Broodiness and broody control.

Broodiness is a nuisance and sometimes a serious problem in some poultry. The behaviour is recognized as persistent nesting, usually associated with cessation of egg laying. Broodiness has two components: incubation and brooding behaviour; the former can only be induced in laying hens, while the latter can be induced in both laying and non-laying hens and in males, by forced fostering of chicks. The motivation to incubate wanes after 4-5 weeks sitting on infertile eggs, while brooding behaviour gradually declines as the chicks become independent and egg laying resumes. Incubation and brooding behaviour are associated with low concentrations of plasma luteinizing hormone (LH) and ovarian steroids, and with high and low concentrations of plasma prolactin, respectively. The preoptic area of the hypothalamus (POA) forms part of the neural circuitry controlling both incubation and brooding behaviours. High concentrations of prolactin and low concentrations of plasma LH in incubating hens are a consequence, respectively, of increased vasoactive intestinal polypeptide (VIP) and decreased gonadotrophin-releasing hormone (GnRH) release from the hypothalamus. Changes in the release of these two neuropeptides from the hypothalamus are, in part, a consequence of changes in their rates of synthesis. The release of VIP in incubating hens is stimulated by dopaminergic input from the POA, which in turn is stimulated by a serotonergic input. Incubation behaviour is a form of extended nesting behaviour induced by the synergistic action of oestrogen and progesterone. It is transformed into incubation behaviour by increased prolactin secretion stimulated by neural information transmitted through the brood patch from a clutch of eggs to hypothalamic VIP neurones. Broody control is best achieved by good husbandry directed to discouraging the behaviour. Pharmacological and immunological methods can be used to control or prevent broodiness but for most practical purposes are uneconomic and some may delay resumption of egg production. Broodiness is a polygenic trait, and quantitative trait loci analysis can be applied to identify regions of the genome controlling broodiness. Identification of markers in, or linked to, these genes and associated with broody behaviour is likely to be of value in breeding programmes incorporating DNA marker-assisted selection.