What hay do you use? I know some UK websites that sell herbal hay.
I limit the amount of pellets my chins get to make sure they eat lots of hay.
Surprisingly, they all gained weight when I reduced the amount of pellets from one tablespoon to one teaspoon. I offer a diverse diet: organic herbal hay that contain various grasses and herbs, some alfalfa hay and red clover + herbal supplements. This way chins can better meet their need for nutrients and regulate their intake of toxins (all plants contain toxins). Chins who constrained to a singe food, do not get this chance, even if the food they get is "complete" and "balanced."
I'm going to copy and paste from another question I answered regarding hay:
Herbivores thrive on good hay and herbs; just look at a 2000 pounds bison. It's important to offer a diet that closely resembles the composition of the foods that chinchillas consume in the wild. Many captive animals suffer from diseases that wild animals do not suffer from or they are not common in wild animals. Many domestic chins suffer from dental problems because people decided to change their diet and offer pellets instead of various grasses and herbs. Chins are born to slice through coarse vegetation with front teeth, and their back teeth are designed to break up grass by grinding action. Pellets are just crushed between teeth – crashing action does not wear down molars properly and fast enough. As a result, they can become misaligned and can cause the misalignment of the incisors as well. Another possible problem is the development of sharp spurs when the back teeth do not wear each other down any more. Pellets also aggravate the condition in chins with teeth problems. It's not only about tooth wear; teeth also grow faster on high energy diets. "Examination of chinchillas in the United Kingdom has shown a 35% incidence of tooth elongation in apparently healthy animals, whilst almost all animals diagnosed with dental disease had marked tooth elongation." (Crossley, 2001)
"Chinchillas are herbivorous rodents with teeth that all grow continuously. In captivity they are commonly affected by dental disease. Since the range of dental disease occurring in wild chinchillas is unknown, the dentition of museum specimens originally obtained from the wild was assessed and compared with specimens prepared from captive bred animals.
Skulls from wild-caught chinchillas showed minimal evidence of dental disease and the teeth were all short, cheek tooth lengths averaging 5.9 mm. Cheek tooth lengths in zoo specimens (average 6.6 mm), clinically normal (average 7.4 mm) and captive bred animals with dental disease (average 10 mm) were significantly elongated by comparison (P < 0.0001). Captive-bred specimens showed a wide range of tooth-related lesions.
These results suggest that some aspect of captivity is responsible for the development of dental disease in chinchillas. It is suggested that the diet (its physical form and composition) is the main etiological factor, and that provision of a diet closely matching that of wild chinchillas should significantly reduce the incidence of dental disease in captive chinchillas." (Elsevier Science).
Chinchilla's diet in the wild is diverse; they eat lots of herbaceous plants to get all the nutrients they need. Here are some plants they consume (with pictures):
puya berteroniana
bridgesia incisifolia
pasto rey
pingo-pingo
oxalis carnosa
glandularia sulphurea
moscharia pinnatifida
adiantum chilense
heliotropium stenophyllum
lobelia polyphylla
proustia cuneifolia
porlieria chilensis
lycium chilense
adesmia microphylla
astephanus geminiflorus
Chins in Las Chinchillas National Reservation especially enjoy
Berberis glomerata (blue berries)!
Unfortunately, many pet owners think that chins need a bagged, "complete" diet as they have been conditioned by manufactures who claim that they know what we should feed our animals. As a result, pet owners think that a bagged diet is better than a natural diet and afraid to follow the logic. Surprisingly, we do not think that a pelleted diet that consists of about 4 ingredients and lots of synthetic nutrients is good for us or for our children.
Here is the truth about animal nutrition. Feeds have been developed by trial and error as we lacked detailed knowledge about diets of wild animals. The scientific study of diet is young, and it's not an exact science; new substances are being discovered all the time. Pelleted feeds lack enzymes and important elements, including secondary compounds that possess medicinal properties (in the wild, nutritional = medicinal). A pelleted diet can not replace a natural diet and goodness that Mother Nature offers.