Fat intake and apperent digestibility of fibre in horses and ponies

Abstract

Performance horses are frequently given high-fat diets with fat contents up to 130 g/kg dry matter. The addition of extra fat raises the energy density of feeds. Diets with a high energy density facilitate a high-energy intake, which is advantageous for horses with high-energy requirements. High-energy diets also allow a reduction in total feed intake, which lowers the weight of gastrointestinal contents, this effect being considered beneficial to performance horses. The first hypothesis tested was that the intake of extra fat at the expense of an iso-energetic amount of nonstructural carbohydrates reduces fibre utilization in horses. In a crossover trial, 6 mature trotting horses were given either a low or high-fat diet. The high-fat concentrate was formulated to contain 37% of net energy in the form of soybean oil. The control concentrate contained an iso-energetic amount of cornstarch plus glucose. The concentrates were fed in combination with the same amount of hay so that the control and test diet contained 25 and 87 g crude fat/ kg dry matter, respectively. Apart from the amounts of fat and nonstructural carbohydrates the 2 diets were identical. The high-fat diet reduced the apparent total tract digestibilities of crude fibre, neutral and acid detergent fibre by 8.0 (P=0.007), 6.2 (P=0.022) and 8.3 (P=0.0005) percentage units, respectively. Since fat in the first experiment was substituted for nonstructural carbohydrates, including starch, the specific effect of fat could not be ascertained. It could not be excluded that starch also inhibits fibre digestibility so that the earlier observed fat effect would be underestimated. In the second study, the intakes of iso-energetic amounts of either soybean oil, cornstarch or glucose were compared as to fibre digestibility. Unlike starch, glucose is fully absorbed by the small intestine and thus is not expected to influence fibre fermentation in caecum and colon. Six trotters were fed rations either high in soybean oil (158 g/ kg dry matter), high in cornstarch (337 g/ kg dry matter) or high in glucose (263 g/ kg dry matter) according to a 3 X 3 Latin square design. Apparent crude fibre digestibility was similar for the rations with cornstarch (70.7 ± 3.06 % of intake, mean ± SD, n=6) or glucose (71.0 ± 1.90 %), but was significantly depressed by fat feeding (56.5 ± 7.65%). Similar observations were made for apparent digestibilities of neutral and acid detergent fibre and cellulose. It is concluded that the addition of fat to the ration of horses has an independent, inhibitory effect on fibre utilization and thus reduces the amount of energy provided by dietary fibre. An increase in fat intake by horses has been shown to decrease the apparent digestibility of the various dietary fibre fractions, but the mechanism was unknown. It was hypothesized that extra fat intake depresses the caecal and/ or colonic microbial degradation of fibre, leading to a decrease in fibre digestion. Literature data indicate that bile acids and linoleic acid may inhibit growth of pure cultures of microorganisms. In the present series of experiments the hypotheses tested were that after extra fat intake as soybean oil more bile acids and linoleic acid would enter the caecum which depresses microbial growth and thus also fibre fermentation. On the basis of measurement of faecal bile acid excretion in horses, no evidence was obtained for a higher influx of bile acids into the caecum after iso-energetic substitution of dietary soybean oil for starch plus glucose. When dietary palm oil was replaced by soybean oil, which caused a six-fold increase in linoleic acid intake, fibre digestibility in horses was not lowered. The infusion of linoleic acid into the caecum of fistulated ponies significantly increased apparent crude fibre digestibility. The process of fermentation involves a series of energy-yielding reactions catalyzed by microbial cells in which organic compounds act as both oxidizable substrates and oxidizing agents. Gas output from in vitro fermentation of forage can be used to measure both digestibility and the kinetics of microbial digestion. Under conditions that nutrient availability is not limiting, gas production is a direct measure of microbial growth. It was hypothesized that extra fat intake depresses the caecal and/ or colonic microbial degradation of fibre, leading to a decrease in fibre digestion. The hypothesis was tested using ponies fed either low-fat or a high-fat ration. In the ponies, the high-fat ration lowered apparent crude fibre digestibility by 13.5 percentage units. The ponies were euthanised and intestinal contents isolated to determine in-vitro gas production after incubation with various substrates. In general, groups mean maximum gas production from either cellulose or xylan by caecal, colonic 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 significantly 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 significant 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 fibre digestibility. An attempt was made to quantify the effect of extra fat intake on fibre utilization in horses. In a cross-over trial, eight mature trotting horses were given four diets. The concentrates were formulated to contain either soybean oil or an iso-energetic amount of glucose or combinations. The concentrates were fed in combination with the same amount of hay so that the whole diets contained 30, 50, 77 or 108 g crude fat/ kg of dry matter. Apart from the amounts of fat and glucose the four diets were identical. With an increase of 10 g /kg dry matter of soybean oil the apparent total tract digestibility of crude fibre was reduced with 0.9 percentage units. It is suggested that a high-fat intake by horses may increase the amount of fat entering the large intestine to levels that depress fermentation by cellulolytic bacteria. The ratio of calculated net energy intake (NEi) to calculated net energy requirement (NEr) can serve as an indicator of the efficiency of dietary energy utilization. The ratio was determined for 93 horses and ponies from 10 riding schools. For each animal with assumed constant bodyweight, energy intake and energy requirements were assessed. The estimated NEi on average was 14 % greater than NEr. There was a significant, negative association between crude fibre intake and the NEi : NEr ratio. Dietary fat concentration was found to range from 32 to 52 g/ kg dry matter (5 to 6 g/ MJ net energy), but on basis of controlled digestibility trials this range would be too narrow to influence the NEi : NEr ratio as was indeed found in this survey. This thesis shows that assessment of the efficiency of dietary utilization, under practical conditions, by using the NEi : NEr ratio is fraught with uncertainty.