Equine Science Update
Reports from the world of  equine research.
Foetal sex determines
weaning time for Zebra.
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What factors influence the age at which foals are weaned?

Many lactating mares are also pregnant. So it is often necessary to balance the needs of the sucking foal with those of its future sibling and the mare herself. It is important to allow the mare enough time after weaning to devote her resources to her next foal.  

The actual time of weaning is often laid down by tradition. Current equine stud practice is to wean foals at about 5 -6 months of age. The timing is often determined by the need to prepare foals for sale. Sometimes early weaning may be required. Foals with developmental orthopaedic disease may have to be weaned early to reduce their food intake.

But what would happen if mares were left to decide for themselves when to wean the foal?  In feral horses, weaning occurs shortly before the mare gives birth to the next foal, when the foal is up to a year old. If the mare is barren the foal may stay on her for up to two years.

According to studies in captive zebra mares in the Czech Republic, one factor that can influence the time of weaning is the sex of any unborn foal the mare is carrying. Dr Jan Pluhácek, of the Research Institute of Animal Science at Praha-Uhrineves, and his colleagues studied three herds of captive plains zebra at the Dvur Králové Zoo,

The gestation period of the plains zebra is longer than that of the domestic horse, ranging from 360 - 385 days. As in mares a “foaling heat” occurs after about a week after foaling. Most zebra mares conceive when they are nursing a foal - so mares often provide for two offspring at the same time.

The researchers looked at various factors - such as the sex of the weaned foal; the parents’ age; size of herd; and number of other foals in the herd.

They found that pregnant mares weaned their foals at least 50 days earlier than did non-pregnant mares.
Mares carrying a male foetus weaned their foal earlier than mares carrying a female foetus. The sex of the foetus was the most significant factor affecting time of weaning of the current foal.

Dr Pluhácek points out that this is the first time the sex of the foetus has been shown to influence weaning age in hoofed mammals.

For more details see:

Sex of the foetus determines the time of weaning of the previous offspring of captive plains zebra (Equus burchelli).
Jan Pluhácek, Ludek Bartos, Miroslava Dolezalova, Jitka Bartosova-Vichova
Applied Animal Behaviour Science (2007) 105, 192 - 204.
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