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Removing horses from affected pastures is the most effective management practice in reducing the incidence of mare reproductive loss syndrome (MRLS). This condition is primarily linked to exposure to environmental factors, particularly the toxic effects of certain plants, including black walnut trees. When horses have access to contaminated pastures where these toxic plants are located, they are at a higher risk of developing MRLS, which leads to significant reproductive issues such as abortions and stillbirths.

By relocating horses away from these affected areas, the exposure to the harmful substances that contribute to MRLS is minimized, thereby substantially lowering the incidence of this syndrome. This approach is proactive and directly addresses the environmental risk factor associated with the disease.

The other management practices, while potentially beneficial in other contexts, do not specifically target the root cause of MRLS in the same way. Vaccination might help with other reproductive diseases but does not prevent MRLS. Cutting down all black walnut trees would not be practical or feasible in many situations since it may be difficult to ensure all trees are removed and the risk persists if any remain. Progesterone supplementation could help support pregnancies once they are established, but it does not prevent the onset of MRLS or address the environmental triggering factors directly.