Comparative parameters of digestion and N metabolism in Mong Cai and Mong Cai*Large White cross piglets having free access to sugar cane juice and duck weed

The hypotheses to be evaluated in this study were: (i) Mong Cai pigs would eat greater amounts of duck weed (Lemna minor) and use it more efficiently than exotic pigs such as those of the Large White breed; and (ii) duck weed grown in ponds fertilized with biodigester effluent would be a satisfactory source of supplementary protein in a low protein basal diet of sugar cane juice. Four Mong Cai male piglets (5-10 kg) were obtained from the local market; four Large White male piglets (12-1 kg) from a nearby State farm and four Mong Cai*Large White piglets (2 male and 2 female) (9-14 kg) from a litter (Mong Cai mother; LW sire) born at the farm. They were housed in metabolism cages made from bamboo and wood (floor area 70*70 cm) with freedom to move around. The fresh sugar cane juice (19-210Brix) and fresh duckweed (31-35% N*6.25 in DM), from ponds fertilized with biodigester effluent, were offered in separate containers in discrete meals at frequent intervals (cane juice at 12.00, 15.00, 18.00, 20.00 and 22.00 hr; duck weed at the same times with additional meals at 07.00 and 10.00hr). The juice was removed between meals and during the night; the duck weed was available continuously.

The reaction of the exotic purebred pigs to the feeding regime was quite different from that of the Mong Cai and crosses. They would only consume (reluctantly) the duck weed when it was mixed with the cane juice; they were notoriously more aggressive attempting to climb out of the cage and becoming extremely vociferous when the time of feeding was near. Eventually they were eliminated from the experiment.

Relationships between the percentage of diet dry matter consumed as duck weed (X) and apparent digestibility (DM and N) and N metabolism (retention as % of intake and digested N) were derived for the combined data for the 2 breed groups (8 pigs) and expressed as linear equations the regression coefficients of which were all significantly different from zero.

The data infer that with no duck weed in the diet the DM digestibility would be 97% which is in agreement with the fact that sugar cane juice is comprised almost entirely of soluble and digestible sugars. With 40% duck weed (a dietary protein level of 13% in DM) the prediction is a DM digestibility of 83%. Apparent N digestibility of the mixtures of juice and duck weed was in the range 63 to 73%. The N balance (g/day) increased linearly with increasing duck weed in the diet. With 40% of duck weed in the diet DM, the N retained as percent of N intake was 41% and of digested N (57%). There were no significant differences between Mong Cai and Mong Cai*Large White crosses for any of the parameters.

The results indicated that the nutritive value of duck weed is high when fed to indigenous pigs and their crosses as a supplement in a basal diet of sugar cane juice. Fifty percent of the pigs were able to consume fresh duck weed in proportions sufficient to provide a diet with more than 10% of protein in the dry matter. Purebred exotic pigs (Large White) appeared to be less adapted to consume the duck weed.

Duck weed has mostly been used as a feed for fish (Skillicorn et al 1993; Hassan and Edwards 1992) and ducks (Becerra et al 1995; Men et al 1995). The only report on its use for pigs is that of Haustein et al (1992)(cited by Leng et al 1995). They dehydrated the duck weed and fed it at levels of up to 10% of a complete mixed diet given to fattening pigs. Growth rates and feed conversion deteriorated with increasing level of duck weed in the diet.

The strategy of "matching systems with available resources" (Preston and Leng 1987; Preston and Murgueitio 1994) implies that these resources are used close to the sites of production and processing is the minimum required to achieve a satisfactory utilization. Freshly harvested duck weed contains more than 90% of water which makes drying difficult and almost certainly uneconomic. It was also observed that pigs preferred to consume the duck weed when it was fresh and that intake fell when partially wilted material was offered.

In the typical management systems of local indigenous breeds such as the Mong Cai in Vietnam the sows are either allowed to roam freely, scavenging for a wide range of feeds of plant (vegetable wastes, grasses, weeds and water plants) and animal (earth worms and insects) origin, or are confined permanently, but receive large quanties of vegetables and other sources of green feed. Thus it could be expected that these animals might eat more readily (would be already adapted to) a resource such as duck weed that grows spontaneously on contaminated water surfaces as are found in many households in Vietnam.

Decisions that had to be made in planning the experiment concerned the selection of the:

Sugar cane juice was chosen as the basal diet because, being composed of almost entirely of soluble sugars with zero fibre, it would not contribute to digesta fill, thus leaving "room" for the "bulky" duck weed; and being almost 100% digestible would contribute minimum amounts of faeces of both "dietary" and "metabolic" origin. The fact that it contains only 1% protein in the dry matter also meant that almost all the dietary protein would be supplied by the test supplement - the duck weed.

It was decided to work with recently weaned pigs of the Mong Cai, Large White breeds and crosses of the two. In fact, because of severe difficulties encountered in adapting the purebred exotic pigs to the duck weed, the experimental findings were confined to the Mong Cai and their crosses.

Metabolism cages are traditionally designed with the primary objective of achieving an efficient separation and collection of the urine and faeces, to which end the animals are restrained by bars and partitions so they are forced to face in only one direction and have minimal opportunity to move around. The stress this causes usually results in less than normal feed intakes during the period the animals are in the cages. For the present study, it was decided to break with tradition and design the cage from the outset taking into account the need to provide a comfortable environment in which the pig could move freely.

Traditional methodology for doing balance trials with animals usually requires that feed offered is restricted so as to minimize selection and avoid differences between feed offered and refused. A feeding system comprising a liquid (cane juice) and a high moisture, voluminous supplement (duck weed) raises other questions (to mix or not to mix? to restrict, and if so on what basis?) . As nothing is known about the relative palatability of fresh duck weed when fed with sugar cane juice, it was decided to make both the feeds available on a simulated free choice basis; offering each feed several times per day.

The experiment was done at the "Finca Ecologica" on the University Campus, a small farm established by one of the authors (TRP) to demonstrate integrated farming systems with perennial crops, multi-purpose trees, local breeds of livestock, low-cost plastic biodigesters and duck weed ponds.The area is close to sea level with ranges in temperature from 24 to 380C, and relative humidity in the range 40 to 100%.

Four Mong Cai male piglets (5-10 kg) were obtained from the local market; four Large White male piglets (12-17 kg) from a nearby State farm and four Mong Cai*Large White piglets (2 male and 2 female) (9-14 kg) from a litter (Mong Cai mother; LW sire) born at the farm.

The cages were made from bamboo fixed to a wooden frame in a composite unit (2.8 m length and 0.9 m wide) for 4 animals. The floors were composed of lengths of bamboo poles of 1-2cm diameter with 1 cm spacing to facilitate passing of the faeces. The cages were fitted with automatic water drinkers and the cane juice and duck weed were offered in ceramic dishes. Area per pig was 0.5m² (70*70cm) and the divisions of half bamboo sections were 50cm high. The faeces collector, suspended 5cm below the floor, was a rigid sheet of plastic net used traditionally for sealing windows against entrance of insects. Urine fell through the net and was collected over a sheet of polyethylene suspended in the form of a shallow"V" with the lowest point emptying into a filter placed in a funnel suspended over a plastic bucket.

Table 1. Mean values for dry matter and N in offer and residue of duck weed and sugar cane juice in experiments with Mong Cai and Mong Cai*Large White crosses

	MC	MC*LW	SE/Prob

Duck weed, %DM
Offer	7.11	6.33	±0.24/0.05
Refusal	7.93	6.92	±0.18/0.04
N	5	5.65	±0.17/0.03
N*6.25	31.3	35.3
Brix in juice
Offer	19	20.9	±0.19/0.01
Residue	18	19.9	±0.28/0.01
N in juice DM	0.32	0.31	±0.019/0.72

Two units were constructed. The total cost each unit for 4 animals (materials and labour) was US$60.00.

The breed groups were put into one of the cage units in successive periods (Mong Cai and crosses). The purebred exotics were housed in the second unit so as to prolong the period of adaptation, but even this was insufficient (see next section). As the relative amounts of cane juice and duck weed that were consumed varied among animals, regression analysis was used to describe treatment effects, using the proportion of diet dry matter consumed as duck weed as the independent variable.

The sugar cane was crushed every morning using a 3-roll mill and a buffalo to provide traction. The average extraction rate was 50 litres from 100 kg of cane stalk. Sugar cane was purchased from local farmers at 2-3 weekly intervals and kept under shade until it was processed. Daily observations of brix values of the juice showed negligible change in this parameter during storage periods of up to 4 weeks. The duck weed was harvested daily from the experimental ponds in the farm fertilized with biodigester effluent (Rodriguez et al 1996). When supplies from these were insufficient it was purchased in the market. The supply for one day was mixed before preparing individual feeds. During the first 5 days of the adaptation period both cane juice and duck weed were offered ad libitum. For the rest of the adaptation period and during collection, the sugar cane juice and duck weed were offered in separate containers in discrete meals at frequent intervals (cane juice at 12.00, 15.00, 18.00, 20.00 and 22.00 hr; duck weed at the same times with additional meals at 07.00 and 10.00hr). The juice was removed between meals and during the night; the duck weed was available continuously.

Table 2: Comparative parameters of digestion and N metabolism in Mong Cai and MC*LW piglets (5-16 kg LWt) having free access to sugar cane juice and duck weed (n=4/breed)

	MC	MC*LW	SE/Prob

Duck weed as % DMI	23.8	26.7	±6.4/0.77
Duck weed N as % total N	77.6	85.7	±5.3/0.32
Digestibility, %
Dry matter	90	85.9	±1.2/0.07
Nitrogen	67.8	67.7	±1.7/0.94
N retained, %
N intake	33	30.5	±1.7/0.34
N digested	48.2	44.7	±2.3/0.44

The pigs had free access to water from automatic drinkers strategically located outside the cages to avoid leaks into the faeces or urine.

During the experimental period intakes of sugar cane juice and duck weed were measured daily (by weighing amounts offered and refused). Samples of sugar cane juice offered and residues were collected daily to measure the brix content (total dissolved solids) and the N content. Samples of duck weed offered and refused were collected daily for DM determination; a sample of the offered duck weed was analysed for N. Dry matter was determined by weighing before and after drying for 48 hr in an oven at 1000C and nitrogen by Kjeldahl. Samples of duck weed and especially the refusals were analysed immediately to get accurate data because when the samples were stored in a refrigerator there were obvious physical changes in their appearance and it was considered this might affect the results.

Urine was collected and weighed daily and H2SO4 was used to keep the pH below 4 in order to preserve the N. The total daily quantities per piglet were stored in a large bucket until the end of the period when the contents were mixed and a representative sample taken to analyse for N. Faeces were collected and weighed daily. The total amount of faeces/piglet was stored in a freezer (-180C) and at the end of the period was mixed, ground and representative samples taken for analysis of N (on the fresh faeces) and for dry matter. Brix of the cane juice (total dissolved solids) was measured with a hand refractometer.

Data on composition of the duck weed and the sugar cane juice are in Table 1. The values of 31.3 and 35.3 % N*6.25 in the duck weed dry matter are in the median range of values reported by Leng et al (1995) and Rodriguez et al (1996). Brix readings of 19 and 21º in the mixed cane juice indicate close to optimum values for this parameter.

Individual values for the experimental parameters for each pig in each breed group are shown in Annex Table 1. Relationships between the percentage of diet dry matter consumed as duck weed (X) and digestibility and N metabolism parameters (Y) were derived (Figures 1 to 4) for the combined data for the 2 breed groups (8 pigs) and expressed as linear equations:

All the regression coefficients were significantly different from zero. Agreement between the two breed groups was sufficiently close to justify the decision to derive the regressions from the combined data.

It can be inferred from equation (..i) that with no duck weed in the diet the DM digestibility would be 97% which is in agreement with the fact that sugar cane juice is comprised almost entirely of soluble and digestible sugars. With 100% duck weed the prediction is a DM digestibility of 61% which also appears a reasonable estimate on the basis of its mode of growth and composition, relative to other leafy materials of tropical origin (Solarte et al 1994). Bui Huy Nhu Phuc et al (1995) reported a DM digestibility for a mixture of cassava root meal (57%) and dried cassava leaves (40%) of 73%. Comparable data in this experiment (40% duck weed of diet DM) would be 83%.

The apparent digestibility of the N of the mixtures of juice and duck weed was in the range 63 to 73% (...ii). Apparent N digestibilities, comparing our data with those of Bui Huy Nhu Phuc et al (1995), are 72% (40% duck weed in the diet) and 27.7% (40% cassava leaf meal in the diet).

The N balance (g/day) increased linearly with increasing duck weed in the diet (...iii) which was to be expected in view of the high digestibility and good amino acid balance of the duck weed protein (Hillman and Culley 1978). In contrast, in their diet of cassava root meal and cassava leaf meal (40%), Bui Huy Nhu Phuc et al (1995) reported a N balance of only 0.29 g/day which can be compared with the predicted value (40% duck weed) from our data of 3.95 g/day. The N retained as percent of N intake (41%) and of digested N (57%) are also vastly superior to those reported for cassava leaf meal at similar dietary concentrations (1.2% of N intake and -0.31% of digested N). When cassava leaf meal was only 28% of the diet and soya bean meal was 8%, the comparable values were 19.1% and 42.3%).

Allowing for the higher digestibility of cane juice compared with cassava root meal, it would appear that the duck weed used in our experiment was of superior nutritive value to cassava leaf meal. However, comparisons of the two protein sources using similar basal diets are necessary before definitive conclusions can be drawn.

The original hypothesis concerning the relative merits of indigenous and exotic pig breeds to utilize diets with duck weed as the sole protein source was not tested adequately, although the fact that the exotic pigs refused to eat the duck weed infers a definite comparative advantage for the Mong Cai breed. However, the crosses of exotic boar (Large White) with Mong Cai, responded similarly to the purebred Mong Cai (Table 3.2) in terms of intake of duck weed, digestibility and N utilization. We believe that when unconventional feed resources are to be used, it may be important to offer them to pigs at an early stage of their lives and that this will facilitate their acceptance later. In this respect the Large White*Mong Cai crosses had access to duck weed from birth as this was the sole protein source of their mother. In contrast, the exotics had been exposed only to cereal-based concentrates prior to the experiment. It is not known precisely what had been the feeding regime of the Mong Cai pigs prior to purchase although the tradition in the village where they were raised is to feed rice soup and vegetables..

The results of this preliminary trial with local pigs and their crosses fed a basal diet of sugar cane juice with duck weed as the only protein source indicated that the nutritive value of the duck weed was high in this particular feeding system, and superior to that of cassava leaf meal fed in combination with cassava root meal.

Fifty percent of the Mong Cai pigs and their crosses with Large White were able to consume fresh duck weed in proportions sufficient to provide a diet with more than 10% of protein in the dry matter (Figure 4). Purebred exotic pigs (Large White) appeared to be less adapted to consume the duck weed and would only eat it when mixed with sugar cane juice, and even then with reluctance. Early exposure to duck weed may be an important factor in later acceptance of this feed.

Appreciation is expressed to the Swedish Agency for Research Cooperation with Developing Countries (SAREC) for financing most of this study. The University of Agriculture and Forestry of Ho Chi Minh city made available the site for the Finca Ecologica on the University campus. Special thanks go to Nguyen van Lai for advice on analytical procedures and assistance in the laboratory, and to Vo Than Hai for help with the day to day management of the feeds and the pigs. Nguyen Van Chin and his assistants made an important contribution by constructing the metabolism cages in a process of participatory design and development using their skilled knowledge of working with low-cost local materials.

Annex Table 1: Parameters of digestion and N metabolism in Mong Cai and Mong Cai Large White crosses

Breed	MC	MC	MC	MC	MC*LW	MC*LW	MC*LW	MC*LW
Expt	1	1	1	1	2	2	2	2
Pig No	1	2	3	4	1	2	3	4

LW,kg	5.65	8.00	7.30	9.55	13.50	11.60	9.30	12.30
DMI(g/d)	256.00	417.00	321.00	346.00	607.00	447.00	423.00	560.00
LemnaDM(g/d)	31.00	188.00	91.00	34.00	144.00	156.00	132.00	95.00
Lemn%DMI	12.11	45.08	28.35	9.83	23.72	34.90	31.21	16.96
DigDM%	94.10	82.60	89.10	95.30	87.80	85.98	80.19	87.90
Nintake,g/d	2.28	10.23	5.32	2.73	9.60	9.80	8.50	6.80
N*6.25%of DM	5.57	15.33	10.36	4.93	9.88	13.70	12.56	7.59
N Bal,g/d	0.57	4.65	1.90	0.61	3.20	3.10	3.20	1.55
Nbal%Int	24.82	45.45	35.71	22.34	33.33	31.63	37.65	22.79
%digN	64.10	72.70	67.10	65.80	70.40	72.36	69.80	59.70
Nret%dig	38.82	62.50	53.30	34.04	46.81	44.30	53.84	38.20
N Lem g/day	1.55	9.50	4.58	1.73	8.20	8.80	7.50	5.38
% N as Lemna	67.98	92.86	86.09	63.37	85.42	89.80	88.24	79.12