Unless you have a climate controlled rabbitry, the feeding demands to keep your animals in top condition changes from summer to winter. The more pronounced the climate change in your area, the more important it is to adjust your feeding pattern to fit the season.
Dramatic drops in temperature require the rabbit to ingest additional feed just to maintain the conditioning seen with less feed in the summer months. One should constantly evaluate your animals to ensure they are maintaining body condition.
If body condition is starting to rough over the back and spine in commercial type animals, you probably need to increase feed. In fancy breeds, where fleshing condition is not as critical, look to see that the animal is maintaining the condition shown in the summer.
You may find you need a slightly higher protein level in the winter feed, but remember that rabbits are like humans in the fact what fattens one individual may starve another. That is why it is so important to take your rabbits out and evaluate them on an individual basis to insure their needs are being met.