516 Effects of increasing hybrid rye silage as a replacement for barley silage for growing beef heifers

Abstract The objective of these studies was to evaluate effects of increasing hybrid rye silage (HRS; harvested at boot stage in Study 1 and soft-dough in Study 2) inclusion as a replacement for barley silage (BS; harvested at soft-dough stage) on dry matter intake (DMI), ruminal fermentation, and growth performance of growing beef cattle. In Study 1, ruminally cannulated Hereford × Simmental heifers (n = 8) with an initial body weight (BW) of 519 ± 25.8 kg were used in a replicated 4 × 4 Latin square design with 28-d periods including 21 d of dietary adaptation and 7 d of data and sample collection to assess DMI and ruminal fermentation. Treatments included a control diet (DM basis) that contained 59.62% BS, 38.60% barley grain, and 1.78% of a vitamin and mineral supplement. Hybrid rye silage replaced 33, 67, and 100% of the BS on a DM basis. In Study 2, steers (n = 384; BW = 343 ± 20.2 kg) were stratified by BW and allocated to 32 pens in a randomized complete block design over 2 yr (4 pens/treatment/yr) to evaluate DMI, growth, and gain:feed (G:F). Steers were fed similar diets as Study 1 for 84 d in both years. In Study 1, increasing HRS linearly (P < 0.01) decreased DMI from 12.0 to 9.2 kg/d, and linearly increased (P = 0.02) water intake from 18.6 to 22.5 L/d. Increasing HRS linearly increased mean (P < 0.01) and minimum ruminal pH (P < 0.01). There was no effect of HRS inclusion on total short-chain fatty acid 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, and < 0.01, respectively) with increasing HRS inclusion. Moreover, increasing HRS inclusion linearly increased the molar proportions of isobutyrate and valerate (P = 0.06, and < 0.01, respectively) while caproate decreased linearly (P = 0.02). The ammonia-N concentration linearly increased with increasing HRS inclusion. In Study 2, DMI was 9.23, 9.10, 8.82, 8.08 kg/d (quadratic, P = 0.02) for diets containing 0, 33, 67, and 100% HRS, respectively. Likewise, ADG decreased (1.45, 1.36, 1.30, 1.08 kg/d; quadratic P < 0.01) as HRS inclusion increased and G:F ratio decreased linearly (P < 0.01) from 0.158 with 0% HRS to 0.134 g/kg when diets contained 100% HRS. Final BW was 462, 456, 450, and 432 kg when HRS was included at 0, 33, 67, and 100% of the silage, respectively (quadratic, P = 0.04). In conclusion, replacing BS with increasing proportions of HRS reduced DMI resulting in greater ruminal pH, altered molar proportions of individual SCFA, and reduced growth performance and G:F during the growing phase.