Impacts of Time-Fed Concentrate-Based Diets on Plasma Metabolites, Rumen Histology, and mRNA Expression of Hepatic Enzymes of Wethers

SIMPLE SUMMARY: In modern ruminant meat production systems, wethers and steers are commonly fed diets containing 80% grain, or more. These diets are commonly fed to increase fat deposition in meat-producing ruminants. However, when wethers and steers are not appropriately transitioned to grain-based diets, they can experience metabolic and inflammatory conditions that negatively affect health and production. It is still unclear how meat-producing ruminants adapt to grain-based diets over time. This study evaluated the effects of an abrupt dietary change, from 80% forage to 80% grain, on rumen, plasma, and liver metabolism in growing wethers and monitored their ability to adapt over time. The results of the study suggest that wethers fed an 80% grain diet adapt over time by altering the expression of key enzymes involved in the systemic inflammation, iron metabolism, and cholesterol and glucose synthesis. This study provides novel insight into the physiology of fattening ruminants that have been abruptly fed grain-based diets and highlights the fact that meat-producing animals can be fed grain-based diets to meet increasing human meat requirements. ABSTRACT: Transition to grain increases inflammation and causes parakeratosis, which can decrease growth performance in fattening animals. It is unknown if ruminants adapt to these inflammatory responses over time. In a three-phase, 49-day experiment, all wethers (n = 13, BW = 50.6 ± 4.7 kg; 4.9 ± 0.3 months of age) were fed an 80% forage diet during P1(day 0 to 21). On day 21, 4 wethers were slaughtered to obtain baseline liver and rumen tissue. During P2 (day 22 to 25), the remaining wethers were fed an 80% concentrate diet. Four wethers were slaughtered on day 25 to obtain P2 liver and rumen tissue. During P3 (day 22 to 49), the remaining five wethers were fed 80% concentrate diets and were slaughtered on day 49 to obtain P3 liver and rumen tissue. Rumen parakeratosis was greater (p ≤ 0.02) in wethers sampled in P2 and P3 when compared to those sampled in P1. Among positive acute phase reactants, expression of serum α-amyloid (SAA) and haptoglobin (HPT) tended (p ≤ 0.12) to be 6- and 10-fold greater, respectively, in wethers sampled in P2 compared to wethers sampled in P1; however, SAA and HPT expression was not different between wethers sampled in P3 and P1. Plasma glucose and β-hydroxybutyric acid (BHBA) increased (p ≤ 0.03) in wethers sampled in both P2 and P3 compared to the wethers sampled in P1, while total protein and cholesterol decreased (p ≤ 0.06) only in wethers sampled from P2 compared to those sampled in P1. Hepatic acute phase responses suggest that the wethers adapted to an 80% concentrate diet over time.