Genetics and Animal Welfare - Genetics and the Behavior of Domestic Animals - page 439

older genetic lines of pigs with more back fat. In the 1990s, truck drivers
transporting pigs often reported one or two dead pigs on a truckload of
hybrid market animals. These deaths were likely to occur even when pigs
were transported under good conditions. Older genetic lines often had no
dead pigs on a truck. Many of the truckloads with high death losses con-
tained pigs sired by PSS-positive boars. Murray and Johnson (1998) reported
higher death losses in market pigs with the halothane PSS gene. In the
2000s, the use of PSS genetics declined and death losses are now lower.
Recently, USDA figures show that dead pigs arriving at the slaughter plants
dropped from 0.29% in 2000 to 0.17% in 2010.
Although the PSS problem had been reduced and the U.S. Pork Industry
has now almost eliminated PSS genetics, other problems associated with a
demand for lean meat and larger loin eyes are occurring. Beta agonist feed
additives (ractopamine) are now used to enhance the growth of lean muscle.
High doses of ractopamine can cause weakness, downed pigs, hoof cracking,
and increased aggression ( Grandin, 2010; Marchant-Forde et al., 2003;
Poletto et al., 2011 ). Feeding the beta agonist ractopamine can suppress the
expression of serotonergic genes in the brains of the pigs ( Poletto et al.,
2011 ). Problems corrected by eliminating PSS genetics may now be repeated
by using drugs.
As the U.S. industry was phasing out PSS genetics, new problems with
poor leg conformation and lameness started to occur. Single-minded selec-
tion for large loin eye size, thin back fat, and rapid growth resulted in a
neglect of selecting breeding stock with good leg conformation. In the early
2000s, the first author observed approximately 50% lame market-weight pigs
in some herds. Today, breeders have added standards for structural correct-
ness of feet and legs, which has reduced lameness.
High death loss in ultralean hybrids was a more serious problem in the
United States than in Europe. Death losses in Europe were much lower for the
same hybrid pig lines from the same commercial breeding company. The rea-
son for this may be that European pigs are fed a limited grain ration and are
grown more slowly. Slower growth results in older market-weight pigs that
have a more mature skeleton. In the United States, market pigs are fed ad lib.
grain and attain market weight at an earlier age. Another factor influencing
increased death losses and non-ambulatory pigs in the United States is higher
slaughter weights compared to pigs in Europe. The first author has observed
that heavier animals are more likely to collapse during physical exertion.
PROBLEMS CAUSED BY GENETIC SELECTION FOR APPETITE
Animals genetically selected for rapid weight gain or production of large
quantities of milk, eggs, or meat require a tremendous appetite and food
intake in order to produce large amounts of these products. Selection for
rapid muscle growth correlates highly with selection for increased appetite.