Effect of type of grain and feed processing on gastrointestinal retention times in horses.

Gastrointestinal retention time may affect digestive processes in the horse. To evaluate the effect of processing of grains on mean retention time in different segments of the gastrointestinal tract, 4 Norwegian Cold-blooded trotters (cecally cannulated, approximately 500 kg of BW) were used. Barley, maize, and wheat were all ground, pelleted, extruded, and micronized to create a total of 12 processed grains. After an adaptation period of 5 d, each horse was given 0.2 kg of Yb-mordanted grain together with their morning meal, which consisted of 2 kg of hay and 1 kg of one of the grains. Fecal samples were collected 5, 6, 7, 8, 9, 10, 11, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 48, and 52 h after administration of the marker dose. The samples were analyzed for Yb, and values were used for a 2-compartment nonlinear passage model to calculate the retention times in different segments of the gastrointestinal tract for each type of grain and each processing method. Among grains, maize had a longer retention time in the time-dependent compartment (believed to be cecum) than barley and wheat (P < 0.05) and hence a decreased passage rate out of this compartment (P < 0.05). For the feed processing treatment, ground grains had a longer compartmental retention time than those grains processed with the high temperature (extruded and micronized; P < 0.05), but the total mean retention time was not affected by treatments. In conclusion, feed processing affected passage rates and compartmental retention times, but did not affect the overall retention time in the gastrointestinal tract of the horse.