The effect of substituting a basal diet for growing pigs with fresh forages on apparent digestibility and nitrogen retention

Nguyen Nhat Xuan Dung, Luu Huu Manh, Truong van Phuoc^*, Peter Udén^ and Brian Ogle^

Department of Animal Sciences, Cantho University, Vietnam
nnxdung@ctu.edu.vn; lhmanh@ctu.edu.vn
**Department of Animal husbandry and Veterinary, Tien Giang University, Vietnam*
phuoctruong76@yahoo.com
**Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences

Abstract

The effect of substituting a basal diet with fresh duckweed (DW) (Lemna minor), Para grass (PG) (Brachiaria mutica) and water spinach (WS)(Ipomoea aquatica) on the apparent digestibility and nitrogen retained was determined in growing pigs. Eight Baxuyen crossbred castrated male growing pigs (Ba Xuyen x Yorkshire) were used in two experiments to evaluate the total tract apparent digestibility of dry matter (DMD), crude protein (CPD), neutral detergent fiber (NDFD), acid detergent fiber (ADFD) and nitrogen retained (NR). In the first experiment, 4 pigs were offered a basal diet at 4% of body weight and the fresh forages were offered ad libitum after the pigs had consumed the basal diet.

Total DM intake was higher for the diets with DW and WS than for the PG and basal diets (P<0.05). There were no significant differences in DMD, CPD, NDFD, ADFD among the 4 diets (P>0.05). N-retained was higher for the diet with WS compared with the other three diets (P<0.05). In the second study, 4 pigs were offered a basal diet at a level of 3% of body weight after consuming the fresh forages. The dry matter intakes of the DW, WS and PG were 11, 7 and 19% of total DM intake, respectively and total DM intake was highest for the basal diet, followed by diets WS, DW and PG (P<0.05). The digestibility of DM, CP, NDF, EE and ADF was not different among diets (P>0.05). N retained was highest for the diet with DW and lowest for PG (P<0.05).

Of the forages evaluated, water spinach and duckweed were more palatable than Para grass and supplementing with the fresh forages did not affect total dry matter or nutrient digestibility.

Key words: Digestibility, duckweed, intake, nitrogen retained, Para grass, pigs, water spinach

Introduction

In recent years, pig producers in the Mekong delta have been faced with an imbalance between feed and animal product prices. This has resulted in reduced economic returns for smallholder farmers. Many feed factories provide concentrates for pigs that are available in both urban areas and even in more remote rural areas. This has contributed to the development of feeding systems that are expensive, and not based on locally available and cheap feed resources.

Water spinach (Ipomoea aquatica) and duckweed (Lemna minor) are water plants that have high biomass productivity and are very abundant in the Mekong Delta. They are high in protein and fairly low in fibre content, and are recognised as good protein sources for non-ruminant animals (Manh et al 2002; Ly 2002; Chhay Ty and Preston 2005).

Para grass (Briachiaria mutica) is commonly grown on waterlogged soils, and is used for feeding cattle and buffaloes. However, the ruminant population of the Mekong Delta, and particularly in the lowland areas, is now rather small. The part of the Para grass plant used for ruminants is high in neutral detergent fiber (NDF) and low in protein content. However, the young tops of Para grass contain less (65%) NDF than the lower leaves and stem, and up to 20% crude protein (CP), which pigs have no difficulty in digesting.

The aims of the study were to determine the digestibility, nitrogen utilization and intake in growing pigs of a basal diet fed restrictedly and fresh duckweed, water spinach and Para grass, offered ad-libitum.

Materials and methods

Experimental pigs

In Experiment 1, four crossbred (Yorkshire x Baxuyen) castrated male pigs of average initial weight of 53 kg±1 were used. In the second experiments, four crossbred (Yorkshire x Baxuyen) castrated male pigs of average initial weight of 70kg ±1 were used .The pigs were de-wormed prior to the start of the study; all pigs were housed individually in metabolism cages.

Feeds, diets and management

In Experiment 1, all pigs were fed a basal diet (BS₁) containing16 % crude protein (CP), which was formulated by diluting with rice bran and broken rice a 35% CP concentrate purchased from the "CP VINA Feed Company, Thailand". The forages consisted of duckweed (Diet_DW1), Para grass (Diet_PG1) and water spinach (Diet_WP1) offered ad libitum. The daily allowance of BS₁ was calculated at 4% of body weight. The pigs were offered the fresh forages after consuming the basal diet.

In Experiment 2, the pigs were fed a basal diet (BS₂) containing14% CP, which was restricted to 25% of the daily allowance on an air dry basis, plus ad libitum fresh duckweed (Diet_DW2), Para grass (Diet_PG2) and water spinach (Diet_WP2). Daily allowance was calculated at 3% of body weight, in order to meet the maintenance requirement and still stimulate the pigs to eat the forages. The pigs were offered the fresh forages before being given the basal diet.

Duckweed was harvested daily from a pond at the experimental farm. Water spinach foliage, consisting of leaves and stems, was collected daily in the same area. Young tops of Para grass (30-40cm in length) were also cut daily around the experimental farm. All fresh forages were washed to remove sand and soil; the water spinach and Para grass were chopped into 5 cm lengths before feeding. Diets were provided three times per day (8:00, 14:00 and 18.00 h) and the pigs had free access to water. During the collection period, offered feeds and refusals were recorded daily. The composition of ingredients and basal diets are presented in Tables 1 and 2.

Experimental design and collection procedures

The pigs in each experiment were randomly assigned to the four treatments in a 4 x 4 Latin square design, with four 16-day periods, comprising 12 days of adaptation, followed by 4-day periods of quantitative collection of faeces and urine. All faeces and urine (to which was added 4% of HCl) were collected and weighed every morning for each animal before sampling, and a 200ml sample of urine was frozen and stored. Faeces of each pig were mixed daily, and a sub-sample of 200g taken. All samples were frozen at -18^oC. Prior to analysis, faeces were thawed, homogenized and sub-sampled. Apparent digestibility of dry matter (DM), organic matter (OM), crude protein (CP), ether extract (EE), acid detergent fibre (ADF) and neutral detergent fibre (NDF), and nitrogen retained, were calculated.

Chemical analysis

Diets and dried faeces were ground (1 mm screen) before analysis. DM, EE, CF and ash were analysed according to standard methods (AOAC 1984). Analyses of crude protein (CP = N x 6.25) in faeces were carried out on fresh samples to avoid losing ammonia. Neutral detergent fibre (NDF) was analysed according to Robertson and Van Soest (1991), but instead of boiling for one hour, samples were incubated in an oven at 90^oC overnight according to Chai and Udén (1998). Acid detergent fibre (ADF) was determined according to Goering and Van Soest (1981). Total carotene was analysed by a spectrophotometric method, using n-hexane and aluminum oxide as solvent and absorbent (AOAC 1984).

Statistical analysis

The data for each parameter were analysed as a 4 x 4 Latin square using the General Linear Model of Minitab version 13, and least square means were compared using the Tukey test.

The model used was: Y_ijk = µ + P_i +S_j + A_k + e_ijk

where Y_ijk is the dependent variable, µ is the overall mean; P_i is the effect of period, i = 1, 2, 3, 4; S_j is the effect of feed source, j = 1, 2, 3, 4; A_k: effect of animal, k= 1,2,3,4; and e_ijk represents random error.

Table 1. Formulation of the basal diets used in Experiments 1 and 2
Ingredients, %	Basal diet 1 (BS₁)	Basal diet 2 (BS₂)
Rice bran	30	43
Broken rice	40	37
Concentrate^*	18	-
Fish meal	5	13
Coconut meal	5	5
Bone meal	1	1
Oyster shell	1	1
^*Concentratewas purchased from the CP VINA Feed Company, Thailand, and its composition is shown in Table 2.

Table 2. Chemical composition of feed ingredients and diets in Experiment 1 and 2 (% of DM)
Composition, %	Concentrate	Basal diet 1 (BS₁)	Basal diet 2 (BS₂)	Duck- weed	Para grass	Water spinach
Dry matter	88.9	88.4	88.4	5.76	15.2	8.09
Crude protein	34.6	16.9	14.4	31.2	20.4	29.6
Ether extract	2.79	12.9	8.91	9.78	7.45	9.49
Crude fibre	2.55	5.32	6.50	12.2	28.6	16.2
NDF	-	20.5	23.7	37.0	65.2	34.9
ADF	-	11.2	12.2	28.3	35.7	28.3
Ash	20.7	10.4	7.85	19.2	10.5	12.9
Ca, %	-	1.44	1.49	3.27	0.71	1.03
P, %	-	1.15	0.94	1.43	0.39	0.59
Carotene, mg/kg				801	133	675
ME, MJ/kg *	13.32	13.38	13.36	9.10	4.50	8.30
^Metabolisable energy (ME, MJ/kg DM) was calculated by using the equation described by Just et al (1984) for pigs: ME = 0.0197CP+ 0.0194 EE -0.0125CF +0.0158 NFE*

Results and discussion

Experiment 1

The ash content in duckweed (19.2 %) was higher than in Para grass (10.5%) and water spinach (13.0%). The values confirm the conclusion of Ravindran and Blair (1992) that the limitation of aquatic plants is their high ash content. In the present study, the WS partly floated on the surface of a river, while the DW grew in a shallow pond. Duckweed is known to be rich in calcium oxalate in the form of crystal idioblasts (Ahmed et al 2003). This could be one reason why the voluntary feed DM intake of the DW was lower than that of WS (Table 3), although the high moisture content of the DW could also have been a contributing factor.

Supplementing the basal diet with DW and WS significantly increased total dry matter (P<0.01) and crude protein intake (Table 3). Among the forages, WS was more palatable than DW and PG. The pigs consumed very little fresh PG (198g day^-1), but considerable amounts of WS (1854g day^-1) and DW (1215g day^-1), although on a DM basis daily intakes were not so different (70, 30 and 150 g^-1 day of DW, PG and WS, respectively). Calculated daily metabolisable energy intakes were not significantly different among treatments (P>0.05). The high palatability of fresh water spinach has been demonstrated by Chhay Ty and Preston (2005). Para grass is a hair-grass, with a lower protein content than duckweed (31% CP) and water spinach (29% CP). The NDF content of Para grass (62 %) was almost double that of duckweed and water spinach (37 and 35% of DM, respectively) and this could have contributed towards the low palatability.

Table 3. Dry matter and metabolisable energy intake for dietary treatments (Diet_DW2), (Diet_PG2) and (Diet_WS2) (Experiment.1)
	Basal diet 1 (BS₁)	Diet_DW	Diet_PG	Diet_WS	P	SEM
Intake, g day^-1
Total dry matter	1960^b	2030^a	1990^b	2101^a	0.01	0.01
Forage DM	-	70	30	150
Fresh forage	-	1215	198	1854
ME, MJ day^-1	33	35	34	36
ab Means on the same row with subscripts that do not have a common letter differ significantly, P<0.05. SEM Standard error of the mean

Apparent digestibility of DM and nutrients was not influenced significantly (P>0.05) by diet, but most nutrients tended to be better digested with the inclusion of fresh forage compared to the basal diet (Table 4). Nitrogen intake was significantly (P<0.01) influenced by forage type, and was highest for the WS treatment and lowest for the basal diet. Nitrogen in urine was highest in Diet_PG and lowest in Diet_WS(P<0.01). Nitrogen retention was highest in Diet_Wsand lowest in Diet_PG (P<0.01) while N-retained as a proportion of N-intake was significantly higher for the WS treatment (P<0.05) compared to the other three treatments, but differences between the basal diet, Diet_DWand Diet_PGwere not significant (P>0.05).

According to Manh et al (2002), replacing the protein in a concentrate by water spinach at levels of 20 and 40% increased the digestibility of NDF and ADF, and did not reduce digestible energy, live weight gain and feed conversion ratio of growing pigs. Similar results on pigs were reported by Men et al (2005) when water spinach replaced 5% of a basal diet.

Table 4. Apparent digestibility and nitrogen utilisation in pigs fed diets supplemented with fresh forages (Experiment 1)
	Basal diet 1 (BS₁)	Diet_DW	Diet_PG	Diet_WS	P	SEM
Digestibility, g /kg
Dry matter	697	726	711	738	0.29	14.3
Crude protein	766	768	774	780	0.96	23.3
Ether extract	702	722	674	723	0.48	23.9
NDF	475	502	550	528	0.18	35.4
ADF	385	335	353	329	0.44	16.7
N utilization, g /day
N intake	53.3^c	56.7^b	54.3^c	60.0^a	0.01	0.46
N in faeces	8.54^b	9.39^a	8.68^b	9.60^a	0.35	0.46
N in urine	12.0^b	9.84^b	14.0^a	7.14^c	0.01	0.93
N retention	32.8^bc	37.5^b	31.6^c	43.3^a	0.01	1.09
N retention (proportion of N intake)	0.62^b	0.66^b	0.58^b	0.72^a	0.02	0.02
abc Means on the same row with subscripts that do not have a common letter differ significantly, P<0.05. SEM Standard error of the mean.

Experiment 2

Total dry matter intakes of diets with inclusion of forages were reduced (P<0.05) in comparison with the basal Diet_BS2 (Table 5). This was due to the high water content of duckweed (94%) and water spinach (92%; Table 2), as although the pigs consumed a high amount of fresh duckweed (3540 g^-1day) and water spinach (4080 g^-1day), dry matter intake was very limited and amounted to only180 g^-1day of duck weed and 330g^-1day of water spinach (Table 5). The result of present study is in agreement with Kyriazakis and Emmans (1995), who stated that the water holding capacity (WHC) of bulky fibrous feeds is responsible for limiting feed intake. However, the nitrogen intake of Diet_DW2 and Diet_WP2 was similar to the basal Diet_BS2, because water spinach and duckweed are high in crude protein content. Moreover, duckweed and water spinach have a reasonably good amino acid balance and are particularly high in carotene.

Table 5. Dry matter protein and digestible energy intake of fresh duckweed (Diet_DW2), Para grass (Diet_PG2) and water spinach (Diet_WS2) (Exp.2)
	Basal diet (BS₂)	Diet_DW2	Diet_PG2	Diet_WS2	P	SE
Intake, g^-1 day
Dry matter	2090^a	1594^c	1524^c	1740^b	0.01	0.03
Forage DM	-	184	115	330
Fresh forage	-	3200	755	4080
ME, MJ day^-1	28.43	19.30	19.28	19.17
abc Means on the same row with subscripts that do not have a common letter differ significantly, P<0.05.

The apparent digestibility of nutrients and nitrogen utilisation are shown in Table 6. Providing fresh forages did not significantly affect the digestibility of DM, CP, ether extract, NDF and ADF (P>0.05). The digestibility of DM and protein in Diet_WS was lower than the values reported by Chhay Ty and Preston (2005) for water spinach. Nitrogen retention was significantly different among diets (P<0.05), with highest value found in Diet_WP2 (30.0g day^-1) and the lowest in Diet_PG2 (19.3g day^-1). Nitrogen in faeces was lowest in the Diet_PG2, while nitrogen in urine was not affected by forage type (P>0.05). Nitrogen retention as a proportion of nitrogen intake was highest for Diet_DW2 and lowest for Diet_PG2 (P<0.05; Table 6), probably because of the superior amino acid balance of duckweed compared to Para grass.

Table 6. Apparent digestibility and nitrogen utilisation in pigs fed diets supplemented with fresh duckweed (Diet_DW2), Para grass (Diet_PG2) and water spinach (Diet_WS2)(Experiment 2)
Digestibility, gkg^-1	Basal diet (BS₂)	Diet_DW2	Diet_PG2	Diet_WS2	P	SEM
Dry matter	789	739	746	752	0.66	27.7
Protein	802	797	797	796	0.99	30.4
Ether extract	795	760	814	749	0.96	42.5
NDF	558	513	506	548	0.85	89.7
ADF	432	388	358	422	0.61	59.7
N utilization, g day^-1
N intake	48.2^a	41.9^b	37.5^b	48.1^a	0.01	0.94
N in faces	7.32^ab	6.70^b	6.00^b	7.70^a	0.01	0.15
N in urine	12.5	7.36	12.2	10.4	0.10	1.33
N retention	25.9^ab	27.8^a	19.3^b	30.0^a	0.02	1.70
N retention (proportion of N intake)	0.57^ab	0.67^a	0.51^b	0.62^ab	0.05	0.03
ab Means on the same row with subscripts that do not have a common letter differ significantly, P<0.05.

Conclusions and implications

Using fresh forage plants for pigs is convenient and saves energy. Most of the fresh forages in the Mekong Delta have a high water content, especially the aquatic plants, and it is an advantage to present them in a succulent and palatable form. If they replace the basal diet at a high rate, the water content in the plants is an important limitation, reducing feed intake. However, these plants are high in carotene and protein content, and the nitrogen retention values in the present study showed that the protein in the duckweed and water spinach is of a good quality.
Supplementing with fresh forages should reduce the risk of obesity in fattening pigs and sows.
Total feed intakes are improved if the basal diet is offered before the fresh forages but an alternative could be to mix the basal diet with chopped fresh forages, as farmers generally do.
Adaptation is very important and the pigs should be allowed access to fresh forages when they are still young.

Acknowledgements

The authors wish to thank the SAREC MEKARN (Mekong Basin Animal Research Network) project for supporting this research

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