____________ allows reproduction by older, less fertile mares by inserting the embryo of a more genetically superior mare.

The correct answer is embryo transfer, which is a reproductive technology that enables breeders to use a less fertile mare to carry an embryo from a more genetically superior mare. In this process, an embryo is created through the fertilization of a superior mare's egg and is then implanted into the uterus of a recipient mare, which may be older or less fertile. This allows the valuable genetic traits of the superior mare to be propagated, despite the recipient mare's lesser fertility.

This method is especially advantageous as it allows breeders to maximize the genetic potential of a limited number of exceptional mares. The recipient mare can gestate the embryo, giving birth to a foal that inherits desirable traits from the genetically superior mare, without compromising the superior mare's own reproductive resources.

While artificial insemination refers to the introduction of sperm into the reproductive tract of a mare to achieve fertilization, it does not specifically involve the use of embryos from superior mares. Frozen semen technology enables the preservation of stallion sperm for future breeding but does not involve embryo transfer. Stallion collection relates to obtaining semen from a stallion, which may be used for artificial insemination but is also not specific to the embryo transfer process.