GAURA, Indonesia, April 4, Reuter - With bloodcurdling yells and a hail of lances the mounted warriors of Sumba battle to spill blood and ensure the success of the harvest.

"How can we satisfy the gods if we don't kill someone or at least spill some blood? You have to have blood, it's only proper," said one young warrior on this island at the remote eastern rim of Indonesia.

Wheeling their stocky steeds around in great circles, young men perform the pasola, their ritual version of human sacrifice, before rice planting begins each year.

Some years ago the national government, reluctant to disrupt local culture but wanting to minimise the gore, insisted all lances used in the ritual battles be blunted.

"Check the lances, check the lances," squawked an officious old man long past the age of fighting. "We don't want any of that Lamboya or Wanokaka nonsense here." He spat in disgust at the thought of the savage pasolas of neighbouring districts.

In Lamboya one man was killed this year, a pointed spear between the eyes.

The Wanokaka pasola was broken up by police after rival villagers drew knives, abandoning the ritual in favour of genuine battle, a local boy whispered delightedly.

A small band of withered elders, tracing the movements of the moon and the sea, determine the timing of the pasola.

Varying slightly across western Sumba's four battle sites, the start of the pasola coincides with the one day in the year when sacred sea-worms swarm to the beach.

Warriors and villagers then mass on the beach they have avoided for a month for fear of annoying the gods by their bathing or fishing.

They scoop up the red, green and white worms, inspecting them carefully as omens of the harvest before taking them home to make into soup.

Long, healthy worms mean a good rice crop. If they are few, villagers expect a drought. If the fragile creatures are damaged, the crop will rot in the fields from too much rain.

In Gaura, most remote of the pasola villages, the sea-worms are collected a few days before the battle. "Very good this year. As long as this," said the local village elder indicating his wizened forearm. "But we've eaten them all now."

On battle day, young fighters troop down to a flat stretch of grass and loiter under the shady trees, chewing on the mildly narcotic betel nut that reddens the teeth and trying not to appear too interested in one another's lances.

Some, mostly excited boys too young to fight, charge up and down on the bare backs of their bony horses.

Horses are decked out in bright woven bridles with lucky charms -- an old Dutch coin here, a plastic cameo brooch there.

Warriors wear the famous fabrics of east Sumba, woven to represent the skull-trees of their headhunting ancestors.

Two camps of fighters, fierce Kodis from the remote west of the island and disdainful Gaura locals, eye each other from either side of the pasola field.

Emerging from their camps three lances in hand, the riders sweep in toward each other. The trick is to hurl one lance, twirl the others to fend off attacks and veer quickly out of range.

Great cheers rise as a warrior hits an opponent. "We'll show those arrogant Kodi boys what we're made of. They'll see who's boss."

No hit goes unnoticed. The victorious warrior charges toward his supporters with a spine-chilling yell, fist raised. He screeches to a halt just a few feet from the crowd.

There are no real winners or losers in this elaborate fertility rite, the warriors say.

"As long as someone gets hurt the gods will be happy, the crop good and then we all win," said a veteran of several decades of battles and harvests.