by Dr Derek Cuddeford, Royal (Dick) School of Veterinary Studies, University of Edinburgh
The word youngstock is a “catch-all” term that includes fillies, colts and geldings less than three years of age so in order to discuss the feeding of these animals we must consider the period from birth of the foal up to it developing into an adult at three. It is necessary to separate breed types within this grouping of youngstock since they are fed and managed very differently. Basically we must consider the “light” horses (Thoroughbreds/Arab horses, Half Arabs and Anglo Arabs, Quarter horses) as distinct from “heavy” (draft) horses, Warmbloods and ponies. Light horses are mostly bred and fed for either racing purposes or other high speed activities. Furthermore, these horses mature early and thus possess the potential for very rapid growth early in life. Thoroughbreds in particular foal outwith the natural breeding season whereas ponies, Warmbloods and draft types foal in late spring/summer taking advantage of naturally available feed. They are slower to mature and the emphasis is not on achieving rapid growth.
Thoroughbreds are probably the best example of light horses in the UK. Most young Thoroughbreds are reared with the objective of sending them to the Yearling Sales, held principally at Deauville (France), Keeneland (Kentucky), Goffs (Kildare) or Tattersalls (Newmarket) where they are bought with the purpose of putting them into training. They will be prepared for racing as two year-olds. Thereafter, if good enough, they will remain in training and race as three year-olds and beyond in the well-known classics on the flat or as steeplechasers where they take part in a different style of racing. A March-born foal will become a Yearling when it is just over 9 months old compared to a January-born foal which will be nearly 12 months old since they are “aged” from the January in the year they are born. This disparity (~90kgs=18/20% of adult weight) confers a benefit to the January-born foal in terms of physical development. Thus, not all Yearlings are equal in terms of chronology or physiology. Clearly this difference persists into the second year of life so that some two-year olds are not ready to race until towards the end of the season. It will be apparent from the foregoing that there is pressure to make sure that young Thoroughbreds are born early in the year and to grow as quickly as possible in order to hasten growth so that firstly, they sell well at the Yearling sales and secondly, that they mature quickly so that they can race sooner. Some recent Japanese research monitoring growth rates amongst Thoroughbred foals has emphasised the importance of two measurements: bodyweight and withers height to monitor development. Because foals respond to season in terms of growth rate (faster in summer, slower in winter) their birth date can have a significant impact on their individual growth curves. Thus nutritional needs will vary between foals according to when they were born as evidenced above by comparing the January-born foal with those arriving in March. Based on this information it would be prudent to individualise feeding programmes for foals as far as possible to accommodate these differences and to plot their growth curves (weight/withers height) as a further guide to management. One can easily imagine the great difference between foals of different ages (weight/withers height) as they make the transition from winter to spring and thus the large differences in their nutritional needs at this critical time.
Nursing Thoroughbred foals will grow at a rate of 1 to 1.5kg/day which is very fast, allowing these animals (in common with other light horses) to achieve 90% of their mature height by 12 months of age. Initially this growth will be supported by the mare in terms of the milk she produces and foals may consume 15 to 25% of their bodyweight in milk per day during their first week of life. The mature height for Thoroughbreds can be assumed to be 162 cm for stallions and 160 cm for mares. By eighteen months of age Thoroughbreds have nearly achieved full height but only four-fifths of mature weight. What is crucial to these animals is that the bone is fully mineralised and able to carry the additonal musculature that develops with training. A study of nearly two thousand foals on a Thoroughbred stud in Canada showed that on average, foals gained 110 kg during the first 90 days after birth, 75 kg during the second 90 day period, 60 kg during the third 90 days and only 45 kg during the fourth 90 day period. Clearly, the greatest amount of bone elongation occurs during the first months of life emphasising the need for a balanced supply of nutrients. Because of the foals rapid growth and bone elongation these animals are susceptible to skeletal defects broadly known as Developmental Orthopaedic Disease (DOD) that includes a range of different conditions including physitis, angular deformities, osteochondrosis dissecans, malformation of the cervical vertebra (“wobblers”) and carpus. It would seem that these conditions are becoming more common and a number of different factors have been implicated including nutrition, heredity, exercise and conformation. Although inbreeding within the Thoroughbred industry may improve the chances of producing horses of high athletic ability, a consequence of this approach to breeding is that other less desirable traits may be concentrated in the offspring.
It seems that the key to minimizing developmental problems is appropriate management-sounds simple but in reality requires a lot of effort. This does not mean making great changes to what is fed to the growing animals since almost all commercially produced diets (creep feed, youngstock cubes, yearling diets, etc) for these animals are usually of excellent quality. The goal has to be to regulate concentrate intake and to maintain the animals on a smooth growth curve with no decline in growth rate that might result in compensatory growth spurts. This requires the introduction of creep feed well before weaning, which usually takes place at 4 (~35% of mature size) to 6 (~46% of mature size) months of age, in order to maintain growth rate at, during and after weaning. Furthermore there is strong evidence to show that controlled exercise throughout the growth process enables the formation of healthy bones and joints. A “rule of thumb” guide to the amount of creep to feed is 450g per month of age. Maximum rates of gain are not desirable in terms of bone quality as it may be compromised by too rapid growth. Table 1 provides average values for Thoroughbreds with an expected mature weight of 550kg.
Table 1: Average bodyweights, daily gains and likely food intake at 3, 6, 9, 12 and 24 months of age based on published data.
Nutrient content of the concentrate varies according to age. Initially milk replacer pellets may be fed that use whey as a protein source and contain ~20% protein overall. Over the period 1 month to 3 a creep feed containing ~18% protein based on soya would be appropriate. From 4 to 6 months a ~14% protein creep should be used. Obviously all of these feeds will be carefully balanced with respect to the absolute amounts and ratios of minerals and vitamins that are suitable for the different growth stages of the developing foal.
In contrast to the January-born Thoroughbred foal, those horses and ponies (basically all non-TB animals) that foal during the normal summer period will depend initially on their mother’s milk, gradually making the transition to a grass-only diet to meet all of their energy and nutrient needs. Natural weaning will normally take place when the foal is a year or more old or when the mare foals again. Providing mares and foals with good quality pasture will ensure adequate energy and nutrient intakes but if in doubt, it is always possible to supply a low volume concentrate that supplies just essential minerals and vitamins to guard against any deficiency. Essentially, those native animals and others kept outside are controlled by the seasons in terms of their growth rates. Foals born in the summer should have plenty of food to support good growth and to develop reserves to carry them overwinter. However, late-born foals may not grow sufficiently well to be able to survive their first winter and will die due to a combination of malnutrition and inclement weather. By the way, this can also be the fate of many Roe and Red Deer kids in their first winter. Those foals that survive the winter will grow very little over this period but once winter gives way to spring and summer their growth speeds up responding to the additional food energy that becomes available to them. This see-saw progression in development will continue until they are mature. Thus the growth of these animals is significantly affected by season unlike that of the Thoroughbred.
In conclusion, it is important to define the goals in feeding youngstock. The targets for liveweight gain and ultimate bodyweight determine the feeding programme. This is most critical for Thoroughbreds where there are well-defined aims (Yearling sales, etc) but for the remaining horse population, natural rhythms take over, usually with a better biological outcome! Apart from Warmbloods, not many ponies, draft horses, etc suffer from DOD…!!!