Carlos F. Boschini
Experimental Station Alfredo Volio Mata
University of Costa Rica
Tres Ríos, Costa Rica
INTRODUCTION
Mulberry, originated from temperate zones in Asia, has now spread throughout the world (Benavides, Lachanx and Fuentes, 1994). In tropical Central and South America, it has adapted excellently (Rodríguez, Arias and Quiñones, 1994). It is a perennial tree or shrub that is easily propagated, with fast growth and vigorous shooting. It develops a strong vertical and profuse horizontal root system (Paolieri, 1970). These features improve physical soil conditions and allow better water conservation. As forage, mulberry has shown excellent organoleptic intake for livestock (Benavides, Lachanx and Fuentes, 1994; Ortiz, 1992; Castro, 1989).
Protein content varies from 14 to 22 percent on a dry matter (DM) basis (Piccioni, 1970) and in vitro digestibility between 70 and 80 percent (Ortiz, 1992).
Under tropical conditions, where conditions favour plant growth, mulberry culture has been practised under various densities and cutting frequencies, which result in large yield differences. In Guatemala, Blanco (1992) obtained 19 tonnes of DM/ha in four cuts every nine weeks, with 30 cm between plants and a cutting height of 75 cm. In Costa Rica, Benavides, Lachanx and Fuentes (1994) used densities of almost 23 000 plants per ha, with DM yields from 21 to 28 tonnes/ha. Rodríguez, Arias and Quiñones (1994) worked in Guatemala with planting distances of 60 and 80 cm between plants and harvesting frequencies of six, nine and 12 weeks, with DM yields of 1 to 4.6 tonnes/ha/harvest. These results suggest that yield increases with density (Benavides, Borel and Esnaola, 1986).
In Japan two planting densities are used, 10 000 and 20 000 plants/ha for the traditional and intensive production systems, respectively (IFA, 1992).
This paper presents the experimental results of the effects of planting densities, cutting heights and frequencies of harvest on DM production in a high altitude tropical environment.
ENVIRONMENTAL CONDITIONS AND ESTABLISHMENT
The study was conducted at the Dairy Cattle Experimental Station "Alfredo Volio Mata" at the University of Costa Rica, located at 1 542 m above sea level, with 2 050 mm of annual rainfall (May to November), mean temperature of 19.5ºC and relative humidity of 84 percent. The soil is volcanic, classified as typic distrandepts (Vásquez, 1982), with medium depth, good drainage and medium fertility (Ca 7.7; Mg 3.0; and K 1.54 cmol/l; P 10.0; Cu 28.8; Fe 234; Mn 6.3; Zn 2.6 mg/l). The pH is 5.9. The zone is considered a Low Mountain Humid Forest (Tosi, 1970, cited by Vásquez, 1982).
The mulberry plantation was established with young stakes of 1-2 cm in diameter, 40 cm in length and with at least three buds, at 5-8 cm in depth at three planting densities: 60, 90 and 120 cm between rows and plants, equivalent to 27 777, 12 345 and 6 944 plants/ha. It was fertilised with a 10-30-10 formula at the rate of 120 kg of P205/ha/year. In July and October ammonium nitrate was applied in two equal doses to complete 150 kg of N/ha/year. After one year, half of the plot was cut at 30 cm in height and the other half at 60 cm. From that time, cuts were made every 56 days (six cuts); 84 days (four cuts) and 112 days (three cuts) for 336 days.
Weeding was done after each harvest, leaving cut weeds between rows. Ammonium nitrate was applied when the shoot reached 3-5 cm (approximately two weeks post-harvest) at the rate of 300 kg N/ha/year (Rodríguez Arias and Quiñones, 1994).
Leaf and stem samples were dried at 60oC for 48 hours, and after grinding DM determinations were made at 105oC.
FORAGE PRODUCTION
The Table presents DM yields by density, height and cutting frequency (P = 0.0l). Density strongly influenced yields. Yields dropped 37 percent (10.2 tonnes/ha) from planting densities 60 to 90 cm and 13 percent (2.2 tonnes/ha) from 90 to 120 cm. Leaf:stem proportion did not change among planting densities despite yield differences. More DM was produced at the 60 cm cutting height (P < 0.01) because more leaves.
TABLE 1
Annual dry matter production (tonnes/ha) of mulberry depending on planting density, cutting height and frequency
|
|
Cutting |
Fraction |
Leaf: |
|||
|
Spacing (cm) |
height (cm) |
Frequency (days) |
whole plant |
Stems |
Leaves |
stem ratio |
|
60 |
30 |
56 |
18.3 |
7.1 |
11.2 |
1.60 |
|
60 |
30 |
84 |
25.1 |
11.4 |
13.7 |
1.20 |
|
60 |
30 |
112 |
40.6 |
21.6 |
19.0 |
0.88 |
|
60 |
60 |
56 |
24.4 |
9.2 |
15.3 |
1.75 |
|
60 |
60 |
84 |
35.8 |
16.3 |
19.5 |
1.19 |
|
60 |
60 |
112 |
30.9 |
15.8 |
15.1 |
0.94 |
|
90 |
30 |
56 |
10.2 |
4.0 |
6.2 |
1.63 |
|
90 |
30 |
84 |
16.5 |
7.8 |
8.6 |
1.11 |
|
90 |
30 |
112 |
26.8 |
14.5 |
12.3 |
0.85 |
|
90 |
60 |
56 |
11.2 |
4.5 |
7.3 |
1.64 |
|
90 |
60 |
84 |
21.5 |
9.3 |
12.2 |
1.32 |
|
90 |
60 |
112 |
20.8 |
10.3 |
10.4 |
1.04 |
|
120 |
30 |
56 |
10.0 |
4.2 |
5.9 |
1.40 |
|
120 |
30 |
84 |
15.5 |
7.5 |
8.0 |
1.08 |
|
120 |
30 |
112 |
19.2 |
10.4 |
8.8 |
0.86 |
|
120 |
60 |
56 |
10.1 |
39.7 |
60.8 |
1.63 |
|
120 |
60 |
84 |
15.8 |
7.2 |
8.6 |
1.22 |
|
120 |
60 |
112 |
20.9 |
10.9 |
10.0 |
0.96 |
|
Means |
|
|
|
|
|
|
|
60 |
|
|
27.5 |
12.3 |
15.1 |
1.36 |
|
90 |
|
|
16.3 |
7.5 |
9.0 |
1.35 |
|
120 |
|
|
14.1 |
6.6 |
7.5 |
1.27 |
|
|
30 |
|
18.4 |
8.7 |
9.8 |
1.26 |
|
|
60 |
|
20.1 |
8.9 |
11.3 |
1.38 |
|
|
|
56 |
14.0 |
5.5 |
8.7 |
1.61 |
|
|
|
84 |
21.7 |
9.9 |
11.8 |
1.19 |
|
|
|
112 |
26.5 |
13.9 |
12.6 |
0.93 |
Height by frequency interaction was significant (P < 0.01) for all DM yields but not for leaf:stem ratio, despite the wide range (0.96 to 1.68) observed. At 30 cm of cutting height DM increases with cutting length, but at 60 cm height, DM yields do not raise further 84 days. Similar effects are seen with leaf and stem yields.
The three-way interaction - height, frequency and density - was significant for the three variables measured, but not for leaf:stem ratio.
DISCUSSION
Forage mulberry plantations had been studied for the humid tropics in Costa Rica (Benavides, Lachaux and Fuentes 1994) and for the dry tropics in Guatemala (Rodríguez, Arias and Quiñones, 1994). The present study was conducted in a high environment (1 542m) with excellent solar radiation throughout the year. Annual DM yields differed greatly depending on density, cutting frequency and height, which should be considered during establishment and management. Planting distance was responsible for 39.5 percent of total variation in DM yields, cutting height only 0.80 percent and frequency 30.64 percent.
Individual plant DM yields were 0.99, 1.31 and 2.03 kg/year for planting distances of 120, 90 and 60 cm, respectively. In China, traditional density is 10.000 plants/ha (IFA, 1992; FAO, 1988) and 25 000 for intensive cultivation. Lin, (1996), Yu and Hsieh (1994), studied spacing effect (1.5 x 0.6, 1.8 x 0.60, 1.5 x 0.75, 2.25 x 0.60, 1.80 x 0.75 and 1.8 x 0.9 m) on leaf production, and found similar results to those of the four times per year harvest of the present experiment. Higher shooting per plant does not compensate for higher shooting per hectare at higher densities. In general terms, larger spacing reduces light competition (González, 1951) and high-density plantations respond to this competition. The long-term effects on the root system and leaf yield are unknown.
Cutting height had a little effect on yield, increasing by 1.7 tonnes DM/ha/year from 30 to 60 cm. Both of these heights are considered low for Chinese practices (FAO, 1988). This increment was due to the leaf fraction. In Turrialba (Costa Rica) annual plant yields were 2.32 kg cutting at 50 cm and 2.12 kg at 100 cm in low-density plantations (Benavides et al, 1986). Some reports recommend low cutting heights (< 70 cm) for 30 000 plants/ha; medium height (70-170 cm) for 12 000 plants/ha and high cutting (> 170 cm) for densities below 6 000 plants/ha (FAO, 1988). Greater leaf production at 60 cm suggests that leaf and stem yields should be studied at various cutting heights under high light competition in dense plantations.
Frequency of defoliation had a marked effect on yield. Greater yields were observed at longer cutting frequencies. This is related to the regrowth delay period and to plant nutrient reserves. The results clearly show that annual production is lower at short intervals. Similar responses were obtained by Rodríguez, Arias and Quiñones (1994) when studying frequencies of harvesting of six, nine and 12 weeds in two different periods in Guatemala. In the second year the yield was triple but the frequency effect was maintained. When studying plant growth post-harvest, it was observed that stem buds do not regrowth immediately. It takes four to ten days for the shoot to reach 1 cm. Afterwards, the re-growth continues with two to three buds on side branches. The density of re-growth seems proportional to the number of cuts. These observations indicate that plant structure and morphology, reflected on leaf and stem proportion, change with cutting frequency. After each harvest there was a little flux of sap in each cut. The results of this experiment show that plant stress (Taiz and Zeiger, 1991), caused by cutting frequency has, in the short term, a negative effect on annual biomass yield.
In Asian countries, often leaves are only picked every two to three months, and there is an annual pruning. In more intensive systems, leaf harvest/high pruning in the spring is combined with leaf harvest/low pruning in the winter (FAO, 1988).
IMPLICATIONS AND RECOMMENDATIONS
Annual biomass yield increased with planting density. This implied lower individual plant yield, indicating competition for basic ground space (root growth and nutrition) and aerial space (gaseous exchange and photosynthesis). Cutting height had little effect on biomass production. This should be considered for mechanical harvesting.
Biomass, stem and leaf yield rose with longer cutting intervals. With frequent harvests, forage had a larger leaf proportion. Leaf:stem ratio reached 1 at about 100 days, then forage became lignified, losing nutritive value.
This study found the largest leaf production (19.0 tonnes) at 60 cm spacing and 112 days cutting frequency. Similar spacing is recommended for an 84 days interval, with only slightly less leaf (-0.5 tonnes) and stem (-5 tonnes) production, with cutting height of 60 cm and leaf:stem ratio of 1.19. With a cutting height of 30 cm, 112 days frequency and 60 cm spacing, the total yield reached 40 tonnes/ha/year, but leaf yield was 18.7 tonnes/ha/year. Forage with more leaves saves on labour and transport cost per unit of feed, and animals can derive a greater intake.
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