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Trainer Nathan Williamson found his best horse to be lame due to a hind suspensory ligament injury after routine trackwork on the eve of Saturday's Northern Southland Cup day.
The exact amount of time the speedy trotter will spend away from the races is unknown and could depend on how well her injury starts to heal in the next month.
''She came in on Friday night as she always does and she was favouring a leg.
''In her work on Friday morning she didn't do a lot but she has obviously picked up the injury there.
''At this stage we are not sure, but it will be a year before she races again.
''It just depends on how the injury recovers.''
Dark Horse was about to embark on an autumn campaign which was to culminate in a start in the group 1 Rowe Cup at Alexandra Park.
The mare was right on track for that campaign after spelling following her disappointing run in the Dominion at Addington in November.
''Everything was just spot on. She was going to race at Invercargill and then we were looking to go to Addington for the Trotting Championship and then possibly Auckland for the Rowe Cup.''
Dark Horse joins a select group of some of the country's leading trotters who have been sidelined.
The country's best trotter, Monbet, has not raced since winning the 2016 Dominion.
The 2017 New Zealand Trotting Free For All winner, Great Things Happen, will not be seen again this season after suffering a quarter-crack injury.
Last season's Rowe Cup runner-up, Habibti Ivy, has not raced since October.
The return of last season's star 3yr-old, Enghien, should boost the depleted trotting ranks.
He returned to racing at Addington earlier this month when galloping and finishing last.
Another Southland trotting mare has also exited the Southland trotting ranks.
Gore squaregaiter Golden Gate has been retired.
The John Ryan-trained trotter signs off after amassing 10 wins and more than $80,000 in stakes.
The 10yr-old is in foal to top trotting sire Love You.
Today's Oamaru trial fields.-
Race 1, qualifying trot, 2500m: Deborahs Gem, Abby May, Riteur.
Race 2, 2yr-old mobile trot, 2000m: Mercadante, Christmas Babe.
Race 3, qualifying mobile, pace 2000m: Geordies Boy, Emerald Green, Out Of The Shadows.
Race 4, pace, 2000m: Out Of The Shadows, The Doorman, Rockin Rod, Two Ply, Coolhand Easton, Bobbie's Medley.
Race 5, mobile pace, 2000m: Bettor Sensation, Talkalot, Its Official Babe, Arizona Highway, Lucca Radich.
Race 6, trot, 2000m: Cosmic Sun, Still Eyre, Majestic Rose, Katiki Lad, Bonny Reece.
Race 7, trot ,2600m: Holdon Toyaspurs, Sunny Demon, Castleton Tui, Bourbon And Dry, I've Got This, One Over Da Stars, Neverneverland, Red Hot Poker.
Race 8, ffa mobile pace, 2000m: Linton Shard, Elusive Flight, Motu Speedy Star.
Race 9, pace, 2600m: Bite The Dust, Changesaw, Arma Twospoons, The Edge Of Reason, Boomer Bailey.