Abstract
This experiment was conducted to investigate the relationship between fibre degradation kinetics and chemical composition of forages and by-products using nylon bag technology. Four rumen fistulated goats fed a medium concentrate diet were used to measure degradation kinetics of dry matter (DM), neutral detergent fibre (NDF) and acid detergent fibre (ADF). Fourteen feed samples included 6 forage species (ryegrass, coronilla, clover, alfalfa, corn plant and wheat straw) and 2 by-product species (cottonseed hulls and corn cobs). Nylon bags were ruminally incubated for 0, 6, 12, 24, 48 and 72 h. NDF content of these feeds ranged from 35.7% to 93.8% of DM. Effective degradability of DM (DMD), NDF (NDFD) and ADF (ADFD) ranged from 6.0% to 76.3%, 4.2% to 52.5% and 3.2% to 41.1%, respectively. DMD, NDFD and ADFD were positively correlated with crude protein content (P < 0.05) but negatively with the content of NDF, ADF and lignin (P < 0.05). NDF content was the best feed composition for predicting DMD, NDFD and ADFD of forages and by-products. These results suggested that chemical analysis could give a satisfactory prediction towards degradable characteristics of fibrous feedstuffs in ruminants.
Introduction
Fibre is a main composition in diets fed to dairy ruminants and usually comprises more than 25% of total mixed rations. The extent of rumen fibre digestion impacts feed energy value available for animals (NRC Citation2001; Robinson et al. Citation2004). Fibre degradable characteristic also relates to feed intake and performance of high-producing ruminants (López et al. Citation1998). Predicting fibre degradability could be useful information to provide guidance to nutritionists for meeting the fibre and energy needs especially in ruminants fed diets containing low quality forages in developing countries.
NRC (Citation2001) uses lignin content to predict fibre rumen degradability. Jung et al. (Citation1997) found the relationship between lignin and fibre degradability differs between forage species. Several researches found a poor relationship between fibre degradability and lignin concentration (Nousiainen et al. Citation2004; Robinson et al. Citation2004). Objectives were: (1) to evaluate relationship between degradation kinetics and chemical composition using nylon bag method and (2) to develop the model to predict fibre degradability in common forages and by-products high in fibre.
Materials and methods
Feed description
A total of 14 individual feeds covering 6 forages (ryegrass, coronilla, clover, alfalfa, corn plant and wheat straw) and 2 by-products (cottonseed hulls and corn cobs) were used in the present study. The samples represent main ruminant forages and feeds high in fibre in the Northwest of China. Before the chemical analysis and in situ degradation, samples were dried at 55°C for 48 h, and ground through a 1.0 mm screen.
Chemical analysis
Dry matter (DM) and crude protein (N × 6.25) were analyzed according to the procedures of the AOAC (Citation1990). Neutral detergent fibre (NDF), acid detergent fibre (ADF) and lignin were determined using filter bags (Ankom Technology). Heat-stable amylase and sodium sulfite were used in the NDF procedure and the results are expressed with residual ash. Acid detergent lignin (ADL) was determined by the methods of Van Soest and Robertson (Citation1985).
In situ degradation
Four rumen fistulated goats (31.5 ± 3.6 kg) were used to measure the degradation kinetics of DM, NDF and ADF of different forages and by-products. Goats were placed in individual metabolism crates (0.75 m × 1.5 m) under continuous lighting with free access to water throughout the experiment. Goats were fed a medium concentrate diet (the ratio of corn silage to grain mixture is 50:50) three times daily at 8 am, 2 pm and 8 pm. Approximately 2.0 g of DM of test feeds was weighed into the artificial nylon bags (6 cm × 9 cm) with a pore size of 50 μm. The bags were removed after intervals of 0, 6, 12, 24, 48 or 72 h and washed in cold running water until the washing ran clear.
Calculation and statistical analyses
Degradability coefficients of DM, NDF and ADF were calculated by fitting the data to the model of Wang et al. (Citation2004):
The effective ruminal degradability values were calculated:
where t = incubation time, L = discrete lag time, P = the cumulative percentage degraded at time t, A = the readily soluble fraction, B = the fraction potentially degraded in the rumen, C = the constant rate of degradation of B, ED = the effective ruminal degradability and K = ruminal outflow rate.
A non-linear regression method of SAS (SAS Inst., Inc., Cary, NC) was used to estimate degradability coefficients. A simple correlation analysis and linear regression analysis were used to establish the relationship between chemical composition and degradability coefficients.
Results
Chemical composition of forages and by-products
Chemical composition of forages and by-products is presented in . Crude protein (CP) content of all species ranged from 1.9% to 21.7%. The minimum CP content was recorded for corn cobs and the maximum CP was obtained with fresh alfalfa. The results of fibre measurements showed very high fibre content for corn straw and wheat straw, cottonseed hulls and corn cobs, and lowest fibre content was recorded for fresh ryegrass. Lignin ranged from 2.5% to 17.4%. Fresh ryegrass had the lowest lignin, while cottonseed hulls had the highest value. Within alfalfa and corn plant, CP content declined, and fibre and lignin content increased with advancing maturity.
Table 1. Chemical composition of forages and by-products.
Degradation kinetics of DM
In situ DM degradation kinetic parameters of forages and by-products are presented in . There was large variability in lag time (L), soluble fraction (A), potentially degraded fraction (B), degradation rate (C) and indigestible fraction (IG). The highest ED of DM (DMD) was obtained from fresh clover (76.3%), while the lowest value was obtained from corn cobs (6.0%). Within alfalfa and corn plant, ED declined with advancing maturity, and the most mature forage always produced the lowest ED.
Table 2. Degradation kinetics of dry matter of forages and by-products.
Degradation kinetics of fibre
In situ NDF and ADF degradation kinetic parameters of forages and by-products are presented in and , respectively. In general, there was a low (2.1% ± 1.7%) soluble fraction for NDF and ADF degradation data. ED of NDF (NDFD) ranged from 4.2% to 52.5%, and that of ADF (ADFD) ranged from 3.2% to 41.1%. Corn cobs had the lowest ED and highest IG.
Table 3. Degradation kinetics of neutral detergent fibre of forages and by-products.
Table 4. Degradation kinetics of acid detergent fibre of forages and by-products.
Relationship between fibre degradation kinetics and chemical composition
Pearson correlation coefficients between degradation kinetics and chemical composition are shown in . CP content negatively correlated (P < 0.05) with NDF, ADF and lignin content. The correlation found between DMD and CP content was positive (P < 0.05); DMD negatively correlated (P < 0.05) with the content of NDF, ADF and lignin.
Table 5. Correlation coefficients (r) for nutritional variables in forages and by-products.
In the present study, we observed that NDFD and ADFD were positively correlated with CP content but negatively with the content of NDF, ADF and lignin (P < 0.05).
There were significantly positive correlations (P < 0.05) among DMD, NDFD and ADFD. In present study, NDF content has stronger correlations with DMD, NDFD and ADFD than either CP, ADF or lignin.
Discussion
Fibre is important to maintain rumen health and as an energy source for ruminants. Rumen fibre degradability largely determined available energy value of fibrous feedstuffs and milk fat synthesis of dairy ruminants (NRC Citation2001; Robinson et al. Citation2004; Xu Citation2007). It is very important to predict fibre degradability in feeding strategies of ruminants.
Fourteen feedstuffs represented main forages and by-products which supply fibre for ruminants in the northwestern China in the present study. Whole cottonseed is popular in dairy diets, but in this study cottonseed hulls were selected to avoid potential negative effects of fat in whole cottonseed on rumen fibre degradation. Ground corn cob is often added into beef diet. Corn straw silage is made of green mature corn plant taken off corn cob and is widely used in ruminant production in this area. NDF content has increased by approximately 15% units in corn straw silage relative to corn silage of normal maturity. Nutrient contents of other test feedstuffs were in agreement with NRC (Citation2001).
Increases in NDF, ADF and lignin content with advancing maturity of alfalfa and corn plant are supported by several researchers (Hoffman et al. Citation1993; Coleman et al. Citation2003; Chaves et al. Citation2006). Negative correlation between CP content with NDF, ADF and lignin is in accordance with several findings (Coleman et al. Citation2003; Kamalak et al. Citation2005, Citation2006; Arzani et al. Citation2006).
DMD, NDFD and ADFD were calculated from their degradation characteristics. The ruminal outflow rate was assumed to be 2 %/h which is an average value for animals fed at approximately maintenance level. Low DMD (3%–15%) for corn straw (mature) and corn cobs was related to high content of cell walls (80%–94%). As the process of plant maturity, decreases in DMD, NDFD and ADFD are in agreement with the findings of Coblentz et al. (Citation1998) and Chaves et al. (Citation2006).
Long lag time for corn straw (mature), wheat straw and corn cobs was observed in both DM and fibre degradation. Wang et al. (Citation2004) and Xu et al. (Citation2007) also reported long lag time of 5–15 h for cereal straw.
Several researches demonstrated that DMD is positively correlated with CP content in forages and agro-industrial by-products (Hoffman et al. Citation1993; Kamalak et al. Citation2005; Kamalak Citation2006). Abundant data show that DMD has negative correlations with both NDF and ADF content in forages (Hoffman et al. Citation1993; Arzani et al. Citation2006; Damiran et al. Citation2008), tree or shrub leaves (Kamalak et al. Citation2005, Citation2006; Bakshi & Wadhwa Citation2007; Gurbuz Citation2007) and by-products (Mekasha et al. Citation2002). The correlation of DMD was positive with CP content but negative with NDF, ADF and lignin content as expected in this study.
It can be seen from this study that NDFD and ADFD were positively correlated with CP content but negatively with the content of NDF, ADF and lignin. This result is in agreement with findings of Hoffman et al. (Citation1993) who found that in situ NDF degradability was positively correlated with CP content. Previous researches showed that NDFD was negatively correlated with cell walls in different type, maturity and cutting frequency forages (Hoffman et al. Citation1993; Spanghero et al. Citation2003). Through using near-infrared reflectance spectroscopy, several studies also have demonstrated that NDF degradable characteristics linearly decreased with the increasing of NDF content of forages (Nousiainen et al. Citation2004; Nordheim et al. Citation2007). Predicting fibre degradability from NDF could provide guidance to Chinese nutritionists for meeting energy requirement and determining feed quality in ruminants fed diets containing fibrous forages and by-products.
Lignin is the cell wall constituent that binds cell wall carbohydrates. A higher lignin content or lignin to NDF ratio implies more lignifications of the plant cell wall, and logically would reduce the fibre degradability. As a result, NRC (Citation2001) predicts fibre degradability using lignin to NDF ratio. Some researchers argued that there are no strong correlations between fibre degradable characteristics and lignin content or lignin to NDF ratio (Coleman et al. Citation2003; Nousiainen et al. Citation2004; Robinson et al. Citation2004). In the present study, we have observed that NDFD and ADFD had a medium correlation (R 2 = 0.54–0.67) with lignin content.
Positive correlation among DMD, NDFD and ADFD is supported by Hoffman et al. (Citation1993), López et al. (Citation1998) and Coleman et al. (Citation2003). In the present study, DMD, NDFD and ADFD were better related to NDF content than other chemical compositions. Selected regressions for prediction of ruminal degradability were stated in .
Table 6. Prediction regressions of DMD, NDFD and ADFD by NDF and CP in forages and by-products.
Present data suggest that effective ruminal degradability of DM, NDF and ADF was positively correlated with CP content but negatively with the content of NDF, ADF and lignin. NDF, ADF and CP content could predict DMD, NDFD and ADFD of forages and by-products. In conclusion, chemical analysis is essential not only for understanding nutrient contents, but also for predicting rumen degradable characteristics in common fibrous feedstuffs.
Disclosure statement
No potential conflict of interest was reported by the authors.
Additional information
Funding
References
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