Evaluation of silages of hybrids of napier grass and sorghum in the low country wet zone of Sri Lanka

The field experiment was conducted in the Faculty of Agriculture research farm, University of Ruhuna, Mapalana, Kamburupitiya, Sri Lanka, started from February 2017 (low country wet zone, 58 m above mean sea level, latitude and longitude: 7.8731° N, 80.7718° E, mean annual rain fall: 2,400 mm, and mean annual temperature: 27–31°C). The experimental site was cropped with Calliandra (Calliandra calothyrsus) for 3 years before the establishment of the present experimental plots. The site was ploughed with a mold board plough and harrowed and leveled with a tine tiller. The soil type was red-yellow podzolic. Soil samples were randomly collected from the experimental location before planting and analyzed for physiochemical properties (pH = 5.8, bulk density = 0.8 g/cm³, soil texture: sandy clay loam) using standard methods (Dharmakeerthi et al., 2007). The basal fertilizer application was given at planting and consisted of urea (200 kg/ha), triple super phosphate (120 kg/ha), and muriate of potash (100 kg/ha).

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, sorghum), three cutting intervals (4, 6, and 8 weeks), and three blocks. The plot size was 5 m × 5 m. A total of 27 plots were used. Two stem cuttings of hybrid napier grasses having two nodes were planted with 1 m × 1 m spacing in one spot on February 14, 2017. An equalizer 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 equalizer cut was taken. Three seeds of sorghum were planted manually in one spot with 45 cm × 15 cm spacing. After germination, two healthy seedlings were selected from each spot and other seedlings were thinned out. All plots were irrigated when required to ensure adequate soil moisture for plant growth. Weeds were slashed after 1 month of establishment.

All three forages were harvested at three cutting intervals of 4 (June 12, 2017), 6 (June 26, 2017), or 8 weeks (July 10, 2017) at 7.5 cm height above ground level. Initially, guard rows were discarded. Subsequently, the net plots were harvested and fresh weights were recorded. Neither wilting practice nor additives were used in any of the silage type. The silage types were as follows:

Hybrid napier var. CO-3 (100%)

Strong high-density black polythene bags to hold 5 kg (laboratory silo) were used as fermenting unit. The quantity used for silages depended on the fresh matter yield of different forages (Table 1) as well as based on the fact that one cow should be fed one bag a day in addition to existing feeds as a green fodder supplement. All bags were carefully packed with forage without making any holes (5 kg fresh matter, chopped at 1–2 cm length) gently but firmly squeezed the bag by hand to expel air, vacuumed, sealed, and stored in a dark room for 6 weeks at room temperature (around 30°C). Silage samples were prepared at subsequent harvest as well in the same manner. In addition to the silages prepared from three main crops, Napier CO-3, Napier Co-4, and fodder sorghum, two combinations of silages were also prepared. Forage materials were taken from the same harvests in a same manner to prepare the additional two mixed silage types without deviating from the experimental design. There were 5 silage types × 3 cutting intervals × 2 harvests × 3 replicates = 90 samples. Following observations were performed after opening the silos: physical parameters such as texture, color, aroma, and molds present or not were determined by following the silage note prepared by New South Wales Dept. of Primary Industries (2008). The dry matter (DM) content of silages was determined according to Association of Analytical Chemists (1980). Chemical parameters such as pH (AFIA, 2011), lactic acid (Bernett, 1951), ammonia nitrogen (Zaklouta, 2011), and SCHO (AFIA, 2011) were determined immediately after opening of laboratory silos.

The General Linear Models procedure was used to compare the mean differences between five silage types and three cutting intervals. Duncan's multiple range test was used to test the differences between means. Differences between means were considered significant if p-values were less than 0.05. The data set was analyzed using SAS University Edition.

It was observed that both grass silages (hybrid napier var. CO-3 and hybrid napier var. CO-4) had wet and leafy texture, which was more typical for tropical grasses. Both grass silages have olive green color, falling within the normal color range for grass silages. Both had little fruity smell, which may be associated with lower fermentation especially from crops with low sugar content. Sorghum silage had a leafy and soft texture likely to be associated with higher digestibility. The silage falls into the normal color range, whereas mild, high fruity smell proved its desirable character with normal lactic acid fermentation. Both mixed silages (hybrid napier var. CO-3 (50%) + sorghum (50%), hybrid napier var. CO-4 (50%) + sorghum (50%) have soft but medium wet appearance. It is pale to light green color fallen within the normal color range for grass and cereal silages. Both had medium fruity smell, which may be associated with fermentation especially from crops with medium sugar content. All silages prepared were free from molds (Table 2).

The sorghum var. Sugar graze silages had a higher DM content when compared with the silages of hybrid napier var. CO-3 or var. CO-4 mixtures with 50% sorghum (Table 3). Silage made from hybrid napier var. CO-3 exhibited the lowest DM concentration among all silages tested. Generally, the DM content of all silages was lower than that of expected values. Bandara et al. (2016) reported DM percentage of 24.1 and 22.1, respectively, for hybrid napier var. CO-3 and sorghum var. Sugar graze.

Furthermore, the DM contents of hybrid napier var. CO-3 or var. CO-4 silages were lower in the current study at 8-week cutting intervals (56 days after regrowth) when compared with the values of dos Santos et al. (2013) for silages prepared with dwarf elephant grass clones. They indicated that dwarf clones are more likely to become dehydrated after cutting when compared with tall elephant grass varieties. Therefore, it could be suggested that, the lower DM contents of hybrid napier var. CO-3 and hybrid napier var. CO-4 silages could have been partly due to no wilting and no addition of additives as well as due to prevailed rainy conditions during the whole experimental period. Furthermore, hybrid napier var. CO-3 and var. CO-4 and sorghum were introduced newly to the low country wet zone of Sri Lanka and the plants were only 4–5 months old when harvested for silages. Therefore, it can be suggested that the performance of all three varieties was suboptimal when compared with other studies conducted with same species.

When considering the cutting intervals, all silages produced significantly higher DM with longer cutting interval. Forages with greater potential for ensiling must have an adequate DM for fermentation. Therefore, sorghum with high DM concentration at 6 or 8 cutting intervals are more suitable for ensiling when compared with hybrid napier var. CO-3 or hybrid napier var. CO-4. McDonald (1981) reported that plant cells were broken down and plant juices (SCHO) were released by plasmolysis, which was a pre-requisite for the development of lactic acid bacteria during the early stages of ensilage.

Silages prepared with sorghum revealed satisfactory pH values at all cutting intervals (Table 3) when compared with mixtures and two grass silages. In this study, all the silages had pH greater than 4 and could be classified as fairly good. Thus, the obtained pH values indicated that all silage types did not have sufficient availability of SCHO for adequate fermentation. However, pH cannot be used as a sole fermentative indicator, because other factors, such as the ammoniacal nitrogen, lactic acid, butyric acid, and homo/hetero fermentation characteristics, are important to measure fermentation quality; indeed these variables should be used as indicators to measure fermentation quality (Jobim et al., 2007; dos Santos et al., 2013)

The respective ammoniacal nitrogen (NH₃-N/TN), lactic acid content, and water-soluble CHO content decreased as the cutting interval increased from 4 to 8 weeks. In relation to NH₃-N/TN content, all silages can be considered as of good quality, because all the values were below 10% (Table 3). Ammoniacal N is a product of clostridial fermentations, and the ammonia content must be less than or equal 12% of the total nitrogen (TN) for grass (McDonald et al., 1991).

Highest lactic acid content was observed in all silage samples at 6-week cutting interval with a maximum of 4.14% ± 0.77% for sorghum silage. Having higher percentage of lactic acid (>3%) is a good indicator of quality silage, and sorghum silage could be considered as the best among tested silages. Lactic acid concentration of all silages decreased with advancement of maturity of all fodders. However, Bandara et al. (2016) reported that the lactic acid content of sorghum at 50% flower initiation stage was higher 5.90% ± 0.46 % when compared with the value obtained in the current study.

Highest SCHO was observed in all silage samples at 6-week cutting interval with a maximum of 2.63% ± 0.57% for sorghum. SCHO concentration of all silages decreased with the advancement of maturity of all fodders, whereas fiber content of the grasses increased with maturity. Bandara et al (2016) reported that the SCHO content of sorghum at 50% flower initiation stage, which was comparable with the values in the current study.

Lowest ash content was observed for hybrid napier var. CO-4 silage, whereas sorghum silage had the highest ash content at 4-week cutting interval. Ash content of all the silages was decreased when the cutting interval increased (Table 4). More or less similar values were reported by Santos et al. (2012) for elephant grass varieties.

Although not significant, all silages had highest CP content under 4-week cutting interval and decreased toward increasing cutting intervals (Table 4). The CP content of sorghum silage at cutting intervals of 6 and 8 weeks was higher when compared with the silage of both hybrids of napier grass with the mixed silages showing intermediate values. Furthermore, the values were similar to those that have been previously reported in the literature for elephant grass silage (Pinho et al., 2008). All silages contained more than 7% CP to provide sufficient nitrogen for effective microbial fermentation in the rumen (Church, 1988). The percentage of CP was not modified with the ensiling process. That could be proved by the low ammoniacal nitrogen content of all silages.

The lowest crude fiber content was observed for sorghum silage, whereas the highest values were observed for hybrid napier var. CO-3 and CO-4 silages. Crude fiber content was clearly increased in all silages when the cutting interval was increased (Table 4). The crude fiber values reported by Bandara et al. (2014) were far higher than those observed in the present study for hybrid napier and sorghum.