It is the usual custom to have the mare covered nine days after she has foaled: not to lose time, and to make all they can from the stud; yet it is certain, that the mare having a foal and foetus to provide for, her strength is divided, and she is not able to give them so much nourishment as if she had only one; it would, therefore, be better, in order to have excellent horses, to let the mares be covered but once in two years; they would last longer, and would not be so liable to drop their foals; for in common studs it is a great thing when, in the same year, half or two thirds produce foals.
The mares, when with foal, can bear to be covered, though there is never any fresh conception: they usually breed till the age of 14 or 15 years, and the most vigorous not longer than 18. Stallions, when they have been taken care of, may engender till they are 20 years old, or upwards. The same remark has been made of these animals as of men, viz. that those who have begun too early are soonest incapacitated; for large horses, which sooner arrive at their growth than delicate ones, are frequently incapable before they are fifteen.
The duration of the life of horses, like that of every other species of animals, is proportioned to the time of their growth. Man, who is above 14 years in growing, lives six or seven times as long, to 90 or 100. The horse, who attains his whole growth in four years, lives six or seven times as long, that is, to 25 or 30. There are so few exceptions to this rule that we cannot draw any precedents from them; and as robust horses are at their entire growth in less time than delicate ones, they also live less time, seldom exceeding 15 years.
It may be easily seen, that in horses, and most other quadrupeds, the growth of the hinder parts is at first greater than those of the anterior, whilst in man the inferior parts grow less at first than the superior; for in a child the thighs and legs are in proportion to the body, much less than those of an adult; on the contrary, the hind legs of a foal are so long that they can touch its head, which they cannot do when full grown. This difference proceeds less from the inequality of the whole growth of the anterior and posterior parts, than from the inequality of the fore and hind feet, which is constantly the case through all Nature, and is most sensible in quadrupeds. In man the feet are larger than the hands, and are also sooner formed; and in the horse the foot forms the greatest part of the hind leg, being composed of bones, corresponding to the tarsus, metatarsus, &c. It is not, therefore, astonishing that this foot should be sooner extended than the fore legs, the inferior part of which resembles the hands, being composed of the bones of the carpus, metacarpus, &c. When a colt is just foaled this difference is readily remarked; the fore legs compared with the hind ones being much shorter in proportion than they are in the sequel; besides, the thickness which the body acquires, though independent of the proportions of the growth in length, occasions more distance between the hind legs and the head, and consequently contributes to hinder the horse from reaching it when arrived at his full growth.