What Makes Junior Grow?
Several Factors Affect His Ultimate Size
Who has the most influence on the growth of the foal, the mother or the father? Is this influence permanent? Does it matter what time of the year the foal is born, or does the mare's age have an influence?
You guessed it! The mare has the most influence over the ultimate size of the foals. An interesting project was done to determine that influence. A draft stallion was bred to a pony mare, and a pony stallion was bred to a draft mare (with some help I suspect!). The resulting offspring were grown out, with frequent measurements taken. We are often asked if a pony mare can be bred to a medium sized stallion, or more commonly we are asked, "If my pony mare was bred to the stallion next door, can she have the foal?" The answer is yes, and more on this later.
The foals resulting from the above crosses were followed into adulthood. The foals from the pony mares were smaller at birth, at one year of age, and at two. They never caught up with the foals from the draft mares. The volume of the abdominal cavity is what determines the size of the foals. The larger the belly (as with the draft mare) the more room the foal has to grow. The smaller belly of the pony limits the size of the foal, and this is why the pony will successfully deliver foals from a much larger stallion. This is also the reason that the first foal a maiden mare of any size delivers will be her smallest foal: she is still “in tone,” and the well-muscled abdomen does not allow much expansion. So we ask ourselves, "If the pregnancy is longer, will the foal will be larger?" This is not the case as those foals born in January and February will be the smallest. To further confuse things, the foals born in July will also be smaller than the average!
Of course, boy babies will be larger than their sisters. This is true at birth and the difference increases with age. It will take them longer to reach their mature size, as well. This is also true of the heavier breeds. The heavier they are, the lower the percentage of their mature weight they will be at any age. For example, at nine months of age an Arabian will have 90% of its height, while the larger Percheron will only be at 87%.
With a balanced ration and the good fortune to avoid a serious illness, all the foals will reach their genetic potential. They do, however, keep us in suspense for a year or two while they are reaching it.
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