Effect of protein supply in cassava root meal based diets on the performance of growing-finishing pigs

This experiment was carried out in order to determine the effect of crude protein (CP) supply derived from soya-bean meal (SBM) on performance and carcass characteristics of growing pigs fed cassava root meal (CRM) ad libitum as the sole energy source. Forty eight commercial type castrates and gilts averaging 31.4 kg initial weight were randomly distributed to one of six treatments with 150, 200, 250, 300, 350 or 400 g CP/day from SBM, with four replicates in a completely randomized design.

Daily intakes of CRM declined with increasing SBM supply (P=0.056), but total DM intakes increased slightly. Daily liveweight gain (Yg=kg/day) and feed conversion ratio (Yfc=kg DM/kg gain) were related quadratically to the supply of CP: Yg = 0.294 + 1.668X - 1.89X² (r²=0.79) and Yfc = 4.56 -7.87X +10.8X² (r²=0.53). As the dietary CP supply increased, dressing percentage and backfat thickness tended to decrease and Longissimus dorsi area (P=0.099) and Iodine index of back fat (P=0.022) increased linearly. Kidney weights increased significantly (P=0.03) with increased dietary CP supply. The apparent digestibility of the diets was slightly higher for finishing than for growing pigs. The lowest dietary cost/kg liveweight gain was for the diet with 200 g/day of CP (P=0.02). It can be concluded that in the conditions of Vietnam, the feeding of pigs throughout the growing-finishing period with CRM ad libitum, combined with 200 g/day of CP gives acceptable growth performance and carcass quality, and the lowest feed costs/kg gain.

Cassava roots are a good energy source but are low in protein, making the economic utilization of roots in animal feeds highly dependent on the incorporation of other protein-rich ingredients (Maini 1978, quoted by Balagopalan et al 1988). According to Preston and Murgueitio (1992) the low protein content of most tropical feeds likely to be used as basal diets in pig and poultry feeding requires that almost all the dietary needs for amino acids must be supplied as an addition to the basal diet. However, as the major part of the supplementary protein can be supplied by high quality sources (eg. soya bean meal, fish meal, leaf and aquatic plant protein) which have a well balanced pattern of amino acids in relation to established requirements, overall amino acid balance of the diet is likely to be better than in conventional cereal based diets, which gives the opportunity to reduce the total protein supply. In this way, the concept of the "ideal protein" can be applied because it implies using a source of protein which supplies amino acids in the proportions required by the pig (Fuller and Chamberlain 1985). Therefore, by using energy-rich tropical feed resources, the total protein needed in the diet can be much less than when cereals are the basis of the diet, and there is less wastage of nitrogen during metabolism (Preston and Murgueitio 1992).

The aim of the present experiment was to test the above hypothesis by studying the effect of different amounts of dietary crude protein on the growth performance and carcass quality of growing-finishing pigs, when fed diets based on cassava root meal and soya bean meal.

This experiment was conducted between March and June 1993, at the research farm of the University of Agriculture and Forestry (UAF), Ho Chi Minh City, Vietnam. During the whole experiment (three months) the average minimum and maximum temperatures in the pig houses were 24C and 38C, respectively, with a mean of 31C; minimum humidity was 40% and the maximum 100%.

Table 1: Ingredient composition of the fortified soybean meal (FSBM) supplement (%, fresh basis)

	Level of protein (g/day)
Ingredients, %	150	200	250	300	350	400

Soybean meal*	63.0	70.0	75.1	78.9	81.8	84.1
Bone meal	14.1	11.8	10.1	8.8	7.8	7.1
Salt	4.6	3.8	3.3	2.9	2.6	2.3
Oil	16.5	12.9	10.3	8.3	6.8	5.6
Premix**	1.8	1.5	1.2	1.1	1.0	0.9

* SBM: 47.5% CP, 92.1% DM, 6.0% EE.
** Premix composition/5kg: Vit A=10,000,000 IU, D3=200,000 IU, B1=1.5g, B2=5 g, B6=1.5 g, E=15 g, B12=0.075 g, K3=0.5 g; Co=0.75 g, Cu=125 g, Fe=120 g, I=0.75 g, Mn=60g, Zn=185 g, Se=0.1 g.

The cassava root meal (CRM) was made by sun-drying and milling chips of peeled cassava roots, and the soya bean meal was fortified (FSBM) with a vitamin/mineral complex. The composition of the supplements is shown in Table 1. Soya bean oil was added at 5% of the total CRM consumed, to ensure no deficiencies in essential long chain fatty acids. Diets were compounded to supply 150, 200, 250, 300, 350 and 400 g crude protein (CP) from FSBM per day per pig, this being kept constant throughout the experiment. The CRM was fed ad libitum as the only energy source. The chemical composition of ingredients, CRM and supplement, is presented in Table 2. The daily supplement allowance was divided in two parts and given twice daily, early in the morning and in the afternoon. CRM was given twice per day (ad libitum) as a wet mash, with an estimated dry CRM to water ratio of about 1:2. The amount supplied was based on mean pen liveweight and calculated to be in excess of intake. Refusals were weighed each morning and feed intakes calculated and recorded every day. Daily feed intakes were calculated using means values per pen. Water was also available ad libitum. The chemical composition of the six test diets is presented in Table 3.

Table 2: Chemical composition of cassava root meal (CRM) and soybean supplement (FSBM) (%, fresh basis)

	CRM	FSBM Supplement
		150	200	250	300	350	400

DM	89.2	94.5	93.6	93.9	92.6	94.0	92.7
N x 6.25	3.05	31.0	33.2	35.7	37.5	38.9	39.9
Lysine, g/kg	0.06	17.0	18.9	20.3	21.3	22.1	22.7
Met+cys, g/kg	0.05	8.80	9.80	10.5	11.0	11.4	11.7
EE	1.90	17.0	15.3	14.4	13.0	10.9	10.1
CF	3.38	3.27	3.64	3.90	4.09	4.24	4.37
Ash	2.70	22.1	19.4	17.4	16.4	14.7	13.6

Table 3: Chemical composition of the experimental diets#

	Protein level, g/day
DM basis, %	150	200	250	300	350	400

N x 6.25	10.5	12.2	14.9	17.2	19.7	20.1
CF	3.35	3.46	3.67	3.66	3.77	3.84
EE	6.38	6.44	6.63	6.89	6.05	6.63
Ash	7.89	7.99	7.69	8.13	8.00	7.76
Lysine	0.52	0.64	0.77	0.90	1.02	1.10
Met+Cys	0.26	0.33	0.39	0.46	0.51	0.56
Met+Cys/Lys	0.51	0.51	0.51	0.51	0.50	0.51

In total, 48 crossbred Yorkshire x Landrace x Duroc pigs were used (24 castrates and 24 gilts). They were vaccinated against swine fever, dewormed, and ear-notched for identification and assigned to six treatment groups of 8 pigs each on the basis of sex, age and initial liveweight, with four replicates per treatment in a completely randomized design (Little and Hills 1984). The average initial liveweight for all treatments was 31.4 kg. The pigs were weighed once every two weeks in the morning before feeding and were slaughtered at a mean liveweight of 91 kg. Daily gains were calculated from the linear regression of liveweight on days on trial.

The pigs were allotted randomly to 24 pens (experimental units) and housed two per pen (1 castrate and 1 gilt). There were four pens per treatment. The pens were washed and disinfected before the start of the experiment and every morning throughout the experimental period. All pens had concrete floors and were equipped with feeding troughs and drinkers.

When the average liveweight of all the pigs on the experiment had reached approximately 90 kg (after 95 days on experiment), the heaviest four on each treatment were sent for slaughter, the remainder being slaughtered one week later when their average liveweight was 88 kg. The pigs were despatched to the abattoir about 24 h after the final weighing, and slaughtered some 10 h later. After dressing and splitting the carcass into two longitudinal halves, carcass weight, carcass length from first rib to pubis bone, average backfat thickness over the first rib, the last rib and the last lumbar vertebra were determined. Loin eye area and weight were measured on loin cuts chilled at 5 ?C. For technical reasons it was not possible to chill the carcasses before taking other measurements.

The weights of some internal organs such as liver, kidneys, and spleen were taken and analyzed after adjusting them for slaughter weight. The carcass was cut and the various cuts were weighed.

Diet digestibility was measured twice during the course of the feeding trial using the total collection method. The pigs were housed two per pen (as above) and they were fed normally. Feed intake was recorded every day, and feed not consumed was collected daily, dried and weighed. Faeces were collected on a 24 hour basis from 18 pens (3 replicates per treatment) for two six day periods when the pigs were 40 - 55 kg and 65 - 85 kg liveweight, respectively. Fresh faeces were collected in plastic buckets and then samples were dried in an oven and weighed. The total collected per treatment for each period was pooled, ground, mixed and sub-samples taken for analysis. Representative feed samples were also analysed.

Feed, Longissimus dorsi muscle and faecal samples were taken for analysis of dry matter (DM), crude protein (CP), ether extract (EE), crude fibre (CF) and ash, at the laboratory of the Animal Nutrition Department of the University of Agriculture and Forestry using AOAC procedures (AOAC 1970). Iodine index was determined from backfat at the Pasteur Institute, Ho Chi Minh City. pH of loin was taken at 3 and 24 h after slaughter.

All data collected were subjected to analysis of variance using the General Linear Model (GLM) procedure of Minitab statistical software (Minitab Inc., 3081 Enterprise drive, State College, PA, 1680-3008, 1991, USA). The effects of treatment, sex and the treatment x sex interaction were used as models and initial live weight, slaughter weight and carcass weight were used as covariates. Regression equations were calculated using the regression procedure of Minitab (Ibid.)

Gross profit and feed costs were calculated considering the cost of piglets and feed as expenses, and price / kg pork as income. Feed costs per day and per kg of daily gain were also calculated. These data were analysed by GLM (as above). Labour and overheads (i.e. electricity, fuel and water costs) were assumed to be similar for all the treatments.

The pigs were generally healthy, with only a few cases of diarrhoea. Based on actual intakes of CRM and supplement the chemical compositions of the complete diets were calculated and are presented in Table 3. Crude protein concentration was 10.5, 12.2, 14.9, 17.2, 19.7 and 20.1 % of dry matter (DM) of the diet for treatments CP150, CP200, CP250, CP300, CP350 and CP400, respectively. Methionine+cystine as a proportion of lysine was calculated to be: 0.50, 0.52, 0.51, 0.51, 0.5 and 0.51, for the six diets. The amount of methionine+cystine as a proportion of lysine for an ideal protein, recommended by Wang and Fuller (1989), is 0.63, thus the protein in all the experimental diets was slightly imbalanced with respect to the sulphur amino acids. This is in agreement with the reports that methionine is the first-limiting amino acid in soya bean meal (Pond and Maner 1974) and also in cassava root meal (Ravindran et al 1983). Methionine is important in diets rich in cassava root because it helps to detoxify the hydrogen cyanide (HCN) (Buitrago 1990; Pond and Maner 1974).

Table 4: Effect of dietary protein supply on pig performance (growing and finishing)

	Protein level (g/day)
	150	200	250	300	350	400	SE/Prob

Liveweight, kg
Initial	31.4	31.2	30.9	31.7	31.1	31.8	±1.26/0.99
Final	82.1	90.8	89.7	91.9	96.2	95.3	±2.20/0.001
Gain, g/d	529	620	615	619	680	655	±18.2/0.001
Intake (kg/d)
Supplement	0.52	0.63	0.74	0.85	0.96	1.06
Cassava	1.52	1.52	1.45	1.34	1.28	1.29	±0.06/0.03
DM	1.85	1.95	1.99	1.98	2.04	2.12	±0.056/0.05
CP, g/day	195	238	297	341	401	427	±2.24/0.001
Feed conversion
(kg DM/kg gain	3.51	3.15	3.25	3.22	3.01	3.24	±0.11/0.11

The relationship between liveweight gain for the whole growing- finishing period, and crude protein intake, was described by the following quadratic regression equation:

The main difference in growth rate (from 529 to 620 g/day) was between the 150 and 200 g/day level of crude protein. A further doubling of protein intake (to 400 g/day) only raised growth rate by 35 g/day to 655 g/day. Dry matter intakes and feed conversion showed the same tendency as growth rate with the only marked difference being between levels of 150 and 200 g protein/day. The relationship between protein supply and feed conversion was described by the quadratic equation:

The amounts of crude protein found to support optimum growth and feed conversion in the present trial were less than those recommended by the NRC (1988) which for pigs in the growing and fattening stages are 285 and 404 g/day. Thus the principal hypothesis was substantiated, namely that on low-protein high- energy diets, as is the case with many tropical feed resources, protein supply can be reduced substantially provided that the supplementary protein source is relatively well balanced with respect to the essential amino acids (Sarria et al 1991; Preston 1995).

Table 5: Effect of dietary protein supply on pig performance (growing stage)

	Protein level (g/day)
	150	200	250	300	350	400	SE/Prob

Liveweight, kg
Initial	31.4	31.2	30.9	31.7	31.1	31.8	±1.634/0.99
Final	49.9	51.5	52.9	54.2	55.1	55.3	±2.14/0.43
Daily gain, g	416	460	500	512	546	530	±20.3/0.001
Intake (kg/d)
Supplement	0.54	0.64	0.73	0.83	0.93	1.02
Cassava	1.22	1.16	1.15	1.11	1.02	1.03	±0.043/0.03
DM	1.59	1.64	1.71	1.76	1.78	1.86	±0.041/0.01
CP, g/day	191	232	287	328	383	404	±2.23/0.001
CP, % of DM	12.0	14.1	16.8	18.6	21.5	21.7
Feed conversion
(kg DM/kg gain)	3.85	3.57	3.42	3.45	3.26	3.53	±0.13/0.10

Table 6: Effect of dietary protein supply on pig performance (finishing stage)

	Protein level (g/day)
	150	200	250	300	350	400	SE/Prob

Liveweight, kg
Initial	49.9	51.5	52.9	54.2	55.1	55.3	±2.14/0.44
Final	82.1	90.8	89.7	91.9	96.2	95.3	±1.88/0.001
Daily gain, g	630	747	700	696	768	743	±35.8/0.161
Intake (kg/d)
Supplement	0.50	0.62	0.75	0.87	0.99	1.12
Cassava	2.03	2.06	1.90	1.63	1.61	1.57	±0.082/0.01
DM	2.28	2.42	2.40	2.26	2.37	2.44	±0.074/0.40
CP, g/day	202	250	313	357	426	457	±2.42/0.001
CP, %	8.86	10.3	13.0	15.8	17.9	18.7
Feed conversion
(DM kg/gain kg)	3.69	3.24	3.45	3.25	3.11	3.29	±0.16/0.256

In general, for all growth performance parameters measured, there were no differences (P>0.05) between male castrates and gilts.

The carcass quality measurements are summarized in Table 7. Carcass traits did not differ (P>0.05) between treatments. Increasing dietary CP supply resulted in slightly decreased backfat thickness (NS) with the lowest value recorded on the CP400 diet. There was a trend (P=0.099) to increased M. Longissimus dorsi areas on the 300, 350 and 400 g CP diets. The area of the loin-eye muscle (LEM) was related to crude protein intake (CPI), according to the following quadratic regression equation:

Table 7: Effect of dietary protein supply on carcass characteristics adjusted for slaughter weight

	Protein level (g/day)
	150	200	250	300	350	400	SE/Prob

Carcass
Weight, kg(#)	67.2	66.9	67.1	67.8	67.8	67.5	±0.51/0.80
Dressing %	78.2	77.9	78.0	78.7	78.9	78.5	±0.61/0.85
Length, cm	77.4	76.6	76.8	76.1	77.1	76.7	±0.67/0.83
Backfat, mm	30.5	32.1	30.4	30.9	28.7	28.2	±1.43/0.43
Longissimus
dorsi area, cm²	37.3	37.2	36.0	39.4	41.1	43.8	1.99/0.099

Weights of valuable cuts adjusted for carcass weight are given in Table 8. Dietary protein levels significantly (P<0.05) influenced lean cuts. The following quadratic regression equation shows the correlation between total valuable cuts (Y, kg) and crude protein intake (X, kg/day):

It is clear that ham and shoulder weights increased when dietary CP supply increased. The following quadratic regression equations show their correlation with crude protein intake:

Y1 = 7.63 + 5.92X + 0.51X² (r² = 0.88, P = 0.042)
Y2 = 0.33 + 55.23X - 80.68X² (r² = 0.92, P = 0.024)

where: Y1 = ham weight, kg and Y2 = shoulder weight, kg
X = crude protein intake, kg/day.

There were no significant differences (P>0.05) between treatments, for side fat, bones and skin weights.

Table 8: Effect of dietary protein supply on weights of valuable cuts adjusted for carcass weight

	Protein level (g/day)
	150	200	250	300	350	400	SE/Prob

Wt of valuable
cuts, kg
Ham	8.94	8.82	9.53	9.64	10.3	10.0	±0.28/0.01
Shoulder	8.13	8.74	9.68	9.69	9.81	8.99	±0.34/0.01
Loin eye	3.23	3.55	3.67	3.76	3.91	3.80	±0.17/0.24
Total	20.3	21.1	22.9	23.1	24.1	22.8	±0.61/0.01
Side fat	8.67	9.01	8.57	7.76	7.50	7.48	±0.61/0.42
Bones	11.1	11.1	11.6	11.8	11.9	12.0	±0.28/0.18
Skin	3.07	3.28	3.78	3.89	4.14	3.94	±0.26/0.10

The chemical composition of the loin eye muscle is given in Table 9. Dry matter content was not significantly affected by treatment. There was a tendency for crude protein to increase with higher dietary level of protein. Ether extract (EE) decreased as dietary CP increased, as is shown by the following linear regression equation:

Table 9: Effect of dietary protein supply on chemical composition of loin eye and Iodine index of backfat, adjusted for carcass weight

	Protein level (g/day)
	150	200	250	300	350	400	SE/Prob

DM, %	26.1	25.9	25.5	25.8	25.4	25.9	±0.35/0.76
CP, %	22.0	21.9	22.4	22.6	22.8	22.7	±0.27/0.20
EE, %	4.2	4.0	3.1	3.5	2.1	3.0	±0.41/0.03
Iodine index	68.7	68.1	71.0	71.6	76.2	74.3	±1.42/0.02

Gomez et al (1982), cited by Balagopalan et al (1988), reported that the moisture, crude protein content and fat content of the meat of cassava-fed pigs did not differ from the control pigs. Iodine index of backfat (Table 9) increased linearly (P=0.022) with increasing CP intake, probably due to the high fat content of the soyabean meal (in this study, fortified soyabean meal) and its high proportion of unsaturated fatty acids, mainly linoleic acid (Pond and Maner 1974).

Table 10: Effect of dietary protein supply on pH of loin eye and incidence of PSE

	Protein level (g/day)
	150	200	250	300	350	400	SE/Prob

pH3	5.3	5.6	5.4	5.5	5.5	5.5	±0.088/0.18
pH24	5.2	5.4	5.3	5.3	5.3	5.3	±0.055/0.34
PSE, %	62.5	12.5	37.5	37.5	37.5	62.5

pH3 3 hours after slaughter; pH24 24 hours after slaughter;
PSE= Pale, soft and exudative muscle.

There was no effect of treatment on the pH of the loin eye muscle (Table 10). Also the incidence of pale, soft, exudative (PSE) muscle was not affected by treatment, but was higher than normal probably due to stress and mishandling in transport to the slaughterhouse and the time between removal from the farm and the moment of slaughter (Whittemore 1993).

Table 11: Effect of dietary protein level on internal organ weights, adjusted for slaughter

	Treatment (CP)
	150	200	250	300	350	400	SE/Prob

Liver, g	1452	1445	1423	1386	1425	1414	44.4/0.93
Spleen, g	170	192	169	241	208	214	28.2/0.49
Kidneys, g	239	287	288	334	379	373	26.5/0.03

Post mortem visual inspection of whole and sliced organs did not reveal any lesions. Only kidney weight was significantly different between treatments (Table 11). Kidney weight was heavier for treatments with higher dietary CP probably due to higher nitrogen content of those diets, increasing the nitrogen circulation in the body and the catabolism of nitrogenous compounds, therefore increasing the work load of the kidneys leading to an increase in size. The correlation between kidney weight (KW) and crude protein intake (CPI), is given by the following quadratic regression equation:

In general for all carcass parameters measured, there were no differences (P>0.05) between castrates and gilts.

Mean values for the apparent digestibilities of the complete diets are presented in Table 12. The values were not significantly affected by dietary protein level except for crude protein (at least for the second trial), where the highest values for both periods were obtained on the CP350 dietary treatment.

Table 12: Apparent digestibility coefficients (%) of the complete Diets

	Protein level (g/day)
	150	200	250	300	350	400	SE/Prob

No of pigs	6	6	6	6	6	6
Trial 1 (40-
55 kg LW)
DM	91.0	92.3	91.6	91.2	91.5	90.5	±1.02/0.87
OM	93.8	94.5	94.0	93.2	93.6	92.5	±0.67/0.43
CP	94.3	96.1	96.5	96.1	97.3	96.8	±0.94/0.38
EE	89.9	91.4	90.3	90.7	89.4	89.2	±1.41/0.88
CF	64.1	74.8	66.6	64.2	73.9	71.6	±6.88/0.77
Ash	56.3	67.7	62.5	68.1	67.7	66.7	±5.25/0.58
Trial 2 (65-
85 kg LW)
DM	91.0	93.2	92.6	93.4	93.1	92.0	±0.58/0.08
OM	93.2	95.0	94.3	94.9	94.7	93.6	±0.44/0.07
CP	94.5	96.2	96.2	97.1	97.3	97.0	±0.32/0.01
EE	87.8	91.3	90.0	91.4	91.2	90.8	±2.16/0.84
CF	72.8	74.8	72.4	74.3	78.4	75.3	±3.55/0.86
Ash	62.0	69.4	68.2	74.6	72.6	71.1	±2.63/0.07

On the low protein diets the proportion of endogenous protein (from bacteria, cell walls, and enzymes) in the faeces is relatively high compared to the high protein diets and therefore the apparent CP digestibility increases with increasing dietary protein supply. DM and OM digestibilities were slightly higher for the heavier pigs probably as a result of the increased CF digestibility. Table 13 shows the effect of growth stage on apparent digestibilities of dietary nutrients, with improved apparent digestibilities for dry matter and organic matter (P<0.05) for the second period, as a result of the development of the digestive tracts. This suggests that optimum utilization of the nutrients occurred when the pigs were in the finishing stage and when they were fed 300 - 350 g dietary CP per day and cassava root meal ad libitum. The generally high values are in agreement with the report of Balagopalan et al (1988), who affirmed that digestibilities of cassava based diets by swine are high, and similar to cereal based diets.

Table 13: Effect of stage of growth on the apparent digestibility (%) of the complete diet

	DM	OM	C	EE	CF	Ash

Growth stage, kg
40 - 55	91.4	93.6	96.2	90.2	69.2	64.8
65 - 85	92.6	94.3	96.4	90.4	74.7	69.7
SE	±0.34	±0.23	±0.29	±0.75	±2.2	±1.7
P value	0.02	0.04	0.62	0.80	0.09	0.06

Table 14 shows the results in economical terms. The retail price for live pigs at the time of sale was very low due to oversupply, therefore the net income for all treatments was negative. Under the conditions of this study, feed costs were significantly different (P<0.05) between treatments. The lowest feed costs were for the diet supplying 150 g dietary CP per day, but per kg of daily gain, the most economical treatment was 200 g CP/day. The most expensive diet in terms of cost/day and cost/kg liveweight gain was the diet supplying 400 g CP/day.

Table 14: Economical analysis of the dietary treatments ('000 Vietnam Dong, VND)

	Protein level (g/day)
	150	200	250	300	350	400	SE/Prob

Income#	784	867	857	878	919	910	±26/0.01
Expenses
Piglets	629	625	618	633	622	636	±25/0.99
Feed	404	451	486	519	558	602	±6.4/0.001
Total	1,033	1,076	1,103	1,152	1,180	1,237	±22/0.01
Profit/pig	-249	-208	-247	-274	-261	-327	±28/0.09
Feed costs/pig
/per day	4.09	4.56	4.91	5.25	5.64	6.08	±0.08/0.01
/kg gain	7.81	7.38	8.03	8.48	8.94	9.23	±0.37/0.02

Prices (VND)/kg of: LW finished pig: 9400 - 9700;
LW piglet: 20000;
CRM, cassava root meal: 1100;
Concentrate: 4400 - 4600
(1 US dollar = 10500 VND)
# Sale of pigs

These results indicate that feeding pigs throughout the growing- finishing period with CRM ad libitum, combined with 200 g/day of CP from soya bean meal gives acceptable growth performance and feed conversion. In the finishing period a daily CP supply of only 200 g/day gave maximum rates of daily gain and the lowest feed conversion, but due to the pigs higher CP requirements in the growing period increasing the CP supply to 350 g/day gave optimum growth rates. Feeding CRM ad libitum combined with 200 g dietary CP/day gave the lowest feed costs per kg gain, and this feeding system can be recommended in areas where cassava is available, abundant and cheap compared to other energy sources.

The authors wish to express their gratitude to the Swedish International Development Authority (SIDA) for financing this work, and the staff of the University of Agriculture and Forestry, Ho Chi Minh City - Viet NAM, for allowing the use of the facilities at the research farm.