Effect of planting season and type of fertilizer on biomass yield and quality of sugar cane

Sugar cane has the twin characteristics of year-round growth but a harvest capability that is often seasonally constrained in order to optimize sucrose yield both in the field and during the process of extraction. For energy, animal feed and protection of soil ecosystems, sucrose yield is less important than a year-round, stabilized supply of total biomass.

Three treatments were compared: planting at monthly intervals from June 1994 through to May 1995; mulching or no mulching with the dead leaves; and organic (cattle) manure or chemical fertilizer. The experimental design employed monthly planting as main plots, mulching or no mulching as split plots and fertilizer type as split-split plots. Harvesting was 12 months after planting.

The densities of mature plants at harvest time varied from 35,000 to 75,000 plants/ha and was higher when planting was from January to April (the traditional system) than from May to August. Plant population was very low for sugar cane planted between September and December .

The plots established between January and July gave higher (P=0.001) stalk yields (67-74 tonnes/ha) than those planted from August to December (37-50 tonnes/ha). Total biomass yield followed the same pattern. °Brix varied markedly with planting date and was highest for cane planted (and harvested) in the period from October to April.

Few, if any, agricultural commodities have the enormous latent potential to produce biomass that characterizes the Saccharum species (Alexander 1985). Sugar cane plant has accomplished botanically over an almost incomprehensible expanse of time its superior capacity for sucrose production and storage and an equally superior expertise to utilize sugar in support of its own growth processes. Whether managed for sugar, or as a total biomass resource, sugar cane has the twin characteristics of year-round growth but a harvest capability that is often seasonally constrained. A new orientation of sugar cane management is needed to increase " total biomass" production and to ensure a year-round, stabilized supply of different components for a biomass consuming industry, for animal feed or to protect soil ecosystems.

Growth-oriented management that aims from the outset to maximize whole cane production all the year round is addressed in this paper.

An experiment was established on a semi-acid (pH = 4.2 - 4.3), medium clay (35%) soil in the upland area of Bavi district and conducted from January 1995 to May 1996. There were three treatments:

The experimental design employed monthly planting as main plots, mulching or no mulching as split plots and fertilizer type as split-split plots.

Sugar cane plots (1,000 m²) were planted every month at 90 cm row distance using the local variety (POJ 30-16). The two fertilizer regimes were: organic (from cattle) manure (20 tonnes/ha) or chemical fertilizers (150 N as urea, 100 P₂0₅, 200 K₂0 kg/ha) as control. Mulching consisted of detaching the dead leaves and leaving them on the soil surface or removing them from the plot at monthly intervals beginning 6 months after planting. The urea was given in three applications (at 1 month, 3 months and 5 months after planting). Phosphorus was supplied at planting. Manure and potassium were in two dressings (75% at planting and 25% at 5 months). The planting material was stem cuttings (30 cm length) in two continuous rows per furrow. Other management practices, such as the method of fertilizer application, cultural practices and use of pesticide, were carried out according to the traditional practices of the farmers.

The plots were harvested monthly after 12 months of growth. The whole plant was harvested by cutting the stalk 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 at the time of harvesting.

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 were taken at 0-20 cm depth 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 roots were weighed.

The data were analysed by Analysis of Variance using the General Linear Model of the statistical software by Minitab (1993, Release 9.2).

Y = Yield of sugar cane, µ = Overall mean, a= Effect of month, b= Effect of mulching,

g= Effect of fertilizer, e = Error, abg = Interaction between month, mulching and fertilizer regime.

The relationships between month of planting (dependent variable) and brix value and total biomass yield (independent variables) were tested using regression analysis.

The average temperature and monthly rainfall in the Bavi area for 1994-95 are indicated in Figure 1. The periods of low average temperature (15-19 ºC) were concentrated from December to March. Rainfall was also low (2.2-67 mm/month) from November to April. Low temperatures and insufficient moisture during the seedling or branching period influence negatively the elongation of the nodes; the top is shortened taking on a fan-shaped appearance and biomass yield will be low.

The numbers of plant/m² for the different treatments are shown in Table 1. The effect of time of planting on plant population was significant (P=0.001). The densities of mature plants at harvest time in the experiment varied from 35,000 to 75,000 plants/ha. The number of plants obtained when planting was at the traditional time (January to April) was significantly higher (7.1-7.5 plants/m²) than when planting was from May to August (5.7-6.8 plants/m²). Plant population was very low for sugar cane planted from September to December (3.5-4.5 plants/m²).

Table 1: Effect of treatments on plant population

Treatments	Plants/m²

Planting month
Jun	6.4
Jul	6.8
Aug	5.7
Sep	4.5
Oct	3.7
Nov	3.5
Dec	4.2
Jan	7.4
Feb	7.2
Mar	7.1
Apr	7.5
May	5.9
SE	±0.29
Prob.	0.001
Mulching
Mulching	5.9
No mulching	5.6
SE	±0.11
Prob.	0.02
Fertilizer regime
Manure	5.9
Chemical	5.7
SE	±0.11
Prob.	0.27

Mulching of dead sugar cane leaves increased the number of plants/m² compared with no mulching (P=0.02), but there was no significant effect (P=0.27) of fertilizer regime.

The yield of edible biomass (stalks, tops and green leaves) which can be used for feeding animals varied among months (Table 2 and Figure 2) and was related with temperate and rainfall during the planting and early growth period. The plots established in the traditional season (January-July) gave higher (P=0.001) yields (67-74 tonnes/ha) than those planted from August to December (37-50 tonnes/ha of stalk). Total biomass yield followed the same pattern.

Table 2: Effect of time of planting, mulching and fertilizer regime on biomass yield of sugar cane

Treatments	Stalk	Top	Green	Total
			leaves

Planting time
Jun	67.2	19.4	10.4	95.4
Jul	73.8	28.0	13.1	114.9
Aug	48.1	16.6	11.1	75.7
Sep	42.1	12.6	8.6	63.2
Oct	36.5	8.8	6.4	51.7
Nov	39.8	12.7	8.5	61.0
Dec	49.5	9.9	8.6	68.0
Jan	72.1	19.6	9.4	110.7
Feb	71.1	19.6	8.4	109.0
Mar	68.1	16.8	9.9	92.2
Apr	72.4	18.9	10.0	101.3
May	73.4	18.5	9.0	101.0
SE	±3.5	±1.24	±0.86	±4.3
Prob.	0.001	0.001	0.001	0.001
Mulching
Mulching	61.3	17.7	9.9	90.3
No mulching	57.6	15.9	9.0	83.7
SE	±1.4	±0.5	±0.35	±1.76
Prob.	0.068	0.016	0.28	0.01
Fertilizer regime
Manure	60.7	17.2	9.8	88.7
Chemicals	58.3	16.4	9.0	85.3
SE	±1.4	±0.5	±0.35	±1.76
Prob.	0.24	0.84	0.10	0.16

Mulching with dead leaves tended to increase stalk yield (P=0.068) and significantly improved yield of tops (P=0.016) and of total biomass (P=0.001) by 11.3 and 7.3%, respectively. This result is in agreement with our earlier reports (Nguyen Thi Mui et al 1996a,b,c) where mulching increased biomass yield in two station and one on-farm experiments by 6.3, 4.3 and 10.6%, respectively. Similar positive effects of mulching on yield were found by Phan Gia Tan (1995), Mendoza (1988) and Ball Coelho et al (1993). It can be expected that the benefit from mulching will be enhanced in successive years due to the improvement in soil fertility that mulching brings about (Nguyen Thi Mui et al 1996a).

The use of organic manure rather than chemical fertilizer tended to enhance yield of stalk (P=0.23) and total biomass (P=0.10) but the effect was small (4-5% increase).

The data for juice extraction rate and °Brix of the juice are shown in Table 3. Both criteria varied with month of planting (42.3 to 54.7% extraction rate, P=0.001; and 14 to 23 °Brix, P=0.001) but there was no effect of mulching or of type of fertilizer.

Table 3: Effect of treatments on juice extraction rate and °Brix value of the juice

Treatments	Juice	°Brix
	extraction (%)

Planting date
Jun	47.2	14.1
Jul	54.7	13.9
Aug	42.3	13.9
Sep	48.5	17.5
Oct	46.8	19.6
Nov	49.3	19.7
Dec	45.3	21.6
Jan	46.8	23.1
Feb	47.6	22.5
Mar	45.1	21.3
Apr	43.5	19.0
May	42.6	15.3
SE	±1.1	±0.29
Prob	0.001	0.001
Mulching
Yes	46.1	18.4
No	47.1	18.5
SE	±0.43	±0.12
Prob	0.11	0.58
Fertilizer
Manure	46.7	18.5
Chemical	46.6	18.4
SE	±0.43	±0.12
Prob	0.93	0.43

The trends in extraction rate and in the °Brix of the juice are shown in Figure 3. The period from October to April was when the °Brix was highest. .

High rainfall and high temperature at planting and during early growth appeared to be the factors favouring high plant populations and high biomass yield of sugar cane harvested after 12 months of growth. The higher °Brix for cane planted between October and April was most probably related to the low rainfall and low temperature at the time of harvest.

Supplementary irrigation would overcome the limitations of soil moisture during the active growth period for cane planted between April and September and would make it possible to study independently the effect of temperature.

For purposes of providing energy (Alexander 1985) the sugar content of the cane is probably not so important but would be expected to influence its value as animal feed. This latter point is addressed in a subsequent paper Nguyen Thi Mui et al 1996d).

This researched was supported by the International Foundation for Science through a grant (B/2291-1) to the senior author.