Effects of feeding hybrid rye silage as a replacement for barley silage on feed intake, ruminal fermentation, and the site and extent of nutrient digestion in growing beef heifers

The objective of this study was to evaluate the effects of feeding hybrid rye silage (HRS) as a replacement for barley silage (BARS) on dry matter intake (DMI), ruminal fermentation, and the site and extent of digestion when fed to growing beef heifers. Eight ruminally cannulated Hereford × Simmental heifers (519 ± 25.8 kg BW) were used in a replicated 4 × 4 Latin square design with 28-d periods, including 21 d for dietary adaptation and 7 d for data and sample collection. Treatments included a control diet (BCON) that contained 59.62% BARS (harvested at the soft dough stage of maturity), 38.61% barley grain, and 1.78% of a vitamin and mineral supplement on a dry matter (DM) basis. The HRS (harvested at the boot stage of maturity) replaced 33% (RLOW), 67% (RMED), or 100% (RHIGH) of the BARS on a DM basis. Increasing HRS inclusion linearly (P = 0.04) decreased DMI, linearly increased water intake (P = 0.02) and mean ruminal pH (P < 0.01) and linearly decreased the duration that ruminal pH was < 5.8 (P = 0.04). There was no effect of HRS inclusion on total short-chain fatty acid (SCFA) concentration (P = 0.46) in ruminal fluid, but the molar proportion of acetate linearly (P < 0.01) increased while propionate and butyrate linearly decreased (P < 0.01). The NH3-N concentration linearly increased (P = 0.03) with increasing HRS inclusion. Increasing HRS inclusion increased ruminal degradability of DM, OM, and aNDFom, while there was no effect on intestinal digestibility of nutrients, either expressed as a percent of intake or percent of digesta flow. Total tract digestibility of DM (P ≤ 0.01), ADF (P ≤ 0.01), and aNDFom (P ≤ 0.01) increased with increasing HRS inclusion, but inclusion did not affect gross energy (GE) digestibility (P = 0.15) and the dietary digestible energy (DE) concentration (P = 0.17). In conclusion, the use of HRS, harvested at the boot stage, as a direct replacement for BARS, harvested at the soft dough stage, is likely to reduce DMI but improve NDF digestibility without affecting dietary DE concentration. As there was no difference in GE digestibility or dietary DE digestibility, these results demonstrate a tradeoff between greater NDF digestibility of HRS compared to BARS but reduced DMI with increasing inclusion.