Does maternal flock age and pre-starter diet composition alter leg strength in Cobb 500 chicks?

While the breeding programs of commercial meat chickens select birds with robust legs (Davis, 2015), poor leg strength and reduced locomotion are frequently observed in late stages of growth. We have previously reported that lower egg shell temperatures during early incubation, which delays chick hatch time, increases bone ash (BA) (Muir and Groves, 2017). Observations from our research and the literature (e.g. Ulmer-Franco et al., 2010) suggest that maternal breeder flock age may influence the amount of some nutrients and minerals within the yolk, which could impact chick bone strength. Additionally, we have proposed that pre-starter diets with varying levels of Ca and P may improve the leg strength of commercial meat chickens (Muir and Groves, 2017). The current experiment evaluated the effect of maternal breeder flock age and pre-starter ration Ca and P levels on chick BA, bodyweight, serum Ca and P levels and 5-week latency to lie (LTL) in Cobb 500 chickens.

Fertile eggs were sourced from a 34 and a 50-week-old Cobb breeder flock and incubated at 37.80 C. At 18 days incubation each egg was transferred into an individual hatching cell in a hatching tray. Chick hatch was observed every 6 hours from 19.5 until 21.5 days of incubation, and was used as a covariate in analyses. At take-off chicks from each breeder flock were randomly allocated to one of three pre-starter diets – standard, low Ca and high P, which they were fed for three days. From 4 days of age all birds were fed a standard ration for the remainder of the 35 day experiment. Bone ash was measured at take-off and day 3. Serum Ca and P was determined at day 3 and, at 5 weeks of age all visibly male birds were assessed for their ability to stand in a LTL test. Bird weights were also measured.

Maternal flock age did not significantly alter BA, day 3 serum Ca and P nor LTL in 5 week old male birds. Chicks arising from the older breeder flock were significantly heavier when 1 and 3 days old (P< 0.05). At 3 days of age chicks fed the high P diet had significantly higher BA than the standard or low Ca fed chicks. Concurrently, serum Ca was significantly lower in chicks fed the low Ca and high P diets, while serum P was lowest in chicks fed the standard ration and, highest in chickens fed high P diet (P< 0.05). No significant effect of diet was observed on 5 week LTL, but, birds fed the high P ration gave the longest standing times. Hatch time did not affect BA at take-off, but, at day 3 BA was significantly higher in later hatching (> 20.75 days) chicks. Similarl,y late hatch birds had a longer LTL time (P=0.108) than earlier hatching birds.

Maternal flock age had no significant effect on the leg strength of their offspring, but, older breeder hens generated heavier chicks at hatch. While the high P diet favoured improved leg strength, chick hatch time had the greatest effects on leg strength, as we have previously reported (Groves and Muir, 2014; Muir and Groves, 2017). A later hatching time increases bone ash in young chicks and, generates longer standing times in 5 week old birds.

ACKNOWLEDGEMENT: This research was funded by AgriFutures Australia.