ⅠIntroduction
As Korean dietary life is changing, meat consumption is gradually increasing. The meat consumption has approached up to a net amount of 2.5 million tons in 2015, which was consistently increased by 0.5 million tons during 5 years from 2011 (MAFRA, 2016). The amount is likely to be continuously increased in domestic.
Livestock needs high protein forage, whereas the grass hay, timothy, Italian ryegrass, and orchardgrass, and whole crop silages, maize and rice, contain 6.6~9.0% CP as reported by various studies (Takagi et al., 2002; Islam et al., 2004; Bal, 2006; Khan et al., 2012). However, the representative forage legume, alfalfa (Medicago sativa L.) hay, cube, and pellet, has a high-protein content, 16.0~17.2% (Khan et al., 2012). As of 2015 in Korea, the net forage amount of 19% was imported from abroad, and the amount of alfalfa as livestock feed among the imported was 186,000 ton (MAFRA, 2016).
Soybean is an annual herbaceous leguminous plant and oil-bearing crop. Soybean seeds generally contain 35 to 38 % protein compared to approximately 20 to 30 % in other legumes (USDA, 2009). Soybean protein contains mostly easily soluble fractions up to 94 % (USDA, 2009). Before World War II, the typical legume crop, soybean, was used as high protein forage with quality similar to alfalfa (Blount et al., 2002). Soybeans as nutritious hay and silage crop are harvested in whole plants at various growth stages from flowering to early maturity (Blount et al., 2002). Days to flowering and maturity of soybean play an important role for harvesting forage, since harvesting stage significantly affects the yield and quality of forage (Acikgoz et al., 2009; Hintz et al., 1992). Various studies (Fehr et al., 1971; Darmosarkoro et al., 2001; Lee et al., 2014; Acikgoz et al., 2013) have recommended forage soybean cultivars with late maturity and harvesting stages from R5 to R7 (beginning maturity) for producing the optimum forage soybean, which generally has the best combination of low fiber, high protein content, and digestible energy.
In domestic, Kim et al. (1982) have evaluated the feed value of soybean straws of the different varieties. Usability of the soybeans as a silage, which contain CP content of 19% and are 7 t/ha in DM yield, was reported by Cho et al. (2003), and three soybean lines suited in intercropped with corn were selected by Shin (2008). Recently, Lee et al. (2014) have evaluated forage yield and quality for the soybean accessions derived from interspecific cross between wild and cultivated soybeans. Moreover, forage soybean varieties which were derived from wild soybean, Chookdu 1 and Chookdu 2, were developed (Lee, 2014). Despite the previous studies that soybean has a greater feed value as forage, commercial cultivation of soybean for livestock forage is few in domestic. In this respect, to increase cultivation and usability of forage soybean, it would be preferentially necessary to develop new soybean varieties with high feed value and forage yield, and to optimize harvesting stage which could be highly suitable for intercropping or multiple cropping systems.
For these needs, we carried out this study to investigate feed value of recent soybean cultivars, and to provide basic information for developing soybean varieties with high feed value and forage yield.
ⅡMaterials and methods
1Soybean cultivation and harvesting methods
Korean soybean varieties were classified into 4 ecotype groups based on seasonal response of flowering and 3 ecotype groups based on maturity (Cho et al., 1994; Chu et al., 1996). For this study, we selected three varieties by each maturity group, a total of 9 varieties (OT89-05, OT93-26, OT93-28, Geomjeongsaeol, Heugsung Joyang 1, Mallikong, Pungwon, Jinpung). The experimental plots for this study were arranged by completely randomized designs. The soybean seeds were sown at 70×15 cm spacing to the plots in the crop experimental field in the National Institute of Crop Science (nICS), Suwon, Republic of Korea. The experimental plots were treated with a basic granular fertilizer at N-P2O5-K2O=40-70-60 kg/ha before sowing and were managed by the crop standard cultural practices of NICS after sowing. Agronomic yield- and quality-related characteristics were measured according to the agricultural science technology standards for investigation of research of RDA, Republic of Korea (RDA, 2012). We harvested soybean plants at R5- and R6-reproductive stages of soybean for analyzing feed value (Fig. 1).
2Analysis of nutrient value
The harvested soybean seeds and whole plants were ground into powder under size of 1.0 mm in ultraspeed-centrifuge mill (ZM 100, Retsch, Germany), and then used for analyzing general composition contents and feed value.
General compositions, crude ash, crude fiber, crude oil, and CP, were analyzed to provide basic nutrient information of the soybeans whole plants according to the recommendation of Association of Official Analytical Chemists (AOAC, 2016). The direct ashing incineration method by heating (600oC) was used for analyzing contents of crude ash (AOAC, 2016). Crude oil was extracted by diethyl ether using Soxhlet extractor (Soxtec System HT 1043 extraction unit, Foss Tecator, Sweden), and then the extract was weighed (Kim et al., 2007). CP was analyzed according to semimicro-Kjeldhl method using Auto Sampler System (Kjeltec 2400 AUT, Foss Tecator, Sweden) (Kim et al. 2007).
3Analysis of feed value
Contents of NDF and ADF also were measured according to the recommendation of AOAC (2016). NDF and ADF of the samples were measured by Goering and Van Soest (1970) and Van Soest and Robertson (1980) modified methods using fiber analyzer (ANKOM2000, ANKOM Technology, USA). The each ground sample of 0.5 g was used for NDF and ADF analysis. When neutral detergent solution (FND20C, ANKOM Technology, USA), sodium sulfate (na2SO3), and thermally stable alpha-amylase were used for analyzing NDF, acid detergent solution (FAD20C, ANKOM Technology, USA) was added for ADF extraction. We modified the arithmetic expression of Lee et al. (2014) to calculate DDM, DMI, and RFV of the samples as following equation: DDM = 88.9 - (0.779 × ADF), DMI = 120/NDF, RFV = (DDM × DMI)/1.29.
4Statistical Analysis
All data were subjected to analysis of variance for each character using SAS 9.2 (SAS Institute Inc., USA) software. The significance of treatment (harvesting stage), main effects (variety), and interactions (environmental conditions) were determined at the 0.05 probability levels. Multiple comparisons between samples were performed by the least significance difference (LSD) method based on three independent biological each (n=3) at the 0.05 probability level.
ⅢResults
1Weather conditions
The accumulated temperatures of Suwon region during whole soybean growing period in both 2015 and 2016 were not significantly different to that in the normal years (1981~2010). However, the respective amounts of precipitation during the periods in 2015 and 2016 were severely declined to 62.3% and 37.9% relative to that of the normal years although the sunshine duration were greater by 258.9 and 104.1 h, respectively (Table 1). In particular, the amounts of precipitation in June and September of both years, which are early stage of growth and maturing stage of soybean, respectively, were significantly lower than those of the normal years (Table 1).
2Evaluation and selection of soybean for forage potential
To select appropriate soybean varieties for analyzing feed value, we investigated characteristics of growth, yield, and CP in seeds of all of the examined soybeans. Days to flowering of OT89-05 in both years were the smallest among the soybeans while that of Jinpung was over 60 days (Table 2). However, the average days to flowering of all of the soybeans in 2015 was longer than that in 2016 although the days to maturity of the soybeans were statistically similar between both years (Table 2). Based on the growth habits, the soybeans were separated into three maturity ecotype groups, early, middle, and late (Table 2). Jinpung in both years had the longest length of stem when yields of Pungwon were the highest among the soybeans (Table 2). Contents of CP in seeds of OT93-26, Geomjeongsaeol, and Pungwon in both years were the highest in each ecotype group (Table 2). Depending on these results, we selected a total of three varieties with high protein content in seeds, OT93-26, Geomjeongsaeol, and Pungwon, because protein is very important as a nutrient to evaluate livestock forage.
3Plant harvesting time
Days to harvesting of the R5 stage-OT93-26 was the shortest, 55 days, whereas that of Pungwon harvested at R6 was 103 days (Fig. 2). The dry matter yields of OT93-26 was the highest at both R5 and R6 stages (Table 3) although the lowest ratio of DM yield was showed in the R5-harvested OT93-26 and Pungwon (Fig. 3). All of three soybeans at R6 stage had a greater DM yield than those at R5 stage (Table 3). We measured R6/R5 ratio of DM to provide the optimum harvesting stage of respective soybean. However, the R6/R5 ratio of dry matter of Pungwon was higher than that of the other soybeans and Geomjeongsaeol showed the lowest ratio (Table 3).
4Analysis of feed value
Contents of crude ash and crude fiber in Pungwon harvested at R5 stage were the highest, but the R5-harvested OT-93-26 contained the lowest (Fig. 4). Crude oil of the R6-harvested soybeans was significantly higher than those of the R5-harvested (Fig. 4). Geomjeongsaeol at both stages contained a higher CP than the other varieties (Fig. 4). Contents of NDF and ADF of Pungwon harvested at R5 stage were the highest whereas the R6-harvested Geomjeongsaeol had the lowest contents (Table 4). On the contrary, Pungwon harvested at R5 showed the lowest DDM, DMI, and RVF while those in Geomjeongsaeol at R6 were the highest (Table 4). DDM, DMI, and RVF of Geomjeongsaeol and Pungwon harvested at R6 stage were higher than those at R5 stage, but in case of OT93-26 those at R6 stage were lower rather than those at R5 stage (Table 4).
ⅣDiscussion
The responses of crops by weather conditions are quite complex and difficult to describe. Previous studies had reported that weather conditions, such as accumulated temperature, precipitation, and sunshine duration, were highly related to various agricultural performances of soybean, growth and development (Purcell et al., 2014; Park et al., 2016), seed yield and yield components (Frederick et al., 2001; Kumar et al., 2008; Park et al., 2016), and oil and protein contents (Piper and Boote, 1999). Our results were similar to the previous results that drought-stress had resulted in decrease of seed yield and yield components (Frederick et al., 2001), and that an earlier flowering and maturity were caused by shorter summer day length and warmer temperatures (McWilliams et al., 2015) although there were different from the other previous report that the increased sunshine duration affects to the increase of soybean yield (Kumar et al., 2008). In addition, stem lengths of the soybean in 2015 and 2016 were not different, but Specht et al. (2001) had reported that the stem length of soybean was decreased under drought stress conditions. However, our results showed that the severely decreased precipitation of 2015 compared to that of 2016 didn’t result in a significant difference of CP contents in seeds between 2015 and 2016. For these results, we assumed that weather conditions are likely to affect various agricultural performances, but could not estimate a critical major environmental factor because agricultural traits were complicatedly affected by diverse weather conditions, especially accumulated temperature, precipitation, and sunshine duration.
Willard (1925), Munoz et al. (1983), and Hintz et al. (1992) had recommended from R6 to R7 as an optimum harvesting stage for forage soybean although Lee et al. (2014) had determined that R6 stage of both lines derived from G. soja × G. max and cultivated soybean is the optimum stage for harvesting based on forage yield and quality. Depending on these recommendations, we harvested at R5 and R6 stages of the soybeans to evaluate feed value and usability of the soybean plants with or without full filled seeds, respectively, which contain high quality nutrients including protein for human being and animals.
Intercropping and multiple cropping are necessary for intensifying usability of farmland to satisfy the increasing demand for food, feed, fiber, and industrial crops. However, harvesting stages of forage crops with high yield and quality are very important to successfully realize the farming systems. All of the soybeans are shorter in days to harvesting than the forage corn and rice, which the optimum harvesting stages are 35~42 days after silking (Chung et al., 2010) and 30 days after heading (nICS, 2006), respectively. This result implied that the triple cropping system using summer forage corn (Lee, 2015) could be substitute for a cropping system using forage soybean, and soybean-used cropping models could be more various than the corn-used.
Ratio of forage DM yield makes a decision an optimum harvesting stage of each forage crop because the dry condition of forage matters highly affects to forage yield and quality, such as thermal denaturation, vitamin A and E contents, nutrient loss, fermentation, digestibility (Chung et al., 2010). In previous studies (Blount et al., 2002; Chung et al., 2010; Lee et al., 2014), the ratio of dry matter yield of soybeans from R5 to R7 were in range of 25~30%. Based on these previous results, we assumed that Geomjeongsaeol as a suitable forage soybean can be harvested at R5 and R6 stage in yield and ratio of DM although the yields at both stages are lower rather than the others, and that R6 of OT93-26 and Pungwon is the optimum stage than R5.
Livestock producers provide legume crops such as alfalfa to increase protein intake (Rotz et al., 1999; Blount et al., 2002). Soybean which is similar to feed value of alfalfa was used as a fodder to provide high-quality protein and digestible energy material for livestock (Blount et al., 2002) because proteins in soybean have high solubility and degradability by ruminant bacteria (Russell et al., 1992). Our data which showed over 21.0% CP in all of the R5- and R6-harvested soybean plants was similar to the results of Blount et al. (2002) and Lee et al. (2014) which CP of the tested forage soybeans was in a range of 16.6 to 25.8%. Therefore, our study showed that soybean could be a better substitute for the imported alfalfa in domestic, and be an alternative protein supplement when other forage crops are unavailable to provide enough protein, since it has comparatively higher CP content.
Based on legume hay grading system developed by Rohweder et al. (1987) and promoted by the America Forage and Grassland Council (AFGC), the 1st grade of legume hay has a feed value of over 19% CP, 65% DDM, 3.0% DMI and 151% RFV, and under 40% NDF and 31% ADF. However, OT93-26 harvested at R5 and R6, and Geomjeongsaeol and Pungwon harvested at R6 correspond to the 1st grade legume hay grading system.
In conclusion, soybeans can be alternative forage with high feed value with or without seed for livestock. Although early-maturing soybean, OT-89-05, was for the 1st grade legume hay, and middle and late-maturing soybeans, Geomjeongsaeol and Pungwon harvested at only R6 stage, were evaluated in the 1st grade, we could not determine whether the flowering and maturity times were linked to the forage yield and quality or not because correlation between the days and its yield or quality was not significant. Nevertheless, we assumed that quality of forage soybean was closely related to its genetic characteristic, namely variety. However, Geomjeongsaeol could be used for developing new forage soybean varieties with high feed value, and R6 stage was optimum in yield and quality to use soybean as forage.
