No, I haven’t gone and acquired some guinea pigs. Yet.
What I have been doing is some productivity calculations. My objective in keeping guinea pigs (aka cavies, aka cuy) is meat; if that squicks you, please stop now and don’t read any further.
Ok then. One young guinea pig (at 4 months old) weighs 0.5 – 1 kg, on average. That obviously includes the bones, organs, skin, etc. which aren’t really edible – although the organs are useful for feeding to cats (and the dog we will eventually get), and pet food is one of the primary drivers here. So at a guess, one guinea pig will produce 300 – 600g usable meat (using a 60% dress-out percentage, since the organs and smaller bones will be useful for cat food).
Guinea pigs reach sexual maturity at 3 – 6 weeks (3 – 5 for males, 4 – 6 for females), and they can breed year round. Females can become pregnant as little as 6 hours after giving birth, although it isn’t good for them. The gestation period averages 63–68 days (but can be anywhere from 59–72 days), and baby guinea pigs are able to eat solid food immediately, although they continue to nurse; they should be weaned by the age of 3 weeks. So, taking all of that into account, and assuming a break between pregnancies, there should be about 79 days between matings for any given female. In other words, a female guinea pig may have 4 – 5 litters per year (depending on the exact length of gestation, and the length of the breaks between giving birth and re-mating). Guinea pig litters normally consist of 1 – 6 pups, with an average of 3.
The recommended ratio for guinea pigs is one male (boar) per 5 – 6 females (sows). So if they were kept in a colony situation, you might keep 5 sows and one boar together, removing babies when they reached about 2 weeks old to a separate pen. Alternatively, you might keep the females in a group pen, and introduce a male when you wanted the females mated, to get more control over the timing of mating and birthing. either way, each group of females would produce somewhere around 80 babies per year. Assuming that you only kept babies as replacements for adults who were getting past their prime (in which case, those adults would be retired – and slaughtered), that means each group of females would produce about 24 – 48 kg meat per year. (Or you could think of each guinea pig as 6 meals for a cat, or 2 meals for a human.)
Since guinea pigs eat grass, hay, and other greens, like any other grazing animal, that’s pretty good. Each guinea pig needs about 6% of its body weight per day in food – slightly less in hay, slightly more in greens because of the water content, and must have greens every day because vitamin C is an essential nutrient for them. So each group (of 6 females and 1 male) would need 420g food per day. Additionally, each group would have an associated group of (on average) 36 juveniles (when the new babies are weaned at 2 weeks, the previous lot of babies will be 10 weeks old, so at any one time there will be 2 age groups of juveniles), which will require 1.6 – 2.2 kg food per day (younger and therefore smaller pigs will need less food than adults). So the total feed requirement for each breeding group of guinea pigs would be 2 – 2.6 kg per day. For comparison, a small bale of hay ($8 – $24, depending where you get it) weighs around 20 – 25 kg, and a (standard 10 L) bucket of cut grass or mulberry leaves would weigh approximately 1 – 1.5 kg. So, about a bucket of cut grass and/or leaves (from good fodder shrubs or trees) and a bucket of hay should be enough food for all those guinea pigs for a day.
The recommended minimum space requirements for guinea pigs are 0.7 sqm for 1 -2 individuals, or 1 sqm for males, and an additional 0.3 sqm per additional animal (some recommendations allow less space than this, suggesting 0.25 sqm per breeding pair, with an additional 0.1 sqm per additional breeding female). Either way, around 2 sqm is sufficient for a breeding group of 7 guinea pigs. For the 36 juveniles, a pen of 7.5 sqm (or another four 2 sqm pens) should be sufficient.
For those who are curious, yes: this does mean that I’m planning on building some guinea pig runs and then getting some guinea pigs in the near future. I really do have to wait til next year for sheep or goats, until the pasture is established, but these grazing animals are plausible much sooner.