According to a Fellow of the Royal Geographical Society in this article they used a mix of outs, corn and a meat-based supplement.
To overcome the horse’s need for bulk grass based feed, Shackleton arranged to purchase ten tons of compressed fodder consisting of oats, bran and chaff. He also took a large stock of corn. Yet upon the advice of the British military establishment, Shackleton decided to enhance his horses’ normal diet with a special meat-based supplement known as “Maujee Ration.” This was a distinctive type of equine pemmican developed at Aldershot, one of England’s most important military establishments.
Sir Ernest recalled, “It consisted of dried beef, carrots, milk, currents and sugar, and was chosen because it provides a large amount of nourishment with comparatively little weight.” (Heart of the Antarctic by Sir Ernest Shackleton, 1909.)
Shackleton set off for the Pole with three comrades and four of the original ten horses. Each of the Manchurian horses pulled a twelve-foot sledge carrying an average of 650 pounds. Like Jackson before him, Shackleton praised his horses.
He wrote, “compared to the dog, the pony is a far more efficient animal, one pony doing the work of at least ten dogs and travelling a further distance in a day…… It was trying work for the ponies but they all did splendidly in their own particular way.”
The harsh weather and unforgiving terrain caused the men and horses to struggle alike through the cold and snow. Nevertheless, Shackleton made a startling observation. The horses preferred to eat the meat-based ration rather than the traditional fodder. They even threw corn out of their nosebags, scattering it on the ground, in anger at being denied the Maujee ration.