Genetics and Animal Welfare - Genetics and the Behavior of Domestic Animals - page 438

has also been reduced by extensive selection for rapid growth—muscle
growth in the breast can make mating difficult. Turkeys selected for large
breasts are unable to breed naturally ( Dinnington et al., 1990 ). Another
example is Belgian Blue cattle. This breed is so heavily muscled that a high
percentage of their calves must be delivered by cesarean section ( Broom
1987 , 1993). Not only do births of such calves require surgical intervention,
their tongue muscles are sometimes too enlarged to suckle, leading to death
( Lips et al., 2001; Uystepruyst et al., 2002 ). Fortunately, some Belgian Blue
genetic lines are now available that have much easier calving. The genetics
of the double muscle trait have been extensively studied. Selection for this
trait is associated with the inactivation of the myostatin gene, coinciding
with reduced organ mass ( Fiems, 2012 ). A mutation causes the myostatin
gene to become inactive, resulting in muscle hypertrophy, and coinciding with
a susceptibility to respiratory disease, kidney stones, lameness, heat and nutri-
tional stress, and dystosia. In addition, Belgian Blue cattle require a diet high
in nutrients, due to a reduced feed intake capacity ( Fiems, 2012 ). In 2012, the
first author visited a farm in the United Kingdom where Belgian Blue sires
were bred to Devon

Limousin cows enabling the production of heavily mus-
cled calves that required no growth promotants. The huge Devon

3

Limousin
cross cows required a high-nutrient diet that worked well in the extensive, lush
pastures in the United Kingdom, but some nutrient-poor pastures in the United
States would not be able to support these cattle. High lean-meat yield is desir-
able, but it is important to find a balance between good performance in
double-muscled animals and acceptable welfare.

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Porcine Stress Syndrome and Leg Problems
In the past, some pork producers deliberately used boars positive for the PSS
(Porcine Stress Syndrome) gene. The crossbred offspring from these boars
have a higher percentage of lean meat and larger loin eyes ( Aalhus et al.,
1991 ). PSS is inherited in a classical Mendelian manner. Pigs that are either
homozygous negative or heterozygous with one PSS gene (carrier state) will
not display the symptoms of PSS when they become excited. Furthermore,
when a homozygous-positive PSS boar is bred to a sow that is free of the
PSS gene, none of the offspring will be homozygous-positive for PSS. To
maximize kilograms of lean meat, swine-breeding companies sell PSS
homozygous-positive boars for breeding to PSS homozygous-negative sows.
A welfare problem is created if a producer breeds PSS-heterozygous sows
(carrier state) to PSS-homozygous boars. Unfortunately, this practice is quite
common because a producer can breed terminal cross gilts from his own
herd instead of buying new gilts that are homozygous-negative and completely
free of the PSS gene.
The first author has observed that some of the lean hybrid pig lines devel-
oped in the 1990s often had five-times as many death losses, compared to