The Australian Olympic equestrian eventing team has lost a member.

Rider Megan Jones had to withdraw her horse Allofasudden Thursday due to a hoof injury. The loss is a challenge for the team, which is a medal favorite when the competition starts Saturday at Greenwich Park.

The Australian team has petitioned Olympic officials to substitute reserve rider Sam Griffiths and horse Happy Times to bring their team back to a full five riders. With only four riders, Australia would be at a disadvantage to better their team silver won at the 2008 Beijing Games.

Australia still has veteran riders Andrew Hoy, Christopher Burton, and the husband and wife team of Clayton and Lucinda Fredericks.

The eventing competition includes a combined score from three phases: dressage, cross-country endurance and show jumping.