Animal welfare at the Grand National came under further scrutiny yesterday after a horse died on the first day of the Aintree meeting.

Battlefront was pulled up, or withdrawn from competition, during the fourth race by jockey Katie Walsh and later collapsed and died. It came after Ms Walsh defended the sport earlier this week, saying that the horses were treated better than "many children".

Battlefront had cleared 10 fences in the John Smith's Fox Hunters' Steeple Chase, the first competitive test of significant course changes and new fence frames designed to improve safety.

The cause of his death was not confirmed but it is thought that he may have suffered a heart attack. A further five horses fell in the race, although none was significantly injured.

Andrew Tyler, director of Animal Aid, said: "The Aintree authorities and the British Horse Racing Authority have been claiming that major new safety measures and efficiencies would eliminate much of the risk associated with racing on the Grand National course.

"But today's Fox Hunters' Chase, in which Battlefront lost his life, was stomach-wrenchingly chaotic from start to finish. Several horses fell or were pulled up, tired and potentially injured. It was both utterly depressing and served as confirmation that the Aintree authorities have got it badly wrong again."

Battlefront was the 23rd horse to die on the Grand National course since 2000, Animal Aid said.

Aintree bosses made significant alterations to the course after last year's big race was marred by the death of two horses, According To Pete and Synchronised. That followed two fatalities in the 2011 race, Ornais and Dooney's Gate.

Old wooden fence frames have been replaced with "Easyfix plastic birch", dressed with spruce. Aintree said the new fences were "kinder if the horse makes a mistake". The height of the fences remains the same.

The start of the Grand National course has also been moved 90 yards further away from the grandstands and the crowds.

John Baker, regional director north-west of Jockey Club Racecourses, said: "You can never remove all risk from horse racing, as with any sport. However, welfare standards are very high and equine fatalities are rare: 90,000 runners each year with a fatality rate of just 0.2%.

"The Grand National is woven into the fabric of British culture and, while the race is designed to be a unique and tough test, Aintree ensures it is as fair and as safe as possible. Nothing has greater importance to Aintree than the safety and welfare of the horses and riders."

More than 150,000 race goers will descend on Aintree over the three-day meeting.