There are several ways to feed guinea pigs during the Autumn/Winter.
This page informs you how to replace nutrients found in pellets during the Autumn/Winter.
Please note: this section just covers the topic of vegetables and does not feature all dietary components.
Guinea pigs that are housed outdoors in the colder season have different dietary requirements to those housed indoors. They use up more energy to keep themselves warm, therefore require higher energy feed to get them through the cold season.
In the warmer season, there is an abundance of wild forage available to collect, which can make up the majority (if not all) of their diet when there is a diverse, continuous amount provided for them. However, during the colder season, there is much less variety to choose from. This means the fresh forage has to be substituted for something else...
AND LOTS OF THEM
Leafy greens should always make up the bulk of the vegetables.
The reason why cabbage valuable in Autumn/Winter is due to the high nutritional value. Cabbages are also a great energy feed which is especially important when it's cold. All types of cabbage can be fed, though it is best to start with the better tolerated varieties (e.g. Chinese, spring greens). Cabbage & other cruciferous vegetables require quick digestion to prevent bloating. Pellets slow down their digestion because they are a really concentrated food source. If you feed cabbage alongside pellets, then chances are that the cabbage will stay in their digestive systems too long which often causes bloat.
This is why there is a widespread myth that cabbage = bloat because most owners feed pellets and then their guinea pig/rabbit gets bloated. It is worse with pellets containing a grain (such as wheat) because gluten upsets their digestion and so do additives like sugar, honey and molasses, but even grain free pellets cause problems.
During Winter you may offer whole cabbages but you must follow the rules!
Root vegetables are also important for outdoor guinea pigs in cold temperatures because they provide lots of useful energy which helps them maintain weight and body heat. You can feed root vegetables as well as, or instead of cabbages. Both are great feed for the colder season. Again, you may offer the root vegetables in large amounts providing they tolerate them well. Carrots in particular are beneficial and you can purchase large sacks from places such as horse suppliers. They have a bad reputation due to the sugar content, but the natural sugars are used effectively during the cold season. Ensure you weigh your guinea pigs regularly, especially in Winter - their weight is a good indicator if you are feeding too much or too little. Root vegetables can be fattening, though a layer of fat helps them to keep warmer. Some root vegetables you can feed include:
parsnips, carrots, celeriac, turnips & sweet potato
Cabbages and some other leafy greens are part of the Cruciferous family, but there are other types of vegetables in this category such as broccoli, cauliflower etc. These are all great Winter vegetables. They should all be introduced very slowly to their diet to prevent bloating and other digestive problems. Guinea pigs who tolerate these vegetables well, may consume them in large quantities. Do not feed these vegetables in larger quantities along with pellets. Examples include:
Broccoli, kale, cauliflower
Often incorporate vegetables from the 'other' category. These provide additional vitamins and minerals so you should aim a variety of these too. Try to alternate the vegetables in each group daily to ensure their nutritional needs are met. In particular Vitamin C rich veggies as they can't synthesise this Vitamin themselves. Examples include:
Pepper, celery, cucumber, cherry tomatoes