Effects of harvest height and time on hay yield and quality of some sweet sorghum and sorghum Sudangrass hybrid varieties

Introduction

The livestock sector has rapidly changed in Türkiye. Cattle breeding has increased to meet demand for meat, milk and dairy products. More cattle necessitate the cultivation of more forage crops in larger areas at lower costs without competing with food production. Sorghum sudangrass hybrid (SSH) cultivars play an important role in animal nutrition because of their ability to rapidly regrow after harvesting. Lignification is the main factor limiting the digestibility of plant cell wall polysaccharides in vitro (Jung, Samac & Sarath, 2012) and also its in vitro digestibility (Reddy et al., 2005). Fresh and dry forage of sorghum varieties are used as silage and haylage as well as for grazing purposes (Undersander, 2003; Avcıoğlu et al., 2009). Sorghum has been characterized as the “camel of the plant kingdom” because of its high tolerance to temporary drought conditions and its ability to regrow after drought conditions have disappeared (Sanderson et al., 1992; Açıkgöz, 2001). Sweet sorghum’s drought tolerance and high-water use efficiency make it a good option for global warming and drought scenarios and an important forage crop for silage production and energy agriculture in Türkiye (Yücel et al., 2017). Since sweet sorghum has a high ethanol yield (Bayram & Turgut, 2015), research about this crop has generally focused on its potential in sugar and ethanol production. To maximize efficiency, it is important to determine the optimal harvesting times of SSH varieties in both fresh and dry forage forms. These varieties can produce 1–1.5 tons of fresh forage/ha with 4–5 cuttings in summer, main crop growing conditions and 2–3 cuttings in second crop growing conditions (İptaş, Brohi & Aktaş, 2001; Salman & Budak, 2015). The reason for investigating different harvesting heights in SS and SSH cultivars is that the harvesting height in plants provides ideas about the development period and its nutritional value of the plant and the amount of the hay it produces. Harvesting time directly affects the quality of hay as well as the regrowth and ultimate yield power. The lower, the plant height at the time of harvesting, the higher the quality of the hay, but the lower or lack of sufficient storage material for regrowth, the slower the regrowth and hence the lower the yield in the second and subsequent harvesting (Ansa & Garjila, 2019). In SSH, there is an increase in the number of tillers produced by the plant due to the increase in the number of harvestings, but the dry matter yield decreases up to 50% due to the decrease in photosynthetic tissues (Sowiński & Szydełko, 2011). The number of harvestings also affects the yield and quality of the hay produced by the plant (Rahman, Fukai & Blamey, 2003). Similarly, there are studies carried out by some researchers showing that dry matter yield decreases with the increase in the number of harvesting (Lee, 2005; Uher et al., 2005). While high quality hay is obtained when harvesting is done in the early periods, as the harvest time is delayed, the hay becomes coarser and its nutritional value decreases (Lang, 2001). Studies on optimal harvesting times, especially those specific to grazing management, are extremely limited. This study was conducted to determine the best harvesting and grazing practices of SS and SSH cultivars for all use cases.

Materials and Methods

Study location

This study was carried out in the Research and Application Field of Canakkale Onsekiz Mart University, Faculty of Agriculture during the 2020 and 2021 growing seasons. The long-term average temperature of Canakkale province is 15.09 °C according to the Turkish General Directorate of Meteorology. Average temperatures on research years were recorded as 17.01 °C in 2020 and 17.58 °C in 2021, which were both above average. Average precipitation in the area from the first week of May to the last week of October is 149.9 mm. Total precipitation was higher than the average for both years of the study, with 157.5 mm of precipitation measured in 2020 and 201.2 mm measured in 2021 in this time period (Fig. 1).

Results

Discussion

Most crops follow a similar growth pattern, beginning with slow growth followed by rapid growth before slowing down again towards the end of their growth cycle (Altın, Gökkuş & Koç, 2011). Since there are fewer chloroplasts at the beginning of the growth cycle there are also fewer assimilation products. But as growth progresses, the number of chloroplasts increases, which increases photosynthetic processes, producing more organic mass. Vegetative growth continuously increases until the generative period. During the generative period, the products of photosynthesis are transferred to generative organs instead of vegetative tissues such as the roots, shoots or leaves (Altın, Gökkuş & Koç, 2011). This limits the forage yield at this stage of development (Chattha et al., 2017). Because of this, the dry forage yields increased regularly and continuously between harvesting heights of 30 cm to the PMS of the crop in this experiment (Table 4). Previous studies have found differing yield values in experiments carried out with sorghum in Türkiye. For example, Aydınoğlu & Çakmakçı (2018) found an average fresh forage yield between 46,000.0–81,880.0 kg/ha and dry forage yield between 11,870.0–20,370.0 kg/ha. A separate study conducted with 13 different sorghum cultivars in Bingöl found an average fresh forage yield of 73,230.0 kg/ha and an average dry forage yield of 13,080.0 kg/ha (Özmen, 2017). Other studies conducted in different regions found average fresh forage yields between 146,410.0–190,300.0 kg/ha in Konya (Acar, Akbudak & Sade, 2002), 22,890.0–47,170.0 kg/ha in Çanakkale (Küçüksemerci & Baytekin, 2017), 67,300.0 kg/ha (Sürmen & Kara, 2022) and 46,500.0–62,600.0 kg/ha in Aydın (Çelik & Türk, 2021). Studies have also found average dry forage yields varying between 13,080.0 kg/ha (Özmen, 2017), 61,00.0–11,830.0 kg/ha (Tosunoğlu, 2014), 16,540.0 kg/ha (Çeçen, Öten & Erdurmuş, 2005), 8,100.0–21,100.0 kg/ha (Kara, Sürmen & Erdoğan, 2019), 13,500.0–28,400.0 kg/ha (Kır & Şahan, 2019) and 14,800 kg/ha (Sürmen & Kara, 2022). Forage yields differ because different varieties have different genetic structures and react differently to environmental factors (Özyazıcı & Açıkbaş, 2019). Differences in forage yield by variety was also observed in this study, with the SS Topper-76 variety yielding significantly less than other varieties (Table 4).

The carbon and nitrogen contents of plants vary by species and varieties, and even between different organs of the same species (Yao et al., 2015; Suhui et al., 2018; Zhang et al., 2018). The nitrogen content of plants is closely related to photosynthesis and plant respiration meaning plants have high nitrogen content at the beginning of the growth cycle because there is more photosynthesis and respiration activity in the leaves, and nitrogen and carbon content decreases as the growth cycle continues (Zhang et al., 2018, 2020). In this study, the carbon ratios of leaves and stalks of different sorghum cultivars differed depending on plant height, with the carbon ratio of the leaves decreasing as plant height increased and the carbon ratio of the stalks increasing as plant height increased. The carbon ratio in the leaves decreased by approximately of 6% from the beginning to the end of the growth cycle, while the carbon ratio of the stalks increased by 4% during the growth cycle. Conversely, the nitrogen ratios in both the leaves and stalks decreased-by 47% in leaves and by 87% in stalks-from the beginning to the end of the growth cycle (Tables 5–8). The nitrogen content of a plant is closely related to its carbon ratio. High nitrogen concentration in the plant results in higher photosynthetic capacity, which leads to higher respiration (Chapin et al., 1987; Lambers et al., 1989). Accordingly, more nitrogen is used in photosynthesis and more carbon in respiration (Poorter & Remkes, 1990). The photosynthetic capacity of plants is higher in earlier growth stages compared to late stages because the number of leaves is higher in the earlier stages, and leaves have high photosynthetic capacity and more nitrogen and nutrient content than the stalks and roots (Poorter, Remkes & Lambers, 1990). The position of the leaves, the age of the plant and certain environmental factors affect a plant’s photosynthetic capacity (Aighewi & Ekanayake, 2004; Hgaza et al., 2009).

In this study, the crude protein ratios of sorghum cultivars showed significant variation between the leaves and the stalks. The 2-year average crude protein content was 11.42% in leaves and 6.75% in the stalks, meaning the crude protein content in leaves was approximately 40% higher than in the stalks. By variety, the highest crude protein content was found in the leaves of the Nutri Honey and Topper-76 varieties and the stalks of the Topper-76 variety. Crude protein contents of leaves and stalks proportionally decreased, by a total of 64% in leaves and 60% in the stalks of sorghum varieties, with increased plant height (Tables 9 and 10). Many factors affect forage quality in forage crops, with nutritional value considered the most important factor. The nutritional value of hay is measured by crude protein content. If the crude protein content of forage plants is 12% or lower, the quality of the hay is considered low. A crude protein content of 15% is considered medium quality hay, and hay with a crude protein content 18% and above is considered to have high nutritional quality (Budak & Budak, 2014; Cherney & Hall, 2014). Plants contain more crude protein in their leaves because of the higher photosynthesis capacity and the higher nitrogen and nutrient content of leaves compared to the stalks and roots (Poorter, Remkes & Lambers, 1990). Previous studies have shown that the crude protein ratios of leaves and stalks decrease with an increase in harvesting heights (Keskin, Yılmaz & Akdeniz, 2005; Karataş & Tansı, 2011). The results of this study align with previous studies; the highest crude protein content was observed at the beginning of the growth cycle when sorghum crops were young. In general, plants possess more dividing cells at the beginning of their growth (Taiz & Zeiger, 2008), and these cells have higher levels of physiological activity. All of the biochemical reactions in living things take place under the catalysis of enzymes. Enzymes are made up of proteins, so the protein ratio is always high at the beginning of the growth cycle in plants (Gökkuş et al., 2011). Crude protein ratio also decreases due to the decrease in physiological activity that occurs with plant growth (Towne & Ohlenbusch, 1992). In this study, the average crude protein contents ranged from 10.67–12.49% in leaves and 5.88–7.43% in stems. A separate study recorded the crude protein content of leaves between 14–15% and between 3–4% in stems (Karataş & Tansı, 2011). One previous study on four different sorghum varieties determined that the average crude protein ratio of the whole plant is around 5% (Keskin, Yılmaz & Akdeniz, 2005), while another study conducted on 13 different sorghum cultivars in the Bingöl province of Turkiye recorded the average crude protein content of sorghum cultivars as 4.81% (Özmen, 2017). The average crude protein ratio has been measured in previous studies as 9.5–10.2% (Kozłowski et al., 2006), 7.2% (Marsalis et al., 2010), 7.1–9.7% (Arslan & Çakmakçı, 2011), 7.2–8.8% (Canbolat, 2012) and 8.3% (Tosunoğlu, 2014).

Crude ash (mineral element) contents of different sorghum cultivars showed significant variation in plant leaves and stalks, and by variety and harvesting height. Generally, the macro and micro element contents of the leaves were found to be higher than that of the stalks. Previous studies have determined that the protein, vitamin and mineral contents of leaves are higher than that of the stalks, while cellulose, hemicellulose and lignin contents are lower (Fales & Fritz, 2007; Jung, 2012; Temel & Keskin, 2020). This explains the high crude ash content of the leaves observed in this study. Crude ash contents of sorghum cultivars also differed significantly from each other, likely because of genetic variations in different varieties of sorghum cultivars (Khan et al., 2006; Kering et al., 2011; Özyazıcı & Açıkbaş, 2019). The SSH cultivars generally contained higher crude ash than that of the sudangrass cultivar (Tables 11 and 12). This is likely related to the amount of nutrients taken from the soil along with the genetic differences of these varieties (Özyazıcı & Açıkbaş, 2020). Previous studies have found that the ratios of P, K, Ca and Mg significantly differ between sorghum cultivars (Başbağ et al., 2011; Gülümser et al., 2017; Gürsoy & Macit, 2017; Başbağ, Çaçan & Sayar, 2018; Polat & Bayraklı, 2019), and even between different varieties within the same cultivar (Lema, Cebert & Sapra, 2004; Markovic et al., 2014; Engin & Mut, 2018; Özyazıcı et al., 2018a, 2018b; Turan et al., 2018; Özyazıcı & Açıkbaş, 2019; Tan, 2019; Özyazıcı & Açıkbaş, 2021). In this study, the crude ash content of SSH and SS cultivars decreased with an increase in crop height, meaning mineral element contents decreased based on the physiological maturity of crops. Crops need high levels of crude ash, especially in times of rapid growth (Tables 11 and 12). Because most of the crude ash is found in the protoplasm, where physiological activities are intense, and less in the cell wall (Spears, 1994), the crude ash content decreases with growth, but the ratio of total organic matter to mineral matter increases because cell wall compounds increase during plant growth. Another reason for the decrease in crude ash content may be the increased amount of dry matter that occurs during plant growth (Kaçar, 2012). Some studies (Dactylis glomerata L., Lathyrus sativus L.) have reported that decreases in macro and micro element contents are correlated with the ripening process in crops (Schlegel et al., 2016; Can & Ayan, 2017; Özyazıcı & Açıkbaş, 2020). According to a study conducted in rangelands, crude ash content decreases because of the physiological maturity of plants (Gökkuş et al., 2011).

In this study, crude fat ratios were significantly different in leaves and stalks, and by cultivar and harvesting heights. Only the fat content of the stalks differed significantly by cultivar, with the stalks of SS cultivars having 11.4% higher fat content than that of the SSH cultivars (Tables 13 and 14). This difference is likely due to the genetic differences of the cultivars (Özyazıcı & Açıkbaş, 2020). In addition to this, the crude fat contents of the stalks and leaves of the crops also differed significantly. Generally, the average crude fat content of the leaves was 37.3% higher than that of the stalks because the metabolic activities in the leaves are faster and more abundant than those in the stalks (Tables 13 and 14). A high leaf ratio has been reported to be important to forage quality in forage crops because there are close relationships between leaf ratios and crude protein, digestible crude protein, ratios of mineral substances and digestibility of dry matter (Açıkgöz, 2001). In this study, the crude fat levels of leaves and stalks decreased by approximately 33.9% and 41.2%, respectively, as the plant height increased. Fats are important to the functions and continuity of the biological membranes in crops, the formation of nutrients stored in the seeds, and the stability of the protoplasm. Fats found in the roots, stalks, leaves, flowers and seeds of the crops protect these organs against external factors. The ratio of crude fat can vary by plant organ, variety, species, genotype, and developmental phase/stage (Pallardy, 2008). In similar studies, the average crude fat content of 58 sorghum genotypes and three sorghum cultivars varied between 2.32–5.74% (Kaplan et al., 2016). Other studies have reported average crude fat content of 5.63% in one sorghum variety (Osman et al., 2000), between 1.5–2.23% in three sorghum varieties (Pontieri et al., 2021) and between 2.6–3.1% in sorghum varieties (Canbolat, 2012).

Conclusions

This study aimed to determine the variation in summer main crop forage production, growth process, carbon and nitrogen content and forage quality characteristics of different harvesting heights, time and different varieties of sweet sorghum (SS) and sorghum sudangrass hybrid (SSH) cultivars. The obtained results of this research were as follows. There has been an increase in hay yields depending on the increase in plant height and time. The highest yields were determined in M81-E, Nutrima and Nutri Honey varieties. Nutrient contents (carbon, nitrogen, crude protein, crude ash and crude fat) of leaves and stalk decreased with the increase in plant height at harvesting period, while only carbon contents of stalk and crude ash contents of leaves increased. Hay yields increased with the increase in plant height at harvesting time, while nutrient contents of hay decreased. It is recommended to cultivated Nutri Honey and M81-E sorghum varieties taking into account the hay yield and the nutritional values of the varieties, and harvest time them when they reach 90 and 120 cm height.

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