Since there are fewer jennets than mares, the gene pool is more limited. Jennets are smaller than mares, and since the foal grows to fit the size of the dam's uterus, hinnies tend to be smaller and lighter than mules. Good mares are cheaper than good Mammoth jennets. Since Mammoth jackstock is classified as endangered, most breeders use the jennets to reproduce their own kind, rather than to produce sterile hybrid mules. Jennets are less fertile than mares, which are not all that fertile to begin with.
Heat is more difficult to detect in a jennet than in a mare.
A jack responds more readily to a mare in heat than a stallion responds to an estrous jennet. The conception rate is the same whether a mare is bred to a stallion or a jack, but is lower in a jennet bred to a stallion. Researchers have found that whenever two species are interbred, fertilization occurs more readily if the dam has more chromosomes than the sire. The gestation period of a jennet is one month longer than that of a mare. As a result, jennets don't foal every year, as a mare might, but will occasionally skip a year.