Genetics and Animal Welfare - Genetics and the Behavior of Domestic Animals - page 451

WELFARE AND GENETIC SELECTION
At what point has genetic selection for a certain trait gone too far? Changing
an animal's diet or housing conditions may provide decent welfare under
some conditions, but not others. There may be a point at which the animal is
so defective that it will suffer even when it is fed and housed in an ideal
environment. If genetic selection for milk production goes too far, it is likely
that Holstein cows may develop serious health problems which cannot be
compensated for by improving their nutrition or housing. The excessive
metabolic drain on dairy cows, and the amount of work required during peak
lactation is so great that a human doing a comparable amount of work would
have to exercise vigorously for 6 hours every day ( USDA, 2008 ).
Animals can be altered by genetic selection to such an extent that serious
structural or neurological defects develop that can cause great discomfort.
For example, animals with bowed legs or other structural abnormalities may
have difficulty walking. Both authors agree that animal welfare is not
acceptable if high-producing meat animals are chronically lame due to weak
legs. It is also our opinion that an adequate level of animal welfare is not
possible if selected traits becomes so extreme that obvious mobility problems
occur, or if the condition is known to be painful in humans. A good example
of this is arthritis.
One may argue that measuring how an animal feels is very difficult.
However, the nervous system of pigs and cattle has the same basic design as
the human nervous system. Careful studies of different mammals reveal that
surgical procedures do cause pain in animals ( Molony et al., 1995; Short and
Poznak, 1992 ). Noxious stimuli that cause pain in people are also likely to
cause pain in animals ( Molony et al., 1995; Short and Poznak, 1992 ). Both
chickens and rats self-medicate to relieve pain by eating or drinking a bitter-
tasting painkiller ( Colpaert et al., 2001; Danbury et al., 2000 ). When a lesion
in their joint heals, they stop ingesting the painkiller and switch to plain feed
or water. This indicates that animals definitely feel pain ( Danbury et al.,
2000 ).
Both authors suggest guidelines for determining the ethical limit for genetic
selection. Animals should have freedom from pain due to structural abnormali-
ties. Over-developed meat-producing muscle that causes fatigue, or an animal
becoming non-ambulatory during handling is not acceptable. Welfare is also
seriously compromised in females unable to give birth naturally and forced to
endure severe dystocias. There should also be low levels of behavior detrimen-
tal to welfare, such as hyperexcitability during handling or high levels of
aggression. Some examples include heart attacks in pigs homozygous for PSS,
hyperexcitable pigs that constantly back up in races and bunch and pile up at
the slaughter plant, and nervous pointers that have difficulty adapting to normal
dog environments. Another example is an inbred strain of mice that does not
habituate to repeated exposure to a novel environment ( Boleij et al., 2012 ).