Cheng, LWoodward, SLDewhurst, RJZhou, HuitongEdwards, GR2017-03-142014-08-192014Cheng, L., Woodward, S.L., Dewhurst, R.J., Zhou, H., & Edwards, G.R. (2014). Nitrogen partitioning, energy use efficiency and isotopic fractionation measurements from cows differing in genetic merit fed low-quality pasture in late lactation. Animal Production Science, 54, 1651-1656. doi: 10.1071/AN141711836-0939AN6RL (isidoc)https://hdl.handle.net/10182/7876The study was carried out to evaluate energy and nitrogen (N) use efficiencies of high and low breeding worth (BW) cow groups relative to N isotopic fractionation (∆¹⁵N). Eight high- and eight low-BW cows (mean BW index ≤ 198 and 57, respectively) in late lactation were used to conduct an N balance study with all cows fed autumn pasture. Individual cow pasture DM intake, N intake and N outputs of milk, urine and faeces were quantified. Plasma sample from each cow was harvested. Feed, plasma, faeces, urine and milk samples were measured for δ¹⁵N and calculated for ∆¹⁵N. Urea N in milk and plasma, and urinary excretion of purine derivatives were also measured. The metabolisable energy (ME) intake, milk energy output, and energy and N use efficiencies of high-BW cows were greater on average than low-BW cows. Conversely, the ratios of urinary N excretion to faecal N excretion and urinary N excretion to N intake were greater for low-BW cows than high-BW cows. There was no effect of BW groups on manure N output, apparent N digestibility, retained N, purine derivatives excretion or ratio of purine derivatives excretion to ME intake. No relationships were found between N and energy efficiencies and δ¹⁵N measurements. Regression analysis with individual cow measurement showed plasma δ¹⁵N - feed δ¹⁵N was negatively correlated with DM intake. N use efficiency was positively correlated with BW. High genetic merit cows are more efficient in N and energy use than lower genetic merit cows when fed low quality pasture in late lactation. Plasma δ¹⁵N - feed δ¹⁵N was proved to be a potential indicator of DM intake for individual cows when identical feed was offered. BW may be used to predict N use efficiency for individual cows. © CSIRO 2014.pp.1651-1656en© CSIRO 2014isotopic discriminationmanure nitrogenmicrobial energetic efficiencymicrobial protein synthesissustainabilityJournal Article10.1071/AN14171ANZSRC::070204 Animal Nutrition1836-5787ANZSRC::30 Agricultural, veterinary and food sciencesANZSRC::31 Biological sciencesANZSRC::41 Environmental sciences