This section is from the book "The Horse - Its Treatment In Health And Disease", by J. Wortley Axe. Also available from Amazon: The Horse. Its Treatment In Health And Disease.
The view may not be universal, but it is generally held that nothing conduces so much to the production of strong, healthy offspring as giving the mare a reasonable amount of work, under judicious management. A certain element of risk, it is true, always attends the active employment of pregnant animals, and especially those engaged in farm-work, but with common care this is far outweighed by the benefits conferred on the dam and produce. When mares have well-proportioned work and a liberal supply of good food, foals are not only dropped bigger and stronger, but they resist the exposure to adverse influences, and thrive and grow much better than those from idle, ill-conditioned mares.
As to the stage of pregnancy when mares should cease to work, different people entertain different ideas, but the question is surely far more one of management than of opinion. It is common enough for mares to work right up to the time of parturition, and especially among little men, who depend for their livelihood on the labour of their mares. But in these cases self-interest lends its force to management, and largely determines the success of the enterprise. As a general system such a course would be fraught with the greatest danger, but there can be no-doubt, where common care is observed in the selection and appointment of work, together with good general management, pregnant mares are all the better for working up to within three or four weeks of the time of foaling. When work is 'discontinued, daily exercise should be substituted, or, if available, some brief, light employment. The late Mr. James Martin - a rare authority, by the way, on blood and breeding - -once observed to the writer: " I have foaled eighteen mares this season. All have worked nearly up to the time of foaling, and without a mishap to either mare or foal." Such a result is not likely to be of common attainment, but it is most assured under the influence of reasonable, well-regulated work, and generous but careful treatment. In-foal mares should, however, be guarded against severe exertion, such as drawing heavy loads in deep ground or on hilly roads, or backing, or trotting at fast pace, especially down hill, nor should they be made to undergo long fasts or suffer fatigue. As pregnancy advances, and the calls of the growing fetus on the nutritive resources of the dam become more and more considerable, so should the amount of work demanded of her be diminished, and the food-ration undergo suitable adjustment. To assert that the observance of such details should be among the commonplaces of every stud is only to suggest a state of things that, to say the least of it, is far from universal, technical education notwithstanding.
 
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