Meeting the mineral nutritional needs of foals
Minerals, though they make up only a small percentage of the equine diet, are critical — especially for the health of foals.
To meet its mineral requirements, the foal is first dependent on the concentration of those nutrients in the mare's milk. Prior to weaning, Thoroughbred foals consume about 15 kg of milk daily. Because the mare's need for minerals is likewise critical, equine caregivers should be aware of the mineral composition of solid feeds.
In addition, creep feeding and pasture intake provide some of a foal's early nutrition. Most foals begin to eat solid feedstuffs within weeks of birth. According to the National Research Council's (NRC) Nutrient Requirements of Horses (2007), 1-week old foals spend only about 8 per cent of the day eating grain/hay/pasture, but by 21 weeks spend about 50 per cent of daylight hours eating solid feeds.Minerals contribute to foals' health in many ways, constituting part of structural entities such as bones and teeth; and playing a physiological role in proper acid-base balance; as portions of enzymes, vitamins, hormones and amino acids; and for physiological functions of certain tissues. These include the roles of calcium, sodium and postassium in nerve transmission, and that of calcium for muscle contraction.
Not only must foals have the proper amounts of individual macrominerals, microminerals and trace minerals, but the minerals' ratio to each other is important, affecting absorption, metabolism and excretion of each other and of other nutrients. The macrominerals include calcium, chlorine, phosphorus, magnesium, sodium and sulfur. Microminerals include cobalt, copper, iron, iodine, manganese, selenium and zinc. Trace minerals include chromium, fluorine and silicon.
Calcium is among the most essential minerals for foals, comprising about 35 percent of equine bone and teeth and contributing to muscle contraction, blood coagulation, the function of cell membranes and regulation of enzymes, such as stomach gastrin. A deficiency of calcium in the developing foal can lead to osteopenia, a condition characterized by poor mineralization of the osteoid tissue and the probability of enlarged joints and crooked long bones.
Calcium absorption efficiency is around 70 percent in young horses, and is affected by the presence of vitamin D (though vitamin D's regulatory role in horses compared to in other species may be different) and by dietary concentration of calcium, phosphorus, phytate and oxalate. Higher dietary concentration of calcium decreases absorption; increased magnesium increases calcium absorption; while, competitively, higher dietary phosphorus decreases calcium absorption. All this is evidence of the interaction between minerals.
Increased content of oxalate in certain grasses may dramatically decrease calcium absorption from the small intestine to the point of deficiency.
As growing foals are accreting bone, they are in positive calcium balance. As stated by the NRC 2007, a 215-kg foal gaining 0.85kg/d with a 50 percent calcium-absorption efficiency would require 27.2 g/d of dietary calcium for skeletal growth plus 8.6 g/d to account for endogenous losses. According to other study data, the daily calcium requirement for growing horses may be up to 42.6 grams.
Phosphorus makes up to 17 percent of bone and is necessary for energy transfer (ATP and ADP) and for synthesis of phospholipids, nucleic acids and phosphoproteins.
Phosphorus absorption, normally about 35 percent to 50 percent, is decreased by high levels of dietary calcium and by increased amounts of oxalate content of grasses. Phytate phosphorus is poorly absorbed by horses, and has been reported to have little or no benefit in feeding. The NRC 2007 notes that "phosphorus absorption is assumed to be higher by foals consuming milk than it is in mature horses," though it is suggested that "creep feed be fed to nursing foals, as milk may be insufficient in phosphorus (as well as in calcium) for optimal growth of foals."
Inadequate dietary phosphorus produces rachitic-like changes in foals, while excess amounts may produce nutritional secondary hyperparathyroidism. Foals fed excessive levels of phosphorus showed severe lesions of osteochondrosis. A calcium to phosphorus ratio of at least 1:1 is as important as the amounts fed, and with growing foals a 2:1 ratio is even safer.
Even if the diet is adequate in calcium, excessive phosphorus may lead to skeletal abnormalities. According to the NRC 2007, using the proper estimate for endogenous phosphorus losses, a 215-kg foal requires 23.7g phosphorus daily.