The effect of feeding order of forage and oats on metabolic and digestive responses related to gastric emptying in horses

Feeding order of forage and concentrate might affect gastric emptying and subsequently digestion in horses. The objective of this study was to measure gastric emptying in combination with metabolic and digestive responses in the plasma and caecum, respectively, when changing the feeding order of oats (O) and hay (H) (oats first then hay: O-H vs. hay first then oats: H-O). Four caecum cannulated horses were used in a 2×2 crossover design, with two 12-day periods consisting of 10 d of diet adaptation and two days of data collection. Hay was fed at 0600, 1400 and 2000 h, while oats were fed in the morning either 15 min before or 1 h after feeding hay. On days of data collection, baseline samples were collected before feeding 1.4 kg DM hay and 474 g DM oats (0.4 g starch/kg BW), and data was collected until 8 h after feeding. Gastric emptying of oats was estimated using the 13C acetic acid breath test, where breath samples were analysed for 12C:13C ratio after administration of 13C acetic acid mixed with oats. Gastric emptying coefficient (GEC), time where half of the total cumulative recovery of 13C was excreted (t1/2), and time where the maximal amount of 13C was excreted (Tmax) were calculated. Samples of blood and caecal fluid were collected at hourly intervals. Blood plasma was analysed for glucose and insulin and baseline concentrations, peak concentrations, time of peaks and area under the curves (AUC) were calculated. A pH probe was placed in the caecum measuring pH every min to find minimum pH and time to reach minimum pH. Hourly caecal samples were analysed for pH and short-chained fatty acids (SCFA) concentrations. Results from the 13C acetic acid breath test indicated that feeding order affected gastric emptying, as Tmax was longer (P = 0.004) when feeding H-O (2.18 h) than O-H (1.09 h), but there was no effect on the GEC and T1/2. No effect of feeding order was found for plasma glucose and insulin measures. Feeding order had no effect on minimum pH, but time to reach minimum pH increased (P = 0.014) from 170 min for O-H to 280 min for H-O, and average pH was lower in the intervals 0 to 170 min (P = 0.006) and 170 to 280 min (P = 0.006) for O-H than H-O. In general, time of sampling had a larger effect on SCFA concentrations than feeding order. In conclusion, this study indicates that feeding order affected gastric emptying, and the digestive and metabolic responses were more clearly reflected in caecum pH than plasma glucose and insulin.