Effect of inclusion level and adaptation duration on digestible energy and nutrient digestibility in palm kernel meal fed to growing-finishing pigs.

OBJECTIVE: An experiment was conducted to evaluate effects of inclusion level of palm kernel meal (PKM) and adaptation duration on the digestible energy (DE) and apparent total tract digestibility (ATTD) of chemical constituents in diets fed to growing-finishing pigs.
METHODS: Thirty crossbred barrows (Duroc×Landrace×Large White) with an average initial body weight of 85.0±2.1 kg were fed 5 diets in a completely randomized design. The diets included a corn-soybean meal basal diet and 4 additional diets in which corn and soybean meal were partly replaced by 10%, 20%, 30%, or 40% PKM. After 7 d of adaptation to the experimental diets, feces were collected from d 8 to 12, d 15 to 19, d 22 to 26, and d 29 to 33, respectively.
RESULTS: The DE and ATTD of gross energy (GE), dry matter (DM), ash, organic matter (OM), neutral detergent fiber (NDF), acid detergent fiber (ADF), and crude protein (CP) in diets decreased linearly as the dietary PKM increased within each adaptation duration (p< 0.01). Diet containing 19.5% PKM had less DE value and ATTD of all detected items compared with other diets when fed to pigs for 14 days (p<0.05). The ATTD of CP in PKM calculated by 19.5% and 39.0% linearly increased as adaptation duration prolonged from 7 to 28 days (p<0 .01).
CONCLUSION: Inclusion level of PKM and adaptation duration had an interactive effect on DE and the ATTD of GE, DM, OM, and CP (p<0.01 or 0.05) but ash, NDF, and ADF in diet (p> 0.05). Considering a stable determination, 21 days of adaptation to a diet containing 19.5% PKM is needed in pigs and a longer adaptation time is recommended as dietary PKM increases.

INTRODUCTION

Palm kernel meal (PKM) is a by-product after solvent extraction of the oil from the oil palm nut kernels [1]. Compared to conventional feedstuffs, PKM is cost effectiveness and mostly used in broiler and pig’s feeds, especial in the tropical regions of the world: Asia, Pacific, South America, and Africa [2]. Some nutritive evaluation studies for PKM have been done on chickens [3,4]. The digestible energy (DE) and metabolizable energy (ME) of PKM on pigs were previously reported by several studies [1,5,6]. However, most of those studies focused on only one substitution rate of PKM in experimental diet. The main limitation for optimum utilization of PKM in non-ruminant animal’s feed is the high fiber content in it. The higher substitution rate of evaluated feedstuff, the more accuracy of DE or ME can be get in it according to the difference method [7]. However, the higher level of dietary fiberous ingredient, the higher fiber content and which can affect energy and the other nutrient digestibility in diets when fed to pigs [8-10]. In addition, the adaptation duration of a new diet especial in high fiber also can affect the evaluation values of experimental diets and their nutrients digestibility [11-13]. Accurate assessment of the nutritive value of PKM is important for it’s optimum utilization in non-ruminant livestocks. There were no studies on investigating the interaction effect of inclusion level of PKM and the adaptation duration on the energy and other nutrient digestibility in diet. Therefore, the objective of the current experiment was carried out to test the hypothesis that there is a combination effect of graded increasing inclusion level of PKM and different adaptation duration of diets on the energy and nutrient digestibility in diet and PKM fed to growing-finishing pigs.

MATERIALS AND METHODS

The protocol for all animal procedures were approved by the Institutional Animal Care and Use Committee at China Agricultural University, Beijing, China.

Animals and housing

Thirty crossbred barrows (Duroc×Landrace×Large white) with an initial body weight (BW) of 85.0±2.1 kg were used. During experiments, pigs were housed individually in stainless-steel metabolism crates (1.7×0.9×0.7 m2). The crates had adjustable sides to accommodate animal welfare and were located in rooms with temperature controlled at 22°C±2.5°C. Humidity varied from 35% to 59% during the trial and the light and dark time was controlled at 12 h vs 12 h automatically. An adjustable tray that was used for total feces collection was placed under each cage, which permitted collection of feces. All pigs had ad libitum access to water via a drinking nipple.

Experimental diets

Thirty pigs were allotted to 5 diets in a completely randomized design according to their initial BW and with 6 pigs fed each diet. Diets included a corn-soybean meal basal diet containing 76.6% corn, 21.0% soybean meal (SBM), 1.0% limestone, 0.6% dicalcium phosphate, 0.3% salt, and 0.5% vitamin-mineral premix and 4 additional diets were formulated by including 10%, 20%, 30%, and 40% PKM at the expense of corn and SBM, and therefore, 9.8%, 19.5%, 29.3%, and 39.0% PKM in final experimental diets, respectively (Tables 1, 2). Vitamins and minerals were supplemented in all diets to meet or exceed nutrient requirements of pigs according to NRC [14]. The composition of PKM in the experiment was also analyzed (Table 1).

Table 1

Analyzed chemical composition of palm kernel meal (%, as-fed basis)1)

Item	Palm kernel meal
Dry matter	91.7
Crude protein	14.7
Ash	5.7
Organic matter	86.0
Neutral detergent fiber	48.7
Acid detergent fiber	26.8
Gross energy (MJ/kg)	17.76

All data are the results of a chemical analysis conducted in duplicate.

Table 2

Ingredients and analyzed chemical composition of experimental diets (as-fed basis, %)1)

Items	Basal diet	Level of palm kernel meal in diet (%)
		9.8	19.5	29.3	39.0
Ingredients
Corn	76.6	68.9	61.3	53.6	46.0
Soybean meal (dehulled), 48%	21.0	18.9	16.8	14.7	12.6
Palm kernel meal	-	9.8	19.5	29.3	39.0
Limestone	1	1	1	1	1
Dicalcium phosphate	0.6	0.6	0.6	0.6	0.6
Salt	0.3	0.3	0.3	0.3	0.3
Vit-Min premix, 0.5%2)	0.5	0.5	0.5	0.5	0.5
Analyzed chemical composition
Dry matter	88.4	88.1	88.6	89.0	88.9
Crude protein	16.8	16.2	16.0	15.8	15.6
Ash	4.0	4.4	4.7	5.0	5.3
Organic matter	84.4	83.6	83.9	84.0	83.6
Neutral detergent fiber	10.1	19.2	22.0	26.5	26.2
Acid detergent fiber	3.5	7.7	9.6	13.9	13.1
Gross energy (MJ/kg)	16.2	16.3	16.5	16.7	16.8

All data are the results of a chemical analysis conducted in duplicate.

Provided the following quantities per kg of complete diet: Mn, 50 mg (MnO); Fe, 125 mg (FeSO4·H2O); Zn, 125 mg (ZnO); Cu, 150 mg (CuSO4·5H2O); I, 50 mg (CaI2); Se, 0.30 mg (Na2SeO3); retinyl acetate, 4,500 IU; cholecalciferol, 1,350 IU; DL-α-tocopheryl acetate, 13.5 mg; menadione sodium bisulphite complex, 2.7 mg; niacin, 18 mg; vitamin B12, 27.6 μg; thiamine, 0.6 mg; pyridoxine, 0.9 mg; riboflavin, 1.8 mg; D-Ca-pantothenate, 10.8 mg; nicotinic acid, 30.3 mg.

Experimental design and sample collection

All pigs were acclimation to the new environment for 7 d and fed a standard corn-soybean meal diet before the start of the experiment. Pigs were weighed at the beginning of the experiment. Experimental feeds were fed in a meal form and offered to pigs at a level of 3% of individually initial BW of pig and were fed in 2 equal daily meals at 0730 and 1630 h, respectively [15]. Feed refusals were collected and weighed daily.

There were 4 different adaptation duration of 7, 14, 21, and 28 d were set in the current study. After 7 days of adaptation to experimental diets fed to pigs, feces were collected from d 8 to 12, d 15 to 19, d 22 to 26, and d 29 to 33, respectively. Each feces collection day was 24 h of full collection and no any marker was used. One stainless feces collection tray was put under the metabolism cage at 4 pm on day 7, so the first whole collection day was end till 4 pm on day 8. The first five collection days were end at 4 pm on day 12. Same method for the other three collection periods. Each pig was fed the same diet through the whole experiment and with ad libitum tab water intake. Fresh feces were stored at −18°C immediately after collected. At the end of each collection duration, the collected feces of 5 d from each pig were weighed, dissolved, and then homogenized thoroughly. A 600 gram sub-sample was dried in a forced-air oven at 65°C for 72 h. Dried feces were ground through a 1-mm screen for chemical analysis.

Sample analysis and calculations

Samples were analyzed for gross energy (GE) via an adiabatic oxygen bomb calorimeter (Parr Instruments, Moline, IL, USA), dry matter (DM) (AOAC procedure 4.1.06; 2000), crude protein (CP) by Kjeldahl N method [16], ash (AOAC International method, 942.05, 2005). Neutral detergent fiber (NDF) and acid detergent fiber (ADF) were determined using fiber bags and fiber analyzer equipment (Fiber Analyzer, Ankom Technology, Macedon, NY, USA) following a modification of the procedure [17].

The DE and ATTD of chemical components in diets and PKM were calculated according to the difference method described by Adeola [7]. In which method, it was supposed that the digestibility of the basal diet was same in different experimental diet and the DE and ME of tested ingredient were calculated by subtracting the DE and ME in the basal diet from corresponding items in test feed according to the contribution of tested ingredient. Organic matter (OM) was calculated as the difference between DM % and ash %.

Statistical analysis

Normality was verified and outliers were identified using the UNIVARIATE procedure (SAS Inst. Inc., Cary, NC, USA). An observation was considered an outlier if the value was more than 3 standard deviations away from the grand mean. Data were analyzed by analysis of variance using the PROC MIXED of SAS in a randomized complete block design with the pig as the experimental unit. The statistical model included inclusion level, adaptation duration, and their interaction as fixed effect and pig as random effect. Treatment means were separated by using the LSMEANS statement and the PDIFF option of PROC MIXED if the interaction between inclusion level and adaptation duration was significant. Contrast statements were conducted to test linear and quadratic effects of inclusion level and adaptation duration. Statistical significance and tendency were considered at p<0.05 and 0.05≤p<0.10, respectively.

RESULTS

Energy and chemical composition of palm kernel meal and experimental diets

On an as-fed basis, PKM detected in the current trial contained 91.7% DM, 14.7% CP, 5.7% ash, 86.0% OM, 48.7% NDF, and 26.8% ADF (Table 1). The concentration of GE in PKM was 17.76 MJ/kg (as-fed basis) and the analyzed chemical composition of diets were showed in Table 2.

Effect of inclusion level of palm kernel meal on energy and nutrient digestibility in diet

Significantly interaction effects between the inclusion level of PKM and adaptation duration of diets on DE and the ATTD of GE in diet were observed (p<0.01, Table 3). Significantly interaction effects were also observed between the inclusion level of PKM and the adaptation duration of diets on the ATTD of DM, OM, and CP in diets (p<0.05). No interaction effects were observed between the inclusion level of PKM and adaptation duration of diets on the ATTD of ash, NDF, and ADF in diets (p>0.05). Therefore, the effect of inclusion level and adaptation duration on the ATTD of ash, NDF, and ADF were showed in Table 4 and 5, respectively.

Table 3

Effect of inclusion levels of palm kernel meal and the adaptation duration on the digestible energy (DE, as fed-basis) content and the apparent total tract digestibility (ATTD, %) of chemical composition in diets fed to growing-finishing pigs1)

Item	DM	OM	CP	ADF	NDF	Ash	GE	DE (MJ/kg)
Adaptation duration, 7 days
0%	92.3a	93.8a	93.1a	61.2bcde	68.5defg	60.6a	92.3a	14.93a
9.8%	88.3c	90.2c	89.3c	68.5a	75.4a	53.7bc	88.2c	14.37c
19.5%	86.8d	88.7de	87.6def	62.0abcde	73.7abc	53.0bcd	86.5d	14.25c
29.3%	83.4ef	85.6g	83.8gh	64.2abcde	70.9abcdef	46.6efg	83.4ef	13.94d
39.0%	80.5hi	83.1i	80.2j	50.4f	65.0ghi	40.0hi	80.6ij	13.57fg
SEM	2.03	1.85	2.24	2.99	1.85	3.50	2.01	0.23
Adaptation duration, 14 days
0%	91.3ab	92.9ab	92.1ab	68.5a	68.9defg	56.1ab	91.4ab	14.78ab
9.8%	88.3c	90.3c	89.2cd	67.2ab	75.4a	51.6bcde	88.3c	14.38c
19.5%	84.6e	86.8f	85.3g	58.4e	70.5bcdef	44.8fgh	84.4e	13.91de
29.3%	82.5fg	84.8gh	83.7gh	60.4cde	67.9efgh	42.7fghi	82.7fg	13.82de
39.0%	79.5ij	82.1j	80.4j	47.5f	62.3ij	38.4ij	79.5j	13.40g
SEM	1.76	1.61	2.05	3.76	2.11	3.17	1.75	0.24
Adaptation duration, 21 days
0%	90.6b	92.4b	91.5b	66.7abc	66.4fghi	52.0bcde	90.7b	14.67b
9.8%	87.8cd	89.7cd	88.7cde	64.8abcde	73.7abc	50.7bcde	87.9cd	14.31c
19.5%	86.4d	88.4e	87.2ef	67.9a	72.4abcde	51.6bcde	86.5d	14.25c
29.3%	82.9fg	85.3gh	83.8gh	63.9abcde	69.7cdefg	42.7fghi	83.0efg	13.87de
39.0%	80.7h	83.2i	81.8i	48.9f	63.5hij	40.9ghi	81.2hi	13.67ef
SEM	1.76	1.63	1.73	3.45	1.88	2.39	1.71	0.18
Adaptation duration, 28 days
0%	90.8b	92.6b	91.9ab	66.8abc	67.2fgh	52.3bcde	90.8b	14.69b
9.8%	87.3cd	89.4cde	88.7cde	65.8abcd	73.3abcd	48.5def	87.4cd	14.24c
19.5%	86.4d	88.4e	86.9f	63.2abcde	74.5ab	49.9cde	86.6d	14.26c
29.3%	81.9gh	84.4hi	83.5hi	59.9de	68.5defj	39.2hij	82.2gh	13.74ef
39.0%	78.9j	81.7j	80.3j	45.6f	60.5j	34.5j	79.5j	13.40g
SEM	2.10	1.92	2.02	3.85	2.48	3.42	1.99	0.22
p-values
Inclusion level	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Adaptation duration	<0.01	<0.01	0.12	0.34	0.18	<0.01	<0.01	<0.01
Inclusion level×Adaptation duration	<0.05	<0.05	<0.05	0.06	0.68	0.06	<0.01	<0.01
Inclusion level
Linear	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Quadratic	0.78	0.93	0.74	<0.01	<0.01	0.28	0.92	0.86
Adaptation duration
Linear	<0.01	<0.01	0.22	0.81	0.07	<0.01	0.01	0.01
Quadratic	0.24	0.23	0.54	0.50	0.32	0.36	0.26	0.27

ATTD, apparent total tract digestibility; DM, dry matter; OM, organic matter; CP, crude protein; ADF, acid detergent fibre; NDF, neutral detergent fibre; GE, gross energy; DE, digestible energy; SEM, pooled standard error of the mean.

Data are least square means of 6 observations for all treatments.

Values within a column lacking a common superscript letter are different (p<0.05).

Table 4

Effects of inclusion levels of palm kernel meal on the apparent total tract digestibility (ATTD, %) of ash, neutral detergent fiber (NDF), and acid detergent fiber (ADF) in diet, regardless of the effect of adaptation duration1),2)

Item	Inclusion levels of palm kernel meal in diet (%)					SEM	p-value
	0	9.8	19.5	29.3	39.0		ANOVA	Linear	Quadratic
Ash	55.3a	51.1ab	49.8b	42.8c	38.4d	1.50	<0.01	<0.01	0.28
Neutral detergent fiber	67.7b	74.4a	72.8a	69.2b	62.8c	1.10	<0.01	<0.01	<0.01
Acid detergent fiber	65.8ab	66.6a	62.9ab	62.1b	48.1c	1.48	<0.01	<0.01	<0.01

SEM, pooled standard error of the mean; ANOVA, analysis of variance.

Data are the means of 6 replicates.

The ATTD of ash, NDF, and ADF were calculated by using the difference method described by Adeola [7].

Means with the same row without common superscripts differ significantly (p<0.05).

Table 5

Effects of adaptation duration on the apparent total tract digestibility (ATTD, %) of ash, neutral detergent fiber (NDF), and acid detergent fiber (ADF) in diet, regardless of the effect of inclusion level of palm kernel meal1),2)

Item	Adaptation duration, days				SEM	p-value
	7	14	21	28		ANOVA	Linear	Quadratic
Ash	50.8a	46.7bc	47.6b	44.9c	0.94	<0.01	<0.01	0.36
Neutral detergent fiber	70.7	69.0	69.1	68.8	0.77	<0.01	0.07	0.32
Acid detergent fiber	61.3	60.4	62.4	60.3	1.05	<0.01	0.81	0.50

SEM, pooled standard error of the mean; ANOVA, analysis of variance.

Data are the means of 6 replicates.

The ATTD of ash, NDF, and ADF were calculated by using the difference method described by Adeola [7].

Means with the same row without common superscripts differ significantly (p<0.05).

A linearly decreased were observed in DE and the ATTD of GE, DM, ash, OM, NDF, ADF, and CP in diets as dietary PKM increasing from 9.8% to 39.0% within each adaptation duration (p<0.01). The greatest (p<0.05) concentration of DE and the ATTD of GE, DM, ash, OM, NDF, ADF, and CP was observed in diet containing 9.8% PKM, and the diet containing 39.0% PKM had the least DE and the ATTD of all detected items (p<0.05). For 7 days of adaptation, no differences were observed in DE value and the ATTD of all detected items between diets containing 19.5% and 9.8% PKM (p>0.05). However, after 14 days of adaptation, diet containing 19.5% PKM had a less DE and ATTD of all detected items compared with 9.8% PKM diet (p<0.05). The ATTD of GE and OM in diet containing 29.3% PKM was greater than that in diet containing 39.0% PKM (p<0.05), but less compared with diet included with 19.5% PKM (p<0.05) within each adaptation duration. The ATTD of CP in diet containing 29.3% and 39.0% PKM was less than that in diet containing 9.8% and 19.5% PKM, respectively within each adaptation duration (p<0.05).

The ATTD of ash, NDF and ADF in diets linearly decreased as the dietary PKM increasing from 9.8% to 39.0% (p<0.01, Table 4), regardless of adaptation duration effect. Diet containing 29.3% PKM had a greater ATTD of NDF and ADF than that in 39.0% PKM diet (p<0.05), but no differences compared with diet containing 19.5% PKM within each adaptation duration (p>0.05).

Effect of adaptation duration on energy and nutrient digestibility in diet

The adaptation duration had no effect on DE and the ATTD of all detected items in diets contaning 9.8%, 29.3%, or 39.0% PKM (p>0.05), respectively. As for diet containing 19.5% PKM, it’s DE and ATTD of GE, DM, OM, and CP was less when fed to pigs for 14 days compared with 7, 21, or 28 days of diet adaptation, respectively (p<0.05).

A linearly decreased in the ATTD of ash in diet was observed as the adaptation increasing from 7 to 28 days (p<0.01, Table 5), regardless of inclusion level effect. The ATTD of ADF in diet containing 19.5% PKM was greater when fed to pigs for 21 days than for 14 days (p<0.05), but no differences compared with 7 or 28 days of adaptation (p>0.05, Table 3). The adaptation duration had no effect on the ATTD of NDF in diet containing 19.5% PKM (p>0.05).

Effect of inclusion level of palm kernel meal on digestible energy and the apparent total tract digestibility of gross energy and crude protein in palm kernel meal

Significant interaction effect was observed between the inclusion level of PKM and adaptation duration on DE and the ATTD of GE and CP in PKM (p<0.05 or p<0.01, Table 6). After fed to pigs for 7, 21, or 28 days, the DE content and ATTD of GE in PKM calculated by 19.5% was greater (p<0.05) than that calculated by 9.8%, but no differences compared with that calculated by 29.3% or 39.0%, respectively (p>0.05). After fed to pigs for 7 days, the ATTD of CP in PKM with inclusion rate of 19.5% in diet was greater than that in PKM calculated by 9.8% but no differences compared with that calculated by 29.3% or 39.0%, respectively (p>0.05). A linearly reduction in the ATTD of CP was observed in PKM as its inclusion rate increased in diet when fed to pigs for 14, 21, or 28 days (p<0.01).

Table 6

Effects of inclusion levels of palm kernel meal (%) and adaptation duration (days) on its concentration of digestible energy (DE, as fed-basis) and the apparent total tract digestibility (ATTD, %) of gross energy (GE) and crude protein (CP) fed to growing pigs1),2)

Item	GE	CP	DE (MJ/kg)
Adaptation duration, 7 days
9.8%	54.3f	57.1h	9.64f
19.5%	66.7bce	67.0efg	11.84bce
29.3%	67.2abcd	63.5fh	11.93abcd
39.0%	66.6abcde	62.2gh	11.83abcde
SEM	3.40	3.86	0.60
Adaptation duration, 14 days
9.8%	62.8cde	85.0a	11.15cde
19.5%	60.1df	68.7efg	10.68df
29.3%	66.9abcd	71.0deg	11.88abcd
39.0%	65.4abcde	67.8efg	11.61abcde
SEM	2.34	3.30	0.41
Adaptation duration, 21 days
9.8%	64.4cde	85.5a	11.44cde
19.5%	72.7a	80.7abc	12.92a
29.3%	69.3abc	72.6bcde	12.31abc
39.0%	70.3abc	72.2cdef	12.48abc
SEM	3.10	3.67	0.55
Adaptation duration, 28 days
9.8%	59.1ef	81.6ab	10.49ef
19.5%	72.4ab	77.8abcd	12.86ab
29.3%	66.4abcde	70.7deg	11.79abcde
39.0%	66.1abcde	67.9efg	11.74abcde
SEM	1.79	2.06	0.32
p-values
Inclusion level	>0.05	<0.05	<0.05
Adaptation duration	<0.01	<0.01	<0.01
Inclusion level×Adaptation duration	<0.01	<0.05	<0.05
Inclusion level
Linear	<0.01	<0.05	<0.05
Quadratic	0.73	0.05	>0.05
Adaptation duration
Linear	<0.01	0.11	<0.05
Quadratic	0.25	0.37	0.19

SEM, pooled standard error of the mean.

Data are least squares means of 6 observations for all treatments.

DE and the ATTD of GE and CP were calculated by using the difference method described by Adeola [7].

Values within a column lacking a common superscript letter are different (p<0.05).

Effect of adaptation duration on digestible energy and the apparent total tract digestibility of gross energy and crude protein in palm kernel meal

Palm kernel meal with 9.8% inclusion rate in diet had a greater DE value and ATTD of GE and CP when fed to pigs for 14 days than that in PKM after fed to pigs for 7 days (p<0.05), but no differences compared with that in PKM as adapted to pigs for 21 or 28 days, respectively (p>0.05). The DE and ATTD of GE and CP in PKM calculated with 19.5% was greater when fed to pigs for 21 days compared with 14 days of adaptation (p<0.05), but no differences compared with an adaptation of 28 days (p>0.05). A greater ATTD of CP in PKM calculated with 29.3% was observed when fed to pigs for 14 days compared with 7 days (p<0.05), but no differences compared with that in PKM after adaptation for 21 or 28 days (p>0.05), respectively. However, as for PKM with 39.0% inclusion rate in diet, the ATTD of CP in which was greater when fed to pigs for 21 days compared with 7 days (p<0.05), but no differences compared with that in PKM after adaptated to pigs for 14 or 28 days (p>0.05), respectively.

DISCUSSION

Energy and chemical composition of palm kernel meal

The GE in PKM was in agreement with the values previously reported [2,18]. The CP content in PKM was within the range of values reported by Stein et al [19], but greater than the those previously values [2,20]. A greater concentration of ash but less NDF and ADF were observed in PKM than those previously values [6,14,20]. The above differences were possibly due to the different species of palm nut, method of oil extraction, and the residual shell in the meal after oil extraction [21].

The linearly decreased in DE and the ATTD of energy in diets as the dietary PKM increasing was in agreement with that increasing the fiber content in the diet negatively affected the fecal digestibility of energy [22,23]. The reason for the reduction maybe due to the high insoluble dietary fiber in PKM as: most of the total carbohydrates in PKM are in the form of non-starch polysaccharides (NSP), and β-(1,4)-D-mannans are the main form of the NSP [24] and the high amount of lignin in PKM [25]. Different effects of dietary PKM on DE and the ATTD of detected items between diets within each single adaptation duration may be mainly due to the interaction effect between the inclusion level of PKM and adaptation duration of diets especial between the inclusion level of 19.5% and 14 days of adaptation. Furthere more, 21 days of adaptation can stable the digestibility of energy and other macronutrients in diets containing high fiber ingredient. Those observations indicated that the interaction effect of inclusion level and adaptation duration had a similar effect on those detected items. The decreased ATTD of CP in diet as deitary PKM increasing was in agreement with several previously studies reporting that dietary fiber’s increasing has negative effect on protein digestibility [9,23,26]. The negative effect of dietary fiber on CP digestibility maybe firstly due to increasing endogenous excretion of N [27]. Increasing passage rate of digesta through the entire gastrointestinal tract [28,29]. Not mainly but as partly reason was that the excretion of NDF-binding N increased in upper gastrointestinal tract as dietary NDF increased [9,27]. In addition, the decreased ratio of urinary to fecal nitrogen excretion induced by high dietary fiber may be also contributed to the decreased ATTD of CP in diet [26,30].

The decreased ATTD of NDF and ADF as dietary PKM increasing was in agreement with previously studies [9,23]. This can be easly explained by that the NDF and ADF had a lower digestibility than the other macronutrients and the higher level, the less digestibility [12]. The decreased ATTD of ash can be explained by the increasing excretion of ash in fecals caused by increased dietary fiber [22]. The decreased effect of inclusion level on the ATTD of DM in diet was in agreement with previously studies [8,26].

The adaptation duration had no effect on DE and the ATTD of all detected items in all but diet contaning 19.5% PKM indicated that an interaction effect between the adaptation duration and inclusion level of 22.0% NDF took place. The quadratic effect of the adaptation duration on DE and the ATTD of GE in diet containing 19.5% PKM showed that the DE value and ATTD of energy firstly decreased when fed to pigs for 14 days and then increased to the same level as 7 days of adaptation after fed to pigs for 21 days and 28 days. This was inagreement with that at least of 23 days was needed to detected the greater ATTD of OM, CP, and starch between diets containing corn digestible starch and resistant starch [31]. The currently results indicated that at least of 21 days of adaptation on diets with 19.5% PKM (22.0% NDF in diet, as fed) are necessary for stability of measurements of the digestibility of fiber and most of macronutrients. This implication is conflict with that of adaptation duration of 7 and 26 days had no effect on the ATTD of NDF [32]. Decreased ATTD of ash and CP in diet when fed to pigs for 14 days may be mainly due to the increased endogenous counterparts before the ileum of pigs caused by the high dietary fiber [22].

The DE concentration in PKM was in agreement with the reported value [18], but less compared with those previously reported values [2,6]. When fed to pigs for 7 days, the less DE and ATTD of GE in PKM with 9.8 inclusion rate in diet may be mainly due to the big error caused by the low inclusion level since the more inclusion level of ingredient, the more calculation accuracy according to the difference method [7]. As time prolonged to 14 and 21 days, the differeces diminished. This may be mainly due to the interaction effect between inclusion level and adaptation duration. However, the DE and ATTD of GE in PKM with 9.8% inclusion rate in diet still less than that in PKM with 19.5% or 29.3% inclusion rate in diet, but no significantly difference compared with that in PKM with 39.0% inclusion rate in diet. This difference firstly may be due to the interaction effect between inclusion level and adaptation duration and another reason may be the higher calculation error caused by the lower nutrient contributions from only 9.8% PKM in diet according to the difference method [7,9]. The linearly decreased ATTD of CP in PKM within each single adaptation duration was may be first due to the decreased ATTD of CP in diet according to the calculation method [7]. Another important reason may be due to the increasing excretion of less digestible NDF-binding N in fecals [9].

Adaptation duration had no effect on DE and the ATTD of GE and CP in PKM calculated by 9.8% indicated that 7 days of adaptation to a new diet with low fiber may be enough for stable digestibility of N and energy [11,32]. Significantly increased in DE and ATTD of GE and CP in PKM with 19.5% inclusion rate in diet when fed to pigs for 21 days compared with 14 days maybe due to the interaction effect between inclusion level and adaptation duration. Also, adaptation duration had no effect on DE and ATTD of GE in PKM calculated either by inclusion of 29.3% or 39.0% indicated that longer time is needed for stable digestibility of energy, may be especial for high fiber digestibility. This difference was in consistent with previous reported and in which 35 days of adaptation to a high fiber diet to pig was suggested [11]. Castillo et al [32] reported that time of adaptation of microbiota was shown by xylanase and cellulase activities that were not detected until day 7 and 42 days and could be even up to 6 weeks. This maybe the main resson for that 28 days of adaptation in the current study is not enough to detect the adaptation effect on digestibility of fiber and so for DE and ATTD of energy in PKM since at least 5 weeks of adaptation of growing pigs was required to reflect the whole-tract digestibility of purine and short-chain fatty acid [33].