ABSTRACT
Effect of season and plant species on in sacco dry matter (DM) and crude protein (CP) degradability of five plant species were investigated. Plant species were Acacia natalitia, Acacia nilotica, Dichrostachys cinerea, Scutia myrtina and Chromolaena odorata. Leaves were harvested during dry, early wet and late wet seasons, subjected to degradation in cows’ rumen using nylon bags technique. Season affected potential DM degradability and effective degradation of DM and CP. Species affected all parameters except slowly degradable fraction of CP. Interaction between season and species affected the parameters except potential and slowly degradable fraction of CP. Chromolaena odorata had highest estimated parameters of degradation among seasons compared to others. Based on potential and effective degradation, plants followed this decreasing order: C. odorata, A. nilotica, A. natalitia, S. myrtina and D. cinerea. These plants have a potential as feed supplements. C. odorata has the highest potential as feed protein source in ruminants. It concluded that season and species affected in sacco degradability of DM and CP of browse species.
Key words: Dry matter, crude protein, Chromolaena odorata, ruminants, nutritive value.
Smallholder farmers in subtropical savannah of Africa keep different ruminant species, most of which survive on natural pastures (Ugwu, 2007). The productivity of these ruminant species depends on quantity and quality of feeds (forage), which is affected by seasonal fluctuations (Abusuwar and Ahmed, 2010). The lowest quantity of forage occurs during dry season and may limit feeding and production of livestock. One strategy to increase value is the use of trees and shrubs as a sufficient source of food for ruminants. Some of forages are legumes, and legumes offer important sources of protein to maintain ruminant production in tropical savannah (Belachew et al., 2013; Gusha et al., 2013). Browse and shrub fodders are essential because they reduce seasonal limitation in ruminant feed (Balgees et al., 2013; Belachew et al., 2013). However, the distribution of tannins and other phenolic compounds in shrubs and tree leaves limits their utilization as animal feed (Belachew et al., 2013). Hence, evaluation of nutritive value of browse trees becomes important only when browse species is used as ruminant feeds.
In sacco degradability is a main evaluation technique of nutritive value of forages (Ørskov and McDonald, 1979). It is a useful method for ranking browse trees in terms of quality (Mehrez and Ørskov, 1977), and for evaluating the digestive abilities of ruminant species (Migongo-Bake, 1992). In sacco estimation has a benefit of estimating the degradation of particular constituent of feed such as dry matter (DM), crude protein (CP). In sacco technique also does not only determine the extent of degradation, but the part that degrades fast and its rate (Ørskov and McDonald, 1979). Moreover, estimation of soluble and slowly degradable fractions is necessary for dietary protein, thus estimating dietary protein used by rumen microbes, and that which bypass the rumen and become available for digestion in the small intestine.
The objective of this study was to determine the effect of season and plant species on in sacco degradation characteristics of dry matter and nitrogen on edible forage of browse species in sub-humid subtropical savannah.
Plant samples and their collecting area
Leaves of five plant species (main in the field and goat’s diet) selected by goats were sampled during dry (June/July 2008), early (November/December 2008) and late wet (February/March 2009) seasons at the Owen Sitole College of Agriculture (OSCA), Empangeni, South Africa (Basha et al., 2012). The mean annual rainfall of OSCA is 1022 mm and temperature is 26°C, and the type of soil is Mayo/Tambankulu. The early and late wet seasons are each part of the wet (rainy) season. January is the middle of the wet season. Plant species were Acacia natalitia (Mely), Acacia nilotica (L. Willd ex Del), Dichrostachys cinerea (L.Wight Arn), Scutia myrtina (Burm. f.) and Chromolaena odorata (King and Robinson). Browse samples were randomly sampled by collecting leaves 1.0 to 1.5 m above ground from three non-browsed trees per species per season. Once collected, leaf samples were kept in paper bags and air dried prior to oven drying at 60°C for 48 h. Part of dried samples was ground through 1-mm mesh sieve (Retsch GmbH & Co. KG 5657 HANN 1, West-Germany) for chemical analysis (Table 1). Other part of dried samples was milled through a 2-mm mesh sieve for in sacco purpose. All ground samples were stored in sealed plastic bottles until used.
Samples were analysed for chemical composition on dry matter basis using pseudo replicates (3 replicates). Nitrogen (N) concentration was determined based on AOAC (Wendt, 2003) using a LECO, FP2000, nitrogen analyser. Crude protein (CP) was calculated as 6.25 × N concentration. Neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL) were determined according to Van Soest et al. (1991) using ANKOM Technology Technique. The NDF was determined with α-amylase. The acid-butanol proanthocyanidin assay (Porter et al., 1985) was used to determine condensed tannin (CT) (Makkar, 1995). Cellulose was calculated as the difference between ADF and ADL, while hemicellulose was derived from the difference between NDF and ADF.
In sacco degradability
The experiment was conducted at the Livestock Section of the University of KwaZulu-Natal Research Farm (Ukulinga), South Africa. Three rumen-fistulated cows (average weight: 350± 45 kg LW) were used. Cows were each fed 2 kg of Lucerne hay per day, with ad libitum access to hay, water and a mineral lick. Cows were adapted to the diet for one week before being used in the experiment. The experiment followed the nylon bag technique described by Mehrez and Ørskov (1977).
Three grams of each dry sample per incubation period replicated thrice (three nylon bags) were incubated for 0, 3, 6, 9, 12, 24, 48 and 72 h in three fistulated cows (one bag/cow). The whole bag size was 18×8 cm with pore size of 40 to 60 µm. Samples were incubated in four batches; all samples within a batch were withdrawn simultaneously. Withdrawn samples were cleaned with water and kept in a refrigerator till washing date, when all samples were washed together including zero hour ones. Washing occurred in a semi–automatic washing machine (Hoovermatic model T4350, South Africa) in 6 cycles of 5 min each. The washed bags were dried in an oven (LABCO, model 5SOE1B, P.O. Box 155, Maraisburg 1700) at 60?C for 48 h, cooled in a desiccator and weighed. Residues were analysed for nitrogen using a LECO, FP2000, nitrogen analyzer. The DM and CP degradation data were fitted to the exponential equation (McDonald, 1981):
Y = a + b (1 - e-c(t-lt))
where, Y is the degradability of DM and CP at time (t), a is the soluble fraction which is rapidly washed out the bags, b is the insoluble fraction which is potentially degradable by micro-organisms, c is the degradation rate of fraction b per hour and lt is the lag time.
The effective degradability (ED) of DM and CP were calculated at a rumen out flow rate (r) of 0.03 h-1 using the following equation:
ED = a + b*c/(c+r).
Statistical analysis
All degradation variables are followed by subscript ‘dm’ or ‘n’ to indicate the nutrient being degraded (dry matter or nitrogen). Data were subjected to analysis of variance (ANOVA) using the general linear models (GLM) procedure of SAS (2002) in a 3 seasons ×5 feeds factorial design with three replicates. The model used was:
Yijk = µ + si + pj + (sp)ij + εijk;
where, Yijk is the observation, µ is the population mean, si is the season effect (i = 1–3), pj is the plant species effect (j = 1–5), (sp)ij is the interaction between season and plant pecies and εijk is the residual error. Statistical significance was declared at P<0.05. Means were compared by least significant difference (LSD). Correlation was used to test the relationships between in sacco degradability and chemical variables of browse species.
Effects of season and plant species on in sacco dry matter degradation
Table 2 shows the effect of season and plant species on in sacco dry matter degradation. Season affected (P<0.001) the soluble fraction (adm), potential degradability (PDdm), effective dry matter degradability (EDdm) and lag time (ltdm). Browse species and its interaction with season affected (P<0.001) all variables (adm, bdm, cdm, PDdm, EDdm and ltdm). Among the three seasons, the soluble fraction was higher in the dry season than in the early wet and the late wet seasons. The PDdm and EDdm were higher in the dry season than in the early wet and late wet seasons. The ltdm was longest in the late wet season and shortest in the dry season.
Among five plant species, A. natalitia had the highest soluble fraction (adm) and C. odorata had moderate adm, while A. nilotica., D. cinerea and S. myrtina had similar adm. C. odorata had highest insoluble degradability (bdm), while A. natalitia and S. myrtina had similar and moderate bdm, and A. nilotica and D. cinerea had lowest bdm. C. odorata had the fastest degradation rate, followed by A. natalitia, A. nilotica, D. cinerea and S. myrtina in this order. The PDdm and EDdm showed similar trend among the species, C. odorata had highest values followed by A. nilotica, A. natalitia, S. myrtina and D. cinerea in this order. Acacia natalitia had the longest ltdm, whilst S. myrtina had the shortest ltdm.
Interaction between season and browse species showed different trends for these variables. For the five species, except D. cinerea, soluble fraction decreased from the dry to the early wet seasons then decreased in the late wet season (A. natalitia, A. nilotica and C. odorata) or increased in the late wet season (S. myrtina), while the soluble fraction of D. cinerea increased from the dry to the early wet seasons from where it decreased in the late wet season. The degradation rate (cdm) of all five species except C. odorata were low and did not change throughout the three seasons, while degradation rates of C. odorata decreased from the dry to the early wet and late wet seasons in this order.
The PDdm for A. natalitia decreased from the dry season to the early wet and late wet seasons in this order. For A. nilotica, the PDdm was similar between the dry and the early wet seasons but decreased in the late wet season. The PDdm of D. cinerea did not change during the three seasons. For S. myrtina and C. odorata, the PDdm decreased (S. myrtina) or increased (C. odorata) from the dry to the early wet and late wet seasons which were similar.
The EDdm for A. natalitia, A. nilotica and S. myrtina decreased from the dry to the early wet and late wet seasons in this order. For D. cinerea and C. odorata the EDdm increased from the dry to the early wet seasons then decreased in the late wet season (D. cinerea) or remained similar between the early wet and the late wet season (C. odorata).
The lag time (ltdm) for A. natalitia and S. myrtina increased from the dry to the early wet and late wet seasons in this order. For A. natalitia, D. cinerea and C. odorata the ltdm increased from the dry to the early wet seasons from where it decreased in the late wet season.
Effects of season and plant species on in sacco nitrogen degradation
Table 3 shows the effect of season and plant species on in sacco nitrogen degradation. Season affected (P<0.001) only the soluble fraction (an) and EDn. Species strongly affected (P<0.001) an, cn, PDn and EDn, weakly affected (P<0.05) the ltn. Interaction between season and species affected (P<0.001) an, cn and EDn. Among these three seasons, an and EDn were higher in the early wet season than in the dry and the late wet seasons.
Among the five plant species, C. odorata had the highest soluble fraction (an), while A. nilotica and S. myrtina had moderate (an), and A. natalitia and D. cinerea had similar and low an. C. odorata had the fastest degradation rate (cn) followed by A. nilotica, A. natalitia, D. cinerea and S. myrtina in this order. C. odorata had the highest PDn and EDn followed by A. nilotica, A. natalitia, S. myrtina and D. cinerea in this order. A. nilotica had the longest ltn, whilst S. myrtina had the shortest ltn.
According to interaction between season and browse species, the an fraction in A. natalitia, S. myrtina and C. odorata decreased from the dry to the early wet seasons which was either similar to the late wet season (A. natalitia and S. myrtina) or lower than in the late wet season (C. odorata). For A. natalitia and D. cinerea, an increased from the dry to the early wet seasons then either decreased in the late wet season (A. natalitia) or remained similar to the late wet seasons (D. cinerea).
During the dry season, the degradation rate (cn) was slowest with D. cinerea, A. natalitia and S. myrtina; intermediate with A. nilotica and fastest with C. odorata. During the early wet season, the degradation rate was slowest for A. natalitia and S. myrtina; intermediate for A. nilotica and D. cinerea; and fastest for C. odorata. For the late wet season, the degradation rate was fastest for C. odorata followed by A. nilotica, A. natalitia, D. cinerea, and S. myrtina in this order. Among seasons, A. Nilotica for which the degradation rates were similar between the dry and the early wet seasons but higher during the late wet season. The degradation rate of D. cinerea was fastest during the early wet season and slowest during the dry season. For C. odorata, the degradation rate was fastest, moderate and low during the dry, early wet and late wet seasons, respectively.
The effective nitrogen degradability (EDn) for A. natalitia and S. myrtina decreased from the dry to the early wet seasons from where it remained similar (A. natalitia) or decreased in the late wet season (S. myrtina). For A. nilotica, D. cinerea and C. odorata the EDn increased from the dry to the early wet seasons then decreased in the late wet season (A. nilotica and C. odorata) or remained similar between the early wet and the late wet seasons (D. cinerea).
Correlation between chemical composition and in sacco degradability
Table 4 presents the correlations between the CP, NDF, ADF, ADL, CT, cellulose (Cell) and hemicellulose (Hcell), and DM and nitrogen degradation parameters. Crude protein was strongly and positively correlated (P<0.001) with degradation rate of bdm and bn, and negatively correlated (P<0.05) to CT. Crude protein was strongly and positively correlated (P<0.001) with EDn and moderately and positively correlated (P<0.01) with EDdm and an. Crude protein was weakly and positively correlated (P<0.05) with bdm, PDdm and PDn. The soluble fraction (adm) was strongly and negatively correlated (P<0.001) to fibre fractions (NDF, ADF and ADL) while an was moderately and negatively correlated (P<0.01) to NDF and ADL as well as an was weakly correlated (P<0.05) to ADF. Neutral detergent fibre was strongly and negatively correlated (P<0.001) with PDdm and EDdm which were moderately correlated (P<0.01) with ADF and ADL. Effective N degradation (EDn) and ltn had negative correlations (P<0.01) with NDF; and weak negative correlations (P<0.05) with ADF, ADL and CT. The PDn was negative correlated (P<0.05) to NDF. Condensed tannins had negative correlation (P<0.05) with adm, an and EDdm. Cellulose negatively correlated (P<0.05) with adm and ltn.
Seasonality was hypothesized to affect rumen degradability due to variation in forage quality among seasons. Results supported the hypothesis. The in sacco degradability (adm, PDdm and EDdm) of browse species was lower during the wet season than the dry season in agreement with a previous finding (Camacho et al., 2010). This decrease in DM degradability can be attributed to the effects of CT on accessible N, which can decrease ammonia concentrations and microbial growth in the rumen (Salem et al., 2007). Van Soest (1994) suggested that lignin and its cross-linkage to hemicellulose, polysaccharides and proteins could also depress digestibility. High an and EDn of browse species during the early wet season is partly in agreement with Ramírez-Orduña (2003) who reported high EDn of browse plants during autumn and winter at Baja California Sur, Mexico. There may be variation in climate factors between the locations of two studies. Ramírez et al. (2000b) suggest that plants may response to produce new foliage with highly soluble CP due to warm temperatures and wet climate that arise sometime at the end of winter at Northeastern Mexico.
Rumen degradability was hypothesized to vary among plant species due to their variation in chemical composition and results supported the hypothesis. Consistent with our results, Melaku et al. (2003), Anele et al. (2009) and Balgees et al. (2013) reported significant variations in DM and CP degradation parameters of multipurpose trees. The DM and CP potential degradability in the current study overlapped the range of 720 - 914 and 546 – 949 g/kg, respectively, reported by Melaku et al. (2003). The current study had values that are higher than the range of 362 - 673 for the PDdm reported by Anele et al. (2009). Based on potential and effective degradation of both DM and CP, the plant species followed this order: C. odorata, A. nilotica, A. natalitia, S. myrtina and D. cinerea. These differences in degradation may be associated to the structural and non-structural protein and carbohydrate fractions (Belachew et al., 2013). Previous reports suggested that the variation in the degradation parameters of the browse species may be due to the variation in chemical composition (Kamalak, 2006; Belachew et al., 2013; Gusha et al., 2013). Furthermore, the variations in chemical composition between seasons or among plant species have been reported (Gusha et al., 2013) which indicate the variation in degradation material of browse species. These variations in PDdm and PDn in the rumen have been reported as a result of variations in fibre content and tannins levels (Gusha et al., 2013) or due to other factors such as ash (Benjamin et al., 1995) or maturity (Kamalak, 2006; Gusha et al., 2013) . Moreover, PDdm and PDn were negatively correlated with NDF, ADF and CT (Kamalak, 2006). Acid detergent fibre (ADF) and tannins were negatively correlated with PDdm (Vadiveloo and Fadel, 1992). With regards to C. odorata, there has not been any previous report on the PDdm and PDn.
The soluble fraction of DM (adm) and CP (an) varied within and among plant species, the highest adm in A. nilotica during the three seasons while the highest an values recorded in C. odorata in the dry and the late wet seasons and A. nilotica in the early wet season. Comparable to the others, differences among these species may be because of variation in the type of carbohydrates (in term of structure and content). The adm and an were negative correlated with ADF, NDL and CT of browse species (Ramírez et al., 2000a).
Melaku et al. (2003) also reported that adm and NDF were negatively correlated, and agree with the negative correlation in this study between adm and an, and fibre fractions and CT. In addition, these results are consistent with the lowest values of adm and an in A. natalitia in the late wet season and in the two wet seasons, respectively, and D. cinerea in the late wet season and in the dry season. These species had higher fibre fractions in these seasons.
The greatest value of the slowly degradable fraction of DM (bdm) and CP (bn) in A. natalitia in the dry season and C. odorata during the two wet seasons and the lowest values of bdm and bn recorded in D. cinerea in the three seasons and in the early wet season, respectively. These parameters were not related to any measured chemicals in this study except bdm was positive correlated to CP. On the other hand, Ramírez et al. (2000a) reported that the slowly degradable fraction of plant cell wall was limited by ADL and tannins and by other factors not measured in the current study such as organic matter, ash and insoluble ash. Many studies reported that the extent of degradation of DM or CP was negatively correlated with NDF, ADF, ADL and CT (Melaku et al., 2003).
Markedly higher degradation rate of DM (cdm) and protein (cn) in C. odorata and the slowest rates (cdm and cn) observed with S. mytrina in the three seasons; reflect differences in chemical composition between the plant species. For instance, C. odorata had higher CP and lower CT contents in the three seasons whilst S. mytrina had lower CP and higher CT. Results showed cdm and cn are positively correlated with CP but negatively correlated with CT and is consistent with findings of Kamalak (2006). Balgees et al. (2013) reported that the rate of degradation of protein (cn) was negatively correlated to NDF and ADF concentrations. Melaku et al. (2003) found negative relationship between cn and CT, and between cdm and neutral detergent fibre bound nitrogen (NDF-N) and ADL, and positive relationship between cn and NDF-N.
The effective degradability of DM (EDdm) and CP (EDn) were positive correlated with CP, but negatively related with fibre fractions and CT. This is in agreement with results of previous studies (Kamalak, 2006; Gusha et al., 2013) reporting that EDdm and EDn were negatively correlated with NDF and ADF concentrations and EDdm was positive correlation to CP concentration. Melaku et al. (2003) found negative relationship between EDn and ADL. The differences in EDdm and EDn may be attributed to structural and non-structural CP and carbohydrate fractions, which affect protein solubility and bio-availability (Belachew et al., 2013).
A significant variation in in sacco degradability parameters were reported among seasons and different browse species harvested from sub-humid subtropical savannah of South Africa, during dry, early, and late wet seasons. These variations were more related with fibre fractions than with tannins content. Fibre concentration appears to be the main factor limiting in sacco degradability. It is suggested that the dilution rate and other factors in the rumen may limit tannin effects on degradability. Based on potential and effective degradability, the plant species can be placed in the following decreasing order: C. odorata, A. nilotica, A. natalitia, S. myrtina and D. cinerea. Consequently, C. odorata is the best supplementary protein source like high-quality leguminous forages.
The authors have not declared any conflict of interest.
This research was funded by the National Research Foundation, South Africa.
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