Forage growth, yield, and quality responses of two hybrids of Napier and sorghum at three cutting intervals in the southern province of Sri Lanka

The field experiment was conducted at the Faculty of Agriculture Research Farm, University of Ruhuna, Kamburupitiya, Sri Lanka, starting from February 2017 to May 2019 (low country wet zone, 58 m above mean sea level, latitude and longitude: 7.8731° N, 80.7718° E, mean annual rainfall: 2400 mm, mean annual temperature: 27°C–31°C). The experimental design was a factorial arrangement in a randomized complete block design having three forages (hybrid Napier var. CO-3 and var. CO-4 and fodder sorghum var. sugar graze), three cutting frequencies (4, 6, and 8 weeks), and three blocks. The plot size was 5 × 5 m. A total of 27 plots were used. Two stem cuttings of hybrid Napier grasses having two nodes were planted with 1 × 1 m spacing in one spot on the February 14, 2017. An equalizing cut was performed for hybrid Napier grasses at a standardized height of 5 cm after 3 months of planting (May 14, 2017). Sorghum seeds were planted on the date of equalizing cut. Three seeds of sorghum were planted manually in one spot with 45 × 15 cm spacing. After germination, two healthy seedlings were selected from each spot and the other seedling was thinned out. All plots were irrigated (three to four irrigation at 10–12 days interval) when it was not raining to ensure adequate soil moisture for plant growth. Weeds were uprooted manually after 1 month of establishment.

During the first year, three plants from each variety from the middle of each net plot (16 m²) were randomly selected to measure the growth. Plant height, number of leaves, number of tillers, leaf length, leaf width, and stem diameter were determined at weekly intervals up to eighth week of growth. The tallest tiller of each plant was used to measure the plant height, stem diameter, leaf length, and leaf width. Height of the plant was measured by using a flexible ruler from the base where it touched the soil to the vertically held leaf tip. Number of tillers and number of leaves were counted manually. Damaged and dried leaves were discarded for calculating the total number of leaves. A ruler was used to measure the length and width of a randomly selected leaf from the tallest tiller in each plant, and an averaged value was calculated. Diameter of the lowest node was measured with a vernier calliper.

All three forages were harvested at three intervals at 4th, 6th, and 8th weeks up to 2 years. Plants were clipped at standardized height of 3–5 cm from the ground level. First, guard rows were discarded and net plots were harvested to record fresh weights. Representative samples were taken from the harvested herbage and chopped into 1–2 cm pieces. The chopped samples were dried at 60°C in an oven for 48 h to a constant weight. This procedure was repeated for the three harvesting frequencies throughout the year for the three varieties. Harvest dates are given in Table 1.

Dried samples were ground to pass through a 1-mm sieve. Samples were stored in labeled glass bottles for further nutritional analysis. DM, CP (by Kjeldhal method; Kjeltec TM 8400; FOSS Analytical Company, DK Hilleroed, Denmark), and crude fiber (CF; acid and alkali method) were determined according to the Association of Official Analytical Collaboration (AOAC (2005) procedure.

Calculations

Annual dry matter yield (t/ha) = annual fresh herbage yield (t/ha) × dry matter content (%)

Annual crude protein yield (t/ha) = annual dry matter yield (t/ha) × crude protein content (%)

Total monthly rainfall data were extracted from the meteorological station of the Department of Agricultural Engineering at the University of Ruhuna, Mapalana, Kamburupitiya. Annual rainfalls during 2017 and 2018 were 1935 and 1983 mm, respectively (Figure 1). It was lower than the average annual rainfall of 2400 mm. Rainfall was very low in February 2017 at the establishment of forages. However, it was increased dramatically during the growing period of forages and reached a lower peak during the period of equalizer cut in May 2017 and October 2018. A higher peak was observed during November 2017 and May 2018 (Figure 1).

Figure 1

Generalized Linear Model with two-factor analysis of variance (ANOVA) was used to compare the mean differences and interaction between cultivars and three cutting intervals. Tukey's Least Significant Difference (LSD) test was used to test the differences between means. Differences between means were considered significant at p < 0.05. Statistical analysis was done with Minitab version 16.

The effect of cultivar and cutting interval on plant height, leaf length and width, number of tillers, number of leaves, and stem diameter and their interaction effects are presented in Tables 2 and 3, respectively. Significant interactions between cultivar and cutting interval were found for plant height, stem diameter, leaf length, and leaf width. In accordance with the main effects, for cultivars, plant height followed the order sugar graze > CO-4 > CO-3, while stem diameter for CO-3 was greater, which was followed by CO-4 and sugar graze. Cutting interval had similar effect on plant height and stem diameter. Plant height increased progressively when the cutting interval increased. At 4 weeks, CO-4 had the lowest plant height, with no significant difference found between CO-3 and sugar graze, while no significant difference was recorded in all varieties at 6 and 8 weeks. Sugar graze had the lowest leaf length and highest leaf width at all cutting intervals, and there was no significant difference between hybrid Napier varieties. There was no significant difference in the number of tillers and leaves between CO-3 and CO-4 varieties across all cutting intervals. Stem diameter at 6 and 8 weeks cutting interval exceeded that of 4 weeks interval. At all cutting intervals, CO-4 gave the highest number of tillers and was followed by CO-3.

The effects of cultivar and cutting interval on ADM and nutritive composition are presented in Tables 4 and 5, respectively. Among the main effects, cultivar had a significant effect on ADM production (CO-3 > CO-4 > sugar graze) and cutting interval had a significant effect too (8 weeks > 6 weeks > 4 weeks) during both years. Cultivar had a significant effect on herbage CP concentration, with sugar graze exceeding (p < 0.05) CO-4 and CO-3. Herbage CP concentration declined dramatically with increase in cutting interval from 20.4% at 4 weeks to 13.9% at 8 weeks during the first harvesting year and from 20.7% at 4 weeks to 13.6% at 8 weeks during the second harvesting year. Annual CP yields of three cultivars followed the same pattern as ADM yield during both years. Cultivar had a significant effect on CF concentration, wherein sugar graze had the lowest effect compared with CO-3 or CO-4.

Interaction effects of variety and cutting interval on ADM and nutrient composition of first and second harvesting years are presented in Tables 6 and 7, respectively. ADM production during both years was higher at longer cutting intervals (8 weeks) for all varieties (Figure 2). Repeated cutting at short intervals (4 and 6 weeks) resulted in low herbage yields, compared with longer (8 weeks), uninterrupted periods of growth. Although DM productions of both Napier varieties were superior at shorter cutting intervals, sugar graze out yielded both varieties at 8 weeks during both years. The pattern of the ADM yield was not much different during both production years for the three varieties (Figure 2). All three varieties showed the highest value for CP percentage at 4 weeks cutting interval during both years. In both years, sugar graze had higher CP than hybrid Napier varieties at all cutting intervals. Even though there was no significant difference in CP contents of CO-3 and CO-4 at the first year, it was higher (p < 0.05) in CO-3 at 6 and 8 weeks of cutting intervals in the second year. CP yield followed the same trend and increased when the cutting interval increased. CP yield of sugar graze was superior at 8 weeks during both years. Sugar graze recorded the lowest CF at all cutting intervals and there were no significant differences between the hybrid Napier varieties at 4 and 6 weeks during both years.

Figure 2

Growth characteristics of the two Napier grass cultivars and sugar graze responded to the varying cutting intervals, demonstrating that defoliation impacts the morphological development and forage quality (Manyawu et al., 2003). Sugar graze had the highest plant height, which is a result of elongation of stem internodes due to narrow spacing as suggested by Pushparajah and Sinniah (2018). Generally, stem elongation occurs due to narrow spacing as a result of competition to obtain sunlight (Pushparajah and Sinniah, 2018; Lamana, 2003). Taller plants lead to narrower stem diameter as well. The plant height of sugar graze at 8 weeks of growth in the present study was slightly deviated from the findings of Epasinghe et al. (2012) who reported 218 cm at the 9^th week of growth when grown in a wet zone in Sri Lanka. Furthermore, Samini and Premaratne (2017) reported 231 cm of plant height in sugar graze and 145 cm of plant height in CO-3 at the 8th week of growth when grown in a northern region of Sri Lanka. This variability may be due to the diverse climatic and soil conditions of the different regions of Sri Lanka. According to Ekemini et al. (2012), there is a positive correlation between plant height and fodder yield. However, greater height of sugar graze compared to CO-3 at the 8th week verified the suitability of sugar graze as good forage. According to Amin (2011), leaf area of forage increases with the leaf length and leads to higher forage yield attributing to efficient photosynthesis. Pahuja et al. (2016) reported a leaf length of 95 cm for sugar graze in India for the first cut at 50% of flowering and a spacing of 15 × 45 cm. Leaf length of CO-3 is higher compared with the findings of Samini and Premaratne (2017) at the 8th week and Mounika et al. (2015) at the 13th week of growth (85.6 and 82.3 cm, respectively). The variability of the leaf length in the current study may be attributed to growth due to maturity and the climatic conditions in different agro ecological zones.

Higher number of tillers and leaves in hybrid Napier varieties demonstrates the profuse tillering capacity of hybrid Napier varieties (Premaratne and Premalal, 2006). By confirming the findings of the present study, Mounika et al. (2015) reported that number of tillers and number of leaves per plant were always higher in CO-4 compared to CO-3 at all growth stages. According to literature, when plants have more number of tillers per plant, it has a positive relationship with DM yield (Bandara et al., 2016). Ibrahim et al. (2014) stated that the number of leaves per plant is a very important parameter for calculating the growth and forage yield of fodder species. Furthermore, leaves have higher nutritive values compared with the stem of the plant. Variation in results may be due to the differences in maturity level, plant spacing, and climatic conditions. The higher stem diameter and profuse tillering suggested that lower plant densities should be employed for the cultivar CO-3.

DM yield is the basic component related to forage production. Based on the results obtained at all harvesting intervals, it may be envisaged that all tested fodder species can produce higher DM yield than traditional fodder species. The lower ADM yields of hybrid Napier varieties at 4 weeks during the second harvesting year may be attributed to immature tillers of hybrid Napier varieties at early growth stages. The higher ADM yield of hybrid Napier varieties at 8 weeks confirms the statement of Wadi et al. (2004) that the number of regrown tillers tends to be higher in Napier plants cut at 60-day interval. However, the present study results agree with the statement of Premaratne and Premalal (2006) that higher production could be obtained from hybrid Napier up to 2 years under good management practices. The present study observed no significant difference in DM yields of CO-3 and CO-4, as evidenced by the absence of significant differences in tiller number and leaf number. In contrast, Singh and Garg (2015) observed that CO-4 had higher DM yield compared with CO-3 in Central Gujarat, India. The highest ADM yield of sugar graze at 8 weeks cutting interval of second year may be due to the ability to produce very high DM yield in sorghum. The DM content of sugar graze at 8 weeks was higher than the results of Pushparajah and Sinniah (2018), that is, 17.9 of DM content at 60 days after planting, and of Samini and Premaratne (2017), that is, 15.6 of DM content at 8 weeks of growth. This may be attributed to the variation in rainfall of different climatic zones.

The CP content of sugar graze harvested at 6th week in the present study was relatively higher than that reported in some other studies. Epasinghe et al. (2012) and Moran (2005) reported a CP content of 17.1% and 12.2%, respectively, for sorghum harvested at the 7th week of maturity.

The CP content generally declines with maturity; this could be the reason for the lower CP content reported by Samini and Premaratne (2017), that is, 14.3% of CP at the 10th week of harvest, and Bandara et al. (2016) demonstrated a CP content of 11.6% at 50% of flowering. The CP content of CO-3 at 8 weeks in the present study was comparable with the findings of Bandara et al. (2016) who indicated 13.3% at 50% of flower initiation after establishment. Whereas Premaratne and Premalal (2006) reported a CP content of 15%–16% for CO-3 when harvested at the pre-blooming stage. The variations in the present values may be attributed to the different stages of maturity and variations in soil and climatic conditions. Among several nutrients, CP is an important nutrient, and according to Qureshi (1992), forage with higher CP has superior feeding quality. Thus, CP content of forages can be used as a good indicator to measure the quality of fodder (Bogdan, 1977).

The CF content the in present study was incomparable with the results of Premaratne and Premalal (2006), who reported 34%–37% for CO-3 when harvested at the pre-blooming stage. Bandara et al. (2016) reported a CF content of 37.8% for CO-3 and 36.8% for sugar graze when harvested at 45 days. However, Pushparajah and Sinniah (2018) reported comparable results with the present study, that is, 29.22% of CF for sugar graze when harvested at 60 days after planting. The deviations may be attributed to the differences in soil fertility, climatic conditions, and stage of maturity of forages. Relatively lower CF content together with higher CP of sugar graze could be having comparatively higher digestibility than CO-3 and CO-4.

In accordance with the reports of Wangchuk et al. (2015) and Tessema et al. (2010), we found that a short cutting interval of 4 weeks seriously reduced the DM yields of both hybrid Napier cultivars. While the protein concentration was very high, this would scarcely compensate for the greatly reduced forage production. Similar patterns for DM and CP were observed for sugar graze as well. General decline in CP concentration with increasing cutting interval found in the present study is in agreement with numerous works (Manyawu et al., 2003; Peiretti et al., 2015). We consider that cutting interval of 8 weeks appears optimal for the environmental conditions in the southern low country of Sri Lanka. This is supported by our results revealing DM and CP yields and acceptable CP and crude fiber concentrations at 8 week cutting interval for all three cultivars.