Cow–calf Efficiency of Beef Cows Grazing Different Herbage Allowances of Rangelands: Hepatic Mechanisms Related to Energy Efficiency

Context Improvement in efficiency of energy utilisation of cow–calf systems could benefit beef production profitability and sustainability. Energy efficiency has been associated with mitochondrial function; therefore, hepatic mitochondrial function and oxidative stress could play a key role in energy efficiency of beef cows. Aim We evaluated the effect of two herbage allowances from rangelands (Campos biome) on cow–calf energy efficiency, hepatic mitochondrial density and function, and oxidative stress of purebred (Hereford and Aberdeen Angus) and reciprocal F1 crossbred beef cows. Methods Mature cows (n = 32) were used in a complete randomised block design with a factorial arrangement of herbage allowance (2.5 vs 4 kg dry matter/day; low vs high) and cow genotype (pure vs crossbred) over 3 years. At the end of the third year, cows were slaughtered at 190 ± 10 days postpartum. Liver was dissected and weighed, and samples were collected and snap-frozen pending analysis of mitochondrial density and oxidative stress markers. Estimated cow–calf energy efficiency was calculated by using total cow estimated metabolisable energy intake as input and calf energy retained at weaning as output. Key results Cow–calf energy efficiency was greater (P ≤ 0.07) for high than low herbage allowance and for crossbred than purebred cows. Mitochondrial density biomarkers (hepatic citrate synthase enzyme activity, citrate synthase mRNA, and mitochondrial:nuclear DNA ratio) were greater (P ≤ 0.03) for high than low herbage allowance. Plasma pro-oxidants and plasma antioxidant capacity were greater (P ≤ 0.07) for crossbred than purebred cows. Plasma oxidative stress index and expression of hepatic 4-hydroxynonenal protein adducts were affected (P ≤ 0.06) by herbage allowance × cow genotype interaction. Conclusion Greater cow–calf energy efficiency was associated with greater hepatic mitochondrial density without differences in mitochondrial function. Contrary to expectation, greater efficiency of crossbred than purebred cows was associated with increased hepatic oxidative damage, which probably reflects greater liver metabolic activity in crossbreds. Implications Herbage allowance and cow genotype affect cow–calf efficiency, hepatic mitochondrial function and oxidative stress markers. Greater efficiency of crossbred cows seems associated with increased hepatic oxidative damage.