Whilst the eggshell colour is affected, breeding for a more robust laying hen has minimal negative consequences on production, the researchers concluded in the journals PLOS ONE and Poultry Science.

In 2012, housing of chicken in individual cages was banned in the EU, because of welfare issues. Nowadays, chickens are group housed on sand with space to move around freely.

However, this new system also brings new challenges to keeping poultry: in this new system chickens get more easily infected through sick flock mates, or through the dirtier environment. Preventing or treating these infections is becoming more difficult, due to stringent legislation and limitations set on the use of antibiotics.

There is an increased need for a robust laying hen: a chicken that maintains egg production and health, under varying and challenging environments. The scientists said breeding for improved general disease resistance could be that strategy to get this robust chicken.

Natural antibodies

Antibodies are proteins present in sick animals that attack the disease causing the sickness. These antibodies are only produced after the animal becomes diseased. However, natural antibodies (NAb) are antibodies in healthy animals without presence of a disease.

NAb were only discovered relatively recently in livestock animals. These antibodies prevent and combat diseases upon entry by sticking to them.

In this way NAb prevent further spread in the body, but also flag the diseases to activate other parts of the immune system. NAb are thus an essential component of the immune system.

Previous studies showed promising results: high levels of NAb were associated with survival in laying hens. Breeding for natural antibodies could be the strategy to improve general disease resistance. However, to be economically viable, selection on NAb should not lead to reduced production.

Breeding for disease resistance

To investigate the potential of breeding for increased general disease resistance, the researchers measured NAb levels of almost 3,700 purebred ISA laying chickens. They measured total NAb levels, and NAb levels for different forms of antibodies.

Heritabilities were estimated to be 0.12 for total NAb levels, and between 0.07 and 0.14 for the different NAb forms. In addition, relations between the different forms of antibodies were very high.

Tom Berghof, involved in the studies, explained: “This showed that there is genetic potential for selection on NAb levels. Also, it is possible to select for different forms of NAb simultaneously, because they seem to be mainly regulated by the same genes.”

Effect on production

Relations between immune characteristics and production characteristics have been described before in several species, and are often negative. This is in line with the resource allocation theory, which states that if an animal spends more energy on immunity, it cannot then spend it on growth.

But most production characteristics were found not to be regulated by the same genes as NAb in almost 2,400 hens studied. A small negative relation was found between egg production efficiency and one NAb form.

Mr Berghof and colleagues conclude that selection for NAb might have some negative effects on production, but these negative effects seem to be small.

Eggshell colour

Remarkably, one form of NAb was found to be associated to the mother’s eggshell colour and egg breaking strength. Additional analyses were done in order to get more insight in the origin of these maternal effects.

Mr Berghof states: “If eggs had a whiter eggshell, than NAb levels in the offspring were higher. If eggs had a stronger eggshell, than NAb levels in the offspring were lower.

"So it seems that there is some kind of relation between the eggshell and the immune system, especially for the eggshell colour.”

At the moment the researchers are investigating this relationship, since it has never been seen before.

Applications and future plans

Free-range housing systems in poultry production increase the risks of spreading infectious diseases. Breeding for natural antibodies could be a strategy to get a more robust laying hen, which could cope more easily with the increased challenges of a free range environment.

“But before this strategy can be used in commercial breeding programs, more knowledge is needed about the genes underlying NAb levels,” said Mr Berghof.

“However, we did already start selecting chickens for high or low total NAb levels on a small scale, resulting in a high and a low line. Eventually, we will test these lines under real life conditions, to evaluate whether NAb indeed lead to a higher general disease resistance with minimal negative consequences on production.”