Description of study locations

Arba Minch site is located at 06°06′ N latitude and 37°35′ E longitude, while the Derashe site is located at 05°30′ N latitude and 37°12′ E longitude. Arba Minch and Derashe sites are laid at an altitude of 1,206 and 1,260 m above sea levels, respectively. The two experimental sites have a bimodal rainfall pattern. Monthly distributions of total rainfall and average maximum and minimum temperatures of Arba Minch and Derashe during 2018 and 2019 are presented in Figures 1a, b and 2a, b, respectively (NMA, 2021). A composite soil (0–30 cm) sample collected from Chanomile was characterized as sandy loam with pH 6.2, available phosphorus 14.47, total nitrogen 0.29, organic carbon 1.19, and organic matter 1.63, while the soil of Derashe is characterized as a sandy loam with pH 6.15, available phosphorus 14.5, total nitrogen 0.31, organic carbon 1.22, and organic matter 1.72 (Laboratory report).

Figure 1. Total monthly rainfall and average minimum and maximum temperature during 2018 (a) and 2019 (b) in Arba Minch.

Figure 2. Total monthly rainfall and average minimum and maximum temperature during 2018 (a) and 2019 (b) in Derashe.

Experimental planting materials

Six dual-purpose sorghum varieties: Chelenko, A-2267_2, and NTJ_2, Dishkara, Konoda, and Rara (local check) were collected from Melkasa Agricultural Research Center (MARC) and farmers of Derashe special district. Chelenko was released in 2005 from MARC A-2267_2 and NTJ_2 also collected from MARC and all three materials adapted to <1,600 m above sea level (Ministry of Agriculture (MOA), 2016). Dishkara and Konoda are the varieties of farmer and adapted to the altitude of 1,200–1,700 m above sea level. Rara (local check) was collected from the farmer store and adapted to the altitude of 1,200–1,700 m above sea level.

Experimental design and procedures

The experiment was laid out in a randomized complete block design with four replications. The gross and net sizes of experimental plots were 2.4 m × 3 m (7.2 m²) and 1.2 m × 3 m (3.6 m²), respectively. Spacing between experimental plots and replications was 1 and 1.5 m, respectively. After plowing the selected experimental plots with oxen, the plots were prepared and leveled manually with the help of necessary farm tools. The six sorghum varieties were allocated to the experimental plots randomly using a randomized complete block design method. Seed sowing was carried out from late March to early April 2018 and 2019. Seeds were sown in a row at inter- and intra-spacing of 60 by 25 cm, respectively. To avoid the risk of failing seedling emergence, two seeds were planted per hill and the weak seedlings were thinned out after 40 days of planting to maintain only a single plant per hill. Experimental plots were fertilized with NPS (19% N, 37% P₂O₅, 7% S) at the rate of 100 kg/ha during planting time, and with urea (46% N) at the rate of 100 kg/ha in two splits as the first half top-dressed on the 45th days of planting and the remaining half applied after initial harvest for ratoon initiation. Experimental plots were kept weed-free until harvesting with frequent hand weeding. No irrigation was supplemented for this trial. No any chemical was applied throughout the experimental period. Harvesting was made by hand cutting the head to easy collection of the grain.

Growth and yield parameters

Data collection for forage and grain yields from the main crop was performed by cutting plants at ground level in the net plot area after physiological maturity. The second harvesting from the ratoon crop was done after 105 days of the first harvest of the main crop. Both fresh and dry biomass yields (DBYs), as well as grain yield obtained from the net plot area (3.6 m²), were converted into a hectare basis. Apart from the collection of forage and grain yields, data on vegetative grow parameters were collected timely. Plant height (PH cm) was measured at a date of 50% flowering from ground level to the tip of the plant with the linear meter. The number of leaves per plant was counted, as well as length and width of leaves in the middle of the plants were measured with linear meters at the forage harvesting time of both ratoon and main crops. The number of tillers per plant was counted also from both main and ratoon crops just before harvesting. To compute leaf to stem ratio, the cut pieces were collected from 12 plants to make a composite sample of 500 g for leaf and stem independently. Green forage yield per net plot area was measured using a spring balance and expressed as fresh biomass yield per hectare (FBY t/ha). The sample was taken to the laboratory and subjected to oven drying at 65°C for 24 hr to get constant dry weight. After cooling, the samples were weighed with sensitive balance and expressed as DBY (t/ha). DBYs were estimated by multiplying FBYs with the dry matter percentage of respective samples. Dry matter of the samples of both main crop and ratoon crops harvests was determined using the following formula:

$ DM\%=\frac{ODW\;(gm)}{FW(gm)}\ast 100 $, where DM% is the dry matter percent, ODW is the oven-dry weight, and FW is the fresh weight of a sample (500 g).

$ DBY\;t/ ha= FBY\;t/ ha\ast DM\% $, where DBY is the dry biomass yield, FBY is the fresh biomass yield, and DM% is the dry matter content in percent.

Disease monitoring

Anthracnose incidence and severity assessment were started on 40 and 45 days after planting at Derashe and Arba Minch sites in 2018 and 2019 cropping seasons, respectively, when the first symptom of anthracnose appeared on plant leaves within the plot. Twelve randomly selected and tagged sorghum plants from the central rows of each plot were used for disease assessment, and a total of six assessments were made per location per season. Incidence (%) was determined by the rating of diseased plants per total number of plants assessed within the plot. Anthracnose severity was visually assessed from 15 pre-tagged plants per plot following the scale devised by Thakur et al. (Reference Thakur, Reddy and Mathur2007), where 1 = no visible symptoms or presence of chlorotic flecks, 2 = 1–10% leaf area covered with hypersensitive lesions without acervuli, 3 = 11–25% leaf area covered with hypersensitive and restricted lesions with acervuli in the center, 4 = 26–50% leaf area covered with coalescing necrotic lesions with acervuli, and 5 = ≥50% leaf area covered with coalescing necrotic lesions with acervuli. Severity scores were transformed into percentage severity index (PSI) for analysis using the formula stated below (Wheeler, Reference Wheeler1969).

$$ \mathrm{PSI}\hskip0.35em =\hskip0.35em \frac{\mathrm{Sum}\;\mathrm{of}\ \mathrm{numerical}\ \mathrm{ratings}}{\mathrm{No}.\hskip0.35em \mathrm{of}\ \mathrm{plants}\ \mathrm{score}\mathrm{d}\times \mathrm{maximum}\ \mathrm{score}\hskip0.35em \mathrm{on}\hskip0.35em \mathrm{scale}}\times 100. $$

Area under disease progress curve (AUDPC) (the development of disease on a whole plant or part of the plant during the epidemic periods) was estimated from PSI (anthracnose) and mean (turcicum leaf blight) values assessed at different days after planting for each sorghum varieties within the plot using the formula mentioned by Campbell and Madden (Reference Campbell and Madden1990) as below.

$$ \mathrm{AUDPC}={\sum}_{\mathrm{i}=1}^{\mathrm{n}-1}0.5\;\left({X}_{\mathrm{i}}+{X}_{\mathrm{i}+1}\right)\;\left({t}_{\mathrm{i}-1}-{t}_{\mathrm{i}}\right), $$

where n is the total number of disease assessments, t _i is the time of the ith assessment in days from the first assessment date, and x _i is the PSI of disease at the ith assessment. AUDPC was articulated in %-days since severity (X) is expressed in percent and time (t) in days.

Growth performance

Plant height

Plant height of sorghum varieties for main plant was significantly (p < .001) varied for the interaction of variety × location × year (Table 1). The tallest plant height of 430 cm followed by 410.8 cm was recorded at Chano Mille subresearch substation during 2018 and 2019 planting season for the variety Chelenko while the lowest plant height of 174.3 cm was at Derashe in 2019 for the variety NTJ_2. Ratoon crops plant height was also significantly (p < .001) varied among sorghum varieties. Dishkara (227.7 cm) recorded the highest plant height at Arba Minch followed by A-2267_2 (203.9 cm) among other varieties while the lowest plant height was at Derashe for Rara (68.0 cm).

Table 1. Growth performance of main crop as influenced by sorghum varieties, experimental years, and locations

Abbreviations: CV%, coefficient of variation; LL, leaf length; LNPP, leaf number per plant; LSD_0.05, least significant difference at p < .05; LW, leaf width; PH, plant height; TNPP, tiller number per plant.

The superscript footnote descriptions indicate mean values sharing common letters in the column are not significantly different.

The plant height of the ratoon crops is presented in Table 2. Dishkara variety recorded significantly (p < .05) the tallest ratoon crop (196.65 cm) among the sorghum varieties while the variety Rara recorded the shortest (110.45 cm). Moreover, the average plant height of main crops (264.3 cm) was greater than the ratoon crops (159.61 cm).

Table 2. Dry biomass yield and growth performance of ratoon crop as influenced by sorghum varieties and experimental locations

Abbreviations: CV%, coefficient of variation; DMY, dry biomass yield; LL, leaf length; LNPP, leaf number per plant; LSD_0.05, least significant difference at p < .05; LW, leaf width; PH, plant height; TNPP, tiller number per plant.

The superscript footnote descriptions indicate mean values sharing common letters in the column are not significantly different.

The plant height recorded in the present study is generally greater than the plant heights of sorghum varieties reported by other researchers where the plant heights of the main and ratoon crops were 147 and 129 cm, respectively (Hassan et al., Reference Hassan, Mohammed and Yagoub2015). Plant height contributes and plays a great role in aboveground biomass accumulation (Halim & Idris, Reference Halim, Shampazuraini and Idris2013). This may be due to the taller a plant, the higher the amount of light energy absorbed and the higher the rate of photosynthesis and consequently the amount of assimilate produced by the leaves (Ngo, Reference Ngo2017). Vinutha et al (Reference Vinutha, Kumar, Blümmel and Rao2017) reported the higher average plant height of ratoon crops (259 cm) than main crops (228 cm) for 36 sorghum lines, which may be due to the variation among genotypes and other management options.

Internode length

The result of internodes presented for sorghum varieties is presented in Figure 3. There was significant (p < .01) variation of internode length among sorghum varieties in the main crop. Chelenko had wider internodes (16.73 cm) while Dishkara (8.4 cm), Konoda (8.4 cm), and NTJ_2 (9.53 cm) had the shortest internodes. Internode length contributes to the DBY where varieties with the longer internodes gave higher DBY. The varieties with taller juicy stems with longer internodes are characterized as forage sorghum (Havilah, Reference Havilah2017). Stem internodes of sorghum genotypes with 5 cm fully grown stem reported previously (Kebrom et al., Reference Kebrom, McKinley and Mullet2017) while the current varieties have longer internodes of 16.73 cm.

Figure 3. Length of internodes of main crop as influenced by sorghum varieties.

Dry biomass yield

The results of DBY of ratoon and main crops of sorghum varieties are presented in Tables 2 and 3, respectively. DBY of the main crop was significantly (p < .001) different among sorghum varieties, location, and years. The highest main crop DBYs were recorded by Chelenko variety at Derashe site during the 2018 growing season (42.2 t/ha) and at Arba Minch site during the 2019 (38.41 t/ha) and Rara variety at Arba Minch site during the 2019 (37.33 t/ha), which were statistically similar. The lowest DBY was recorded by variety Konoda grown at Derashe site during the 2019 growing season (Table 3).

Table 3. Dry biomass yield of main crop as influenced by sorghum varieties, experimental years, locations, and their interaction

Note. Means with common letter (s) are not statistically different (p.05).

Abbreviations: CV%, coefficient of variation; DMY, dry biomass yield; LSD_0.05, least significant difference at p < .05.

DBYs of ratoon crop harvested at 105 days after main crop were significantly (p < .001) varied among varieties and locations. Variety Dishkara demonstrated the highest total (DBY of main crop + ratoon crop) DBY (41.67 t/ha) at Arba Minch site while Rara (2.5 t/ha)) and NTJ_2 (2.34 t/ha) varieties recorded the lowest DBYs at Derashe site (Table 2).

As indicated in Figure 5, Dishkara variety produced significantly (p < .05) higher total (yield of main crop + ratoon crop) DBY (45.3 t/ha) followed by Chelenko variety (33.3 t/ha) while the lowest DBY was obtained from Konada variety (18.5 t/ha).

Figure 4. Standard mean value of main crop grain yield as influenced by sorghum varieties.

Figure 5. Total dry biomass yields of sorghum varieties used in the present study.

Ratoon crops are very important for contribution of dry lowland forage production system where dual-purpose sorghum varieties could generate both grain and forage production. Forage DBY parameter is an important agronomic trait in forage crops production (Lauer, Reference Lauer2006), especially for the production of dual-purpose sorghum varieties (Chen et al., Reference Chen, Zhu, Liu, Zhang, Lv, Liu, Xiao, Yuan and Cai2020). The higher DBYs in the main crop than in the ratoon could be associated with the change in seasonal conditions for growth of the crops and probably depletion of nutrient levels in the soil (Vinutha et al., Reference Vinutha, Kumar, Blümmel and Rao2017). To boost the production, it needs the amendment of the nutrient depletion during the harvesting of main crops and ratoon crops (Afzal et al., Reference Afzal, Ahmad, Zamir, Khalid, Mohsin and Gillani2013).

Grain yields

The mean values of grain yields for sorghum varieties are presented in Figure 4. Grain yield was significantly (p < .01) varied among sorghum varieties. Variety NTJ_2 (3.89 t/ha) followed by Konada (3.77 t/ha) demonstrated the highest grain yield than other varieties while variety Chelenko (1.74 t/ha) gave the lowest yield. Grain yields of A-2267_2, Chelenko, Dishkara, and Rara were not varied significantly. Varieties in the present study producing higher DBY gave lower grain yield and vice versa. For example, Konoda and NTJ_2 demonstrated higher grain yield with lower DBY than other varieties in the test. This result is in agreement with the findings of Borghi et al. (2013) where sorghum DBY was reduced by increased grain yield. The extent of grain yields of dual-purpose sorghum varieties recorded in the present study was in line with findings of other researchers (Mahfouz et al., Reference Mahfouz, Mohamed Ali, Megawer and Mahmoud2015). Sorghum varieties used in the present study generally gave relatively higher mean grain yield (2.58 t/ha) compared to reports for dual-purpose sorghum genotypes, which recorded grain yield of 0.62 t/ha in winter and 0.55 t/ha in summer production (Hassan et al., Reference Hassan, Mohammed and Yagoub2015).

Anthracnose incidence, severity, and AUDPC

The incidence, severity, and AUDPC were significantly (p < .05) varied among the tested sorghum varieties at Arba Minch and Derashe districts in the 2018 and 2019 cropping season (Table 4). The highest mean anthracnose incidences of 98.90 and 100% were recorded from A-2267_2 variety grown at Arba Minch site during the 2018 and 2019 cropping seasons, respectively. Similarly, highest anthracnose severity of 43.67 and 40.36% and AUDPC of 860 and 1,085.27%-days were recorded from the same variety grown at Arba Minch site during the 2018 and 2019 cropping seasons, respectively. The lowest mean anthracnose incidence, severity, and AUDPC were recorded from Konada variety during the 2018 growing season. Similarly, the lowest incidence (on in 2019), severity (in 2019), and AUDPC (in 2019) were recorded from varieties Rara, Konada, and Konada grown at Arba Minch site. The highest mean (100%) anthracnose incidence was noticed from genotype A-2267_2 in 2018, while in 2019 the highest mean anthracnose incidence (100%) was recorded from A-2267_2 and NTJ_2 varieties at Derashe with the lowest mean anthracnose incidence was noted from Rara (61.15%). The highest mean anthracnose severity was recorded from the varieties A-2267_2 (32.02%), NTJ_2 (31.98%), and Dishkara (26.81%) in 2018, while in 2019 the highest mean anthracnose severity was noted from all varieties except for Konada and Rara varieties at Derashe. AUDPC values were recorded higher from A-2267_2 (829.92%-days) in 2018, while the highest mean AUDPC values were noticed from A-2267_2 (829.92%-days), Dishkara (696.22%-days), and NTJ_2 (741.55%-days) in 2019 at Derashe. Anthracnose is the most severe and distressing sorghum disease in terms of dry biomass and grain yields in the study areas (Getachew et al., Reference Getachew, Tariku, Melese, Selamawit and Dizgo2021). The plant disease epidemic development is highly affected by availability of optimum temperature, relative humidity, host tissue, levels of host resistance, and other factors during the growing periods of the crop (Campbell & Madden, Reference Campbell and Madden1990).

Table 4. Incidence, severity, and AUDPC of sorghum varieties to Anthracnose at different sites during the 2018 and 2019 main cropping seasons

Note. Means within each column for each parameter that are not followed by the same letter(s) are significantly different (p ≤ .05), while those followed by the same letter(s) are not significantly different (p > .05).

Abbreviations: AUDPC, area under disease progress curve; CV, coefficient of variation; LSD, least significant difference at a 5% probability level; PDI_f, percent disease incidence at final date; PSI_f, percent severity index at final date.

Correlation analysis of growth and yield parameters as influenced by sorghum variety, location, and years

The correlation of yields (dry biomass and grain) with growth and disease assessment parameters for main and ratoon crops are presented in Tables 5 and 6, respectively. DBY of main crops was positively correlated with plant height, tiller number leaf length, leaf number, and internodes. The correlation of DBY was significantly (p < .001) strong with internodes (0.946) of main crops. Tiller number and leaf length of the main crop had weak relationship with DBY of sorghum varieties. Day biomass yield of the main cropping season was negatively correlated (−0.785) with grain yield. DBY of main crop was the function of internodes of the stem in the present study. DBY of ratoon crops was positively correlated with plant height, tiller number, leaf length, leaf width, and leaf number. The positive correlation of biomass yield of ratoon crop with AUDPC showed anthracnose disease not affected the growth and development of ratoon crops in this study. Positive correlation of parameters either for main or ratoon crops of sorghum varieties indicates that selection on any one of the traits will increase in the other traits, thereby improving biomass yield in sorghum. The phenotypic correlation of plant height of sorghum varieties with biomass yield was reported as 0.349 (Narkhede and Seeds, 2020) while the correlation of plant height for the present study was as higher as 0.804 for main crop and 0.426 for ratoon crop (Tables 5 and 6).

Table 5. Correlation of dry biomass and grain yield with growth and disease assessment parameters for main cropped sorghum varieties

Abbreviations: AUDPC, area under disease progress curve; DMY, dry biomass yield; GY, grain yield; LL, leaf length; LNPP, leaf number per plant; PH, plant height; TNPP, tiller number per plant.

*p < 0.05; **p < 0.01.

Table 6. Correlation of dry biomass yield with growth and disease assessment parameters for ratoon cropped sorghum varieties

Abbreviations: AUDPC, area under disease progress curve; DMY, dry biomass yield; LL, leaf length; LNPP, leaf number per plant; LW, leaf width; PH, plant height; TNPP, tiller number per plant.

*p < 0.05; **p < 0.01.