Protein Supplementation during Mid-Gestation Alters the Amino Acid Patterns, Hepatic Metabolism, and Maternal Skeletal Muscle Turnover of Pregnant Zebu Beef Cows

SIMPLE SUMMARY: Pregnant beef cows raised on extensive pasture systems in tropical regions commonly experience nutrient restriction during gestation. Herein, it was postulated that a strategic maternal protein supplementation (PS) might reduce maternal tissue mobilization intensity. Therefore, this study aimed to assess how protein supplementation (~40% of crude protein at the level of 3.5 g/kg BW/day, achieving 12% CP) for cows fed low-quality forage (corn silage + sugarcane bagasse, achieving 5.5% of crude protein) affects the maternal blood profile and the mRNA abundance of skeletal muscle turnover markers and glycogenic enzymes in the liver. This study also aimed to assess if these responses were related to the fetal sex. The PS resulted in a greater nutritional status, which in turn was demonstrated by the greater insulin, IGF1, and glucose concentrations. The effects of PS in mid-gestation persisted into late pregnancy. Protein supplementation also enhanced hepatic gluconeogenesis from AA and altered skeletal muscle turnover. In conclusion, protein supplementation during mid-gestation promotes a greater hepatic glucose production from AA, preserving the maternal tissue reserves and benefiting the longevity of the cows of the herd. ABSTRACT: From 100 to 200 days of gestation, 52 cows carrying male (n = 30) or female (n = 22) fetuses were assigned to CON (basal diet—5.5% of CP, n = 26) or SUP (basal diet + protein supplement [40% CP, 3.5 g/kg BW]—12% of CP, n = 26) treatments. Glucose concentrations decreased at 200 (p ≤ 0.01; CON = 46.9 and SUP = 54.7 mg/dL) and 270 days (p ≤ 0.05; CON = 48.4 and SUP = 53.3 mg/dL) for CON compared to SUP. The same pattern occurred for insulin (p ≤ 0.01). At parturition, the NEFA concentration was greater (p = 0.01, 0.10 vs. 0.08 mmol/L) for CON than for SUP. Total AA increased in SUP (p ≤ 0.03) at mid- and late-gestation compared to CON. At 200 days, CON dams carrying females had less essential AA (p = 0.01) than cows carrying males. The SUP dams had greater expressions of protein synthesis markers, namely eIf4E and GSK3β (p ≤ 0.04), at day 200 and of MuFR1 (protein degradation marker, p ≤ 0.04) at parturition. Supplemented cows had higher hepatic pyruvate carboxylase expressions (p = 0.02). Therefore, PS alleviates the restriction overload on maternal metabolism.