Finding a good cavy feed can be challenging at best. What one breeder swears by may not work for you at all. It takes time and patience to find a feed that is readily available in your area and that your cavies do well on.
There is no "perfect feed" out there. And even though the feed bag has the same name on it (like Manna Pro), does not mean it has the same stuff inside. Many feed companies will swap out ingredients if something cheaper comes along, so stay in touch with you feed Rep. and look at the ingredients tags to see if they have changed something. Some feeds (like Manna Pro Rabbit Gro) use different mills in different parts of the country. Some mills are known for having poor quality feed, some for high quality. So just because your breeder friends in New York have great luck with a feed, does not mean you will have the same results with the same feed in California, even though it has the same name on the feed bag. Do your research first! Ask the feed reps where the feed was milled.
Call the manufacturers (all of them that you can find) and ask to talk to someone who really knows guinea pigs, and see why they think you should use their feed. Let them "prove" to you that their feed is worth your business. Then ask them if they know about the "other guy's" feed. Compare the feeds as much as possible. You will learn fascinating things about each feed company doing this. You may also find that you will be offered "free" feed or coupons, to try their feed out.
Also, find out who your feed store gets their feed from (which large distributor). My feed store did not even know they could get Oxbow Cavy Performance. I called "their" supplier/wholesaler and asked, the supplier said they could easily send a bag if it to the feed store in their weekly order. The feed store was surprised when I told them this, but when they called their wholesaler, sure enough, the Oxbow was readily available. So...you may have more feeds available to you than you think (more than even your own feed store thinks/knows about!)! It took a couple days of calling around, but at least I found some alternatives to the poor quality cavy chow that my feed store regularly carries.
Probably the best thing to do is research all available cavy feeds in your own area. Find one that is readily available (you may have to drive a bit to find it!), made with good ingredients (no animal products, no corn if possible, etc), preferably hay based (usually alfalfa), has Vit C, and is fairly fresh (within 3 months). You may have to try it out for a few months before deciding if it is the right feed for you. It can be a challenge finding a good cavy feed in your area, keep calling, be willing to drive a bit or even have it shipped in if necessary. Consider combining your feed needs with other breeders in your area, you may be able to get a good quality feed shipped in if enough people commit to purchasing it.
When evaluating the feed you have chosen to go with, make sure it looks and smells fresh. It is better if it has been stored inside, as heat or vermin can ruin a good bag of feed. After using it for a while, your animals should appear healthy, bright eyed, have shiny coats, and nice hard bodies. Some good indicators that the feed you are using is working: high conception rates, healthy animals, low incidence of pregnancy toxemia, low stillborn rate, nice sized babies, and comments from the judges that your animals feel great and have great coat and body condition.
Some indicators that you may need to switch feeds or at least supplement you current feed: poor condition (flabby bodies, too fat, etc), dull or thinning hair, low conception rates, sows dumping their litters, baby small and weak, high stillborn rate, babies not thriving after weaning, many cases of pregnancy toxemia, neural tube defects in newborns (folic acid deficiency), difficulty walking/bleeding gums (or other signs of Vit C deficiency), sows dying during childbirth or right after (calcium deficiency?), animals not making senior weight (if it's in their genes to be large) and judges making comments about the poor body/coat condition.
May everyone find a feed that works for them!