Otherindependent cross-checkabstract only checked I-INTA-11 in-depth review

Why do modern chickens grow so fast?

Growth, efficiency, and yield of commercial broilers from 1957, 1978, and 2005.

Overview

Zuidhof MJ et al.. Poultry science 2014 · PMID 25260522 · DOI 10.3382/ps.2014-04291

Modern broilers grew over four times faster than 1957 birds on the same feed, showing the role of breeding.

Does the study support the specific claim? Partly · Methodological quality: moderate

Study explained simply

What did the researchers want to find out?: Broiler genetics and growth
How was the study done?: Strain comparison on identical diet
How many people, animals or samples were studied?: 180 chicks per strain
What was actually found?: Growth +400% from 1957 to 2005
How large was the observed effect?: FCR -50%
What do the results mean in everyday life?: Fast growth is mostly bred into the birds
What are the study's limitations?: One experiment
What does it say about Elawady's specific claim?: Qualifies 'antibiotics make chickens grow' (C-RES-05)
Do the results apply to Germany and Europe?: High
What can explicitly not be concluded from this study?: Whether antibiotics are still used in Egypt

Effect of commercial genetic selection on growth, efficiency and yield of broilers

Study design: Other – Controlled comparison of broiler strains (1957, 1978, 2005) raised on the same current nutrition
Population: Broiler chicks: University of Alberta control strains unselected since 1957 and 1978, and Ross 308 (2005)
Country: Canada
Period: Grow-out to 56 days
Sample size: 180 mixed-sex chicks per strain
Inclusion and exclusion criteria: Not applicable

Exposure / intervention: Genetic strain
Comparator: Older unselected strains
Endpoints: Body weight, feed intake, feed conversion, carcass yield
Statistical methods: Nonlinear mixed Gompertz growth model; nonlinear ANCOVA

Results: From 1957 to 2005, broiler growth increased by over 400% with a 50% reduction in feed conversion ratio (compound annual increase in 42-day live weight of 3.30%). Pectoralis major yield increased by 79% in males and 85% in females.
Absolute risks: Not applicable
Authors' conclusion: Almost 50 years of quantitative genetic selection achieved the intended changes; musculoskeletal and immune challenges may need attention.

Large effect sizes attributable to genetics under identical feeding.

Shows modern broiler growth is mainly due to genetic selection, not antibiotics.

Antibiotic growth promoters were historically used (I-RES-04, A-ABX-03).

High (same commercial genetics worldwide).

Strong evidence for a genetic basis of fast broiler growth.

Multidimensional assessment
Multidimensional assessment
Authenticity of the source	verified
Methodological quality	moderate
Risk of bias	not assessable
Precision of results	high
Directness	indirect
Reproducibility	unknown
Consistency with the overall evidence	consistent
Regional transferability	high
Adequacy of the interpretation (by the archive)	not applicable

Contribution of feed additives in Egypt.