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Coefficient of Inbreeding (COI) and Planned Breeding

A much-debated topic of our time is that of ‘inbreeding’. This article explains the BKKPS approach to inbreeding, the use of the COI for responsible breeding.

What is inbreeding?

Inbreeding is the pairing of animals that are related – they will share some common ancestors in their last 5 generations. It has been widely done in small rare breed herds out of necessity, but it has also been done by breeders wanting to breed in desirable traits such as short noses, specific colours, small ears etc.

What is wrong with inbreeding?

Whilst inbreeding has sometimes been used to reinforce desirable traits, it can lead to other, undesirable and often hidden, effects. It has been scientifically proven that inbreeding in pigs can cause:

  • Pigs to be smaller (up to 10%, by bodyweight, per generation)
  • Unhealthy characteristics (e.g. short noses lead to breathing difficulties and sunken eyes)
  • Inherited deformities
  • Inherited diseases
  • Lower birth rate
  • Reduced fertility
  • Miscarriage/abortion
  • Stillbirths (and defects in the stillborn piglets)
  • Mummified piglets
  • Reduced vitality (less vigorous piglets that grow more slowly and are not as healthy, growing into unhealthy adults).

Is it easy to spot inbreeding from a pedigree?

At a recent AGM I asked some experienced breeders to use the BKKPS pedigree cards to look at specific trial pairings. They were asked if they would advocate the pairs I had suggested. I managed to catch most of them out – but I was trying very hard to!! It demonstrated how even the most experienced people can be confused by what is on a pedigree card.

Myth. More than one instance of the same boar or sow bloodline on the pedigree card equals inbreeding.

Myth busted. There are now so many branches on the family tree that boars and gilts of any given bloodline can be so far apart they are no longer ‘related’ in terms of inbreeding. This means, two Tutaki boars could be less related than a Te Whangi and a Tutaki – the devil is in the detail!

For this reason I would urge even the most experienced and savvy of us to use the online breed planning page the BKKPS now offers to its members as you might find some surprises!

How to get an objective measure of inbreeding

You can now log onto the members’ section of the BKKPS website and access a new area called ‘Plan Breeding’. This new facility has been set up so:

  • Breeders can test pairings to see if pigs are genetically compatible and that the resulting litter will not be too inbred.
  • Buyers looking for breeding stock can see if a pig they are buying is compatible with their existing pigs.

In addition, buyers can use the ‘Family Tree’ function on the website to look up a pig's pedigree and note the COI to see if what they are buying is inbred and to what degree. Once on the page you can type in the Herd Book Number (HBN) of any two pigs and it will give you three important pieces of information. These numbers are the Coefficient of Inbreeding (COI), Relationship Coefficient (RC) and the Ancestor Loss Coefficient (ALC). Towards the end of this article I will explain a little more about what these numbers are but for now let’s look at how we will use them.

What is an acceptable level of inbreeding?

The BKKPS has made an initial declaration of what is an acceptable level of inbreeding and will review this after it has been used for a period of time. This allows breeders and the BKKPS to make decisions about how inbred a piglet from a pairing will be and whether or not a litter is acceptable for birth notification. To make things easier to understand we have added a traffic light system to help. This is how it works:

Range Traffic Light Comments
0% to 3% Green Good pairing from a COI point of view. (eg. 1 common great grandparent = COI of 3%)
over 3% to under 10% Amber Not recommended – The system will suggest that you contact the Registrar for advice before pairing. (eg. 2 common great grandparents = COI of 6.25%)
10% and over Red Not recommended. The online system will automatically prevent birth notifications for offspring from this pairing (eg. Common grandparent = COI of 12.5%). Exceptions may be agreed with the Registrar, but this should be done PRIOR to pairing.

Note to breeders

The Society will continue to reject inbreeding such as Mother to son, Father to daughter and brother to sister (all have a COI of 25%). However, it is our aim to work with our breeders to support them in making the right choices for their herd and the overall breed population, so any birth notifications that are in the red range will be referred to the Registrar. We can then discuss with you why this pairing is not acceptable to the Society, check the health and standard of the parent pigs and the piglets in the resulting litter and make a decision on whether to accept the birth notification.

Above all else, our breeders MUST remember that this is only one tool in helping select the right pigs for pairing, buying or registering. The health and standard of individual pigs is still paramount. A new film will be published by the Society later in June, which is all about selecting piglets/pigs and it is hoped that this will further support you in the choices/decisions being made.

One final thought

In the wild, inbreeding occurs naturally – I have seen this quoted as a reason for it being acceptable. However, what is not revealed is the fact that, in nature, the weak do not survive and the strongest fight for their right to breed, successfully moving from one herd to another to spread their genes. We have to be mindful of this lesson from nature!


The following gives you some brief information about the three coefficients that the Society is using. This link relating to dog breeding, is helpful in understanding more about inbreeding in scientific and practical terms.

Coefficient of Inbreeding (COI)

Is a measure of inbreeding. It measures the number and location of common ancestors within a family tree. The result is a percentage: eg 25% for brother & sister and 12.5% for cousins. We use Wright’s Coefficient of inbreeding formula as defined in his 1922 paper.

Relationship Coefficient (sometimes known as Mean Kinship Coefficient)

Is a measure of how closely related an individual is to the rest of the population. A COI is calculated against all other individuals in the population, and the average of these is the mean kinship coefficient. For the purposes here ‘population’ is defined as any pig born in the 12 month period before the target pig’s birth. We expect this figure to naturally decrease as the population increases. Whilst we might mainly use COI to check we are not inbreeding, the relationship coefficient is a measure of how genetically 'unique' a pig would be within the entire herd. It’s better for the breed if we encourage diversity, so this is another factor in the decision-making process when planning breeding.

Ancestor Loss Coefficient

Is a measure of how many unique ancestors a pig has compared to the number it should have without any duplicates in the family tree.

Article by: Sam Jones, The Rushbury Herd.