Abstract
Inclusion of extra fat in the diet of horses, at the expense of an iso- energetic amount of non-structural carbohydrates, has been shown to decrease the apparent digestibility of the various dietary fiber fractions, but the mechanism was unknown. It was hypothesized that inclusion of extra fat in the diet depresses the caecal and/or colon microbial degradation of fiber, leading to a decrease in fiber digestion. The hypothesis was tested using six ponies fed either low fat or a high fat ration. In the ponies, the high fat ration lowered apparent NDFom digestibility by 0.089. The 12 ponies were killed and intestinal contents isolated to be used as inoculum with in vitro gas production technique. In general, groups mean maximum gas production from either cellulose or xylan by caecal, colon and faecal bacteria was lower when the ponies had been fed the high fat diet. Cumulative gas production by caecal fluid with xylan as substrate after 20 h of incubation was depressed when the donor animals had been fed the high fat diet. With cellulose as substrate, gas production by caecal contents was lowered by on average 20% after fat feeding of the ponies. There was a diet effect of cell-free caecal fluid on gas production from cellulose by a standard inoculum: fat feeding had an inhibitory impact. It is concluded that fat feeding in ponies inhibits microbial activity in the caecum, which in turn leads to a decrease in fiber digestibility.