Younger horses definitely have different nutritional needs to adult horses, one significant being more protein in the diet. Balanced nutrition is important to avoid illnesses like Developmental Orthopaedic Diseases (DOD).
I cannot cover all scenarios, so are focusing on yearlings and extrapolating for your horse, as rising 3 yo.
The feed requirements vary from the first year, while being fed by mum to a yearling and then the following years towards adulthood, 5 year old, gradually merge to match an adult diet.
Any nutritional needs will vary if a horse is in work and the intensity of the work.
The key for horse is to get the right balance of calcium, phosphorus, magnesium, zinc and copper, plus amino acids, often available in protein. Key to this is that the bulk of any horse's diet is forage - pasture, hay or other fibre sources, like the beet.
On her current diet, I would feed lucerne/alfalfa hay (same thing), cut out the beet, it's better if she has the hay to munch on overnight. Then add a supplement designed for your local area. This depends on your soil and pasture. It would likely contain copper, zinc, magnesium, plus other nutrients. For example, where we live the soil is high in iron, which requires more supplementation of copper and zinc, plus we have pasture that is high in oxalates which prevents calcium absorption, so extra calcium is required on top of the usual balance. You could use a balancer pellet, which would probably require something in the vicinity of 0.5 - 1 kb per day depending on the manufacturer. Always check the pack.
If you can feed hay over bagged feed, hay is always the better choice, as it suits the horse's digestive system. I only supplement with hard feed in my horses that are elderly. However I do provide all my horses with supplements designed to complement the pasture.
From the MSD Veterinary Manual:
NUTRITIONAL REQUIREMENTS OF HORSES
Current recommendations are that horses receive at least 1.5%–2% of
their body weight in forage or forage substitutes such as hay cubes or
other high-fiber source daily. The average maximum daily dry matter
intake is 2.5%–3% body wt (although some breeds and age groups,
notably ponies and weanlings, can exceed those maximums if on good
pastures); therefore, forage or forage substitutes should be the major
components of an equine ration.
These are for 500 kg horses, which is the average horse and your appaloosa may come in around that weight as a mature horse.
Estimated Daily Nutrient Requirements of Growing Horses and Ponies
Estimated Average Daily Nutrient Requirements of Mature Horses and Ponies
Between the two table from 24 months to adult you can see the following differences:
Digestible Energy: 18.7 - 16.7
Crude Protein: 770 - 630
A rising 3 will still need to gain weight and have greater energy and protein requirements.
From the following articles by Kentucky Equine Research:
Choices in Feeding Young, Growing Horses
For a yearling:
... for owners who want to feed the absolute minimum amount of grain
necessary to their horses on good-quality pastures, it is possible for
the yearling in this example to consume enough pasture to satisfy
requirements for digestible energy and protein to maintain a moderate
level of growth. However, the concentration of critical nutrients
(calcium, phosphorus, copper, and zinc) is often inadequate in a
pasture-only diet. To properly balance the diet in this situation, it
is estimated the yearling will consume nearly 16 lb (7 kg) of pasture
dry matter/day along with 1.25 lb (0.5 kg) of a ration
Protein-to-Energy Ratio in Young-Horse Diets
Young horses need high-quality protein to supply adequate lysine and
other amino acids for growth. In addition to protein, the young
growing horse has a requirement for energy in its diet. These
requirements are closely linked and a deficiency of either will result
in a reduced growth rate. In fact, protein and energy are so closely
linked that one should not be considered without the other in rations
for growing horses.
In other words, it is the ratio of protein to energy that is important
for growth rather than either the protein percentage in a ration or
even the daily intake of protein, because it is the energy that
provides the potential to grow new tissue that will then require
protein. If there is an excess of protein supplied with an inadequate
amount of energy, the protein will be oxidized to produce energy. .../...
Different Horse Feeds for Different Needs
By the end of its yearling year, a horse will have obtained 90% of its
adult weight. The demands for protein, vitamins, and minerals still
remain higher in the yearling than in the adult horse. While yearlings
can eat more than weanlings, they still require a feed which is more
concentrated than feed intended for adult horses. Typical yearling
feeds have 14 to 16% protein and are fortified similarly to the
concentrate designed for broodmares. Balance in the diet, particularly
of energy and minerals, is especially important during the yearling
year because this is when many of the signs of developmental
orthopedic diseases (DOD) such as epiphysitis and osteochondritis
dissecans (OCD) begin to appear.
When a young horse begins training, horse owners and managers must
realize the horse is still growing. The dual demands of training and
growth make it especially important to pay attention to proper
nutrition. During the training process, the bones will undergo
constant remodeling to adapt to the stress of work. A balance of
vitamins and minerals in the diet will aid in minimizing the amount of
stress these changes cause. Further, adequate dietary protein is
essential as greater muscle breakdown is a physiological consequence
of increased work. Protein for growth and work can usually be supplied
with a 14% protein concentrate, or 12% if fed with alfalfa or alfalfa