Reproduction in farm animals

The reproductive performance of cattle, sheep, goats, pigs, horses and poultry determines how many young are born, how quickly genetic improvement spreads through a population and, ultimately, how much food a farm can produce. Farm animals have also served as research models for human reproduction. Research in this area combines basic physiology, such as how seasons and social cues control breeding, with applied work on reproductive technologies and their side effects.

Artificial insemination and frozen semen

Artificial insemination (AI), in which collected semen is placed in the female reproductive tract without mating, is the most widely applied and technically mature reproductive technology in livestock. In country reports compiled by the Food and Agriculture Organization of the United Nations (FAO) for its third global assessment of animal genetic resources, 96 percent of reporting countries indicated at least some use of AI. Insemination methods are adapted to each species’ anatomy, and fertility results are often good but vary considerably, which has turned attention towards semen quality control.

AI became far more powerful once semen could be stored. In 1949, researchers in London reported that fowl spermatozoa treated with glycerol could survive freezing to very low temperatures and recover their activity after thawing. This finding opened the way to frozen semen banks, allowing a single valuable male to sire offspring across countries and long after his death. Frozen semen and embryos are also used to conserve the genetic resources of rare breeds.

Embryo technologies and cloning

Embryo transfer offers a second route to multiplying valuable animals. Hormone treatments can make a genetically valuable female release several eggs at once, and the resulting embryos are collected and transferred to recipient females, a combination known as multiple ovulation and embryo transfer (MOET). In vitro fertilisation and embryo culture are now widely used, particularly in cattle, where transfer of such embryos can give success rates similar to AI. Because one straw of frozen bull semen can be used to fertilise many eggs in the laboratory, a single round of IVF can potentially yield dozens of calves.

Techniques differ between species. In sheep and goats, embryos have traditionally been recovered surgically; newer non-surgical methods that pass through the cervix give results approaching surgery in goats, whereas in sheep the narrow, folded cervix means surgery often remains the main option. When a valuable male dies unexpectedly, sperm can still be recovered from the epididymis and frozen for later use, an approach FAO highlights for conserving native breeds.

Embryo culture also revealed unexpected risks. Calves and lambs produced through some assisted reproduction procedures may show an overgrowth condition known as large offspring syndrome. Studies comparing it with Beckwith–Wiedemann syndrome, an overgrowth disorder in children, have found similar disturbances of imprinted genes. In one such study, seven of 27 cattle fetuses produced by assisted reproduction were above the 97th percentile for body weight, and some showed features such as an enlarged tongue or umbilical hernia; the cattle condition is now studied partly as a model for the human one. This links livestock research closely to the genetics and epigenetics of the early embryo.

The best-known landmark is Dolly the sheep. In 1997 researchers in Scotland reported lambs born after transferring nuclei from cultured cells, including cells from an adult mammary gland, into eggs whose own nuclei had been removed. The result showed that specialisation of an adult cell does not permanently alter its genetic material. FAO describes cloning and genetic engineering as among the fastest-moving areas of livestock reproductive technology.

Seasons, males and social cues

Many farm species keep traces of the breeding strategies of their wild ancestors. Sheep are seasonal breeders: ewes normally stop cycling as day length increases in spring and summer, a state called anoestrus. Photoperiod, nutrition, stress and social interactions all feed into the hormonal axis that controls reproduction.

A classic example of a social cue is the “ram effect”. Introducing a sexually active ram to anoestrous ewes, or even exposing them to his fleece, raises the pulse frequency of luteinising hormone; oestradiol then rises and, as in the breeding season, triggers the surge that leads to ovulation. The smell-detecting systems of the brain are essential to this response, and its study has shed light on how the reproductive axis is regulated more generally. Similar effects of male odours and pheromones in pigs and rabbits appear in the older parts of the archive.

The dairy cow fertility problem

Gains in productivity have sometimes come at a reproductive cost. In the United States, milk production per cow rose steadily through better management, nutrition and intense genetic selection, while reproductive efficiency fell. The highest-producing cows tend to have the highest incidence of infertility, although epidemiological studies point to other contributing factors as well. Proposed research priorities have included control of the oestrous cycle, the metabolic effects of lactation, links between disease and reproduction, and early embryonic loss. The energy side of this problem is described on the nutrition, metabolism and lactation page.

What is established and what is still debated

• Established: AI with fresh or frozen semen is routine worldwide; embryo transfer and in vitro embryo production are widely used, especially in cattle; nuclear transfer can produce live offspring from adult cells; seasonal and social signals strongly influence breeding in species such as sheep.
• Still debated or under study: the causes of variable fertility after AI; how embryo culture conditions alter imprinting and fetal growth; how to restore fertility in high-yielding dairy cows; and how to deploy reproductive technologies without eroding the genetic diversity of livestock breeds, a concern repeatedly raised by FAO.

The archive’s articles on farm animals range from 1960s field studies of oestrus in ewes and reproduction in alpacas to research on embryo culture, horse and pig reproduction, and poultry genetics in the 2000s and 2010s.