Keywords: chicken, residual feed intake, resource allocation, immune response, Salmonella
Enteritidis, energy partitioning.
The continuous selection in farm animals for efficient production and high production levels may have led to animals that are "programmed" to put a lot of resources in production processes, at the expense of resources for maintenance processes, among which the immune system. When efficiently and non-efficiently producing animals in a population are discriminated, it is hypothesized that non-efficient animals are better able to reallocate resources from production processes to maintenance processes than efficient animals. Non-efficient animals may, thus, be better off than efficient animals when maintenance processes are under pressure.
Residual feed intake is used as a trait to discriminate efficient and non-efficient animals. It is defined as the difference between observed feed intake and expected feed intake; in this thesis, expected feed intake is based on metabolic body weight and growth. Animals that eat more than expected have a high residual feed intake and are considered non-efficient, whereas animals that eat less than expected have a low residual feed intake and are considered efficient.
The research described in this thesis was carried out with pullets. Pullets are young, growing female chickens that do not produce eggs yet. Pullets were rated from high to low residual feed intake as a phenotypic trait; animals with the highest and lowest values for residual feed intake were selected for further research. Immune responses to non-replicating antigens and to live Salmonella
Enteritidis bacteria were investigated.
The results showed that non-efficient pullets had a higher feed intake than efficient pullets, but body weight and growth were equal in efficient and non-efficient pullets. Energy partitioning trials showed that non-efficient pullets spent more energy on maintenance processes than efficient pullets. However, an infection with Salmonella
Enteritidis did not lead to repartitioning of energy from production processes to maintenance processes. It was concluded that a Salmonella
Enteritidis infection is not energetically costly. Efficient pullets had a lower immune status than non-efficient pullets in situations where the animals were not infected with Salmonella
Enteritidis, whereas during a Salmonella
Enteritidis infection the efficient pullets had higher immune responses than non-efficient pullets. It is suggested that efficient and non-efficient pullets, as measured by residual feed intake, may have different "immune coping styles".