The effects of two different amounts of dietary grain on the digestibility of the diet and behaviour of intensively managed horses

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The current experiment examined the effect of switching from a moderate grain diet to a high grain diet, as are typically fed to intensively managed horses, on some behavioural and physiological (oro-caecal transit time [OCTT], digestibility, plasma cortisol concentrations and heart rate) parameters in adult cribbers (n = 5), weavers (n = 6) and non-stereotypic Thoroughbred horses (n = 6). The cribbers and weavers in the study had been known to show stereotypic behaviour for at least 12 months prior to commencement of the study. Switching to a high grain diet reduced explorative behaviour (P < 0.01) but did not affect stereotypic behaviour. Horses took more time to consume the entire high grain ration than the moderate grain ration (P < 0.01), possibly in an attempt to slow the rate of starch delivery to the hindgut. The high grain diet had lower digestibility than the moderate grain diet (P < 0.01). Switching to a high grain diet was accompanied by a reduction in water intake (P < 0.05), though the two diets did not differ significantly in their effects on plasma cortisol concentration, heart rate, or OCCT. Combined, these findings suggest that horses may have altered their feeding habits in response to the change in diet and this change in behaviour could have been an attempt to limit diet-induced acid build-up in the hindgut. Future studies on the relationship between the level of grain in the diet and stereotypic behaviour should consider that horses may show feeding changes to different diets which attenuate diet-induced effects on physiology and other behaviour.