Introduction

Soybean [Glycine max (L.) Merr.] is a major worldwide oilseed crop and an important source of protein and oil for food and feed (Wilson, 2012). Soybean seed contains about 36-44% protein and 17-23% oil. The two products derived from the soybean seed after processing are oil and high-protein meal. The soybean oil is mainly used as human food or to produce biodiesel. The meal is used primarily as a protein source for poultry, beef, dairy, and fish. Soybean meal can also be used to make protein concentrate, texturized protein, and protein isolates that are used in food products for human consumption. Global soybean production continues to expand as demand for soybeans and soybean products increase (Lee and Ahn, 2017). The demand comes from the increasing use of its oil for human consumption and for biodiesel, and an increasing demand for high-protein meal for animal feed in both developed and developing countries. The US is the largest producer of soybean followed by Brazil, Argentina, and China (USDA/FAS, 2017).

Soybean is a new oilseed crop in Sudan that has lately gained significant attention due to its wide adaptability and multi-end uses in human food and livestock feed (Ibrahim, 2016). Domestic demand for soybean has changed dramatically in the past 10 years. These changes have been driven by the increasing demand for soybean meal, which is a major ingredient in poultry feeds. This rapid growth in the demand has promoted a renewed interest in introducing soybean crop into irrigated and rain-fed farming systems. Development of adapted and improved varieties suitable for wide range of agro-ecological zones is a key factor for successful commercial soybean production. Over the past 10 year, soybean breeding program at Agricultural Research Corporation (ARC), Sudan has been highly successful in adapting soybean to irrigated and rain-fed cropping environments in Sudan (Ibrahim, 2016). In 2012, two non-GMO soybean varieties, Sudan 1 and Sudan 2 were released for commercial production in irrigated and rainfed farming (Ibrahim et al, 2012). However, the narrow germplasm base on which ARC soybean breeding program rests may complicate the ability of breeders to deliver continued yield advances (Ibrahim, 2016). This narrow base has led to a current consensus that an expanded ARC soybean germplasm pool through germplasm introduction and introgression of potentially useful genes from exotic germplasm is needed for applied breeding efforts (Ibrahim, 2016). Soybean germplasm with specific adaptation traits such as tolerance to drought and extreme temperatures and less sensitive to short day-length conditions was introduced from the International Institute of Tropical Agricultural (IITA), Nigeria; the USDA Soybean Germplasm Collection, USA; the World Vegetable Center (AVRDC), Taiwan, China; and the Rural Development Administration (RDA), Republic of Korea (Ibrahim, 2011). Broadening the genetic base of the released soybean varieties, Sudan 1 and Sudan 2 through introgression and identification of potentially useful genes from exotic soybean germplasm is a major part of the ongoing ARC soybean breeding program to mitigate the potential problems associated with this narrow breeding pool (Ibrahim, 2016). Artificial crosses were made and Recombinant Inbred Line (RILs) populations were developed for further genotypic and phenotypic evaluation, with the aim of identifying exotic favorable genes that can be associated with specific traits to be used in marker-aided breeding to improve drought adaptation, yield potential, and grain quality in soybean. The objectives of this study were to (i) evaluate yield potential and agronomic performance of fourteen early maturing soybean germplasm under irrigated farming in Sudan, (ii) identify and select early maturing germplasms that are potentially valuable to the ARC soybean germplasm breeding pool.

Materials and Methods

Fourteen varietal soybean introductions with different geographic origins and maturity groups (MG) II, III, IV, V, VI, VII, VIII, and IX were used in this study (Table 1). Field trial was conducted for two consecutive seasons (2013/14 and 2014/15) under irrigated cropping environment at Gezira Research Station, Wad Medani, Sudan (14° 22 to 14° 25 N, 33° 29 to 33° 30 E). The trial was arranged at a randomized complete block design with three replicates. Four-ridge plots, 6 m in long, 80 cm apart and 4 cm plant spacing were used. Sowing dates were June 23, 2013, and June 21, 2014. All the cultural practices were carried out manually. Data were collected on days to 50% flowering, days to maturity, plant height (cm), height to first pod (cm), grain yield (kg/ha), fodder yield (kg/ha), number of pods/plant, and 100-seed weight (g). Grain yield and fodder yield (kg/ha) were recorded from the net harvested plot in the 4-m length of the central two ridges excluding one meter at both ends of each ridges. Statistical analyses were conducted with SAS version 9.1 (SAS Institute, 2003).

Results and Discussion

Fourteen soybean varieties introduced from temperate and tropical regions and belong to maturity groups II, III, IV, V, VI, VII, VIII, and IX were used in this study. Individual analysis of variance revealed highly significant (P ≤ 0.0001) differences among the 14 varieties in eight agronomic traits, in seasons 2013/14 and 2014/15 and over the two seasons (Tables 2, 3, and 5). The highly significant difference among varieties indicates the presence of genetic variability among the varieties for the studied agronomic traits. Table 4 depicts the results of the combined analysis of variance for all studied traits. The combined analysis of variance showed highly significant differences among varieties and variety x year interaction for all the characters, except fodder yield (kg/ha), days to 50% flowering, number of pods/plant, and 100-seed weight (g) which showed a non-significant variety x year interaction. The absence of variety × year interaction for fodder yield, 50% flowering, number of pods/plant and 100-seed weight (g) implied that varieties did not show crossover interactions in the different years (Fox et al., 1997). The significant year effect for all traits except days to 50% flowering and 100-seed weight (g) indicated the sensitivity of varieties to environmental factors that prevailed in the different years (Tefera et al, 2009). In 2013/14 season, grain yield (kg/ha) ranged from 338 to 943 kg/ha. The high grain yield was given by Vietnam coll. 1 followed by N04-9859 and Taekwangkong (Table 2). In 2014/15 season, grain yield (kg/ ha) ranged from 497 to 1000 kg/ha (Table 3). The high grain yield was given by AGS 327 followed by Vietnam coll. 1 and N02-8951. Fodder yield (kg/ha) ranged from 737 to 1455 kg/ha and from 828 to 1438 kg/ha in 2013/14 and 2014/15 seasons, respectively. Vietnam coll. 1 had the high fodder yield in both seasons. Over the two seasons, the high grain and fodder yield were given by Vietnam coll. 1 followed N04-9859, AGS 327, and Taekwangkong (Table 5).

Days to 50% flowering ranged from 25 to 29 days and from 25 to 31 days in 2013/14 and 2014/15 seasons, respectively (Tables 2 and 3). In both seasons, Saeolkong was earliest maturing variety (70 and 73 days). The latest maturing variety (87 days) was N05-7229 in 2013/14 season while N05-7229 and N02-8951varieties were the latest maturing varieties (86 days) in 2014/15 season.

Plant height ranged from 17.7 to 47.5 cm and from 19.9 to 48.5 cm in 2013/14 and 2014/15 seasons, respectively (Tables 2 and 3). The tallest plant height was given by Vietnam coll. 1 (47.5 cm in 2013/14 and 48.5 cm in 2014/ 15) followed by AGS 327 in both seasons (39.5 cm in 2013/14 and 48.5 cm in 2014/15). Vietnam coll. 1 and AGS 327 had the tallest plant height across the two seasons, 2013/14 2014/15 (Table 5). Height to first pod also showed a trend similar to that of plant height. The highest to first pod was given by Vietnam coll. 1 followed by AGS 327 in both seasons. This result indicated that Vietnam coll. 1 and AGS 327 are more adapted to irrigated environment in Sudan comparing to other temperate varietal introductions. It is expected that Vietnam coll. 1 and AGS 327 are more adaptable to growing environments of Sudan than temperate germplasm since they are tropical varieties and belong to maturity group VIII and IX.

Number of pods/plant ranged from 14 to 22 and from 15 to 30 pods/plant in 2013/14 and 2014/15 seasons, respectively (Tables 2 and 3). Pungsannamulkong produced the lowest 100-seed weight in both seasons (13.3 g in 2013/14 and 13.3 g in 2014/15), whereas Daewonkong gave the largest seed size in both seasons (21.4g in 2013/14 and 21.7 g in 2014/15). Pungsannamulkong is a Korean variety has small and good seed quality for use in sprouting soybean (Ghani et al, 2016). Daewonkong is a large seed size variety released with high protein content released in Korea for producing tofu (Ha et al, 2009). Pungsannamulkong and Daewonkong offer valuable resource genetics that both diversify the ARC soybean germplasm pool and provide new nutritional quality traits. As such, these varieties can be useful as parental stock for broadening the genetic base of the released soybean varieties in Sudan.

The low grain yield of almost of the 14 varieties was attributed to sensitivity of the varieties to photo-periods and high temperature and resulted in poor vegetative growth, shorter plant height and height to first pod, thus indicating their poor adaptation to irrigated environment in Sudan. In commercial soybean production, adequate plant height and height to first pod are important for mechanical combine harvesting (Kang et al, 2017 and Saryoko et al, 2017). Adjusting planting density of the promising varietal introductions is also an important tool to optimize crop growth and the time required for canopy closure, and to achieve maximum biomass and grain yield (Liu et al, 2008).

Genetic variability is a key resource for varietal improvement. USDA Southern soybean germplasm has played an important role in adapting and expanding soybean to tropical regions (Camacho, 1981 and Kueneman et al, 1984). In this study, N05-7229, N05-7396, N05-7260, N04-9859 and N02-8951 are USDA-ARS Southern breeding lines derived from exotic germplasm and characterized as having superior agronomic characteristics and slow canopy wilting under drought (Devi et al, 2014). In environments where water is limited, genetic improvement of a crop for drought tolerance is an economically attractive option (Blum, 2002). In this study, the unique pedigree and good agronomic performance of the 14 varietal introduction germplasm will open up new opportunities for ARC soybean breeding in Sudan to utilize these germplasm that may contain valuable alleles for crop improvement. Further studies are needed to identify the varietal introductions with superior agronomic characteristics and yield potential under drought conditions.

Conclusions

High phenotypic variability was detected among the fourteen varieties for eight agronomic traits under irrigated environment in Sudan. The agronomic performance of Vietnam coll. 1 and AGS 327 provides a clear example of the adaptability of tropical soybean germplasm to irrigated environments in Sudan comparing to other temperate germplams. The relative high yield of Vietnam coll. 1 and AGS 327 can be maximized by increasing plant populations/ unit area. The unique soybean germplasm from the southern US and South Korea offer valuable genetics resource that can diversify the ARC soybean germplasm breeding pool in drought-related and nutritional quality traits.

적 요

미국, 한국, 베트남, 대만, 크로아티아에서 도입된 14 종 의 품종 및 유전자원에 대한 수단의 와드 메다니 (Wad Medani), 게지라 시험장(Gezira Research Station)에서 2년 연 속 (2013/14 및 2014/15) 농업적 형질을 평가하였다 . 시험구 는 길이가 6m, 이랑 간격이 80cm, 식물체 간격이 4cm인 4개 의 이랑으로 임의배치 3 반복으로 파종되었다 . 개화기 , 성 숙기 , 초장 , 착협고 ( 첫번째 꼬투리의 지상 높이 ), 종실 수 확량, 사료 수확량, 개체당 협수, 종실의 100립중 등을 2개 년간 조사하였다 . 조사결과 8 가지 농업 형질에 대해 14 가 지 품종간에 유의미한 차이 (P ≤ 0.0001) 가 발견되었다 . 2 개년간 동안 베트남 자원1(Vietnam coll. 1)은 945kg/ha의 최 고의 종실 수확량을 보였으며, 미국 자원(N04-9859, 828kg/ ha)과 대만 자원(AGS327, 798kg/ha)의 순서였다. 생육일수 는 2개년 동안 71일에서 86일 사이였다. 베트남 자원 1과 AGS 327 은 사료 수확량 , 초장 및 착협고에서 최고의 농업 형질을 나타냈다.