant. This variety could be grown in various conditions and types of soil. It is suitable for intensive farming.  Table 7 depicts the production testing of QT55 in the Central Highlands of Vietnam. It had the same medium length of the growing period as the control varieties (CP888 and NK67). QT55 was less infected with the stalk borer and corn earworm as well as other diseases such as the banded leaf, sheath blight and stalk rot. The corn stalk was strong and good at anti-falling; fairly drought tolerant and cold resistant. The yield was recorded at 78.35 quintals ha -1 on average (ranging from 73.84 to 82.85 quintals ha -1 ). This quantity was 6.1% and 4% higher than that of CP888 and NK67 respectively.
In short, production testing of QT55 had been conducted during 13 crops (from 2013-2014 Winter-Spring crop to 2018 Spring crop) in 18 different locations of 5 ecological regions including Red River Delta, Northern midland and mountainous region, North Central, South Central and Central Highlands of Vietnam. The yield of dried kennel ranged from 60.0 to 100.2 quintals ha -1 , 75.59 quintals ha -1 on average, which was 10.2% higher than that of control varieties.

Maize growth criteria during drought experiments
Underwater stress during the period from the 3-leaf to 6-leaf stage, QT55 was 39 cm in height, which was 0.7 cm higher than LCH9a drought-tolerant variety; its root length was 49.8 cm, which was 11.5 cm longer than LCH9; the number of roots found in QT55 is 3 roots less than that of LCH9 (Table 8).
From 3-leaf to 6-leaf stage, D54 (paternal line) was 42.9 cm in height, which was 12.8 cm higher than D1; its root length was 51.9 cm in length, which was 13.2 cm longer than D1; the number of roots was also higher than that of D1 at 11.2 roots. Meanwhile, D4 (maternal line )' height was 31.7 cm, which was 1.6 cm higher than D1; its root length was 47.1 cm (8.4 cm higher than D1) and the number of roots was 5.2 (2.9 roots less than D1) ( Table  8). At 5-6 leaf stage, the weight of dry matter of paternal line, D54, and the maternal line, D4, was 0.466 grams/plant and 0.233 grams/plant respectively, which were higher than D1 at 0.167 grams/plant with a significance level of 95% (Table 10).

IV. CONCLUSIONS
The final result of the study was a single cross maize hybrid, QT55 with medium length of growing period. This variety was 203.8 cm high on average while the average corn insertion height was 104.3 cm. It was a healthy plant which able to grow well in a good shape, the leaves covered all the cob; the grain percentage per cob was 67.8%; 1000-Grain Weight was 306.5 grams on average. The yields (basic testing) in the Red River Delta and Northern midland and mountainous region, North Central, South Central and Central Highlands of Vietnam were 63.67 quintals ha -1 , 62.89 quintals ha -1 , 70.58 quintals ha -1 and 80.62 quintals ha -1 respectively. The average yields (production testing) in the Red River Delta and Northern midland and mountainous region reached 65.46 quintals ha -1 , which was 9% higher than the control variety; the figures recorded in North Central, South Central and Central Highlands of Vietnam were 76 quintals ha -1 , 82.55 quintals ha -1 and 78.35 quintals ha -1 , which were 12.3%, 14.49% and 5.09% higher than the control variety respectively. QT55 was fairly good at drought tolerance, less infected with the stalk borer as well as other diseases such as the banded leaf, sheath blight and stalk rot. The variety was also good at antifalling and suitable for different crops in provinces in the North, South Central as well as the Central Highlands of Vietnam.