Nutritional Assessment of Weanlings and Yearlings

Height or long bone growth is the developmental priority for young horses. Energy, protein, minerals, and vitamins are first directed to maintenance requirements, and any additional nutrients are used for skeletal growth, specifically long bones (limbs for locomotion) and flat bones (skull, ribs, etc. for protection). Additional nutrients above those needed for optimal bone development are used to fuel more rapid growth, first developing muscle and then producing a heavier and more well-developed young horse. Optimal growth rates may vary somewhat between breeds, but all young horses have several critical considerations for bone growth and development. Extremely rapid growth caused by overfeeding (particularly energy) has been implicated in developmental orthopedic disease (DOD) and unsoundness. Periods of slow followed by rapid growth are of particular concern for developmental disorders. Optimal bone development is greatly influenced by nutrition. During the first two months of life, the mare’s milk contains enough energy, protein, and other essential nutrients to meet the needs for growth. Work in Australia (Kohnke et al., 1999) has shown that a horse maturing to 450-500 kg requires approximately nine kg of milk for each kg of gain at seven days of age, 13 kg at one month of age, and 15 kg at two months of age. Thoroughbred foals may consume up to 18 kg of milk per day. The foals require around 16.4 kg of milk per kg of gain, so they should be gaining just over 1 kg per day. Beyond two months of age, there is a decrease in milk production, and additional nutrients must be supplied by creep feeding until weaning. Foals begin to nibble grass soon after birth, but they do not develop a functional hindgut that will allow them to extract significant nutrients from forages for months. In contrast, their efficiency of grain utilization is high at three weeks of age.