| Summary: | This thesis details a series of studies to evaluate the effects of different dietary treatments on production performance and N utilisation in lactating dairy cows, with a particular emphasis on urinary N excretion. Firstly a digestibility trial was performed to investigate the effect of concentrate crude protein (CP) content (14.1, 16.1 and 18.1% CP) and cow genotype (Holstein vs Holstein x Swedish Red) on N and energy utilisation, nutrient digestibility, methane emissions and milk production. Reducing concentrate CP content decreased urinary and milk urea N outputs, without negatively effecting productivity, methane or energy utilisation parameters. However, crossbreeding of Holstein cows resulted in reduced gross energy, digestible energy and metabolisable energy intake and milk energy output. Regression equations were produced for estimating milk urea and urinary N output. The same concentrate feeds were then evaluated over a grazing season on pasture to examine effect on milk production and milk urea N parameters. Findings indicate concentrate CP content was negatively related to milk N/N intake and milk urea N content showed a strong tendency to increase with concentrate CP content. The effect of protein source was evaluated in a second digestibility trial. Specifically the rate of fresh white clover (WC) inclusion (0, 20 or 40% dry matter (DM) basis) in fresh perennial ryegrass based iso-nitrogenous diets on milk production, nutrient digestibility and N metabolism was investigated. White clover had no effect on production parameters despite a quadratic effect on DM and organic matter intake. However, WC inclusion linearly decreased digestible organic matter in total DM and digestibility of fiber, while increased digestibilities of N and gross energy were associated with the low WC (20%) diet. Relative to N intake, the low WC diet decreased fecal and manure N output rates while the high WC (40%) diet linearly decreased milk N output.
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