Leg problems are still one of the most prevalent causes of late mortality for heavy broilers and turkeys. Leg issues and bone weakness have a major impact on welfare, food safety, and production costs.

Developmental disorders of long bones can be affected by genetics, nutrition, infectious diseases, and environmental stressors, among other factors. Ossification of bones in poultry begins during the embryonic period. Consequently, stressful conditions within the incubator machine may affect bone development.

We have evaluated the effects of eggshell properties, especially eggshell conductance, pre-incubation, early and late incubation temperatures, and oxygen concentrations in a series of experiments that examined chicks and poults at hatching as well as in 41- or 56-day-old broilers.

Research has demonstrated that the incubation temperature during the plateau stage of oxygen uptake, which occurs during the last four to five days of incubation, affects yolk utilization, intestinal maturation, thyroid metabolism, heart and muscle development. Incubation temperatures can also affect bone development because thyroid hormones have a critical role in bone growth, especially on growth-plate chondrocyte differentiation. If the lipids, trace minerals and vitamins are not absorbed from the yolk, early bone modeling and remodeling processes can be impaired. Furthermore, pre-hatch stress impairs embryonic development regulation and increases relative asymmetry between limbs and other bilateral traits in chickens, i.e., the individual bones of the two legs are not the same length. As a result, alterations in metabolism, nutrient utilization and development caused by less-than-optimal incubation conditions and hatcher conditions may affect musculoskeletal development and post-hatch gait patterns.

Embryos from different genetic backgrounds endure changes in incubation conditions in different ways. One of the causes of this difference is the eggshell conductance that is characteristic of each breed.

The eggshell conductance determines the timing of the plateau stage in oxygen consumption by limiting oxygen diffusion to the growing embryo. The bone development of embryos from genetic strains with low eggshell conductance can be affected more during incubation.

The results of our research related to temperature and oxygen conditions during the plateau stage of incubation show that high incubator temperatures and low oxygen concentrations can affect bone development by altering bone weight, length, thickness, and relative asymmetry in both chickens and turkeys. Temperatures greater than 101°F and oxygen concentrations of 19 percent or less should be avoided to ensure optimal bone development at hatching. High temperatures depress the expression of collagen Type X and Transforming Growth Factor Beta, which are two important proteins involved in bone ossification. The failure to produce these proteins has been closely related to tibia dyschondroplasia, a common bone developmental disorder in fast-growing poultry. Researchers in Israel and Turkey have observed similar results in tibia dyschondroplasia in broilers when evaluating suboptimal incubation conditions during the first eight days of incubation.

Eggs from 35-week-old broiler breeders were pre-incubated in either non-uniform conditions at 75°F without forced air movement for nine hours or in uniform conditions at 80.1°F with forced air movement for 11 hours. The eggs were then incubated in a similar manner and exposed to elevated temperatures in the hatchers. Results indicate that the treatments did not affect chick body weight or yolk utilization at hatching. Uniform pre-incubation reduced the incidence of twisted legs at 40 days of age.

Our previous data and recent research publications indicate that the effects of incubation can be due to a combination of temperature stress and oxygen conditions during critical periods of embryonic development such as the first three days and the last four. Consequently, another experiment was conducted to evaluate the effects of low temperature (98.6°F) during early and hot temperature (102.2°F) during late incubation, and transportation conditions from hatchery to farm on long-bone development and leg health of broilers. In this experiment, eggs were incubated to achieve a low eggshell temperature or a standard eggshell temperature (99.5°F) during the first week of incubation. During the third week of incubation, the eggs were incubated to achieve the standard eggshell temperature or hot temperature. All eggs were incubated at standard temperature days eight to 17. Hatched chicks were separated into two transportation groups, control and stressed or "suboptimal."

Incubation conditions did not affect chick body weight, or yolk utilization, but hot temperature during late incubation reduced the relative weight of femurs and shanks. Weight of tibias was reduced by the combination of low early temperature and hot late temperature. The relative asymmetry of tibia and shank weights was increased by early low temperature. Shank length was increased by hot late temperature. At 41 days, males had more leg problems. Late hot temperature and "suboptimal" transportation conditions increased incidence of crooked toes and percentage of chickens with gait scores. Walking ability was divided into six categories of gait scores ranging from normal (score zero) to immobile (score 5).

We also conducted six commercial trials with an integrator to evaluate the effects of two incubation profiles on leg health of broilers at eight weeks of age. Eggs from the same breeder flocks were incubated in either single-stage or multi-stage machines. Hatchlings were placed in paired houses on the same farms. At 56 days of age, 200 chickens per house in each of the six farms, were sexed, weighed, and legs inspected for disorders. Gait scores were then assigned. Broilers hatched in single-stage incubation machines were an ounce heavier than those hatched in multi-stage machines.

A higher percentage of birds with crooked toes (0.8 versus 0.1 percent), and gait scores 1, 2 and 3 were observed in broilers hatched in multi-stage machines. Broilers hatched in single-stage machines had higher percentage of birds with gait score zero (63.4 versus 53.2 percent). No significant interactions between incubation profile and farm conditions or sex were observed, indicating that the effect of incubation was similar for both genders and across farms.

The results indicate adequate warm pre-incubation with good air flow are critical for proper bone development. Hot temperature and hypoxia during late incubation reduce bone development. Single-stage incubation can reduce the incidence of leg problems and improve gait scores under commercial conditions.