The regulation of feed intake in broiler chickens in sickness and in health

Abstract

There is an increased interest in the use of alternative feed ingredients in broiler production systems on the grounds of food security and reduced environmental impact. Such ingredients are typically bulky in their nature and consequently lead to feeds whereby the energy and nutrient composition is diluted in comparison to more traditional feeds. Therefore, interest is growing in the ability of broiler chickens to cope with bulky ingredients. Understanding such an ability and the limiting factors of such feeds is vital for the development of accurate simulation models that predict broiler performance and the environmental impact. As broilers are challenged by ubiquitous pathogens during their growth, additional understanding of how birds deal with bulky feeds under such challenge is also relevant to such predictions. There are concerns that modern broilers have lost their ability to regulate their energy intake and that genetic selection for carcass yield has limited the size of their gastrointestinal tract (GIT), thus limiting their capacity to cope with energy dilution. The first two chapters of this thesis investigated the capacity of a modern broiler strain to deal with increasing levels of various bulky ingredients and aimed to identify a feed bulk dimension responsible for limiting feed intake. These experiments also allowed the investigation of the capacity and rate of adaptation of the gastrointestinal tract on these bulky feed ingredients. The results of these experiments showed: 1) Birds showed a remarkable ability to regulate energy intake when feed energy content was reduced up to a point, which was presumed to reflect the maximum capacity for bulk. 2) Further feed dilution with bulky ingredients limited feed intake and penalised performance. 3) The Water holding capacity of the feeds was able to predict the feed intake of birds not previously adapted to bulky feeds, i.e. in the short term. 4) Birds adapted very rapidly on the bulky feeds and the rate of adaptation depended on the bulkiness of the feed, i.e. the bulkier the feed the longer the adaptation. Infection with coccidia was used as the infectious model to investigate the interaction between feed bulkiness and infection. In the third experiment, infected birds were given access to feeds which were progressively diluted with a bulky ingredient, lignocellulose. In uninfected birds feed intake was reduced as feed dilution increased and performance decreased, whereas in infected birds feed intake increased as feed bulkiness increased, and performance was unaffected by feed bulkiness. In the final experiment, the protein content of the feed was diluted by substituting an ingredient with a high protein content for one with a low protein content, whilst maintaining the energy contents of the feeds. In both uninfected and infected birds feed intake increased as the protein level of the feed increased, and performance ii increased. The results from these experiments show that performance during infection is indeed sensitive to feed composition and it may altogether be absent when broilers are offered feeds diluted with bulky ingredients, such as lignocellulose. The findings of this thesis facilitate the development of models to predict the feed intake and performance of broiler chickens offered feeds with alternative, bulky ingredients. Unravelling how feed intake is regulated during Eimeria infection will help to understand how these birds should be fed during the critical stages of infection.