Establishing cover crops in immature oil palm: ground cover strategy for the first three years

No period in an oil palm stand's productive life is more formative for soil health than the 3 years between replanting and canopy closure. During this window, the soil is exposed: to tropical rainfall, to weed competition, to the mechanical disturbance of planting and early maintenance operations. Without active ground cover management, the newly replanted field develops a compacted, weed-choked, structureless soil surface that constrains the young palms throughout their growth period and sets a lower ceiling on productivity for the next 25 years.

Conversely, the immature phase presents the single best opportunity to build the soil organic matter, nitrogen capital, and cover crop community that will sustain productivity for the crop's entire lifespan. The palms are small, interrow access is easy, inputs move through the whole interrow without obstruction, and the soil responds rapidly to organic inputs before compaction and root competition from mature palms develop. The agronomic decisions made in years 1 to 3 at replanting determine more about a field's long-term potential than any subsequent management change.

Species selection for the immature phase

Three leguminous cover crop species dominate Malaysian oil palm immature phase management, each with distinct establishment characteristics suited to different site conditions and management objectives.

Mucuna bracteata is the primary choice for most plantation situations. Its rapid growth, vigorous nitrogen fixation and strong weed suppression make it a high-performance option where establishment conditions are adequate. In a UPM trial on mineral soil, MB planted at 680 seedlings/ha (1.5 m spacing) reached about 100% ground cover at 6 months (Samedani et al., 2015); on deep peat, 320 seedlings/ha gave 31% cover at 6 months and 78% at 24 months (MPOB TT No. 501). In a 15N study under three-year-old oil palm on Jawa Series soil in Malaysia, MB obtained 67 to 84% of its nitrogen from the air (Cheah, Zaharah and Aminuddin, 2010), with no routine annual rate asserted. MB's aggressive vining requires management in the immediate vicinity of young palms to prevent competition with the palm crown, but this is a minor management input compared to the weed control and nitrogen fixation it provides across the wider interrow.

Pueraria javanica is often planted alongside MB as a companion species, or as the primary cover on sites too acid or infertile for reliable MB establishment. PJ establishes more slowly and reaches lower canopy height than MB, but its tolerance of strongly acid soils and its ability to nodulate with a broad range of native rhizobia make it reliable across the range of conditions encountered on Malaysian replanting sites. Recommended seeding rates (sown alone): PJ 10 kg/ha and MB 1 kg/ha, drilled or planted in holes (Chemiseed recommendation, 2026 catalogue). These rates are for one species sown alone. For a mixture, the quantity of each species is set on the quotation for your site. MB can also be raised in a nursery and transplanted; in MPOB's trial on deep peat, 320 seedlings/ha, two per palm point, were raised from about 500 seeds (75 g) (MPOB TT No. 501).

Calopogonium mucunoides and Centrosema pubescens are secondary options that establish a background cover layer beneath the primary species. CM establishes quickly and gives early cover, covering the soil 3 to 6 months after sowing, but it is short-lived and the first species to be shaded out as the palm canopy develops (PROSEA 11). CP tolerates moderate shade and can persist into the canopy closure transition, making it useful in the long-term interrow community after year 3.

Timing: plant with the rain, not against it

All four species germinate most successfully when planted at the onset of the wet season, allowing rainfall to drive establishment without supplemental irrigation. In Peninsular Malaysia, planting at the start of the heavier rains that usually arrive from October gives cover crops time to establish before the next drier spell; the estate's own rainfall records are the best guide. In Sabah and Sarawak, where rainfall is more evenly distributed year-round, planting at any time with adequate initial rainfall is feasible.

Using Seed Activator (a seed treatment that combines a rhizobium inoculant with compost-derived nutrients) is particularly valuable for legume cover crops planted on acid, nutrient-poor replanting sites where the soil environment is challenging for germination. It is intended to support even germination and early root growth, so that fewer gaps open in the establishing stand. To judge the effect on your own site, leave an untreated strip for comparison.

Managing competition between cover crops and young palms

The primary management challenge in immature-phase cover crop systems is preventing cover crops from competing directly with young palms. Young palms (0 to 18 months) have limited root systems and are sensitive to competition for light, water, and nutrients from vigorous cover species. The standard practice is to maintain a cleared circle around each young palm (sized to the estate's standard and widened as the palm grows), removing any cover crop runners that encroach into this zone during monthly field checks.

This circle maintenance is the primary ongoing cost of immature-phase cover crop systems. However, it replaces (not adds to) much of the weed control labour that would otherwise be required across the entire interrow, and as the cover crop closes and shades out weeds, the interrow needs less weeding.

Inoculation: making the nitrogen fixation work from day one

The nitrogen fixation benefit of leguminous cover crops is entirely dependent on successful nodulation: the formation of root nodules housing nitrogen-fixing Bradyrhizobium bacteria. Malaysian replanting soils, particularly those recently burned or chemically treated, may have insufficient native Bradyrhizobium populations to reliably nodulate MB and PJ. Without nodulation, these species grow as non-fixing legumes, providing weed suppression but not the nitrogen benefit that justifies their cost.

Applying a suitable rhizobium inoculant to the seed at planting is the usual way to secure early nodulation where native rhizobia are scarce; MPOB TT No. 501 specifies 5 g of Rhizobium per kg of seed for its legume cover mixture. Seed Activator already contains a rhizobium inoculant together with compost-derived nutrients, so treated seed carries both the bacteria and early nourishment into the soil, giving a sound biological foundation for a productive cover crop system from day one.

Correctly established immature-phase cover crops (proper species selection, timed planting, Seed Activator use, inoculation and circle management) go on delivering weed suppression, nitrogen fixation, erosion control and soil organic matter building through the life of the stand.

Sources

• Cheah, S.S., Zaharah, A.R. & Aminuddin, H. (2010). Biological nitrogen fixation by Mucuna bracteata under oil palm field environments. Oil Palm Bulletin 60: 22-27. Malaysian Palm Oil Board.
• Samedani, B., Juraimi, A.S., Rafii, M.Y., Sheikh Awadz, S., Anwar, M., & Anuar, A.R. (2015). Effect of cover crops on weed suppression in oil palm plantation. International Journal of Agriculture and Biology, 17(2), 251-260.
• Hasnol Othman, Farawahida Mohamad Darus & Zulkifli Hashim (2012). Best management practices for oil palm cultivation on peat: Mucuna bracteata as ground cover crop. MPOB Information Series 588 / MPOB TT No. 501. Malaysian Palm Oil Board.
• Chen Chin Peng & Aminah, A. (1997). Calopogonium mucunoides Desv. In: Faridah Hanum, I. & van der Maesen, L.J.G. (eds), Plant Resources of South-East Asia No. 11: Auxiliary plants (PROSEA 11), pp. 84-86.
• Tropical Forages: an interactive selection tool (CIAT and ILRI, 2020), Neustanthus phaseoloides (Pueraria) fact sheet, www.tropicalforages.info