The body condition (BC) of mares can undoubtedly affect their reproductive efficiency, so to achieve maximum efficiency, mares must be kept in an adequate body condition (score of 5 to 6 on the Henneke Scale - scale of 1 to 9, where 1 is thin / very insufficient CC, and 9 obese / excessive CC).
Scientific research has shown that mares kept in proper body condition ovulate earlier and have shorter calving/conception intervals.
skinny mares
Mares that enter the reproductive phase thin, regardless of whether or not they are being fed with enough energy to gain weight, are less reproductively efficient compared to those that are in an adequate body condition.
fat mares
At the opposite extreme, mares with excessive body condition are shown to remain efficient from a reproductive point of view. Which does not invalidate that it is undesirable, given the reduction of the intrauterine space, which does not favor the development of the fetus, as well as the metabolic problems associated with obesity (common to all horses).
Group dynamics
Since breeding mares are usually found in a group context, assessing group dynamics may be relevant. More submissive mares may find it difficult to feed themselves, given the pressure exerted by the dominant elements of the group. If excessive dominance is noticeable, it may be better to separate some mares to ensure that the thinner ones are able to feed properly.
Like any other horse, a thin mare that does not improve her body condition, being fed for that purpose, should be evaluated by the veterinarian, as there may be an underlying health problem that conditions the weight gain.