Milk 13C and 15N discriminations as biomarkers of feed efficiency and energy status in early lactation cows

Abstract

Measuring feed utilization efficiency (FUE, milk production / feed intake) and energy status of dairy cows is essential to optimize dairy management and reduce feed waste, but FUE measurement is laborious and costly for dairy farms, particularly when cows are fed in groups. Our study was to investigate whether milk 13C and 15N stable isotope discriminations (naturally existing isotope; milk δ13C or δ15N – diet δ13C or δ15N) were used as biomarkers to estimate FUE, energy balance, and milk production of early lactation cows. Ten multiparous lactating cows with similar body weight (mean ± SD; 691 ± 18.0 kg) and parity (1.6 ± 0.5) were fed a homogeneous diet from 4 to 11 wk postpartum. Body fat had 1.5‰ less δ13C (P < 0.001) and muscle had similar δ13C compared to animal diet. Muscle had more than 2‰ greater δ15N than the diet (P < 0.0001). Milk 13C discrimination was reduced (P = 0.04) and milk 15N discrimination was increased (P < 0.0001) as lactation progressed postpartum. In early lactation, milk 13C discrimination was positively correlated with net energy balance (NEl) (linear; R2 = 0.49), and milk 15N discrimination was negatively correlated with NEl balance (linear; R2 = 0.56). Meanwhile, milk 13C discrimination was negatively and linearly (R2 = 0.30) correlated with feed carbon utilization efficiency, and milk 15N discrimination was positively and linearly (R2 = 0.55) correlated with feed nitrogen utilization efficiency. For milk production, milk 13C discrimination was negatively and linearly correlated with the milk yield (R2 = 0.44), milk fat yield (R2 = 0.40) and lactose yield (R2 = 0.46). Milk 15N discrimination was positively and linearly correlated with the milk protein yield (R2 = 0.49). Collectively, our data revealed that milk 13C and 15N discriminations could be as biomarkers to estimate the FUE, energy status, and milk production of early lactating cows. Potentially, measuring milk isotopic discrimination may be also applied to understand energy and nutrient partitioning of internal C and N reservoirs (body fat and muscle) for supporting milk synthesis in early lactation.