Online P&A Jan 30.indd

THE effects of a high concentrate versus high fibre diet on performance parameters and welfare of the horse have been widely studied in the past (Tinker and others 1997, Nadeau and others 2000, McGreevy and others 2001, Ellis and others 2002). Horses fed diets low in ‘structural’ fibre have a greater susceptibility to develop stereotypical behaviour (apparently functionless, repetitive activities, such as wind sucking or crib biting) and stomach ulcers due to a lack of time spent chewing, reduced saliva production and a negative effect on the hindgut environment (Nicol and others 2002, Ellis and others 2003). Gillham and others (1994) reported that stereotypical behaviour significantly increased during and after feeding concentrates. The mechanism by which diet may influence behaviour could be clarified by the hypothesis that diet influences the serotonergic system (Grimmett and Sillence 2005). In human beings, this hypothesis has been confirmed in women but not in men (Walsh and others 1995). The neurotransmitter serotonin (5-HT) is known to be involved in many physiological and behavioural processes, including mood, appetite, sleep, sexual behaviour and cognition, but also in the pathophysiology of psychiatric disorders such as depression, anxiety and obsessive compulsive disorder (OCD) (Lucki 1998). Crib biting and wind sucking in horses have been described as stereotypical behaviours similar to OCD in people (Shuster and Dodman 1998). 5-HT is synthesised via tryptophan (TRP) hydroxylase and therefore TRP levels may be an indicator of 5-HT metabolism. Diets with different levels of protein and carbohydrates could influence plasma TRP levels. Modification of this essential amino acid may influence the rate of synthesis of the corresponding neurotransmitter (Fernstrom 1983). The depletion or increase of TRP in diets has been shown to affect behaviour and mood in experimental