Leg health is an important component of broiler chicken welfare and the economics of broiler production.

This study presents the development of leg health in 3 purebred commercial broiler lines during 25 yr of selection [by the Aviagen UK breeding program] and investigates the genetic background of leg health traits in current populations of these lines. The leg health traits were deformities of the long bones (LD) and crooked toes (CT), recorded since 1985, and tibial dyschondroplasia (TD) and hock burn (HB), recorded since 1990.

The prevalence of CT and HB decreased mainly in the first decade (range among lines −1.2 to −2.3% and −1.3 to −1.5% per year, respectively), after which it stabilized at low levels. The prevalence of LD and TD decreased by −0.6 to −0.9% and −0.4 to −1.2% per year, respectively…1990^–₁₆2006_18ya, TD steadily decreased by −0.6 to −1.8% per year. Its prevalence increased after 2007, linked with the change to a more accurate device, but declined rapidly from 2008 onward in all 3 lines at a rate of −1.0 to −5.2% per year. Similar to CT, HB showed a large decrease, at a rate of −1.3 to −1.5% per year, in the first 10 yr after initiating recording of this trait, after which its prevalence stabilized at very low levels.

Genetic parameters were estimated using data from 4 recent generations. The BW ranged 2.0–2.4 kg at 5 wk of age; the prevalences of LD, CT, TD, and HB 8.6–12.9%, 0.6 to 2.6%, 4.6 to 8.0%, and 4.0 to 12.2%, respectively. Estimates of heritability were h² 0.04 to 0.07 for LD, 0.01 to 0.10 for CT, 0.10 to 0.27 for TD, and 0.06 to 0.09 for HB (all SE ≤0.01). Estimates of the genetic correlations between LD and CT were 0.11 to 0.43 (all SE ≤0.09), between these traits and HB were negligible, and of TD with LD, CT, and HB were −0.26 to 0.16 (all SE ≤0.11). Estimates of genetic correlations between the leg health traits and BW were lowly to moderately unfavorable, ranging from r_g = 0.09 to 0.37 (all SE ≤0.06).

…Considerable decreases in the prevalence of leg disorders have been achieved by a strong focus on accurately scoring selection candidates and a stringent culling policy of discarding any selection candidate with clinical leg defects. In addition, predicted breeding values for candidates with nonclinical leg defects allowed the identification of families that were prone to develop leg issues. Although the heritabilities of all 4 leg health traits were low and their genetic correlations with BW unfavorable but low to moderate, breeding strategies for simultaneous selection for live performance and leg health have been, and continue to be, effective…The differences between the lines suggest that strategies for simultaneous improvement of all traits tailored for each line individually have been effective. This research demonstrates the long-term effectiveness of selection for improving leg health in broilers and highlights that, despite somewhat unfavorable genetic correlations with BW, these traits can be improved simultaneously in a balanced breeding program.

[Keywords: heritability, leg defect, genetic correlation, welfare, long-term trend]

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