The world's largest rhino farm looks like a vast fortress and is guarded by a private army.

At night a helicopter fitted with an infrared camera circles over the 8,000 hectare (nearly 20,000 acre) electric-fenced ranch in South Africa, and by day armed men in military fatigues are on patrol.

Their sole mission: to protect 1,200 rhinos from poachers, who killed 1,175 of the horned beasts across the country last year.

Rhinos are being slaughtered in record numbers to meet the insatiable demand for their horns in countries such as China and Vietnam, often for use in traditional 'medicines'.

The horn is mainly hard keratin, the same substance found in human nails, but on the black market where it is sold in powdered form it is believed to cure cancer and other diseases.

It can fetch as much as $60,000 (55,000 euros) per kilogramme, more than gold or cocaine.

Clad in khaki shorts, blue shirt and sandals, wealthy South African businessman and rhino farmer John Hume says he has bred 600 rhino since 2008, but his target is to breed 200 each year.

- Legalise trade? -

"The way we are going to save the rhino from extinction is to breed more and protect them, and that is what I am trying to do here," said Hume.

His next battle is to secure the legalisation of international trade in horns, which has been outlawed since 1977.

Horns peddled on the black market are from dead rhinos but, if trade is allowed, demand would be fed from live ones. Just like nails, cut horns will regrow.

"We can supply horns from live rhinos, while now every single horn that you are supplying to the demand is coming from a dead rhino. Surely that is stupid. It absolutely flummoxes me," said Hume.

Plans to legalise the trade are controversial, however, and are fiercely debated by conservationists.

Hume opened the farm in 2008 after selling hotels he owned.

Today he employs around 60 full-time staffers plus his "army", whose strength he refuses to divulge on the grounds that it is "too sensitive."

Even the exact location of the farm -- where he spends some $170,000 (156,000 euros) a month in security costs -- is kept secret, to protect it from poachers ravaging game parks elsewhere across the country.

- Harvesting horn -

On the plains of South Africa's North West province where the farm is located, a dozen rhinos were due for dehorning when AFP visited.

Standing at the back of a pick-up truck, Menard Mathe used a pair of binoculars to identify the animals earmarked for dehorning.

In front of the vehicle, veterinarian Michelle Otto drew her gun and darted one animal with a powerful anaesthetic.

A few minutes later, the gigantic animal began to stagger.

Otto cautiously walked towards it, and another worker secured its hind legs with a rope, forcing it to fall limply to the ground.

Quickly the rhino's eyes were covered with a piece of mutton cloth and old socks used for makeshift earplugs.

The horns are measured and a line is marked precisely where it will be cut, making sure blood vessels are not touched.

Then a handheld power saw cuts through the horn. The procedure is painless for the animal.

"We trim their horns for their safety and to deter poaching," said Otto.

Despite the dehorning and the massive security cordon around the ranch, 39 rhinos have been poached there since 2008.

Back at the farm building, the horns are weighed. A total of 23 kilogrammes (51 pounds) have been harvested on this day, said farm general manager Johnny Hennop.

Each horn is then numbered and they are stored in metal trunks where they are wrapped in baby diapers to protect them from moisture while mothballs are strewn around the containers to keep bugs away.

The boxes are then sealed and are ready to be moved to a safe location.

Hume has a stockpile of five tonnes in banks and with private security companies.

It is potentially worth a fortune, but is worthless as long as the ban in international trade in rhino horn remains in place.

The ranch's security chief, Stefran Broekman, who previously worked at private game reserves, says he is "frustrated" that even when poachers are arrested in South Africa, some of them get away with a "small fine".

At the turn of a muddy track, Broekman's face lights up on seeing a newborn calf suckling his mother under a tree. It was born overnight.