Genetic selection to improve bird’s health and welfare

Helping to build a sustainable future for the world’s broiler industry requires making genetic improvements in chickens related to broiler breeder production, efficiency of broiler growth, the environment and, very importantly, poultry health and wellbeing. Poultry breeding and genetics are helping make permanent production and wellbeing improvements in broiler parent performance and the performance of their broiler offspring for customers worldwide. For example, genetic improvements in the Cobb500 over the last 30 years included a 10 percent increase in egg production, a 10 percent increase in carcass yield, an 11 percent increase in breast meat percent of live weight and a .60 reduction in feed conversion ratio therefore producing broilers which require 25 percent less feed per unit gain. In addition, a number of selection traits related to broiler wellbeing and the environment including leg and skeletal health, metabolic disease, broiler livability and feed conversion have been added or expanded.
Introduction
Genetic selection has contributed greatly to the improvement of poultry over the last 50 years as described by Siegel et al. (2006). In fact, comparisons of selected commercial lines versus a non-selected Athens-Canadian random bred control line separated by over 44 years reported by Havenstein et al. (2003) showed that 85-90% of broiler growth rate, carcass and part yield changes appeared to be due to genetic selection in broiler stocks. A large variety of different breeding strategies have been used by broiler breeders to make genetic improvements in individual traits including reproduction, growth rate, feed efficiency, yield, meat quality and heat stress as well as to help reduce metabolic and skeletal disorders and these have been summarized by Thiruvenkadan et al. (2011).
Commercial results
Selections made by commercial broiler breeding companies have been very successful in improving both broiler parent and broiler performance. For example, from 1980 to 2010 U.S. broiler weights for the Cobb 500 product at 6 weeks of age increased from about 2.5 pounds (1.13kg) to 5.5 pounds (2.50 kg), meat yields improved from 64% to 74% of live weight, breast meat yields of live weight improved from 12% to 23 % while feed conversion ratios (pounds of feed per pound of gain) decreased from approximately 2.40 to 1.80. From 1995 to 2011 egg production of the Cobb 500 increased by 15 eggs or by almost one egg per year. Selection continued against birds with physical defects including for example crooked legs, toes, keels or backs, poor feathering, dark feather or shank color, short legs and poor breast conformation. Throughout the 1990’s and in the new millennium Cobb added additional selection traits related to chicken wellbeing including more sophisticated measurements of leg health (e.g. tibial dyschondroplasia (TD) and femoral head necrosis (FHN)), walking ability, cardiovascular fitness (e.g. ascites), skin condition (e.g. foot pad dermatitis) and disease resistance. These trait
measurements are now routine parts of Cobb broiler breeding selection programs and include X-ray testing for TD, FHN evaluation, gait assessment, blood oxygen measurement for ascites control and foot pad dermatitis grading. Associated with this, Cobb incidence of TD in pedigree lines decreased, field reports of ascites declined and company regional managers reported reductions in leg problems in each world region.

In addition, US field results summarized by Agri Stats (2010) documented continued progress in field broiler livability and reductions in percent condemns between 1988 and 2010 reflecting both genetic and management efforts to improve bird health. New extramural research in a variety of health related areas are being supported in hopes of further improving the innate immunity of Cobb broilers and also resistance to Salmonella, Campylobacter, Marek’s disease and Avian Influenza. In addition, genetic improvements in feed conversion are believed to be largely responsible
for many of the positive changes in measures of environmental improvement. Research
conducted by Williams et al. (2006) on the effects of genetic selection on the environment have calculated estimated improvements in broiler related global warming potential (tCO2) of 23 percent over 20 years and broilers continued to show the lowest estimated GWP100 tCO2 values of the major meat producing species in the UK with a value of 4.6 for chickens versus values of 16, 6.4 and 17 for beef, swine and sheep, respectively. In addition, broilers from 2010 when compared with broilers from 1980 were expected to show a reduction in dry manure output of 18,174 tons (16,522 tonnes) for each 52 million broilers processed per year at 5 pounds (2.27 kg) in an analysis based on personal communication with Teeter (2002).

Discoveries in molecular genetics, bioinformatics and genetic statistics are also helping to develop new, more accurate, selection tools for applied selection programs. Many commercial broiler breeding companies, including Cobb, are supporting both external and internal research regarding the effective use of SNP (single nucleotide polymorphism) gene markers and genomics as an aid to traditional selection. These involve the collection of both blood samples and phenotypic trait measurements from pedigree chickens followed by sophisticated DNA and statistical analysis to identify genetic differences related to important production, welfare and environmental traits. It is believed that accuracy of selection and therefore rates of genetic progress will be improved by combining standard phenotypic trait measurements on individual birds with genotypic estimates from these DNA analyses as discussed by Goddard et al. (2010).

Conclusions
Genetic selection for improved chicken performance has been very successful in making permanent changes in breeder and broiler performance related to production and efficiency of growth and meat production. Improvements in the feed efficiency have in turn led to expected improvements in reduction in broiler waste production and also in gasses of global warming potential. Broiler livability has been shown to have improved in the U.S. while broiler condemns have decreased concurrently. Numerous new traits have been incorporated into commercial selection programs designed to help further improve skeletal and leg health, skin integrity, cardiovascular fitness and disease resistance in broilers. Furthermore, research is being conducted to develop additional selection tools for health and welfare including selection for improved innate immunity as described by Swaggerty et al. (2009) and bacterial resistance related to human food safety as discussed by Fife et al. (2010).

Breeding for improved bird health and welfare and also for a sustainable future will require the continued addition and integration of more accurate and effective trait measurements plus new applications of molecular genetics and genomics to help solve the challenges of increasing both production and efficiency in broilers while helping to balance and improve bird health, animal wellbeing and the environment.

References
AGRI STATS. 2010. US Broiler livability and field condemn (%). Vital Signs 1998 through 2010. Unpublished subscription service.
FIFE M.S., HOWELL J.S., SALMON N., HOCKING P.M., VAN DIEMEN P.M., JONES M.A.,
STEVENS M.P. and KAISER P. 2010. Genome-wide SNP analysis identifies major QTL forSalmonella colonization in the chicken. Anim Genet 2010 June 23 [Epub ahead of print].
GODDARD M.E., HAYES B.J. and MEUWISSEN T.H. 2010. Genomic selection in livestock populations. Genetic Res. (Camb) 92:413-421.
HANKE O.A., SKINNER J.L. and FLOREA J.H. 1974. American Poultry History. American printing and publishing, Inc. Wisconsin, USA.
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THIRUVENKADAN A.K., PRABAKARAN R. and PANNEERSELVAM S. 2011. Broiler
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WILLIAMS A., AUDSLEY E. and SANDARS D.L. 2006. Determining the environmental
burdens and resource use in the production of agricultural and horticultural commodities. Main Report. Defra Research Project IS0205. Bedford: Cranfield University and Defra. URL: www.silsoe.cranfield.ac.uk, and www.defra.gov.uk. Accessed: 6-July-2011.

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