Genetics and Animal Welfare - Genetics and the Behavior of Domestic Animals - page 431

Chapter 12
Genetics and Animal Welfare
Temple Grandin * and Mark J. Deesing †
*Department of Animal Sciences, Colorado State University, Fort Collins, Colorado, USA;
† Grandin Livestock Handling Systems, Inc., Fort Collins, Colorado, USA
INTRODUCTION
The productivity of domestic livestock and poultry has almost tripled in the
last 100 years through the use of both improved feeding methods and genetic
selection. Freeman and Lindberg (1993) state that enhancements in genetic
selection in the dairy industry have contributed more to increased milk pro-
duction than improvements in management. The National Milk Producers
Federation (1996) reports that during a 30-year period, milk production in
Holstein cows more than doubled. Since the first edition of this topic was
published in 1998, milk production has had an additional 14% increase from
1999 until 2009 ( USDA/NASS, 2010 ). Data from the United Kingdom shows
that from 1995 until 2012, milk production increased approximately 30%.
Similar gains have been made in poultry ( Gordy, 1974; Maudlin, 1995 ). Due
to genetic selection, the ability of a chicken to gain weight has increased
phenomenally. In 1923, it took 16 weeks to produce a broiler chicken.
In 1993, only 6.5 weeks were required.
Adverse effects on bird welfare are an inevitable and unavoidable conse-
quence of selective breeding for high-production traits. The ancestors of the
modern-day chicken laid about 25 eggs a year; today's laying hens produce
more than 250 eggs. After about a year they are considered “spent.” In the
meat sector, broiler chickens can convert 3 kg of food into 2 kg of meat
( Robbins and Phillips, 2011 ). To put this growth rate into perspective,
Boersma (2001) calculates; “If you grew as fast as a chicken, you'd weigh
349 pounds at age 2”. Selection for rapid growth rate has resulted in decreased
heart and lung size relative to the rest of the body ( Decuypere et al., 2007 ),
and in skeletal defects that affect walking ability ( Corr et al., 2003; Knowles
et al., 2008 ). Although many breeding companies now increasingly incorpo-
rate health and welfare goals alongside economic goals, from a welfare point
of view, growth rate is still seen as a problem ( Cooper and Wrathhall, 2010 ).
For years, both authors have been concerned that in the future, the most
serious animal welfare problems may be caused by over-selection for