The effect of replacing nonstructural carbohydrates with soybean oil on the digestibility of fibre in trotting horses

The hypothesis tested was that the intake of extra fat at the expense of an isoenergetic amount of nonstructural carbohydrates reduces fibre utilisati on in horses. In a crossover trial with feeding periods of;42 days each, 6 mature trotting horses (age 4-12 years, bodyweight 340-476 kg) were given e ither a control or test diet. The test concentrate was formulated to contai n 37% of net energy in the form of soybean oil. The control concentrate con tained an isoenergetic amount of corn starch plus glucose. The concentrates were fed in combination with the same amount of hay so that the control an d test diet contained 25.13 and 86.66 g crude fat/kg dry matter, respective ly. Apart from the amounts of fat and nonstructural carbohydrates, the 2 di ets were identical. The test diet reduced the apparent total tract digestib ilities 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. It i s suggested that a high fat intake by horses may increase the amount of fat entering the large intestine to levels that depress fermentation by cellul olytic bacteria. The observed interaction between fat content of the diet a nd fibre utilisation may have consequences for practical horse feeding in t hat calculating the energy content of test diets on the basis of feedstuff tables leads to overestimating the amount of energy provided by the high-fi bre ingredients of the diets.