Some may be fine and ignore them, younger jacks nay want to play with them and accidentally injure them, or they may see them as small predators and attack them. However, it will make things much easier on the jenny if she is not constantly being pursued by a lovesick jack while she is also being trailed by a demanding youngster. This problem can also arise if a jennet is exposed to two different jacks within a short time period. If the time is less than two months apart on the breeding, it cannot be told for certain which jack is the sire. Jennies usually carry their foals for 12 months, but any are, whether the first-time mother or the most experienced brood jennet, can be off on their timing.
Some studies also show that male foals are carried a slightly different length of time than females.
Bigger, older, more experienced jacks may injure or kill smaller, quieter, or shy jacks. Serious injury could also occur to any animal that got n the middle. Don't believe that keeping a yearling jack in with a herd is safe just because he is less than one year old. Young males have been known to sire foals when they were only 8 or 9 months old. Although it is far less detrimental to the young jack than to the jennet or mare that is bred too young, it is still not a sound breeding practice.