An experiment was carried out with sugar cane at the Goat and Rabbit Research Centre, Hatay, Vietnam, from January 1993 to December 1995 to study the effect of row width (75, 100 or 150 cm), plant material (stem cuttings or tops), and removing or leaving the dead leaves, on biomass yield and juice quality (°Brix).

Yield of cane stalks was higher when stem cuttings, rather than tops, were used as planting material. The effect was most pronounced in the first harvest year and became less with succeeding harvests. The cost of seed was also reduced by 9-10% compared with the traditional way used by the farmers in North Vietnam. Increasing population density by reducing inter-row spacing to 75 cm led to higher yields of biomass with no reduction in?Brix or extraction rate of the juice. Mulching with the dead leaves increased yields, the difference being more marked in the second than in the first year and even more apparent in the third year (6.3, 20 and 30 % increases for mulching in plant, 1st and 2nd ratoons, respectively). Mulching also improved soil fertility and increased the amount of carbon sequestered in the soil, indicating that mulching is a positive alternative to the traditional way of removing the dead leaves. Soil fertility increased steadily with increasing ratoons indicating that the growing of sugar cane does not exploit soil nutrients but, in contrast, has beneficial effects on growth of subsequent crops.

In recent years a considerable amount of research has been directed at increasing biomass quantity and quality of agricultural crops used as animal feed. Sugar cane is a perennial crop which can be used for animal feed as well as sugar production (Preston and Murguetio 1992). It has a high leaf area index and a high photosynthetic efficiency under strong sunshine, more than any other crop in the tropics (Bassham 1978). The individual and combined effects of certain management practices-- planting date, row spacing, planting depth, fertilizer rate, pest control and irrigation -- have a great impact on the growth and yield of sugar cane. The number of plants at harvest time is a major determinant of biomass yield, and a density of 50-70,000 plants/ha was recommended by Tran Van Soi (1988 ). This was a general recommendation to account for factors such as variety, branching ability , climate and soil conditions. If the plants are too close, there may be too many shoots which will reduce the efficiency of the parent plants and class 1 branches while too large a space between rows will lead to a waste of the area and of solar energy (Duong Duc Thang 1991). In the hilly land of Vietnam, higher production was obtained with the same plant density but arranged in narrower rows (Nguyen Huy Uoc 1987) compared to the traditional method of planting. Other researchers also reported higher yields from narrow spacing (90 and 60 cm) compared with wide spacing (130, 140 and 180 cm) (Sharma 1982; Irvine et al 1984; Singh and Singh 1984; Gonzalaz et al 1989; Arvind Misra et al 1990).

Soil organic matter is an important feature of soil fertility. Ways of increasing soil organic matter are by growing a cover crop (green manure) or by mulching with compost or crop residues. These practices also help to control weed growth. Mulching has been shown to increase the yield of tomatoes and tuber plants (Kaniszewski 1994; Pakyurek et al 1994, Khalak and Kumaraswamy 1993 ; Shin et al 1993). The method gave higher yields in fruit trees, vegetables and crops such as mango, sweet pepper, bananas, guava and maize ( Farre et al 1993; Siwek et al 1994; Singh and Singh 1992; Sarad-Gurung et al 1994; Sandhu et al 1992). Mulching by crop residues was superior to polyethylene sheets (used to control weeds) in terms of incremental cost-benefit ratios ( Khalak and Kumaraswamy 1993). There are several reports of improvement of soil fertility as a result of mulching with crop residues (Sandhu et al 1992; Kitou and Yoshida 1994; Arzeno 1992).

The hypotheses to be tested in the present study were:

• Reducing row distance would lead to increased yield of biomass
• Planting with stem cuttings rather than the growing point would increase the longevity and the yield of sugar cane
• Returning the dead leaves to the soil (mulching) would increase biomass yield and soil fertility

The experiment was carried out from January 1993 to December 1995 at the Goat and Rabbit Research Centre in Bavi district in a hilly area some 60 km North-West of Hanoi with an average slope of 3-8^o. The soil composition at the experimental site was as follows: pH 5.3, K₂O 0.06%, P₂0₅ 0.09%, N 0.14%. In general, the soil is of low fertility and the organic matter has been reduced to very low levels by erosion. Drought occurs frequently in the area.

The treatments in a split-plot design with three replications were:

• Planting distances of 75, 100 or 150 cm between rows
• Planting material: Tops ( growing points of sugar cane) or stem cuttings (the stalk was cut into pieces of 50 cm)
• Mulching (returning the dead leaves to the soil surface) or no mulching (taking away the dead leaves)
• Harvesting at 10 months or 12 months after planting

Row spacing was the main plot and plant material and mulching were the sub-plots. The area for each experimental plot was 90, 80 and 75 m² for row spacings of 150, 100 and 75 cm, respectively. The size of the recorded area was 60 m² for all treatments. The total number of plots was 36, giving a total experimental area of 3,569 m² (2,940 m² for recording data). The plant material was set in double continuous rows with 8, 10 and 12 tonnes/ha for 150, 100 and 75 cm spacing between rows, respectively.

Fertilizer was applied according to the normal practice by farmers in the area. The amounts of cattle manure and chemical fertilizers applied are shown in Table 1.

Table 1: Applications of cattle manure and chemical fertilizers
Year	Manure	N	P2O5	K2O
	(tonnes/ha)	------(kg/ha)----------
1993	20	200	100	200
1994	15	150	80	150
1995	10	100	60	100

The dressings of nitrogen were based on government recommendation of N 200 kg, P₂O₅ 100 kg and K₂O 200 kg/ha for 100 tonnes cane stalk. It was also based on results from Kanwar et al (1989), Ravindra et al (1989) and Chapman et al (1992) where regimes of 150-225 kg N/ ha gave highest cane and commercial sugar yields.

The sugar cane variety used was POJ 3016 (originally from Java, Indonesia). There were sporadic infections with insects which facilitated invasion of the leaf blade by black fungi. An insecticide "Wafatox" at 1 g/litre concentration was applied locally to control the insects. Weeds were removed by hand at intervals but the amounts were not recorded.

The dead leaves were collected and weighed every month beginning 6 months after planting. For the mulching treatment the leaves were returned to the soil after they were weighed.

At harvest the experimental plots were divided into 2 parts for harvesting. One half was harvested at 10 months after planting and the other half at 12 months. The whole plant was cut at ground level. The stalk was separated from the whole plant by cutting immediately below the second node measured from the top. The growing points (tops) were then separated from the leaf blades (green leaf). Each component was weighed and sampled for chemical analysis.

Samples of the stalk from each plot (about 10 kg) were crushed by passing them three times through a 2-roll mill driven by a buffalo. On the second and third pass the partially pressed stalks were doubled to maximise extraction of the juice. Extraction rate was expressed as weight of juice as a percentage of the weight of cane stalks. The total soluble solids in the juice (°Brix) were determined using a hand refractometer.

Soil samples (from 0-20 cm depth) were taken from each experimental plot immediately after each harvest. Equal amounts (3 kg) were put into clay pots (about 5 litre capacity) for a biological test of overall soil fertility. Three seeds of maize were planted. After 5 weeks the maize plants were removed from the soil, washed to remove soil from the roots and allowed to dry for 1 hour. The total fresh biomass and the maize roots were weighed (Maria Elena Gomez 1993 personal communication).

Samples of soil (from 20 cm depth) were taken from each plot after the 12 month harvest. The 18 samples corresponding to the mulching treatments were bulked and 6 sub-samples taken for analysis. The samples from the 18 non-mulching treatments were treated in the same way. The sub-samples were analysed for pH, N, P, K and carbon by standard methods (AOAC 1985). Estimates were made of the populations of fungi, bacteria and actinomycetes according to "Standard methods of analysis for Soil, Plant tissue, Water and Fertilizer" (Philippine-Los Baños, Laguna 1980) with asparagine-manitolagar medium for bacteria, glycerol-agar for actinomycetes and peptone-dextrose-agar plus rose bengal and streptomycin for fungi.

The total numbers of stalks in each plot were counted after 3, 6 and 9 months and at harvesting at 10 and 12 months. Beginning in the sixth month (August) after planting and subsequently at monthly intervals, drops of juice were taken from the upper, mid and lower inter-nodes and the three samples mixed for determination of the °Brix value.

Statistical analysis

The data were analysed by Analysis of Variance using the General Linear Model of the statistical software by Minitab (1993). The model used was:

Y_ijk= µ +a_i + b_j + g_k + (abg)_ijk + e_ijk

Y = Yield of sugar cane

µ = Overall mean

a = Effect of spacing

b = Effect of plant material

g = Effect of mulching

e = Error

abg= Interaction between spacing, plant material and mulching

The mean numbers of plants/m² for the different treatments are shown in Table 2. The plant densities increased as distance between rows decreased with the effect especially notable in the first ratoon (the second year) and the second ratoon (the third year). The number of plants was higher in the first year when stem cuttings were used as the plant material. In the first and second ratoons there were no differences. Mulching had little effect on plant density.

Table 2: Effect of treatments on plant density
	Mean plants/m²
	1993	1994	1995
Row spacing(cm)
75	8.2	7.4	7.1
100	7.4	6.5	4.9
150	5.6	4.3	3.0
SE	0.24	0.2	0.5
Probability	0.001	0.001	0.001
Planting materials
Stem cuttings	7.2	5.6	5.3
Tops	6.4	5.3	5.0
SE	0.19	0.16	0.44
Probability	0.05	0.85	0.32
Treatment of dead leaves
No mulching	7.2	5.0	5.3
Mulching	7.0	5.8	5.2
SE	0.19	0.16	0.44
Probability	0.48	0.57	0.58

The densities of mature plants at harvest time in year 1 varied from 56,000 to 82,000 plants /ha which is in agreement with values obtained in the traditional sugar cane plantation (Tran Van Soi 1988), but the populations of 43,000 at 150 cm row distance in year 2 and of 35,000 in the third year are rather low. The population of sugar cane at 150 and 100 cm row distance decreased markedly year by year while at the 75 cm spacing it was more stable and in year 3 it was twice that of the 150 cm treatment.

Analysis of variance of the yields of the components of the edible biomass (stalks, tops and green leaves) which can be used as animal feed (Table 3), showed that decreasing the row spacing led to increases in yield. These results are in agreement with reports of Sharma (1982), Irvine et al (1984), Singh and Singh (1984), Sudama et al (1988), Arvind et al (1990), Patel et al (1990), Bharad et al (1991) and Singh and Singh (1992).

Table 3: Effect of treatments on fresh biomass yield at the 12 month harvest in each of the three harvest years (tonnes/ha)
Components of yield	Cane stalks			Tops			Green leaves
Year	1993	1994	1995	1993	1994	1995	1993	1994	1995
Row spacing, cm
75	134	103	74.4	22.4	15.8	14.9	12.9	11.1	8.8
100	120	92.5	54.3	19.4	12.4	10.4	11	8.9	5.8
150	86.4	62.6	34.8	13.7	8.2	7.7	10.5	5.5	5.2
SE	1.9	2.8	5.25	1.02	0.72	1.17	0.86	0.63	0.84
Probability	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Plant material
Stem cutting	117	87.1	54.4	19.8	11.9	12..3	11	8.5	6.9
Tops	110	85.7	54.6	17.2	12.4	9.7	12	8.4	7.1
SE	1.5	2.28	4.28	0.84	0.6	0.96	0.7	0.51	0.68
Probability	0.05	0.68	0.97	0.037	0.56	004	0.47	0.83	0.71
Treatment of dead leaves
No mulching	110	78.6	47.3	17	11.9	9.6	11.7	8.6	6.5
Mulching	117	94.2	61.7	19.5	12.4	12.4	11.1	8.4	6.7
SE	1.5	2.28	4.28	0.84	0.6	0.96	0.7	0.51	0.68
Probability	0.006	0.001	0.024	0.06	0.66	0.8	0.41	0.12	0.29

Planting stem cuttings, rather than tops, increased yield of stalks in the first year, but had no significant effect in subsequent years. Mulching with the dead leaves increased yield in all years, the difference being more marked in the second than in the first year and even more apparent in the third year (6.3, 20 and 30 % increases in stalk yield in the first, the second and the third year, respectively). Mendoza (1988) in the Philippines also reported a more marked effect of mulching in the ratoon compared with the plant crop. Work in India (Yadav et al 1994) and in Northeastern Brazil (Ball Coelho et al 1993) proved that recycling sugar cane trash to the soil helped to sustain yields of successive ratoon crops in sugar cane. Other favourable effects on cane stalk yield and economic benefits from mulching with dead leaves (trash) were reported by Mondharan et al (1990), Sathyavelu et al (1991), Sinha et al (1991) and Mahadevaswamy et al (1994).

The reduction in yield of total biomass (Table 4) followed closely the trends in plant population. In the third year, yield at 150 cm row distance was only half that of the 75 cm treatment. These data, together with those in Table 3, show clearly why farmers replant sugar cane after the first ratoon when the row width is at the traditional 150 cm.

Table 4: Effect of treatments and of year of harvest on annual fresh biomass yield (tonnes/ha) (combined data for 3 years at the 12 month harvest)
Components	Stalks	Tops	Leaves	Total
Row spacing, cm
75	102.	16.5	10.9	130
100	86.	13.5	8.55	109
150	59.6	9.42	7.10	76.5
SE	2.1	0.60	0.45	2.6
Probability	0.001	0.001	0.001	0.001
Plant material
Stem cutting	83.5	12.8	8.73	106
Tops	82.1	13.4	8.95	105
SE	12	0.4	0.33	1.5
Probability	0.28	0.82	0.69	0.39
Treatment of dead leaves
No mulching	76.9	12.2	8.6	98.8
Mulching	88.7	13.6	9.1	12.1
SE	1.5	0.4	0.33	1.5
Probability	0.001	0.006	0.16	0.001
Year
1993	114	18.5	11.6	144
1994	86.4	12.1	8.57	107
1995	54.5	11.0	7.03	72.6
SE	2.1	0.60	0.55	2.7
Probability	0.001	0.001	0.001	0.001

The monitoring of the °Brix of the sugar cane juice (Figure 1) showed that it increased linearly from August to December. There were no consistent differences between treatments in °Brix value or juice extraction rate (Table 5). Juice extraction rate appeared to be higher in the 1st ratoon compared with plant and 2nd ratoon crops.

Table 5: Effect of treatments on juice extraction rate and Brix value (12 month)
Year	1993	1994	1995	1993	1994	1995
	----Juice extr. %----			--------Brix, %------
Row spacing, cm
75	49	60.8	49.3	18.7	19.0	20.3
100	49.7	61.7	48	18.5	18.8	20.2
150	50.4	61.7	46.1	18.9	19.4	20.6
SE	0.43	0.9	1.37	0.15	0.49	0.4
Prob.	0.07	0.63	0.54	0.09	0.73	0.68
Planting materials
Stem cutting	50.1	62.5	48.2	19	18.9	20.3
Tops	49.2	59.7	47.4	19	19.2	20.4
SE	0.35	0.7	1.1	0.12	0.4	0.33
Probability	0.14	0.015	0.8	0.97	0.64	0.81
Treatment of dead leaves
No mulching	50.4	60.9	45	18.8	18.8	20.5
Mulching	49.3	61.2	44.6	19	19.4	20.3
Probability	0.05	0.8	0.6	0.35	0.35	0.63
SE	0.35	0.7	1.1	0.12	0.4	0.33

Yields of stalk were higher, and of tops and green leaves were lower (Table 6), for harvests made at 12 months compared with 10 months. The °Brix in the juice was lower but extraction rate was higher at the 10 month harvest than at 12 months.

Table 6: Yield and quality of sugar cane at harvesting time (combined data of 3 years)
Harvesting time	Stalks	Tops	Green Leaves	Total	°Brix	Juice
	--------tonnes/ha-------------				------%------
10 months	75.8	15.6	10.4	101.8	15.8	56.8
12 months	84.8	13.9	9.05	108.0	19.4	51.9
SE	1.4	0.43	0.28	1.69	0.16	0.47
Probability	0.001	0.004	0.001	0.015	0.001	0.001

The amounts of dead leaves collected monthly from 6 months after planting to the time of harvest are shown in Table 7. The highest values were recorded for the 75 cm row spacing. There were no significant effects of plant material or mulching practice. According to Patriquin (1982) this amount of leaf trash will support the fixation by soil microbes of about 150 kg N/ha.

Table 7: Effect of treatment on amount of dead leaves collected between 6 months after planting and harvest
Year	1993	1994	1995
	--------tonnes/ha--------
Row spacing, cm
75	38.1	40.1	24.4
100	38.5	34.9	21.0
150	31.8	26.7	19.5
SE	0.82	0.53	1.3
Probability	0.02	0.001	0.04
Planting materials
Stem cuttings	35.8	34.2	21.7
Tops	35.4	33.1	21.5
SE	0.61	0.43	1.1
Probability	0.87	0.26	0.93
Treatment of dead leaves
No mulching	35.3	33.1	20.6
Mulching	36.7	34.6	22.6
SE	0.61	0.40	1.09
Probability	0.34	0.27	0.22

In the biological test of soil fertility (Table 8) there were no significant effects of row width or planting material on the growth of the maize but there was a significant positive effect due to mulching especially by the time of the second and third year. Maize growth in soil on all the treatments increased (P=0.001) with succeeding ratoons indicating a positive effect of sugar cane growing on soil fertility. The effects were more pronounced with mulching, with increases over non-mulching of 8.2, 27.8 and 37.6% in years 1, 2 and 3, respectively.

Table 8: Weights of roots and green biomass of maize plants grown in soil from experimental plots
	1993		1994		1995
	Root	Biomass	Root	Biomass	Root	Biomass
	----------------------------Weight, g/pot-------------------
Row spacing (cm)
75	3.3	12.6	8.75	18	15.2	27.8
100	3.6	15.2	9.6	18	14.2	25.5
150	3.6	14.1	8.1	17.8	13.4	25.5
SE	0.33	1.21	1.33	1.69	1.12	1.60
Prob.	0.86	0.72	0.5	0.53	0.55	0.54
Planting materials
Stem	3.48	13.5	8.2	17.6	14.6	26.5
Tops	3.53	14.4	9.4	18.4	13.9	26.0
SE	0.27	0.99	1.09	1.38	0.92	1.31
Prob.	0.88	0.98	0.47	0.79	0.64	0.79
Mulching with dead leaves
No	2.97	13.4	7.9	15.8	12.0	22.1
Yes	4.04	14.5	9.7	20.2	16.5	30.4
SE	0.27	0.99	1.09	1.38	0.91	1.31
Prob.	0.014	0.5	0.02	0.001	0.002	0.001

Soil fertility was also estimated on samples taken at depths from 0-20 cm (Table 9). The data show a marked improvement in soil fertility as a result of leaving the dead leaves on the soil. There were increases in carbon, pH, N, P and K.

Table 9: Parameters of soil fertility after mulching and no mulching before planting and at the end of 1993, 1994 and 1995
	Prior	1993		1994		1995
Mulching		Yes	No	Yes	No	Yes	No
pH (KCl)	5.5	5.7	5.4	5.6	5.5	5.6	5.5
C, %	0.79	1.73	1.57	1.48	1.33	1.44	1.2
P205, %	0.09	0.126	0.1	0.103	0.09	0.08	0.07
K2O, %	0.06	0.43	0.33	0.55	0.46	0.42	0.33
N, %	0.14	0.14	0.1	0.084	0.08	0.15	0.14
Bacteria (105)				3.2	3.0	90	49
Actinomycetes (103)				5.5	4	na	na
Fungi (103)				15	1.5	82	7.7

na: Not analysed

In general, most of the parameters analysed tended to demonstrate an improvement in soil fertility, especially microbial activity. Actinomycetes and fungi have been shown to be associated with N fixation in sugar cane (Patriquin 1982; Dobereiner 1992). Similar results were reported by Phan Gia Tan (1993) in Vietnam and Yadav et al (1994) in India.

Other effects

It was observed that there were more fallen and broken canes at the 75 cm row distance treatment than at the wider distances. In accordance with farmer beliefs, it appeared that the narrow distance (high plant population) made the sugar cane more susceptible to wind damage. The thinner cane rind at the higher plant population apparently made the cane more susceptible to wind dadmage.

The reported results are part of a long term study (4-5 years) on the effect of management practices on sugar cane grown for livestock feed. The results for the first three years show:

• When sugar cane was planted by using stem cuttings there was a higher yield in the first and the second year. The cost of seed was also reduced by 9-10% compared with the traditional way used by the farmers in North Vietnam.
• Increasing the plant density by reducing row spacings lead to higher yields of biomass with no reduction in °Brix or extraction rate of the juice.
• Leaving the dead leaves on the soil surface increased biomass production, improved soil fertility and increased the amount of carbon sequestered in soil. Mulching was found to be a most positive alternative to the traditional way of removing the dead leaves.
• Soil fertility increased steadily with increasing ratoons. This also indicated that the new way of growing sugar cane does not exploit soil nutrients but, in contrast, has beneficial effects on the growth of subsequent crops.

Sandhu B S, Ravi Dutt, Khera K L, Beri V, Chaudhary M R, Sidhu P S, Pashricha N S and Bajwa M S 1992 Effect of soil temperature, soil moisture and straw mulching on nutrient mineralization and growth of summer crops in Northern India. Proceedings of the International Symposium on Nutrient Management for Sustained productivity, pp. 20-22.

Received 31 May 1996