Effect of expander processing on fractional rate of maize and barley starch degradation in the rumen of dairy cows estimated using rumen evacuation and in situ techniques

Abstract

Effects of heat treatment (expanding) on ruminal and intestinal digestibility of starch in barley and maize grains were studied in an extended 4×4 Latin square experiment. Four lactating Danish Holstein Friesian cows fitted with ruminal, duodenal and ileal cannulae were offered grass-clover silage and grass-clover hay based diets supplemented with soybean meal and either untreated barley, expanded barley (105 °C), untreated maize or expanded maize (95 °C). Ruminal degradation characteristics of starch for untreated and expanded grains were determined in situ using nylon bags with two pore sizes (15 or 36 μm). Rate of starch degradation in the rumen was also determined based on series of total rumen evacuations. In vivo digestibility was estimated by sampling duodenal and ileal contents and faeces. Ruminal degradation of starch determined in situ was higher for barley than maize regardless of heat treatment. In situ studies (36 μm) showed expanding increased effective maize starch degradability in the rumen from 0.60 to 0.72, mainly due to an increased soluble fraction. Effective barley starch degradability, however, was unchanged at 0.96 as an increase in the soluble fraction was counterbalanced by a decrease in rate of degradation from 0.63 to 0.36 h −1 with 36 μm nylon bags. In vivo degradation characteristics based on rumen evacuations showed that heat treatment increased fractional rate of degradation of starch for both barley and maize, resulting in a slight increase in the effective degradability of barley starch from 0.81 to 0.85 and an increase in rumen effective degradability of maize starch from 0.71 to 0.78, when rumen starch pools were corrected for rumen starch outflow. In situ studies seemed to overestimate ruminal degradation of rapidly fermentable starch in barley, and underestimate degradation of slowly fermentable starch in maize, although in situ results were highly dependant on assumptions made on digestion and passage of the soluble fraction. Total tract digestibility of barley starch was not affected by heat treatment (0.99), whereas total digestibility of starch in maize increased from 0.84 to 0.96. Apparent rumen digestibility of starch was higher for barley than maize. Duodenal flow of starch in barley was highest 4 h post feeding, whereas a much larger peak was found for maize at 10–12 h post feeding, which indicates a passage time lag for undegraded maize starch in the rumen, or possibly the abomasum. Fractional rate of passage of starch from the rumen was not constant, indicating that passage of starch does not follow first order kinetics.