Nutritive value of experimental feeds
The present study was designed to determine the effect of native grasses and bamboo leaves on the growth performance and carcass traits of lambs. Concerning the first objective, it was found that the CP contents of native grasses (NPC) from area exclosures and bamboo leaf (BL) were 13.1%, and 12.76%, respectively. This combination is well above the minimum level (7%) required for normal rumen microbial growth and fermentation (Van Soest 1994). Although the experimental lambs in the control group with no BL supplement displayed the lowest growth performance, they showed no negative growth. The possible explanation for this might be that the CP content of NPC was above the maintenance requirement of sheep. Generally, the CP content of NPC was below the recommended 15% of the protein supplements for lambs (Umunna et al. 1995). This suggests that the use of NPC as a sole diet cannot fully cover the growth requirement of lambs, and it is essential to supplement lambs with additional CP sources from locally available feed resources. A previous study (Denbeshu, 2010) that noted the importance of bamboo leaves suggested that it is a potential source of supplement that is locally available, evergreen, and palatable throughout the year. The current study found that BL had CP, NDF, ADF, and IVDMD contents of 12.76%, NDF 66.72%, ADF 33.96%, and 49.85%, respectively. and this result is consistent with that of Gashu G. et al. (2017), who reported a comparable result on the bamboo leaf content which showed a CP of 10.96%, NDF of 69.52%, ADF 31.58% and IVDMD of 45.85%. The slight variation that exists could be attributed to the age of the bamboo culm at harvest and the season of harvest. Despite its free availability and accessibility to local farmers, bamboo leaves alone might not fulfill the expected protein requirement. Surprisingly, when BL was supplemented along with NPC resulted in a better growth performance in lambs than in grasses alone. The NDF contents of both NPC and BL were similar to the report of Gashu G. et al. (2017) on the chemical composition of native grass and highland bamboo leaves. This result is slightly above 65% which limits the digestibility and DM intake of ruminant animals.
The chemical composition of mountain bamboo (Yushania alpina) leaf used as a supplement in this study was CP 12.76%, NDF 66.72%, ADF 33.96%, ADL 9.6%, Ash 10.96%, IVOMD 49.85% ME 7.98 (MJ/KgDM), which is also similar with the result reported by Bezabih et.al, (2016). The probable explanation for the observed variations may be due to differences in location, soil, and age of bamboo leaf used, etc.
Feed intake
In this study, the share of bamboo leaf in the total diet was about 10.77% for 150 g (T2), 18% for 300 g (T3), and 23.31% for 400 g (T4) supplementation level. An important observation from this trial was the absence of any negative effect on feed intake and weight gain when the lambs were supplemented with bamboo leaf up to 23.31% of the total diet DM consumed.
Although, the NDF content of both NPC and BL was in the range (above 65%) that limits digestibility and DM intake in ruminants, the significant increase in the intake of total DM, OM, CP, and NDF of sheep with increasing levels of bamboo supplementation (P < 0.000) is an indication of improvement in the digestibility of the diets consumed. Previous studies (Allen, M.S et al, 1988; Poppi, 2011) have shown that the intake and digestibility of fiber are directly related to the potential digestible portion of NDF (pdNDF) in the diet. Potential digestibility is defined as the NDF fraction which disappears after a long incubation period, leaving the indigestible component of NDF (iNDF) which is unavailable for microbial digestion (Waldo et al. 1972; Wilkins, 1972). The relationship between DM intake and NDF is more than just NDF content in the diet but also dependent on the potential digestibility of NDF (pdNDF) (Poppi, 2011; Waldo et al. 1972, Wilkins, 1969) and the indigestible component of NDF (iNDF) which is unavailable for microbial digestion. The pdNDF fraction is defined as the difference between the NDF and iNDF. It is the iNDF component that is the rate-limiting constituent of forages at higher NDF levels (Lippke, 1986). NDF has been proposed as a reliable predictor of voluntary dry matter (DM) intake under certain conditions (Minson, 1990). The relationship between the NDF content and feed intake is complex and not a linear relationship (Arelovich et al 1990). Tropical forages can have the same NDF content but differ vastly in iNDF (Arelovich et al 1990). The amount and quality of NDF in a diet can either enhance or limit feed intake. This may be the case, that the higher NDF content of the feeds in this study did not affect much the DM intake and digestibility of diets. There may be a lower level of iNDF in both NPC and BL. This requires further research on the iNDF content of NPC and BL.
The improved intake of DM and nutrients in this study is likely due to the increased availability of N and readily available OM or energy in the rumen, which in turn facilitates microbial growth and fiber digestion. The bamboo leaf supplementation did not limit intake as other fibrous feeds do rather it enhanced it. The combined effect of improved rumen environment for microbes and lowered physical limitations of intake must have contributed to the linear increase in the intake of the total DM and nutrients by the lambs. The total DM intake between the higher two bamboo supplementation levels (120 g DM and 180 g DM/day) was significantly different (P < 0.000) signaling that the supplementation of the bamboo leaf did not reach its optimal level, where the balance between microbial growth and fiber digestion reaches its maximum level thereafter, the increase in intake may start to decline. This is the most likely indication that the bamboo leaf supplementation did not reach the possible maximum level.
The treatment groups supplemented with bamboo leaves (T2, T3, and T4) consumed more (P < 0.000) feed than the sheep in the control group (T1). There was no refusal of bamboo leaf record as all supplements had been eaten, which indicates bamboo leaves from young culms (< 1-year age) are nutritious and palatable. The DM, OM, CP, NDF and ADF intakes among the supplemented groups varied significantly (P < 0.000). The highest and significant intake of these was recorded in T4 which also received the highest amount of bamboo leaf. As the amount of bamboo leaf supplement increased, total DM was significantly increased (P < 0.000) indicating that the inclusion of fresh bamboo leaf along with grass promoted more DM intake compared with the control group. Nutritionally, the increase in DM intake along with increased supplementation is a desired effect. This is evidenced by the considerable improvement of FCE from T1 to T4. The total DM intake was significantly different (P < 0.000) in the order of T4 > T3 > T2 > T1. The observed total DM intake as a percent of BW falls within the range of 2.96–3.27% suggested to be consumed by small ruminants (Schoenian, 2003). The estimated ME intake at T1 (8.54 MJ/kg/d) in the present study is considerably above the maintenance energy requirement recommended for yearling lambs (McDonald et al., 2010).
Body weight change and feed conversion efficiency
Growth performance and feed conversion efficiency of Arsi-Bale sheep grazed and fed indoors on native grass cut (NPC) from area exclosures and supplemented with bamboo leaf are presented in Tables 3 and 4. In this study, sheep in the control groups (T1 and T5) did not lose weight but rather achieved a total growth of 2.99 kg (T1), 2.59 kg (T5), and ADG of 33.33g and 29.11g, respectively. This indicates that the native grass in the area exclosures and grazing land had supplied the lambs with nutrients and energy above maintenance level. The average daily gain (ADG) attained in this study for T4 (62.67g) and for T8 (48.67g) were higher than ADG (45.11 g/day) for yearling Farta sheep fed urea-treated rice straw as basal diet and supplemented with 400 g dry Sesbania sesban leaf as reported by Tekliye et al. (2018). And it is lower than 103 g/day for sheep fed diet containing 300 g/day Sesbania sesban foliage (Moges et al. 2008) and 75.71 g/day for Abergelle rams supplemented with 300 g/day DM Sesbania sesban leaf (Tekle et al. 2016).
The highest ADG (62.67 g/day) for the current study was higher than 41.8–56.4 g/day ADG for Blackhead Ogaden sheep fed haricot bean haulm as a basal diet supplemented with the mixtures of wheat bran and 300 g/day brewers dried grain (Emebet, 2008). It was also comparable with the highest ADG (64.4 g/day) for Washera sheep fed grass hay basal diet and supplemented with 350 g/day concentrate (field pea hull, wheat bran, and “Noug” seed cake) (Mesganaw, 2014). Similarly, as Denbeshu D. (2010) reported, Arsi-Bale lambs with similar initial weight and age fed 25% bamboo leaf 75% concentrate (wheat bran and Noug cake in 3:1 ratio) on grass hay basal diet had attained 22.1(kg) FBW, 5.3 (kg) TG and 62.8 (g) ADG. These results are consistent with the current study results in which lambs in T4 which received 180 g DM bamboo leaf supplement attained 22.82 (kg) FBW, 5.62 (kg) TG, and 62.67 (g) ADG.
Carcass characteristics
Carcass characteristics of Arsi-Bale lambs reared under grazing (G) and indoors fed from native grass cut (NPC) and supplemented with bamboo leaf are shown in Tables 5 and 6. In this study, the carcass weight of lambs improved with the increasing addition of bamboo leaves in the ration. The slaughter body weight (SBW), empty body weight (EBW), and hot carcass weight (HCW) were significantly heavier than the control treatments, which is consistent with that observed for TG and ADG. HCW in this study was heavier than Kaffa (5.55–12.4 kg), Washera (9.23–9.8 kg), Arsi Bale (6.97–9.30 kg) and Farta (6.40–10.1 kg) sheep reported by different authors (Dereje et al.2014, Bekele et al. 2013, Worku et al. 2015, and Melaku et al.1998), respectively. The ribeye REMA values obtained in this study (7.49–8.88 cm2) were similar to the lowest values obtained by Melaku et al. (1998) and Tekliye et al. (2018) which were (8.1–11.9 cm2) and (8.41–10.8 cm2) respectively. Worku et al. (2015) reported that supplementation had a significant effect on the ribeye muscle area which is consistent with this study. Generally, the variations in carcass traits in this study and previous studies may be due to variations in initial weight and supplement feeds used for the experiment. In agreement with this, McDonald et al. (2010) noted that nutrition, age, sex, genetics, season, and other related factors affect the growth and carcass traits of animals.
There is significantly higher weight in supplemented groups of T3 and T4 than the control (T1, T5) groups and T6 groups in SBW, EBW, HCW, CCW, and ribeye muscle area (P < 0.05). There is no observed significant difference among T3, T4, T7, and T8 in SBW, EBW, HCW, CCW, and ribeye muscle area (P > 0.05). T2 (indoor fed) and T8 (grazed), received 60g DM and 180g DM bamboo leaf supplement respectively, yet, T8 had shown no significant difference compared with T2 in SBW, EBW, HCW, CCW, and ribeye muscle (P > 0.05). This indicated that the indoor-fed groups performed better than the grazed ones.
The weight of the empty digestive tract of T4 and T8 which received the highest bamboo leaf supplement was significantly heavier than the control groups (T1 and T5) (P < 0.05). The more bamboo leaf is added to the diet the more the digestive tract tissue is impacted by its fibrous nature. Wester et al. (1995), reported, that forage-based diets with large amounts of digesta entering the small intestine give rise to new tissue growth. The same author, studying planes of protein and energy nutrition in visceral organs in lambs, concluded that the digestive tract was more affected by the physical characteristics of the diet than by the level of nutrient intake.