Ms Armstrong, 28, a former stable girl who has two children by the millionaire, was found guilty of cruelty to 11 Arab horses on Gucci's former stud farm near Crawley, West Sussex, last October.
She was prosecuted by the RSPCA, banned from keeping horses for five years and ordered to pay pounds 5,640 costs and pounds 15,573 compensation.
Magistrates at Chichester Crown Court yesterday rejected her appeal and added an extra four month suspended sentence over each cruelty charge. But she was granted an additional 12 months, on top of the year allowed in October, to pay costs and compensation.
The RSPCA was called to the pounds 2.5m estate last January after locals saw a collapsed horse in a field. Inspectors discovered several more starving and emaciated horses. Some were so weak they could hardly walk.
David Buck, prosecuting at the court case, said the animals were suffering from diarrhoea and listlessness. One filly had to be put down immediately, a vet estimated that she had not eaten for two weeks. Eleven others were removed to an animal welfare centre for intensive care but six of them failed to recover.
The horses were part of a group of 100 kept at the stud at Rusper. Ms Armstrong had been living in a house associated with the stud.