Effects of quality and physical form of hay on mean retention time of digesta and total tract digestibility in horses

The objective of this experiment was to investigate the effect of hay quality and form on mean retention time (MRT) of digesta and total tract apparent digestibility (TTAD) in horses fed under ad libitum or restricted conditions using four Hokkaido native horses. Two experiments (Exp.) were conducted using 4 × 4 Latin square designs with treatments arranged factorially with hay quality [high quality timothy hay harvested at late vegetative-stage (HH) or low quality timothy hay harvested at late bloom-stage (LH)] and physical form [chopped to a theoretical 10-mm length (G10H) or ground through a 5-mm sieve (G5H)] as main effects. . 1, horses were offered the experimental diets ad libitum whereas in Exp. 2 horses were offered a restricted allowance of 1.6 kg dry matter (DM) per 100 kg of BW per day. Under ad libitum conditions, hay quality, physical form and their interactions did not affect feed intake. In both Exp., solid phase digesta MRT in total tract and in the first compartment which having the longest MRT were longer (Exp. 1: P=0.08 and P=0.02, Exp. 2: P=0.04 and P=0.04, respectively) for G5H than for G10H, but hay form did not affect MRT in the second compartment which having the second longest MRT. Hay quality did not affect MRT. The MRT through the gastro-intestinal tract of the solid phase was 23.7 h in Exp. 1 and 27.1 h in Exp. 2. The TTAD of DM was higher (P<0.01) for HH than for LH in both Exp. In horses fed HH, TTAD of DM was 0.487 in Exp. 1 and 0.561 in Exp. 2, whereas in horses fed LH was 0.355 in Exp. 1 and 0.359 in Exp. 2. Hay form did not affect TTAD of DM. The effects of diet on TTAD of fibre were similar to those observed for DM. The results indicate that MRT lengthen in horses when the proportions of small particle sizes in the diet are high. Feeding chopped hay instead of ground hay did not decrease TTAD of fibre even if total and/or segmental MRT were shortened.