Influence of hatch time on meat chicken leg strength

Production improvements in commercial meat chickens via genetic selection and improved husbandry have resulted in rapid growing birds. These birds have endemically reduced locomotor ability; a conservative estimate is that 30% of birds in a flock are likely to have poor mobility (Knowles et al, 2008). Leg weakness has a direct impact on production due to lower bird quality, which may result in culls or condemnation of carcasses. While not well understood, early rapid growth can result in various nutritional imbalances, bone undermineralisation, and associated metabolic disease, such as rickets and tibial dyschondroplasia (Angel 2007). These, in turn, predispose birds to bacterial penetration of bone growth plates and femoral head necrosis. The issue has attracted consumer and media attention and is considered an important welfare concern. Any improvement in bird leg strength is highly desirable.

Four trials were conducted in which Cobb 500 eggs were obtained from hatcheries in NSW, from a total of 5 different breeder flocks of varying ages. The basic procedure of each trial was as follows:

Eggs were set amongst six Multiquip E3 incubators and incubated at 37.8°C for 17.5 days. At that point, eggs were removed from the incubators and transferred into hatching trays which were divided into 60 individual hatching cells per tray. This allowed individual identification of chicks when they hatched. The hatcher trays were placed in a randomized fashion into one Aussieset™ incubator (Bellsouth Pty Limited, Victoria, Australia) at an initial temperature of 37.2°C and 60% relative humidity. Temperature was dropped to 36.0°C by 21 days of total incubation and relative humidity raised to 65%.

The time of hatch for each egg was observed and recorded from 468 hours of total incubation (HOI) (Embryonic Day (ED) 19.5) at 6 hour intervals, until 516 HOI (ED 21.5, designated Take off, TO). Chicks were only considered hatched once they had cleared the shell and their down was dry. In total, 2411 chicks were hatched. At 492 HOI (ED 20.5, designated Hatcher sampling, H) a selection of hatched chicks was removed from the incubator. Birds were euthanised and the right leg removed for tibia bone ash assay to determine bone mineralisation.

At TO, remaining birds were removed from the hatcher, identified, wing tagged and weighed. Chicks were selected again and sampled as described above. The remaining chicks were removed from the hatcher and allocated to small cages, at 4-5 birds per cage.

Three days from placement (designated Day 3, D3), chicks were randomly selected from the cages (approximately 2/cage). These birds were sampled in the same manner as above. In the final experiment, all hatched birds were sampled by this point in time.

Period Remaining chicks were moved into a tunnel ventilated floor pen shed, combining cages without mixing treatment groups. Individual bird weight was recorded each week. After two and four weeks from TO, all birds were placed on a standard grower and finisher diet, respectively. On day 35 (Week 5), all visibly male birds, as determined by comb and wattle development and size, were selected from pens to be subjected to a modified latency-to-lie (LTL) test, where they were made to stand in a tub with approximately 3cm of tepid water (30-33°C) and timed until they sat down, up to a maximum of 5 minutes, at which point the birds were considered censored. After completing this test, birds were identified, weighed and euthanised. They were then assessed for foot pad dermatitis (FPD) and hock burn (HB). Sex was confirmed at post mortem. Remaining (female) birds were weighed and sent for commercial slaughter on day 35.

Birds were grouped based on time of hatch – birds hatching before 486 HOI were classified as Early, between 486 and 498 HOI as Mid, and after 498 HOI as Late. Feed intake and FCR could not be calculated as birds were not necessarily grouped per the current hatch group classification scheme in each trial. Birds standing for 5 minutes in the LTL test, the incidence of FPD and HB, and bird weight on D35 were compared via ANOVA with hatch group as the treatment factor. LTL times were compared using Cox’s F-Test. Bone ash results of Early and Mid birds were treated as one group and compared using ANOVA with sample time as the treatment. Late hatching bird bone ash results were subjected to a Student’s T-test comparing TO and D3 results.

Chick hatch times are shown in Figure 1. Approximately 80% of birds hatched before 498 HOI.

Survival curves for LTL time based on hatch groups are shown in Figure 2. Late and Mid hatching birds stood for significantly longer times than Early hatching birds (p = 0.004, 0.03). Late birds tended to stand for a longer time than Mid hatching birds (p = 0.055).

Seven percent of Early hatching birds stood for 5 minutes in the LTL test, compared to 12 percent of Mid and 19 percent of Late hatching birds (p = 0.026). Early hatching birds had higher incidences of FPD and HB, with Mid intermediate and Late the lowest (p < 0.0001). At time of LTL (5 weeks), Early hatching birds were the lightest, weighing 2480g compared to 2600g for Mid and 2654g for Late (p < 0.001).

For Early and Mid hatching birds, bone mineralisation decreased from 25.8% at H to 24.7% at TO, then rose to 32.4% at D3. (p < 0.001). Late hatching birds went from 25.0% bone mineralisation at TO to 32.9% at D3 (p < 0.001).

Across trials, later hatching birds performed better in the LTL test, being most likely to remain standing for 5 minutes, and had fewer incidents of FPD and HB. This is a strong indication that leg strength and mobility was higher in these birds (Groves and Muir, 2016). Interestingly, they were heavier than Early hatching birds. This is important as it shows the impact leg strength has on production values. Lower weight in conjunction with less standing ability may indicate Early hatching birds are less able or inclined to stand and eat. This effect is not as prominent in the Mid hatching birds, however it is justifiable to conclude that hatching at a later time has had a positive effect on bird leg strength.

The advantage of a later hatching time may be due to Late hatching birds spending under 24 hours without feed compared to 24-48 hours without feed for Early and Mid hatching birds. It is possible to delay chick hatching time by reducing initial incubation temperature, which improves bone mineralisation at TO, as well as hatchability (Muir and Groves 2017). If fed between H and TO, bone mineralisation in Early and Mid hatching chicks can increase during this period (Muir and Groves 2017), rather than decrease as seen in unfed birds. Twenty-four hours without feed after hatching also impairs muscle growth (Powell et al 2016).

Strategies to provide early feed access include in-ovo feeding, provision of a prestarter ration inside the hatcher incubator, feeding during transportation, and hatching/brooding systems such as Patio, Hatchbrood and Hatchcare. Identifying hatch time as a factor relating to leg weakness is a key step in improving meat chicken production. Implementing early feed access across the industry is an important reform that will greatly benefit poultry producers, consumers and bird welfare.

ACKNOWLEDGEMENTS: Agrifutures Australia provided project and scholarship funding, and Poultry Hub provided scholarship funding. The PRF team, Sue Ball of Zootechny and the undergraduate students who gave their assistance were invaluable to the project.