Nitrogen partitioning, energy use efficiency and isotopic fractionation measurements from cows differing in genetic merit fed low-quality pasture in late lactation

The 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 d15N 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 d15N measurements. Regression analysis with individual cow measurement showed plasma d15N – feed d15N 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 d15N – feed d15N 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.