Cassava leaf meal was used to replace the protein of soya bean meal at levels of 0, 35, 70 and 100% in a diet for growing pigs based on cassava root meal. The overall protein level was restricted to 11% (DM basis). Four castrated pigs (Large White) of 50 kg were used in a 4 x 4 Latin Square arrangement of the treatments. The experimental periods were 12 days, the first 7 for adaptation to the diet and the last 5 for total collection of faeces and urine. During the adaptation period the pigs were in concrete floor pens while for collection of excreta they were in metabolism cages. Feeding was ad libitum during the adaptation period and restricted during the experimental period to the maximum eaten by the pig on the least palatable diet (usually that with 100% cassava leaf meal). The cassava leaf contained in dry matter: 22% N x 6.25 and 15.5% crude fibre. The HCN content (DM basis) was reduced by sun-drying from 190 mg/kg in the fresh leaf to 20 mg/kg in the leaf meal.

There were significant differences in apparent digestibility of all nutrients with linear decreases as CLM levels were increased. The degree of depression was much more marked for protein and for ether extract than for the other elements. Excretion of N in faeces was significantly higher, and in urine significantly lower, for the three diets containing CLM. Daily N retention did not differ between diets with 100% soya bean meal and 65% protein from soya and 35% from CLM. Diets with higher rates of substitution (70 and 100% CLM) supported significantly lower N retention. When N retention was adjusted for daily amount of N digested, then the diet with 35% protein from CLM was significantly better than the control (0% CLM) and the 100% CLM diet but did not differ from the 70% CLM diet.

The results suggest that nitrogen balance in growing pigs fed a cassava root meal diet is unaffected when cassava leaf meal replaces up to 35% of the protein from soya bean meal. The lower digestibility of the cassava leaf protein, compared with soya bean, is apparently compensated by a superior biological value.

Cassava is an important crop in the small-farm sector in Vietnam and is extremely reliable to grow, especially on sloping rain-fed soils of low fertility. A measure of its level of importance is that in terms of production of food energy for human consumption within the tropics it is the fourth most important crop after rice, maize and sugar cane. In Vietnam it is the second crop after rice. The cassava plant survives drought periods and grows well with limited supplies of water. In addition, it is tolerant of acid soils and yields well on marginal soils without excessive use of costly inputs. These qualities have endeared cassava to small farmers and it is almost exclusively grown by them.

Table 1: Cassava production in Vietnam (1994)
	1985	90	91	92	93
Yield
'000 tonnes	2,939	2,276	2,455	2,568	2,548
tonnes/ha	8.7	8.9	9.0	9.0	9.1
Area, '000 ha Vietnam	335	257	273	284	280

The area of cassava has not increased in recent years (Table 1). Beside the root each hectare of cassava can produce a large amount of leaf. If cassava is established at a density of 12,500 plants/ha, it can give 2.44 tonnes of fresh leaf equivalent to about 613 kg of dry leaf. The protein content in cassava leaf is between 20-23% therefore from one hectare of cassava, it is possible to obtain 140 kg of protein. Cassava leaves are therefore a potential source of protein for livestock in the tropics. However, the emphasis so far has been in using the leaf meal as a source of pigments in mixed feeds for poultry (Ravindran 1991).

In view of the predicted world shortage of cereal grains, because of competing needs for the expanding human population and the diminishing food producing capacity of the earth's surface (Brown and Kane 1994), it is argued that a major priority is to develop livestock feeding systems which do not depend on cereal grains (Preston 1995). Considerable progress has been made in the use of non-cereal energy-rich feeds such as cassava (Machin and Nyvold Solveig 1991), molasses (Preston et al 1968), sugar cane juice (Sarria et al 1990), palm oil and palm fruit (Ocampo et al 1990, Ocampo 1994a,b) and sugar palm juice (Knieu Borin et al 1995). Research with non-conventional sources of protein has been less successful although there are reports of replacing up to 15- 30% of the soya bean meal in sugar cane juice diets with Azolla (Becerra et al 1990) and the fresh leaves of Trichanthera gigantea (Sarria et al 1991; Sarria 1994). Digestibility and palatability are considered to be the constraints to higher rates of useage.

The research to be described here is in response to the challenge to make greater use of local non-cereal feed resources in pig feeding, in this case the roots of cassava. The specific aim of this experiment was to determine the degree to which cassava leaf meal could replace soya bean meal in the diet of pigs fed on cassava root meal.

Cassava leaf meal was used to replace the protein of soya bean meal at levels of 0, 35, 70 and 100% in a diet for growing pigs based on cassava root meal. The dietary ingredients and chemical composition are given in Table 2. The overall protein level was restricted to 11% (DM basis) to ensure that this nutrient would be in limited supply and in view of the opportunities to use lower overall levels of protein in diets based on cassava root meal (Ospina et al 1995).

Table 2: Composition of diets
	N from CLM as % total N
	0	35	70	100
As % of air-dry diet:
Cassava root	77	70	63	57
Soybean meal	20	13	6	0
CLM	0	14	28	40
Salt	0.5	0.5	0.5	0.5
Bone meal	2.5	2.5	2.5	2.5
Premix	0.05	0.05	0.05	0.05
Chemical composition, % of DM:
N x 6.25	11.2	11.3	11.1	10.8
Fibre	4.08	5.54	7.37	8.26
Organic matter	94.5	93.3	93.1	93.0
NFE	77.2	73.5	71.0	71.0
Ether extract	2.04	2.91	3.645	4.79
Ash	5.46	6.70	6.86	7.04

Four castrated pigs (Large White) of initial weight about 50 kg were used in a 4 x 4 Latin Square arrangement of the treatments. The experimental periods were 12 days, the first 7 for adaptation to the diet and the last 5 for total collection of faeces and urine. During the adaptation period the pigs were in concrete floor pens while for collection of excreta they were in metabolism cages. Feeding was ad libitum during the adaptation period and restricted during the experimental period to the maximum eaten by the pig on the least palatable diet (usually that with 100% cassava leaf meal).

During the experimental period, all the faeces and an aliquot of 10% of the urine were collected and stored in a freezer at -18 ºC. At the end of the collection period samples of faeces and urine were taken for chemical analysis. Nitrogen was determined by the kjelhdal method, and dry matter by drying at 105 ºC. A sub-sample of faeces was dried at 60 ºC to determine fibre by the Van Soest method. Ash was measured by incinerating at 500 ºC. Ether extract was done in a Soxhlet apparatus.

The cassava leaf contained in dry matter: 22% N x 6.25 and 15.5% crude fibre. The HCN content (DM basis) was reduced by sun- drying from 190 mg/kg in the fresh leaf to 20 mg/kg in the leaf meal. Increasing the level of CLM in the diet led to increases in ether extract (from 2.04 to 4.79%), in crude fibre (from 4.08 to 8.26 %) and to decreases in NFE (Table 1). These changes reflect the higher concentration of pigments and crude fibre in CLM compared with soya bean meal.

Table 3: Effect on intake and digestibility of replacing soya bean meal with cassava leaf meal (CLM)
	Protein replaced by CLM, %
	0	35	70	100	SE/Prob
Feed intake, g/d	1697	1740	1698	1527	51/.06
Apparent digestibity, %
N x 6.25	84.8	64.6	46.7	27.7	1.5/.001
Dry matter	92.5	85.2	79.6	72.6	0.52/.001
Organic matter	94.4	86.6	80.1	71.4	0.75/.001
Fibre	76.8	61.3	59.2	48.2	1.25/.001
NFE	97.3	94.6	92.1	89.5	0.54/.001
Ether extract	74.6	46.2	33.1	-7.56	8.0/.001

It was intended that feed intake would be constant at the level consumed by the pigs fed the least palatable diet. In fact, there were refusals on the 100% CLM which resulted in a significantly lower intake on this diet compared with the other treatments (Table 3).

There were significant differences in apparent digestibility of all nutrients with linear decreases as CLM levels were increased (Table 3). The degree of depression was much more marked for protein and for ether extract than for the other elements. In the case of protein it is probable that linkages between the protein and secondary plant compounds in the CLM were the factors limiting access by proteolytic enzymes. With respect to the ether extract fraction it is clear that the pigments in the CLM were of vey low digestibility.

Table 4: Effect on N balance of replacing soya bean meal with cassava leaf meal (CLM)
	Protein from CLM,%
	0	35	70	100	SE/Pr
N, g/d
Intake	27.2	28.3	27.3	23.6
Faeces	4.45	10.8	15.0	16.0	1.6/.002
N digested	22.8	17.4	12.2	7.52	1.4.001
Urine	14.0	7.20	6.96	7.24	1.1/.002
Retention
Daily, g	8. 75	9.71	5.22	0.29	1.5/.003
% of digested	37.8	55.8	42.3	-0.31	12.5/.043

Excretion of N in faeces was significantly higher, and in urine significantly lower, for the three diets containing CLM. Daily N retention did not differ between diets with 100% soya bean meal and 65% protein from soya and 35% from CLM. Diets with higher rates of substitution (70 and 100% CLM) supported significantly lower N retention. When N retention was expressed as a percentage of N digested, then the diet with 35% protein from CLM was significantly better than the control (0% CLM) and the 100% CLM diet but did not differ from the 70% CLM diet.

The greater excretion of N in faeces on all CLM diets is proof that the digestibility of the protein in CLM is less than that in soya bean. In contrast, the reduced excretion of N in urine on all CLM diets indicates that the CLM protein that was digested had a superior biological value than the digested protein from the soya bean meal. The soya bean meal had been purchased from a small scale artisan producer and it is possible that in the process of heat treatment some of the amino acids had been denatured. The positive effect of the CLM on net protein utilization indicates that the CLM protein has a well balanced array of amino acids. Analytical data support this contention (Figure 1). These results are supported by the performance data reported by Ravindran et al (1991) who showed that most efficient gains in pigs were obtained when 33% of the protein from coconut meal was replaced by CLM (CLM was 14% of the diet). However, in another trial when CLM was used at levels of 10, 20 and 30% in a maize-soya bean meal diet, pig performance was negatively related to the level of CLM (Ravindran 1991).

The results of this experiment suggest that nitrogen balance in growing pigs fed a cassava root meal diet is unaffected when cassava leaf meal replaces up to 35% of the protein from soya bean meal. The lower digestibility of the cassava leaf protein, compared with soya bean, is apparently compensated by a superior biological value.

(Received 12 October 1995)