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United States Department of Agriculture,2
Abstract
Data are presented on certain aspects of fertility of 209 mares bred to 14 stallions over a 15-year period at the U. S. Range Livestock Experiment Station, Miles City, Montana, in a cooperative study with the Montana Agricultural Experiment Station.
The general fertility level was 59.6 percent (based on the number of mares bred) and 46.8 percent (based on the number of estrus periods in which the mares were bred).
The effect of year was apparently not connected with any trend but there were indications that it was associated with the fertility of the particular stallions used in different years.
Very young mares as well as old mares appeared to have a lower fertility than mares of intermediate ages.
The order of fertility of the mares of the different breed groups was Belgian, Morgan, grade-light and grade-draft. Differences in the stallions used and possibly management appeared to account for much of the difference.
Significant differences were found between the fertility of stallions bred to the same mares in seven out of 15 comparisons.
One set of twins was obtained in 567 foalings.
The sex ratio was 97.6 males per 100 females for all the foals (568).
Eighty-seven and two-tenths percent of the foals born were weaned. The numbers dying of different causes are listed.
1 Bureau of Animal Industry, Washington, D. C. The authors acknowledge assistance of J. R. Quesenberry, superintendent of The U. S. Range Livestock Experiment Station, and E. B. Osborn, who was directly responsible for collection of the data.
2 This work was conducted cooperatively by the Bureau of Animal Industry and the Montana Agricultural Experiment Station.
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