This new fact has emerged from laboratory and field trials involving several hundred thousand birds on farms throughout the UK, carried out by Lohmann Animal Health in conjunction with poultry vets.

“When using a ‘hot’ vaccine, conventional broilers in the UK are normally vaccinated at around 14-16 days when their maternal immunity is waning,” said Adam Goddard, Lohmann’s UK sales consult. “However, we have found this maternal immunity lasts longer in the slow growing breeds that are reared both indoors and out.”

Realistically, the correct day of vaccination for free-range table-birds is more likely to be achieved by using the vaccine date predictions for broiler breeders, rather than that of the broiler, under the Deventer formula, which gives calculations for different types of bird.

Mr Goddard advises producers to consult their vets who can take blood samples to get an accurate prediction regarding optimum timing of vaccination with their particular flocks.

“Our trials have shown that, in general, and depending upon the breed used, the decline in maternal immunity is delayed — perhaps for 4-5 days. So, in order to get a better ‘take’, we recommend vaccinating these birds later or using an IBD [infectious bursal disease] vaccine that ‘takes’ in the presence of high maternal immunity.” said Mr Goddard.

IBD - also known as Gumboro disease - can cause severe losses in chickens, and outdoor-reared birds are more susceptible to the disease due to environmental influences.

Earlier this year Lohmann Animal Health launched a new ‘hot’ vaccine, classified as an ‘intermediate plus’ against the very virulent form of Gumboro disease. Because of its ability to break through high levels of maternal antibodies, it allows vaccination to take place earlier than with other existing vaccines.