For a Mars colony, food will, of course, be a critical issue.
The situation is simple; they can either produce food, or import it from Earth. The latter is very costly, especially long term, due to the price per pound of getting it to the Martian surface.
There are ways to reduce the mass of transported food, such as via dehydration. However, the cost will still be enormous. As Ken said in the post below, around $76,000 per kilogram.
What this means is that colonists will need, in a fairly short timeframe, to produce most of their food.
As has already been noted on this blog, farming should be one of their first tasks. This is a nececity, and fortunatly, there appear to be no show-stoppers in using Martian soil (It might need preprocessing to remove some iron and most perchlorates, both of which are easily accomplished.)
Therefor, within a few months, the colonists ought to be able to be harvesting many fast-growing crops. Yeilds could be increased via the simple expedient of increasing the partial pressure of CO2 (this has been often demonstrated here on Earth in experimental greenhouses).
However, there is a serous issue that I've not yet seen addressed; farm animals. Some will be vital, such as earthworms, though fortunately those are not hard to transport. What is of concern is meat animals; without them, the colonists will be condemned to exist as vegetarians.
During the early days of a colony, they will be unable to afford the production losses of using their farm produce for animal feed, but that should soon change. Then, they will need meat animals.
This begs the question, one I've yet to see raised anywhere; how do you get farm animals to Mars?
For chickens and turkeys, the answer seems obvious; send fertile eggs and incubate them on Mars. However, this is not as feasible as it sounds, due to the time constraints on per-incubation storage. A fertile chicken egg has lessening chances of developing after a few days, and after three weeks, incubation is impossible. Turkey eggs are better, but only by about an extra week before incubation percentages drop to zero.
It takes approximately 21 days of incubation for a chicken egg to hatch, while a turkey egg takes 28. So, if eggs were launched inside an incubation system that held them in cold storage first (for a maximum of about two weeks to maintain a sufficient incubation percentage) your hatchlings would emerge approximately 5 weeks after liftoff (6 weeks for turkeys)
The key problem; a minimum-energy Mars voyage takes about 6 months at minimum.
Can hatchlings survive in zero G, even with humans aboard to care for them?
The answer, sadly, appears to be "no". The Russians performed some experiments on their Mir space station, which included hatching quail eggs. http://finchwench.wordpress.com/2011/09/06/cosmoquails/
Does this apply to chickens and turkeys as well? We don't know, though the evidence from the quail experiments is hardly promising. Even incubation in zero G caused issues.
The zero-g incubation issue can be dealt with via a small centrifuge, but even so, that would require a journey time of no more than 5 weeks. Therefor, it's quite possible that the only way to get chickens and turkeys to Mars is to do it very fast; a small craft on a very high-energy trajectory. The fastest probe ever launched was New Horizons to Pluto, and that took 78 days to cross Mars' orbit. What we need is something that can do it in half that time. This can be done; a very small Mars entry spacecraft launched by a very large rocket with multiple upper stages could attain the needed velocity, though engineering it to enter and land with an entry speed of over 100,000 mph will be an engineering challenge, and very costly.
What of other needed livestock; pigs, for example? Pigs might have a good chance, though they have, as yet, not been flown in space. All we have to go on are data from other animals, such as dogs, cats, and monkeys - and none of that is long-term. Theoretically, If launched young, piglets, if cared for by a human crew, could probably survive months in zero-g. Or, they might not. We simply do not know.
Sending cows to Mars (A single female calf would suffice, along with a stock of frozen fertilized eggs to be implanted) would be harder yet.
There might be one way around the zero-G issue. Artificial gravity. Connect two Mars-bound transport habitats by a long cable, and then spin up the resulting assembly. In this way, the need to make a fast transit could be eliminated. It may, in fact, be the only viable alternative.
The need to send livestock to Mars might be a longer-term concern, but it is something that will need to be done at some point in the early years of a colony.