Before you get your guinea pigs, it would be wise to first:

• learn about the species (it has quirks and may not be something you actually want to deal with);
• research what sales opportunities are realistically available to you;
  
• determine if doing this is actually a good match for your situation (and you personally).

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    Inbreeding is characteristic of show-type hobbyist breeding. I recommend that as a pet breeder, you go the opposite direction, and focus on outcrossing to the best of your ability. The show-type, inbred animals I've owned had health problems, worst among them for pregnant sows being immune system problems that contribute to upper respiratory illness. Immune system weakness can turn what's supposed to be business assets (healthy breeding animals) into financial liabilities (animals that get sick after getting pregnant, which is often ruinous).

    Inbred guinea pig sows are known to be a weak links in production, producing fewer babies and smaller babies within those litters. If there is a "win" for a pet producer in using inbred guinea pigs (especially sows), I don't know what it is.

     Whenever I start using a guinea pig that I know is from pedigree breeding, I treat it as a risk factor. I set up for a long-shot outcross to something as different as I can arrange and hope for the best.

 

I recommend that you not play to low welfare standards in guinea pig breeding - this is the wrong species to cut welfare corners on.      In some areas, my understanding is that the legally-mandated welfare standards for maintaining guinea pigs are locally-determined. Further, being classified as rodents, the legal welfare standards for their care may be lower in some settings than for classical pet species (such as cats and dogs).       Some of your competition may be rolling with lower welfare standards than you are, and utilizing this to boost their bottom line. Maybe your local competition is physically located in a different area (or even in a different country) and they ship to your local pet stores. Maybe your competition is getting away with something they shouldn't be. Who knows.     If you're competing with someone who is more efficient expense-wise for some reason, you need as much of an edge as you can get.      Inbred breeding stock with an inclination toward genetic disease would be the opposite of an edge.      Utilizing outcrossed breeding stock can be one part of minimizing your expenses and losses, regardless of what your competition may be doing differently, or what business advantages they might have.

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    It is likely that you will be getting young sows that you will be starting in their first pregnancies. These sows should weigh between 500 and 600 grams when they are mated by the boar for the first time. Sows will generally be between 2.5 and 3 months old when they attain this weight.

    Although I usually cannot personally arrange this, I think the ideal arrangement for first-time young sows might be to have them be mated by a boar that is their same age. A young boar's sperm count is lower than older boars in their breeding prime, and maybe this would effect fewer babies in that first litter. A sow's first litter having fewer babies is in my experience ideal for various reasons (such as smaller litters being less stress on the sow and considering the young sow's more limited milk supply). If you can't arrange this and will be using an older boar, don't worry about it, its pretty common to do that.

    Sows are sexually fertile for a while before they reach 500 - 600 grams, so beware of a sexually-mature boar being in with sows that are smaller than this.

    If for some reason your young sows are not mated on time and you still want to use them, try to ensure that they deliver their first litter before or at their 6-month birthday at the latest (so, mated by the boar something like a week before their 4-month birthday). It's not a guaranteed catastrophe if they are bred late, but there is a risk. It is possible that a late-bred sow's pelvis may not open up at birthing time (it can fuse closed with increasing age in this species). If a sow's pelvis is fused and there's babies inside of her, its either a euthanasia or an emergency trip to an exotic veterinary surgeon for a c-section (and if you go that  route, hopefully you get a mother that feels good enough to nurse after that, which is not guaranteed).

    Instead of getting young, first-time sows, it would theoretically be possible to use sows that have already had at least one litter. But in practice you won't tend to have access to this. I think it's because sows' usable breeding lifespans don't tend to work out this way.

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    Boars become sexually mature a bit later than sows. Puberty for boars is considered to be on average at 18 weeks, but there could be individual variability, as sperm motility may be seen as early as 11 weeks.

Do not house intended breeding boars on wood shavings. Some boars housed this way will become infertile because of the wood shavings causing issues with their male parts.

    In terms of mature but virgin / inexperienced boars: I have had success with boars that are 1 year old virgins, but if I have a choice for a first-time sow, for timing reasons I prefer an experienced boar or a young one (which will have a lot of drive).

    It is possible for boars to have sexual performance problems. Especially with first-time sows, you want to be sure you're using a boar that does not have this kind of issue.

    Some issues that boars can have are:

• Boar is young, small, and inexperienced
  
• (these do get it right eventually and they have a lot of energy; my concern is getting it done on time if time is tight).
  
• Boar is old:
• difficulty standing tall enough to mount sows,
  
• cannot chase them well,
  
• cannot get it up any more, which is dangerous for the boar's health because his male part could come out but not do much other than hang there, and he would drag it on the cage floor while chasing a sow and injure it.
  
• Boar was a virgin for too long or not sexually active enough, doesn't seem to know how to get the job done.
• keeps doing it wrong, like mounting the sow's head or shoulders
  
• climbs up too far over her back
  
• may dump his load on the cage floor next to the sow
  

    It's much easier to find boars for sale which have breeding experience than sows that have been bred. I've gotten a number of these off of the local equivalent of Craigslist. 2 of them were the kind of outcrosses I like, and one of those was a homogeneous dark-eyed white (which was very nice to have). Most of them were a genetic sense good quality.

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    When you are getting the first guinea pigs, you won't necessarily have much choice about which animals you get. The sources of guinea pigs available in one area are not always numerous.   

    Once you are established, you generally use more sows than boars. One strategy to maintain genetic diversity is to use sows that you breed yourself and bring in new, unrelated boars to cover them.

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Ringworm   If something has this, it will be in localized area(s) on the skin. You may be able to see where it is forming ring-like shapes.   Carefully check anything new you are bringing in. Localized sores or scabby areas could be ringworm. The best treatment I know for this is topical enilconazole (trade name Imaverol Vet) (1ml of Imaverol Vet is mixed into 50ml of water) As of 2024, Imaverol is available world-wide except for in the United States which is inappropriately blocking it. It's illegal to ship this into USA. I've heard you will get big fines if you try to ship it in. I've used other topical antifungals to remove ringworm such as ketoconazole shampoo (Nizoral and cheaper generic equivalents) and non-prescription "Azole" antifungal creams. Its highly contagious and can persist in the environment. Hopefully, you don't get a big problem with this. People can catch this, kids seem more susceptible than adults.

Pregnancy Toxemia There is a catastrophic condition called pregnancy toxemia. The fetuses die quickly, plus you frequently lose the sow (in spite of rehabilitation efforts). The short reason it happens is, the guinea pig's intestine is not processing any food. To my knowledge the cause of this can be: the sow stopped eating, for any reason, food cannot move through the digestive tract for some reason, part of the digestive tract died (such as the intestines) which is of course irreversible. To my knowledge this can happen because some mechanical situation caused compression of important blood vessels in the gut: overhandling incorrect handling fetus weight and position Handling of late pregnancy sows is to be minimized!