Inbreeding within the national pedigree Texel flock
I
nbreeding is something which concerns some breeders more than others, but which, arguably, all breeders should be paying attention to at some level.
As Society technical and data services manager Ed Smith explains: “Inbreeding is essentially the mating of related individuals or of individuals with shared ancestors. “And while linebreeding can be used to establish desirable traits within a family or flock, it is a form of inbreeding and the offspring should be carefully monitored for any undesirable characteristics. “A balance needs to be maintained between selecting animals to optimise rates of genetic gain and minimise levels of inbreeding,” he says. While some level of inbreeding cannot be avoided for most pedigree breeds and inbreeding risk should not be the only selection consideration, reducing the inbreeding load in a population is a valuable goal, says Dr Smith. An individual animal’s level of inbreeding is reported as a percentage and presented in iTexel as the Inbreeding coefficient (or F) value. A minimum of five generations are required to give confidence in the calculated inbreeding coefficients. “This is one of the benefits of the pedigree flock book and iTexel registry service provided by the Society and another reason to maintain accurate records.
Understanding the risks associated with inbreeding and knowing how breeding decisions will impact future generations is being made easier with the Society’s new inbreeding decision calculator.
“Increasing levels of inbreeding can result in inbreeding depression, where performance can suffer. Studies in other sheep breeds have demonstrated that as inbreeding increases ewe breeding weights and fertility are reduced and lamb birth and weaning weights are lower. “It can be argued that any animal is only one mating away from being outbred and there are examples of this in iTexel from well-managed outbred matings, such as the ram lamb below from the Incheoch flock (Fig. 1). The lamb’s sire and dam both had F values higher than that determined in the lamb itself. Indeed, two unrelated animals, each with a high F value could be mated to each other to produce outbred offspring with a low inbreeding coefficient,” adds Dr Smith.
“In general, the lower the level of inbreeding the better, though a threshold value of 7% has been determined to be an acceptable level of inbreeding within a breeding programme,” he explains. The percentage estimate of inbreeding is based on the relatedness of individuals in a pedigree and examples of percentage inbreeding values for different matings are presented in Table 1. Table 1. Expected level of inbreeding from example matings Relationship Father x daughter or mother x son Brother x sister Grandfather x granddaughter or grandmother x grandson Half-brother x half-sister Uncle x niece or aunt x nephew Great-grandfather x great-granddaughter or great-grandmother x great-grandson Half-uncle x niece or half-aunt x nephew First cousins
Inbreeding coefficient (F) 25.0 % 25.0 % 12.5 %
Fig.1. Example of an outbred lamb birth-notified in 2020. Analysis of the average level of inbreeding in birth-notified Texels over the past ten years (Fig. 2) indicates that at the breed level, the level of inbreeding is increasing and was just below 5.5%, on average, for 2020-born animals.
6.25 %
“The overall rate of increase in inbreeding has remained unchanged over the past 10 years. For individual flocks that birth-notified five or more lambs, average flock-level inbreeding coefficients ranged from zero to more than 18%.
6.25 % 6.25 %
“Over the past 10 years the number of flocks that have an average inbreeding coefficient over 7% has risen from 59 (4.2%
12.5 % 12.5 %
Technical Corner
34 Spring 2021
texel.uk
