199. Effects of Rumen Protected Fat and Vitamin E Supplementation on II: Muscle Cellularity and Transcriptomic Regulation in Wagyu X Angus Cattle.

Abstract Angus cows (n = 40) were inseminated with Wagyu ‘Oh-No’ semen and all resulting Wagyu x Angus calves (n = 23; 11 heifers and 12 steers) were included in this study to evaluate the effects of rumen-protected fat and vitamin E supplementation influence on longissimus muscle and intramuscular fat development. After weaning and a 45-d backgrounding period, calves were randomly assigned within sex to either a high concentrate control diet (CON) or a high concentrate diet containing 2.5% Megalac and 0.145% vitamin E (RPE). Calves were housed in four individual pens (5-6hd/pen) and fed using individual SmartFeed (C-Lock) bunks to monitor intake. The production system included three phases: Phase 1 (135-d feedlot with treatments), Phase 2 (78-d pasture without supplementation), and Phase 3 (74-d finishing feedlot with treatments reinstated). Longissimus muscle biopsies were collected from steers at 25 d and 200 d of age prior to supplementation (pretreatment/CON), at the end of phase 1 (417 d of age), and at the end of phase 3 (569 d of age). Biopsy samples were subdivided into a cell-isolation sample placed in HBSS + 5x AB/AM and an RNA sample snap-frozen in liquid nitrogen for downstream RNA extraction and gene expression analysis. During the cell isolation, we utilized a pre-plating technique, and cells were divided into PP1 (pre-adipocytes and fibroblasts), PP2 (myoblasts), and PP3 (satellite cells). Cell isolation data were analyzed with age, treatment and the interaction in the model. RNAseq data were analyzed using Novogene and Novimagic comparing phase timepoints and treatments. The treatment-by-age interaction was significant (P < 0.05) across preadipocyte (PP1) numbers, while myoblast (PP2) numbers were unaffected (P > 0.05) by treatment and age. Satellite cell (PP3) numbers were different (P < 0.05) depending on age but were unaffected by treatment. RNA sequencing results displayed strong developmental regulation of the muscle transcriptome, which was active throughout the study. Between 25 days of age and 200 days of age (pretreatment/CON), 404 genes were upregulated and 1,091 downregulated (Padj < 0.05). At 417 days of age (end of phase 1), 364 genes were upregulated and 128 downregulated between RPE and CON (Padj < 0.05). At 569 days of age (end of phase 3), 645 genes were upregulated and 415 genes were downregulated between RPE and CON (Padj < 0.05). Supplementation of rumen-protected fat and vitamin E during feedlot phases appeared to alter preadipocyte abundance and gene expression in the longissimus muscle of Wagyu x Angus cattle, potentially illuminating mechanisms behind improved marbling throughout the course of the study.