Level of Inbreeding in Yorkshire, Landrace, and Hampshire Breeds
D. L. Lofgren and T. S. Stewart
Department of Animal Sciences
Introduction
Inbreeding is a measure of the common ancestry of the parents of 
an animal, and ranges from 0 to 1 (or 100%).  If a sire and dam are 
related (have genes in common through common ancestors), then 
their progeny are inbred.  The level of inbreeding depends on how 
strongly related the parents are.  Inbred animals are more 
homozygous than noninbred animals; that is, they have the same 
alleles (form of the gene) on both chromosomes for a locus (gene 
location).  High levels of inbreeding generally will result in 
decreased performance, particularly for reproductive traits.  The 
occurrence of these problems tends to become apparent at levels of 
inbreeding above 15%, and becomes progressively more severe as 
inbreeding increases.  Breeders who are trying to improve their 
herds with performance testing and Expected Progeny Deviations 
(EPD) often find that the best animals are related to one another.  
As these superior animals are mated to each other, average 
inbreeding in the herds will tend to increase, and it eventually may 
become difficult for the breeders to avoid high levels of inbreeding.
This study examined the amount of inbreeding, and the change in 
inbreeding over time, in the Yorkshire, Landrace, and Hampshire 
breeds.
Data
Pedigree information in the analysis was for animals that had been 
performance tested, with records submitted to the STAGES (Swine 
Testing And Genetic Evaluation System) program by late 1994.  
Animals included sows with STAGES litter records, their ancestors 
(parents, grandparents, and great-grandparents), and ancestors of 
pigs with STAGES growth records.  Inbreeding was calculated for 
all animals in the data set.  This was summarized overall, and by the 
year of birth of the animal.


Results
Table 1 contains the means and ranges for inbreeding in the three 
breeds.  Most animals were not inbred.  As a result, the means were 
low, ranging from 2.00% for Hampshires to 3.13% for Landrace.  
However, some animals were highly inbred.  The maximum 
inbreeding of an animal was at least 44% for all breeds.  This level 
of inbreeding occurred with several generations of full-sib matings, 
or several generations of parent-offspring matings, and was most 
likely the result of deliberate management decisions to produce 
inbred offspring.
The change in inbreeding over time for Yorkshires is in Figure 1.  
Inbreeding has increased, reaching a peak of 3.7% in 1991, and 
then decreasing to 3.0% in 1993.  The decreasing trend is good.  
However, it will be a challenge for Yorkshire breeders to keep 
inbreeding this low in the future.  A few sires with high EPDs have 
had a large influence in the breed.  They have sired a large number 
of litters themselves, and now have sons and grandsons producing 
litters.  Some breeders may find it hard to avoid inbreeding while 
maintaining selection pressure in their herds.
The trend for Landrace is in Figure 2.  Inbreeding has continued to 
increase over time, to a maximum of 5.4% in 1993.  Although not 
very high, this is still higher than the other breeds, and continues to 
increase.  The Landrace breed has a relatively small number of 
animals, which makes it more difficult to avoid inbreeding.  There 
may be problems with inbreeding in the future for Landrace, unless 
breeders are careful.
The change in inbreeding for Hampshires is in Figure 3.  Inbreeding 
has increased over time, reaching a peak of 3.0% in 1991, and then 
decreasing to 2.3% in 1993.  This decrease is good, and 
Hampshires have the lowest inbreeding of the three breeds.  
Recently, Hampshire breeders have been able to identify the 
superior sires in the breed.  If these become heavily used, there may 
be more inbreeding in the future.  However, for now, Hampshire 
breeders are doing a good job of avoiding inbreeding.


Implications
Inbreeding levels are currently low for all breeds, and decreasing for 
Yorkshires and Hampshires.  However, breeders need to be aware 
of potential problems with inbreeding.  They should continue to 
avoid mating close relatives, so reproductive performance will not 
be affected.  Breeders should search for unrelated sources of 
superior genetics to use in their herd improvement programs.
Table 1.  Average inbreeding % for the three breeds.	
	Breed	Number	Mean	Minimum	Maximum
Yorkshire	100,940	2.76	0	50.00
Landrace	17,509	3.13	0	46.09
Hampshire	33,069	2.00	0	44.53


 
Figure 1.  Average inbreeding by year of birth for Yorkshires.


 
Figure 2.  Average inbreeding by year of birth for Landrace.


 
Figure 3.  Average inbreeding by year of birth for Hampshires.