Effects of Perennial Intercrops on Oil Palm Agronomic and Yield Traits

Oil palm (Elaeis guineensis Jacq.) is a potentially economic oil crop in Thailand (Soonsuwon et al., 2020). In 2019, Thailand had 0.94 million hectares under cultivation with oil palm, of which 86.23 per cent was in the south of Thailand (Office of Agricultural Economics, 2019). In general, oil palms are planted at a triangular distance of 9 x 9 x 9 m with 143 palm trees per hectare (Dariah et al., 2014). In plantations in southern Thailand, the first harvestable bunches appear less than 3 years after planting, but during the period when the palm trees are young, they are intercropped with a number of different short-lived plants such as pineapple, banana, chili, cucumber, cocoyam, soybean, maize and groundnut. Several previous studies have reported that the intercropping of plants has no effect on oil palm. Moreover, some studies have found that intercropping oil palm improves the properties of the soil and aids the development the trees as compared to monocropped oil palm (Erhabor and Filson, 1999; Okyere et al., 2014; Putra et al., 2012; Salako et al., 1995; Dissanayake and Palihakkara, 2019). However, once the trees become mature, 3 years after planting, few plants can be intercropped with oil palm because of lack of sufficient light under the oil palms and restriction on the availability of minerals and water in the soil due to high use by the oil palms (Goh et al., 2011; Dhandapani et al., 2020). Nevertheless, some farmers in southern Thailand have planted perennial plants among mature oil palms and it appears that those plants have not been affected by the growth and cropping of oil palm and have been successfully intercropped with oil palm. However, there have been a few empirical studies reported in this connection. The findings reported will be helpful and informative to farmers and may form a baseline for researchers to further study this subject.

Study site
 
The plantation studied is located at Lam Sin Tambon, Srinagarindra District, Phatthalung Province, southern Thailand (7°30'02.4"N 99°53'58.7"E). This site has a tropical climate with an average rainfall of 2,156.9 mm per year, with the highest rainfall occurring between October and December. The lowest monthly average temperature in December is 26.7°C and April is the hottest month with an average temperature of 29.3°C. The relative humidity ranges between 75 and 83 percent. The area of the plantation studied is around 6 hectares, to which chemical fertilizers are applied every 3 months. The quantity and quality of the fertilizers applied are monitored based on soil analysis and the nutrient demands of the oil palms. In addition, manure is applied twice a year to improve the physical and biological properties of the soil. Harvestable bunches are harvested twice a month and the fronds of the oil palms are cut during the harvesting of the oil palm fruit bunches. The fronds are maintained at 24-32 fronds per plant. The soil properties in the 0-30 cm soil depth are: pH = 6.0, organic carbon = 1.5%, cation exchange capacity = 12 molkg^-1, total nitrogen = 0.15 gkg^-1, available phosphorus = 20.00 mgkg^-1 and available potassium = 30.09 mgkg^-1.
 
Oil palms, Intercrops and parameters measurements
 
This study employed a completely randomized design of 8 treatments as follows: oil palm monocropping (Palm), oil palm intercropped with Intsia palembanica (Palm-A), Hopea odorata (Palm-B), Swietenia macrophylla (Palm-C), Ternstroemia wallichiana (Palm-D), Azardirachta excelsa (Palm-E), Magnolia champaca (Palm-F) and Mesua ferrea (Palm-G). Each treatment consisted of 10 replications. In each replication, a ten-year-old intercropped plant was planted with a twelve-year-old oil palm (tenera variety) as the same row. Their agronomic and yield traits were recorded. For the oil palms, their diameter and height were measured, the number of male and female inflorescences were counted and the sex ratio was calculated as the number of female inflorescences relative to the total number of inflorescences (Rival, 2017). The growth rates (GR) of the stem perimeter and plant height in the observed months were calculated as:
   
where
H₂ and H₁ are the plant height (or stem perimeter) at t₂ and t₁, respectively. For the perennial intercrops, their perimeter and height were measured. The observations were carried out in October 2019, February and June 2020. General information regarding the oil palms and perennial intercrops is shown in Table 1.
 

 
Statistical analysis
 
The variance in the parameters was calculated using ANOVA. The means were compared using Duncan’s multiple range test and differences were reported as significant at or above p < 0.05 (Harter, 1960). All statistical analyses were conducted using the R software program (version 3.6.1) with the Agricolae package (de Mendiburu, 2019).

The agronomic traits and inflorescence yield of oil palm
 
The plant diameter and height, the number of male and female inflorescences and sex ratio of oil palms planted with different intercrops are presented in Table 2. Plate 1 illustrates twelve-year-old oil palms intercropped with some perennial plants. The diameter, height and the number of male inflorescences in all observed months, the number of female inflorescences in June 2020 and sex ratio in October 2019 and June 2020, were not significantly different among the various treatments. The only significant differences were found in the number of female inflorescences in October 2019 and February 2020 and sex ratios in February 2020. Oil palm intercropped with A. excelsa (6.00±0.00) and M. champaca (5.71±1.70) produced the greatest number of female inflorescences in October 2020, but they were statistically at par with monocropped oil palm (3.25±0.38; p < 0.05), oil palm intercropped with I. palembanica (4.20±2.17; p < 0.05), H. odorata (3.55±2.62; p < 0.05), S. macrophylla (3.43±1.40; p < 0.05) and T. wallichiana (4.50±1.91; p < 0.05). Monocropped oil palm (5.13±2.75), oil palm intercropped with I. palembanica (5.40±3.13), H. odorata (5.25±2.65), S. macrophylla (5.57±3.74), T. wallichiana (5.25±2.87), A. excelsa (6.00±1.41) and M. champaca (4.43±2.64) produced a greater number of female inflorescences than oil palm intercropped with M. ferrea (0.50±0.07; p < 0.05) in February 2020. Oil palm intercropped with T. wallichiana (0.93±0.14), A. excelsa (0.92±0.12) and M. champaca (1.00±0.00) produced the greatest sex ratio in February 2020, but they were statistically at par with monocropped oil palm (0.86±0.35; p < 0.05), oil palm intercropped with I. palembanica (0.76±0.43; p < 0.05), H. odorata (0.87±0.25; p < 0.05) and S. macrophylla (0.76±0.27; p < 0.05).
 

 

 
The agronomic traits of perennial intercrops and their growth rates
 
The plant height and stem perimeter of the intercrops are presented in Table 3. M. champaca produced the greatest plant height (15.83±4.62, 18.83±2.99 and 19.83±3.20 m) in all observed months, but they were statistically at par with S. macrophylla (13.67±3.61, 13.87±1.94 and 13.92±1.72 m; p < 0.01), A. excelsa (13.00±3.45, 14.00±1.55 and 15.00±3.25 m; p < 0.01) and M. ferrea (9.50±2.12, 11.00±1.41 and 12.00±1.41 m; p < 0.01). A. excelsa produced the greatest stem perimeter (84.90±4.90, 90.00±3.00 and 93.20±3.20 cm) in all observed months, however, these were statistically at par with M. champaca (74.90±14.27, 80.83±16.23 and 83.30±15.16 cm; p < 0.01) and S. macrophylla (54.55±8.11, 58.92±9.82 and 60.70±10.54 cm; p < 0.01). Growth rates of plant height and stem perimeter of intercrops are presented in Table 4. From October 2019 to February 2020, M. champaca produced the highest growth rate of plant height (55.00±8.74 cm month^-1), but was at par with S. macrophylla, A. excelsa and M. ferrea (29.17±9.21, 25.00±3.56 and 37.50±7.68 cm month^-1). A. excelsa produced the highest growth rate of stem perimeter (2.03±0.15 cm month^-1), but they were statistically at par with S. macrophylla and M. champaca (1.09±0.85 and 1.48±0.85 cm month^-1). From February to June 2020, M. champaca produced the highest growth rate of plant height (35.00±7.91 cm month^-1), but they were statistically at par with S. macrophylla, A. excelsa and M. ferrea (18.75±6.85, 25.00±4.25 and 29.00±5.00 cm month^-1). A. excelsa produced the highest growth rate of stem perimeter (1.05±0.11 cm month^-1) and were also statistically at par with S. macrophylla, T. wallichiana and M. champaca (0.45±0.30, 0.38±0.19 and 0.92±0.42 cm month^-1).
 

 

       
The results are similar to those found in previous studies relating to oil palm, where it has been reported that oil palm can be intercropped with different plants at different growth stages. Salako et al., (1995) studied the effect of intercropping oil palm with cocoyam (Xanthosoma sagittifolium) and the results show that such intercropping did not affect the growth or yield of oil palm. They recommended that owners of oil palm plantations maximize their land use and profit by intercropping oil palm with cocoyam during the first five years after planting oil palm. Erhabor and Filson (1999) investigated the nutrient dynamics in relation to soil fertility management for oil palm intercropped with soybean (Glycine max), maize (Zea mays) and cocoyam during the first three years after planting. The results showed that, there was no significant change in soil pH after three years of using these cropping systems. Moreover, oil palm intercropped with soybean showed a 71 per cent increase in the level of phosphorus in the soil. Putra et al., (2012) determined the effect of soybean and groundnut on oil palm, with neither producing high yields. However, the presence of soybean and groundnut between the rows of oil palms was found not to inhibit the growth rate and development of the oil palms. Okyere et al., (2014) investigated the effects of intercropping four-year-old oil palm with maize, cassava and plantain, finding that there were no significant differences between the vegetative growth and yield of oil palm intercropped with those plants and those of sole cropped oil palm. Moreover, intercropping oil palm with maize, plantain and / or cassava had no adverse effect on the growth, development and yield of the oil palm. Amoah et al., (1995) studied the feasibility of cocoa intercropping with eighteen-year-oil palm which had achieved maximum canopy formation. The results found that, there were no significant differences in oil palm yield between oil palms intercropped with cocoas and monocropped oil palms. Oil palms have a four-level root system, consisting of primary, secondary, tertiary and quaternary roots. The root system of oil palm spreads in both the vertical and horizontal directions and extends to a maximum depth of over 6 meters (Jourdan and Rey, 1997). The number and strength of the roots of oil palms affects their potential to absorb water and minerals from the soil. In addition, intercropping oil palm with other plants could provide a habitat for and increase the activities of soil microorganisms, resulting in the release of fixed minerals, leading to an increase in available plant nutrients in the soil (Belel et al., 2014; Buragohain, 2015; Rahim et al., 2016).

Oil palm can be intercropped with many perennial plants which can be grown under light competition conditions under oil palm shading. Based on the findings of this study, A. excelsa and M. champaca can be grown as intercrops with oil palm. However, more research is needed to determine the long-term effects of intercropping in oil palm.

This study was facilitated by Mr. Thana-art Inpaeng, the owner of the farm. We are grateful to him for helping us to select the plants and record the data and for giving general information relating to oil palms and intercrops. This study was supported by graduate student development grant from National Research Council of Thailand (fiscal year 2020) and the government budget of Prince of Songkla University (NAT620172b).