Because of the increasing demand for raw cuts and processed products, there is a trend to producing very heavy broilers, according to Dr Elisabeth Baéza of INRA in Nouzilly, France and co-authors there and at ITAVI in Nouzilly, Fleury Michon and INRA in Surgères. In the introduction to their paper in Journal of Animal Science, they explain that breeds that are used for such kinds of production have been intensively selected for growth rate and breast meat yield, and birds are reared for a longer period than standard broilers.

Their study aimed to evaluate the effects of increasing slaughter age on technical and economic factors, including production efficiency and environmental costs, bird welfare and breast meat quality in a modern heavy broiler line.

Five groups of 300 male Ross 708 chickens were reared until slaughter ages of 35, 42, 49, 56 or 63 days.

Increasing age at slaughter from 35 to 63 days resulted in a 7.4-fold increase (P<0.01) in mortality rate (5.21 versus 0.70 per cent). It also increased (P<0.001) the slaughter weight and average daily feed intake of birds 2.5– and 1.4–fold, respectively, without affecting their gain:feed ratio.

Under these experimental conditions, economic profit evaluated through the net gain reached a maximum at 42 days.

The moisture and ammonium content of litter increased (P<0.05 and P<0.01, respectively) rapidly during rearing concomitantly with increased (P<0.05) occurrence and severity of contact dermatitis and decreased (P<0.05) walking ability and activity of birds.

Thermal comfort also decreased (P<0.05) greatly as early as 42 days of age.

Changes in carcass quality occurred mainly between 35 and 56 days of age, with a progressive increase (P<0.001) in breast and leg yield, whereas body fatness was barely affected by age.

Major changes in breast meat traits were observed between 35 and 49 days of age, with an increase in muscle pH at 15 minutes (P<0.01) and 24 hours (P<0.001) postmortem and reduced (P<0.001) lightness and drip loss. The protein and lipid content of raw breast meat also increased (P<0.05 and P<0.01, respectively) with age.

Taking into account the main aspects of sustainability, Baéza and co-authors recommend slaughtering chickens of a heavy line at 42 days of age.

Reference

Baéza E., C. Arnould, M. Jlali, P. Chartrin, V. Gigaud, F. Mercerand, C. Durand, K. Méteau, E. Le Bihan-Duval and C. Berri. 2012. Influence of increasing slaughter age of chickens on meat quality, welfare, and technical and economic results. J. Anim. Sci., 90(6):2003-2013. doi: 10.2527/jas.2011-4192

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