Feed restriction reduces short-chain fatty acid absorption across the reticulorumen of beef cattle independent of diet.

The objective of this study was to evaluate the effects of the forage-to-concentrate ratio (F:C) of diets fed before and during short-term feed restriction (FR) on rumen fermentation, absorptive capacity of the reticulorumen, and apparent total tract digestibility. Twenty ovariectomized and ruminally cannulated Angus × Hereford heifers were blocked by BW and individually penned in box stalls (9 m(2)), having free access to water throughout the study. Heifers were randomly assigned to 1 of 2 dietary treatments, receiving either a high forage diet (HF; F:C of 92:8) or a moderate forage diet (MF; F:C of 60:40). Diets were fed ad libitum for 14 d before 5 d of baseline measurements (BASE) followed by 5 d of FR where heifers were restricted to 25% of ad libitum DMI relative to BASE. Dry matter intake was measured daily and ruminal pH was recorded every 2 min throughout the study. Ruminal fluid and blood samples were collected on d 3 of BASE and FR whereas short-chain fatty acid (SCFA) absorption was assessed in vivo using the isolated washed reticulorumen technique on d 5 of BASE and FR. Indigestible NDF was used as a marker to estimate apparent total tract digestibility. Diet × period interactions (P = 0.030 and 0.025) were detected for DMI and ruminal SCFA concentration, respectively. The interaction was the result of greater DMI and numerically greater SCFA concentration for MF than HF during BASE, with a reduction observed for both during FR, although treatment effects were no longer present. Period effects (BASE vs. FR) but not treatment effects (P > 0.05) were detected for mean ruminal pH (P < 0.001) and the total SCFA absorption rate (mmol/h; P = 0.038). During BASE, mean pH was reduced (6.4 vs. 6.9) and the SCFA absorption rate was greater relative to FR (674.5 vs. 554.8 mmol/h). Diet (P < 0.001) and period (P < 0.001) effects were detected for DM and OM digestibility with greater digestibility occurring for heifers fed MF than HF (70.5 vs. 63.3% for DM and 73.0 vs. 66% for OM) and greater digestibility during FR than BASE (69.5 vs. 64.3% for DM and 71.7 vs. 67.2% for OM). During FR, NDF digestibility was also greater than during BASE (P < 0.001; 62.4 vs. 55.8%). The effect of FR on serum NEFA differed by diet (diet × period, P < 0.001) with NEFA being greater for heifers fed HF than MF during FR (474.4 vs. 377.7 μEq/mL, respectively) with no differences observed between HF and MF during BASE. It can be concluded that severe short-term FR had a negative impact on ruminal SCFA absorption and energy balance and that altering the F:C of the diet does not mitigate these effects.