Saliva Production and Short-chain Fatty Acid Absorption in Beef Cattle Fed a Low- or High-forage Diet

Abstract Text: Based on past research, there are indications that a potential decrease in acid removal from the rumen via epithelial absorption during a bout of ruminal acidosis could possibly be compensated for by an increase in salivation. However, there is limited information on whether similar changes in the relative contributions of salivary bicarbonate and passive and/or facilitated absorption of short-chain fatty acids (SCFA) to pH regulation occur when dietary forage content is altered. Therefore, the objective of this study was to determine the effects of feeding a low- (LF) or high-forage (HF) diet on ruminal fermentation, salivation and SCFA absorption. Eight ruminally-cannulated cattle were used in a crossover design with 49 d periods. The treatments were barley silage at 30 (LF) or 70% (HF) of dietary dry matter (DM). The LF and HF diets contained 45.3 and 30.9% starch and 26.4 and 38.3% physically effective fiber (DM basis), respectively. On d 35, ruminal fluid was collected to determine SCFA concentration. Ruminal pH was continuously measured from d 29 to 35. Eating or resting salivation, was measured by collecting masticate (d 39 and 40) or saliva samples (d 42 and 43) at the cardia, respectively. On d 42 and 43, the temporarily isolated and washed reticulo-rumen technique was used to measure total, and chloride competitive (an indirect measure of protein-mediated transport), absorption of acetate, propionate and butyrate. Total ruminal SCFA concentration and osmolality were higher (P < 0.02) in cattle fed the LF compared to the HF diet. Additionally, feeding LF resulted in a longer (P = 0.02) duration (h/d) and a larger (P = 0.05) area (pH × h/d) that pH was below 5.5. Although there was no diet effect on total and chloride competitive absorption (mmol/h and %/h) of SCFA, eating salivation (mL/min) was lower (P = 0.02), whereas resting salivation (mL/min) tended to be lower (P = 0.10) in cattle fed a LF diet. The lower ruminal pH in cattle fed the LF compared to the HF diet could be attributed to the increase in SCFA production and decrease in salivation, which were not compensated for by an increase in SCFA absorption.

Keywords: dietary forage content, saliva production, short-chain fatty acid absorption