Page 81 - The Welfare of Cattle

58 the WeLfare of CattLe

Cow somatic cell score Sire somatic cell score
0.2
0.15
0.1
0.05 0
Somatic cell score –0.05

–0.1
–0.15
–0.2
–0.25
–0.3
1984 1988 1992 1996 2000 2004 2008 2012 2015
Holstein or red & white birth year

Figure 7.4 trend in somatic cell score for holstein or red & White. Calculated december 2016.

how the genetic trend in somatic cell score, an indicator of mastitis (inflammation of the mammary
gland), decreased after inclusion of this trait in the dairy selection index in 1994.
Likewise, because the genetic correlation between body weight and incidence of leg disorders in
broiler chickens is positive, appropriate multi-trait selection indexes have been developed to permit
genetic improvement in leg health concurrently with continued, though more modest, improvement
in growth rate. Cobb-Vantress Inc., a large chicken breeding company based in Arkansas, records
56 individual observations on each pedigree selection candidate in their broiler chicken breeding
program. More than 50% of these 56 individual traits are some measure of health and fitness of
an individual. This underscores the importance of combined selection for many traits, including
robustness, specific and general disease resistance, absence of feet and leg problems, and metabolic
defects in the breeding objectives. In recent years, genetic selection has had a major impact on
decreasing the incidence of skeletal disorders in broiler chickens.
Some breeders have also incorporated behavioral traits into their selection criteria. It may be
considered unethical to select for behaviors that better suit an animal to an agricultural production
environment, with some advocating that the production environment should be modified to suit the
animal. However, it should be recognized that livestock populations have been selected for behav-
ioral traits since their domestication. Consider the behavior of herding dogs. While altering the
environment such as moving to pasture-based systems might be appropriate in some cases, such a
change needs to be considered in the context of undesired negative impacts (e.g., increased exposure
to pathogens, predators, and weather) on other components of sustainability. It is conceivable that
selection to better suit a population of animals to their production environment could improve both
animal welfare and productivity, thereby working toward multiple sustainability goals.
An example from the poultry industry illustrates this point well. A 1996 selection experiment
was performed on a line of White Leghorns to improve adaptability and well-being of layers in
large multiple-bird cages. Feather and vent pecking, and sometimes cannibalism can occur in
multi-pen cages, a problem that can be managed with beak trimming young birds. Using a selec-
tion approach termed “group selection,” offspring from select roosters were housed as a group in
multiple-bird cages, and the group was either selected or rejected based on the productivity of the
group. An unselected control, with approximately the same number of breeders as the selected