There is another option, namely renting out your new slave labour and making the purchasers responsible for feeding them.
This used to happen in South Africa during the great depression with prisoners sentenced to hard labour. The labour was essentially 'sold' to farmers, quarries and the like, and they worked set (long) hours during the day. They were not fed much of a breakfast, nor much of a dinner, because these were provided by the prison. The person renting the labour on the other hand was required to provide them a lunch with a minimum set size and nutritional standard, and this was how the prisoners were sustained.
Prisoners who were not capable of hard labour were often only given light duties in the prison itself because their food allocations were not enough to sustain a heavy workload.
Ultimately, slave labour of any kind is a bad idea, not just because of the humanitarian aspect but because of the economics of it all.
Slaves (if they are to be sustained) require regular good quality food as they are doing physical labour. They require good medical care, and they require security. After all, slaves are highly unlikely to be happy with their lot, and will try to escape and/or kill their masters. In a more modern world, motivation is understood to be a critical element of productivity, and slaves just don't have any to remain in their state.
Even the Romans understood this with their practice of freeing accomplished Gladiators. They weren't accepted into society as such, but as a slave forced to fight for survival, the lure of freedom could be a compelling motivation to fight well and survive for another day.
Also, if your society has access to mechanisation, slaves as a workforce simply can't compete. There's a reason you don't drive past a farm these days and see hundreds of men cutting down sugar cane. A harvester does the job faster, and requires only a single skilled worker. And, the harvester requires less maintenance than a hundred human bodies and the only security you need for it is a lock on your barn door and the keys being put in the farm house.
Let's say though that this is just a mechanism to inflict pain on your enemy to appease your hatred. In such a case, others have provided sufficient answers as to how long your slave workforce would last, which isn't long with a single low quality meal in six being provided. You'd be far better off financially putting them all in an arena and letting them fight to the death; you'd be able to sell tickets to that and it only takes a single meal per slave to support.
NB. Don't try this at home. Slavery is still a horrific and barbarous action and should be opposed by every reasonable person. And remember, if hatred persists, see your therapist.