In situ degradation kinetics of 6 roughages and the intestinal digestibility of the rumen undegradable protein

Abstract

Three ruminally fistulated Xuanhan steers weighting 312.5 (±23.85) kg were used to determine the kinetics of ruminal degradation of nutrients using in situ nylon bag technique, and a modified 3-step in vitro procedure was adopted to estimate intestinal digestibility of 16-h rumen undegradable protein (RUP) of maize cob (MC), distillers grains (DG), spent mushroom substrate (SMS), starch residue of sweet potato (SRSP), citrus pulp (CPP), and rice straw (RS). Samples were incubated for 0, 2, 6, 16, 24, 36, 48 and 72 h. Additional samples were incubated for 16 h in the rumen, and the residues from these bags were transferred to the nitrogen-free polyester bags for determination of intestinal digestibility in vitro. The highest DM disappearance at 6-h incubation was in SRSP (P < 0.01), and that at 36, 48, and 72 h was in CPP (P < 0.01). The lowest DM disappearance at 2- and 6-h incubation was in RS and SMS (P < 0.01), and that at 36, 48, and 72 h incubation was in RS, MC, and DG (P < 0.01). The lowest and greatest CP disappearance was in RS and DG, respectively, at all the incubation times (P < 0.01). There was no difference (P > 0.07) on CP disappearance between DG and MC at all the time points except for 16 and 24 h. NDF and ADF disappearance for SRSP was significantly higher (P < 0.01) than other roughages at all the time points except for ADF at 72 h. The lowest NDF and ADF disappearance was in DG at all the time points (P < 0.01) except 2 and 6 h. The effective degradability (ED) of DM was the highest in CPP (P < 0.01) and the lowest in MC and RS (P < 0.01). The highest and lowest ED of CP was in DG and in RS (P < 0.01), respectively. The ED of NDF was the highest in SRSP (P < 0.01), followed by CPP and RS, and the lowest in DG (P < 0.01). The ED of ADF was the highest in SRSP and CPP (P < 0.05), and the lowest in DG (P < 0.01). For MC, DG SMS, SRSP, CPP, and RS, the intestinal digestibility of RUP was 95.28%, 37.23%, 38.72%, 48.06%, 54.49%, and 37.88%, respectively, and the content of intestinal digestible crude protein (IDCP) was 23.65, 83.63, 35.63, 15.03, 25.60, and 12.03 g/kg, respectively. Distillers grain was considered to be of good quality for the greatest content of IDCP. Although not readily degraded in rumen, CP in MC may be digested well in small intestine.

INTRODUCTION

Roughage was the main source of nutrients for ruminants, accounting for 40% to 80% of ruminant diets. The rumen degradable characteristics and intestinal digestibility of CP in roughage were important indexes in appraising nutritive values. When the amount of rumen undegradable protein (RUP) in feeds is known, diets can be formulated to maximize ruminal microbial protein synthesis without limiting available N, which is important because ruminal microbial protein provides a near optimal balance of essential AA for protein synthesis (Schingoethe, 1996). RUP is used by host animal in small intestine. Owing to the lack of empirical data, NRC (1989) suggested a constant intestinal digestion of RUP of 80% for all feeds, although it was recognized that the value differs among feeds. Obviously using the constant value of intestinal digestibility of RUP will lead to the wrong prediction of nutrient needs. Therefore, intestinal digestibility of RUP has become an important variable in recent protein evaluation systems for ruminants (Hvelplund and Nørgaard, 2003).

Many researchers have studied rumen degradability of protein supplements in both plant and animal feeds (Erasmus et al., 1994; Cozzi et al., 1995). Using nylon bag technique, Kamalak et al. (2005) examined the DM degradability of wheat straw, barley straw, lucerne hay, and maize silage. Gargallo et al. (2006) studied intestinal digestibility of 12 feedstuffs including blood meal, fish meal, green peas, lupin seed, whole cottonseed, corn gluten meal, alfalfa pellets, heat-processed SBM, sunflower seeds, barley-dried distillers grains, corn-dried distillers grains (DG), and corn gluten feed. Calsamiglia et al. (1995) studied intestinal digestibility of soybean meal, blood meal, hydrolyzed feather meal, corn gluten meal, and fish meal. However, there are limited data reported in the literature on intestinal digestibility of the RUP fraction of roughage. This study aimed to estimate rumen degradation characteristics and intestinal RUP digestibility of maize cob (MC), distillers grains (DG), spent mushroom substrate (SMS), starch residue of sweet potato (SRSP), citrus pulp (CPP), and rice straw (RS), using in situ nylon bag technique and modified 3-step in vitro procedure (TSP). The outcome of the study may enrich and improve the database of nutritive value of roughages in beef cattle, which in turn contributes to the improvement of ruminant production.

MATERIALS AND METHODS

This study was conducted at the Teaching and Experiment Station in Sichuan Agricultural University, Ya’an, Sichuan, China. All animals were managed in accordance with the guidelines of the Animal Care and Ethic Committee of Sichuan Agricultural University.

Animals and Diets

Three ruminally fistulated Xuanhan steers with an average BW of 312.5 (±23.85) kg were used to evaluate ruminal degradability of nutrients. Animals were fed twice daily at 8:00 and 16:00 h, and access to water ad libitum. The composition and nutrient level of the basal diet were summarized in Table 1.

Table 1.

Ingredient and chemical composition of the basal diet fed during the in situ experiment (DM basis)

Ingredient	Amount, % of DM
Ingredients
Corn	35.53
Wheat bran	5.13
Alfalfa meal	25.48
Distilled grain	7.06
Rice straw	25.02
NaCl	0.5
NaHCO3	0.58
Premix1	0. 70
Total	100.00
Chemical composition2
DM	88.14
CP	10.40
Fat	2.80
Ca	0.53
P	0.33
NDF	38.77
ADF	21.30

¹Supplied the following per kg of diet: 2200 IU, vitamin A; 2751 IU, vitamin D3; 11 IU, vitamin E; 50 mg, Fe; 25 mg, Cu; 20 mg, Mn; 30 mg, Zn; 0.50 mg, I; 0.10 mg, Se.

²Values for the contents of dry matter, crude protein, neutral detergent fiber; acid detergent fiber; Ca and P were analyzed.

Samples Preparation

MC，DG，SMS，SRSP, CPP, and RS were collected commercially in Sichuan province. The samples were dried at 55 °C for 48 h in a forced-air oven and then milled through a 1-mm sieve for chemical analysis and 3-mm sieve for in situ degradation. DM was determined by drying the samples at 105 °C overnight. Nitrogen (N) content was measured by the Kjeldahl method (AOAC, 1990). CP was calculated as N × 6.25. NDF and ADF contents were determined by the method of Van Soest et al. (1991).

In Situ Nylon Bag Experiment

The in situ DM, CP NDF, and ADF degradation in the 6 roughages were determined according to the procedure described by Mehrez and Ørskov (1977). Five-gram samples dried and milled through a 3-mm sieve were weighed into nylon bags (45-m pore size, 8- × 16-cm bag size) in triplicate and incubated for 2, 6, 16, 24, 36, 48, and 72 h in the rumen of 3 ruminally fistulated steers. After removal of the bags at each time point, each bag was washed in running tap water until the outlet water becomes clear. Bags were then dried to a constant weight at 55 °C for 48 h and weighted. The residues were ground through 1-mm sieve for laboratory analysis.

A Modified 3-Step In Vitro Procedure

The 16-h rumen undegradable samples were obtained from the in situ nylon bag experiment and were then digested using the pepsin and pancreatin digestion steps of the modified TSP described by Gargallo et al. (2006). The 16-h RUR samples were ground to pass a 1-mm screen. One gram of RUP sample was weighed into nitrogen-free polyester bags (3.5 × 5.5 cm, pore size 50 ± 15 cm). The samples were then incubated in a pepsin/HCI solution (pH = 1.9) for 1 h in an incubator (Ankom Technology) and then incubated in a pancreatin/KH₂PO₄ solution at 38 °C for 24 h. After 24-h incubation, the liquid was drained from bottles, and nylon bags were washed in running tap water until the outlet water becomes clear. Bags were dried in a forced hot air oven at 55 °C for 48 h. The weight of samples and bags was recorded, and bags were then opened and samples for laboratory analysis. Digestibility of 16-h RUP was calculated based on the RUP remaining in the bags and that disappearing from the bags.

Calculation and Statistical Analyses

The degradation data were fitted to the following exponential equation:

$$\text{y}=\text{a}+\text{b }(\text{1}-{\text{e}}^{-\text{ct}}),$$ (1)

where y is the nutrient disappearance in rumen at time t, a is the the rapidly soluble fraction, b is the the slowly soluble fraction, and c is the the constant rate of degradation of b (%/h).

Effective degradability (ED) of nutrients was calculated applying the following equation of Ørskov and McDonald (1979):

$$\text{ED}=\text{a}+\left(\text{bc}/\left(\text{c}+\text{k}\right)\right),$$ (2)

where a, b, and c are the same as in equation (1) and k is the rumen outflow rate. The ED of the nutritions was calculated using the outflow rate of 0.031/h at the maintenance level, according to Ørskov et al. (1988).

Pepsin-pancreatin digestion of CP in the 16-h undegraded residuals was calculated as follows:

$$\text{Intestinal digestibility} (\%)=[(\text{amount of CP in, g}, - \text{amount of CP out, g})/\text{amount of CP in, g}]\times 100.$$

The intestinal digestible crude protein (IDCP) content of the original feeds can be calculated from ED (IDCP_ED) and 16-h RUP (IDCP_{16-h RUP}) as follows:

$${\text{IDCP}}_{\text{ED}}\left(\%\text{ of original feed}\right)=(\%\text{ ED}\times 0.85\times 0.775)+(100-\%\text{ ED})\times (\%\text{ intestinal digestibility})÷100,$$

$${\text{IDCP}}_{16-\text{h RUP}}\left(\%\text{ of original feed}\right)=(\%\text{ 16}-\text{h RUP}\times 0.85\times 0.775)+(100-\%\text{ 16}-\text{h RUP})\times \left(\%\text{ intestinal digestibility}\right)÷100.$$

The coefficient of ruminal degraded protein to microbial protein (MCP) was 0.85 (NRC, 2001), and intestinal digestibility of MCP was 0.775 (Storm, 1983).

Analysis of variance (ANOVA) was carried out for chemical composition disappearance and estimated parameters using General Linear Model (GLM) procedure of SAS. Significant differences between the intestinal digestible of samples were identified using Tukey’s Multiple Range Test (Pearson et al. (1966)). Mean differences were considered significant at P < 0.05. Standard errors of means were calculated from the residual mean square in the analysis of variance.

RESULTS

Chemical Composition

Chemical compositions of the 6 roughages are presented in Table 2. DM content did not differ among the roughages (P = 0.668). CP content was the greatest in DG followed by SMS (P < 0.01). CP content in SRSP was the same as in RS (P = 0.686), lower than in other roughages (P < 0.01). NDF content did not differ between MC and RS (P = 0.181), greater than in other roughages (P < 0.01). NDF content was the same in CPP as in SRSP (P = 0.315), lower than in other roughages (P < 0.05). There were no differences on ADF content among MC, RS, and SMS (P = 0.892), and among CPP, DG, and SRSP (P = 0.533).

Table 2.

Chemical compositions of the 6 roughages

	Roughages
Content (%)	MC	DG	SMS	SRSP	CPP	RS	SEM1	P value
DM	92.76	87.23	90.29	89.27	89.84	91.24	0.644	0.768
CP (DM basis)	3.04a	14.20b	7.48c	2.69d	4.49e	2.73d	1.557	< 0.001
NDF(DM basis)	79.25a	42.61b	57.82c	32.79d	31.78d	75.58a	7.218	< 0.001
ADF(DM basis)	45.80a	34.65b	45.51a	27.19b	27.23b	45.71a	3.163	< 0.001

^a-eWithin a row, means with uncommon superscripts differ (P < 0.05).

¹SEM = standard error of the mean.

The Disappearance Rates of Nutrients

Nutrient disappearance at different incubation time is presented in Table 3. The highest DM disappearance was in SRSP (P < 0.01) at 2 and 6 h, and in CPP (P < 0.01) at 48 and 72 h. There was no difference on DM disappearance from 6- to 72-h (except for 24-h) incubation between MC and RS (P > 0.11), which was significantly lower (P < 0.001) than that in SMS, SRSP, and CPP. CP disappearance in MC was the same as (P > 0.07) that in DG at all the time point except for 16 and 24 h, higher (P < 0.01) than that in all the other roughages, and that in RS was the lowest (P < 0.01) at all the time points. NDF disappearance was the highest in SRSP (P < 0.01) at all the time points. NDF disappearance was the lowest (P < 0.01) in SMS at 2 and 6 h, and in DG after 16 h. ADF disappearance was the highest (P < 0.01) in SRSP at all the time points (P < 0.01) and the lowest in DG from 24 to 72 h.

Table 3.

In situ nutrient disappearance of 6 roughages at different incubation times

Incubation time (h)	Roughages
	MC	DG	SMS	SRSP	CPP	RS	SEM1	P-value
DM
2	7.89a	9.03a	2.24b	21.55c	21.16c	2.45b	2.998	< 0.001
6	9.00a	15.43b	8.83a	30.53c	22.99d	8.79a	3.135	< 0.001
16	13.5a	27.86b	19.43c3	7.04d	32.97d	15.36a	3.347	< 0.001
24	17.68a	28.37b	31.67b	42.87c	40.44c	21.17d	3.481	< 0.001
36	31.06a	32.21a	41. 83b	44.96bc	52.41d	28.44a	3.246	< 0.001
48	30.31a	32.81a	46.77b	44.32b	66.84c	30.03a	4.902	< 0.001
72	35.8a	34.35a	49.49b	54.49b	84.31c	33.21a	6.751	< 0.001
CP
2	40.52a	39.50a	25.96b	33.96c	13.30d	8.46e	4.680	< 0.001
6	42.52ab	49.19a	28.69c	38.19b	14.14d	16.16d	4.937	< 0.001
16	59.43a	75.62b	32.82c	43.82d	22.14e	22.12e	7.414	< 0.001
24	63.00a	80.35b	35.75c	48.75d	34.24c	23.71e	7.239	< 0.001
36	72.71a	83.75a	35.59b	48.09c	42.57c	29.02d	7.462	< 0.001
48	73.99a	85.25a	42.44b	44.44b	60.50c	35.05d	6.815	< 0.001
72	78.51a	87.82a	45.87b	44.87b	77.28a	39.27b	7.307	< 0.001
NDF
2	8.56a	5.98b	3.74c	30.15d	5.10e	8.27a	7.620	< 0.001
6	12.12a	8.75b	6.53c	39.53d	7.08c	20.01e	4.381	< 0.001
16	16.94a	13.77b	15.21ab	49.51c	14.41b	27.13d	4.821	< 0.001
24	21.24a	16.78b	27.24c	54.24d	21.39a	36.35e	4.781	< 0.001
36	34.23a	17.40b	36.47a	56.47c	36.00a	47.42d	4.564	< 0.001
48	32.54a	18.11b	41.72c	57.72d	56.95d	54.04d	5.482	< 0.001
72	38.46a	21.65b	44.09a	70.79c	78.47c	57.41d	7.319	< 0.001
ADF
2	9.60a	8.35a	2.20b	27.20c	5.13d	7.11e	3.054	< 0.001
6	14.47a	10.16b	4.68c	38.18d	4.93c	13.93a	4.276	< 0.001
16	20.37a	12.85b	13. 25b	44.25c	13.08b	18.45a	4.181	< 0.001
24	24.85a	14.76b	25.61a	50.91c	23.27a	22.39a	4.263	< 0.001
36	38.74a	15.04b	35.32a	54.48c	37.30a	25.58d	4.588	< 0.001
48	41.06a	17.52b	39.62a	55. 31c	54.35c	31.24d	4.937	< 0.001
72	44.26a	17.76b	42.28a	66.88c	75.11c	34.62d	7.276	< 0.001

^a-eWithin a row, means with uncommon superscripts differ (P < 0.05).

¹SEM = standard error of the mean.

The Degradation Kinetic Parameters and ED

The degradation kinetic parameters and ED are summarized in Table 4. Parameter a was the greatest (P < 0.01) for DM, CP, NDF, and ADF in SRSP, the lowest (P < 0.01) for DM and ADF in SMS, and the lowest (P < 0.01) for CP and NDF in CPP. Parameter b was the greatest (P < 0.01) for DM, CP, NDF, and ADF in CPP, the lowest (P < 0.01) for DM, NDF, and ADF in DG, and the lowest (P < 0.01) for CP in SRSP. The greatest c for DM was in DG and SMS (P < 0.05), for CP was in DG (P < 0.01), for NDF was in SMS and RS (P < 0.05), and for ADF was in DG (P < 0.01). The lowest c for DM was in SRSP and CPP (P < 0.05), for CP was in SMS and CPP (P < 0.05), for NDF was in SRSP (P < 0.01), and for ADF was in SRSP and CPP (P < 0.05).

Table 4.

Ruminal degradation kinetics and effective degradability of nutrients of 6 roughages

	Roughage
Item	MC	DG	SMS	SRSP	CPP	RS	SEM1	P value
a, %
DM	1.03a	9.47b	−11.92c	25.07d	5.97e	0.02f	4.228	<0.001
CP	34.89a	39.82b	25.02b	33.60a	−0.56c	8.31d	5.584	<0.001
NDF	3.10a	6.76b	−10.01c	33.70d	−14.98e	0.03f	5.892	<0.001
ADF	0.00a	5.45b	−5.90c	30.68d	12.43e	5.54b	4.375	<0.001
b, %
DM	37.88a	25.19b	62.58b	37.78a	93.56d	35.05a	8.702	<0.001
CP	45.44a	48.40b	25.90b	15. 56c	00.04d	36.51e	10.171	<0.001
NDF	37.97a	17.10b	55.47cd	51.28c	113.61e	59.63df	11.120	<0.001
ADF	45.88a	12.48b	50.92a	48.48a	111.34c	31.51d	11.477	<0.001
c,h-1
DM	0.034a	0.60b	0.055b	0.020c	0.023c	0.041d	0.006	<0.001
CP	0.044a	0.65b	0.023cd	0.050e	0.020df	0.026c	0.006	<0.001
NDF	0.036a	0.027b	0.052c	0.017d	0.022be	0.049cf	0.005	<0.001
ADF	0.046a	0.60b	0.043ac	0.018d	0.020de	0.35cf	0.006	<0.001
ED, %
DM	20.90a	26.10b	28.15b	39.84c	46.02d	20.20ae	3.633	<0.001
CP	61.57a	72.65b	36.00b	43.22c	38.19bc	24.97d	6.103	<0.001
NDF	23.58a	14.79b	24.72b	51.79c	31.80d	35.56d	4.388	<0.001
ADF	27.48a	13.67b	23.54ac	48.68d	55.83d	22.34c	5.706	<0.001

^a-eWithin a row, means with uncommon superscripts differ (P < 0.05).

¹SEM = standard error of the mean.

The highest ED of DM was found in CPP (P < 0.05), followed by SRSP (P < 0.05), and the lowest was in MC and RS (P < 0.01). The ED of CP was the greatest in DG and MC (P < 0.01), and the lowest in RS (P < 0.01). There was no difference (P = 0.14) on ED of CP between SRSP and CPP. There was no difference on ED of NDF between MC and SMS (P = 0.35), and between CPP and RS (P = 0.16). The highest ED of NDF was in SRSP (P < 0.01), and the lowest in DG (P < 0.01). The highest ED of ADF was in SRSP and CPP (P < 0.05), and the lowest in DG (P < 0.01). There was no difference on ED of ADF between MC and SMS (P = 0.08), and between SMS and RS (P = 0.32).

Intestinal Digestibility of RUP and the IDCP Content of Feeds

The intestinal digestibility of RUP of the 6 roughages and the IDCP content are presented in Table 5. The intestinal digestibility of 16-h RUP of MC was the highest (P < 0.05). There was no difference on intestinal digestibility of 16-h RUP between SRSP and CPP (P = 0.12), and among DG, SMS, and RS (P > 0.55). The content of IDCP was the highest in DG (P < 0.01), followed by SMS (P < 0.01), and the lowest in RS (P < 0.05).

Table 5.

Intestinal digestibility of rumen undegradable protein (RUP) and intestinal digestible CP (IDCP) content of the 6 roughages

	Roughages
Items	MC	DG	SMS	SRSP	CPP	RS	SEM1	P value
CP content %	3.04a	14.20b	7.48c	2.69d	4.49e	2.73d	1.557	< 0.001
ED %	61.57a	72.65a	36.00b	43.22b	38.19b	24.97c	6.103	< 0.001
16-h RDP, %	59.43a	75.62b	32.82c	43.82d	22.14e	22.12e	7.414	< 0.001
Idg of RUP, %	95.28a	37.23b	38.72b	48.06c	54.49c	37.88b	7.70	< 0.001
IDCP16-hRUP, g/kg	23.65a	83. 63b	35.63c	15.03d	25.60a	12.03e	9.09	< 0.001
from RDP, g/kg	12.33a	67.96b	17.74c	7.66d	11.30a	4.49e	8.21	< 0.001
from RUP, g/kg	11.13a	14.46b	18.54c	7.34d	15.12b	7.76d	1.53	< 0.001
IDCPED, g/kg	23.46a	82.42b	36.27c	15.00d	26.42a	12.25e	8.92	< 0.001
Relative error2	−0.0082	−0.0147	0.0178	−0.0019	0.0311	0.0178	–	–

^a-eWithin a row, means with uncommon superscripts differ (P < 0.05).

¹SEM = standard error of the mean.

²Relative error = (IDCP_ED – IDCP_16-hRUP)/IDCP_ED.

Distillers grains provided the vast amount of IDCP, followed by SMS, and the lowest in RS. Among the IDCP contained in DG (83.63%), 81.3% was from rumen degradable protein (RDP). IDCP content in CPP was 25.60%, 59.1% of what was from RUP.

DISCUSSION

Chemical Composition of the Roughages

Chemical composition (CP, NDF, and ADF) of most of the roughages investigated in this study was within the range of other reports (Table 6). It should be noted that CP content in DG was much lower, whereas ADF content was much greater, in this study, than those in Preston et al. (2011) and Spanghero et al. (2010), respectively. Similar circumstances were found in CPP, in which CP content was much less while ADF content was much greater than those reported by Preston et al. (2011) and Pereira et al. (2004), respectively. Chemical composition of roughages was influenced by many factors such as plant varieties, different parts of plant, harvest time, climatic conditions, soil conditions, fertilizer, and maturity level. It is possible that the different raw materials of DG lead to the great discrepancy of nutrient contents between this study and other reports. Spiehs et al. (2002) and Fastinger et al. (2006) reported that the nutritional composition and nutritional value of dried distillers grains with solubles (DDGS) differed greatly due to the different raw materials used in United States. CP and NDF content of SMS in this study was within the ranges reported by Bae et al. (2006). ADF content of SMS was lower than the results of Bae et al. (2006) and Kwak et al. (2008). The chemical composition of SMS was affected mainly by the primary culture ingredient rather than mushroom species. Different components of SMS may lead to such differences in nutrient contents. There was no other report on nutrient content of SRSP.

Table 6.

Nutritional composition of feedstuffs compared with other researches (DM basis)

Feedstuff	CP	NDF	ADF	Researcher
MC
	2.50%	–	45.80%	Kaliyan et al. (2010)
	2.60%	78.80%	43.70%	Spanghero et al. (2010)
	3.04%	79.25%	45.80%	This study
DG
	28.00%	40.00%	18.00%	Preston et al. (2011)
	28.90%	35. 80%	18.60%	Spanghero et al. (2010)
	14.20%	42.61%	34.65%	This study
SMS
	7.2–11.1%	64–78.2%	42. 8–66.3%	Bae et al. (2006)
	5.90%	76.10%	59.10%	Kwak et al. (2008)
	7.48%	57.82%	45.51%	This study
CPP
	7.00%	21.00%	20.00%	Preston et al. (2011)
	6.54%	25.90%	16.10%	Pereira et al. (2004)
	4.49%	31.78%	27.23%	This study
RS
	4.00%	72.00%	47.00%	Preston et al. (2011)
	5.60%	76.90%	44.60%	Eun et al. (2006)
	2.73%	75.58%	45.71%	This study

Nutrient Disappearance of the Feeds

Kamalak et al. (2005) reported that the rate and extent of DM fermentation in rumen were important determinants for the nutrients absorbed by ruminants. DM disappearance of SRSP and CPP was higher than that of other feeds at all incubation times (Figure 1). DM disappearance of DG, MC, and RS tended to be overlap after 36-h incubation and lower than other roughages. These results indicated that SRSP and CPP were more digestible, and DG, MC, and RS were less digestible than other roughages. It should be noted that CP disappearance of DG and MC, especially DG, was considerably higher than other roughages (Figure 2) despite the very low DM disappearance, which illustrated that CP in DG and MC was utilized well in rumen. The low DM disappearance in DG and MC may be due to the low NDF and ADF disappearance (Figures 3 and 4) in these 2 roughages. In this study, NDF and ADF disappearance of DG was the lowest after 16-h incubation. NDF and ADF disappearances were important parameters that reflect the degree of difficulty the feed is digested.

Figure 1.

DM disappearance of 6 roughages in the rumen of Xuanhan steer.

Figure 2.

CP disappearance of 6 roughages in the rumen of Xuanhan steer.

Figure 3.

NDF disappearance of 6 roughages in the rumen of Xuanhan steer.

Figure 4.

ADF disappearance of 6 roughages in the rumen of Xuanhan steer.

The ED of the Roughages

The ED of DM of DG was 48.08% in this study. Batajoo et al. (1998) reported that the ED of DM of distillers dried grain (DDG) was 58.3% at an outflow rate of 0.07/h. The great discrepancy of ED of DM between the present study and other reports was mainly due to the various outflow rates used in different reports (Table 7). Martins et al. (1999) studied the ED of DM of CPP and found that the value was 79.8%, 67.5%, and 61.7%, when the outflow rate was 0.02, 0.05, and 0.08 per h, correspondingly. The ED of DM of DG and CPP in this study were lower than that in the reports of Batajoo et al. (1998) and Martins et al. (1999), respectively.

Table 7.

The ED of DM of feedstuffs compared with other research results

Feedstuff	Breeds	Outflow rate	ED	Researcher
DDG	Holstein cow	0.07	58.30%	Batajoo et al. (1998)
DG	Xuanhan steers	0.031	48.08%	This study
Citrus pulp	Holstein cow	0.02	79.80%	Martins et al. (1999)
Citrus pulp	Holstein cow	0.05	67.50%	Martins et al. (1999)
Citrus pulp	Holstein cow	0.08	61.70%	Martins et al. (1999)
Citrus pulp	Xuanhan steers	0.031	46.02%	This study

The RDP of DDGS was reported to be from 28% (Archibeque et al., 2008) to 44% of CP (Kelzer et al., 2010a), quite lower than the ED of CP of DG in this study. Rumen degradation characteristics of protein contained in DDGS may be influenced by fermentation material (Spiehs et al., 2002). Protein in DDGS originating from yeast may resist rumen degradation (Castillo-Lopez et al., 2010). Bateman et al. (2005) reported that dairy cattle would have a greater passage rate than steers, and therefore, different animal breeds may also contribute to the variability of ED of CP. Satter (1986) concluded that ED of CP negatively correlated to passage rate. The ED of CP of DG in this study was 83.5% higher (Table 8) than that reported by Batajoo et al. (1998). The ED of CP of DG in this study was 61.2% when the outflow rate was 0.07/h, which was still 54.5% higher than the result of Batajoo et al. (1998).

Table 8.

The ED of CP of feedstuffs compared with other research results

Feedstuff	Variety	Out of rate	ED	Researcher
DDG	Holstein cow	0.07	39.6%	Batajoo
DG	Xuanhan steer	0.031	72.6%	This study
DG	Xuanhan steer	0.07	61.2%	This study
DG	Holstein cow	0.07	39.6%	Batajoo et al. (1998)
Citrus pulp	Holstein cow	0.02	70.4%	Martins et al. (1999)
Citrus pulp	Holstein cow	0.05	62.1%	Martins et al. (1999)
Citrus pulp	Holstein cow	0.08	59.1%	Martins et al. (1999)
Citrus pulp	Xuanhan steer	0.031	40.0%	This study

NDF and ADF represent the fraction of feedstuffs uneasily digested by livestock. The facts that both disappearance and ED of NDF and ADF were the highest in SRSP indicated that SRSP was easily degraded in rumen and may have greater digestibility. Similarly, these observations were the lowest in DG reflecting the high level of lignification which should retard the rumen degradation and the further digestibility. But the lignification may not trap CP from microbial degradation because both disappearance and ED of CP were the highest in DG.

The Intestinal Digestibility of RUP

Hvelplund et al. (2000) reported that the intestinal digestibility determined at 16 h could apply to all feeds and reflect the metabolic situation in rumen before the feeds reached the small intestine, and researchers Calsamiglia and Stern (1995) and Promkot and Wanapat (2003) applied rumen undegraded residues from bags of 16-h ruminal incubation for estimating intestinal digestion of protein in ruminants by in vitro pepsin-pancreatin digestion. This study investigated for the first time the intestinal digestibility of RUP in MC, SMS, SRSP, CPP and RS. Negi et al. (1988) reported that the protein escaping rumen degradation was combined with lignin and therefore not easy to be digested in small intestine. The intestinal digestibility of most of the roughages investigated in this study was indeed low (less than 55%), with the exclusion of MC. The RUP intestinal digestibility of MC (95.3%) was higher than all the other roughages, illustrating the RUP in MC could be digested well in small intestine. Because of the high CP content (14.2%) and the high ED of CP (72.65%), DG was considered to be of good quality, though the intestinal digestibility of RUP was low (37.23%). IDCP includes RUP and MCP synthesized from RDP reflecting the total digestible CP reaching the small intestine. The difference between IDCP_16-hRUP and IDCP_ED lies in the calculation of RDP (for calculating MCP production), which was based on 16-h RUP in IDCP_16-hRUP and ED of CP in IDCP_ED. This study revealed the trivial differences between IDCP16-hRUP and IDCPED in all the roughages investigated, with the biggest relative error of less than 3.11% (Table 5). Both IDCP_16-hRUP and IDCP_ED can be used to reflect the total digestible CP through the gastrointestinal tract of ruminants. In this study, DG provided the vast amount of IDCP, followed by SMS, and the lowest in RS (Table 5).

CONCLUSION

SRSP and CPP were easily digested in rumen, whereas DG, MC, and RS were resistant to digestion in rumen. CP in DG could be easily degraded in rumen, but RUP in DG was uneasy to be digested in intestine. CP in MC was utilized well both in rumen and in intestine. Distillers grain was considered to be of good quality for its high IDCP content.

IMPLICATION

Our results revealed that DM degradability may not synchronize with CP (or other nutrients) degradability. The digestibility of CP bypassing the rumen varied greatly with feedstuffs. This pattern indicates highly complicated nutrient utilization in rumen and in intestinal tract, which encourages researchers to make out the different portions of nutrient utilized in different parts of gastrointestinal tract for a better understanding of nutrient supply of feedstuffs. The higher rumen degradability and higher intestinal digestibility of CP in DG suggest that DG feeds the rumen microbes with more nitrogen and subsequently supplies sufficient digestible CP (both from microbial CP and from bypass CP) for host animal.

Conflict of interest statement. None declared.