Feed conversion into milk, nutrient excretion in manure and subsequent environment impacts of manure management are highly influenced by the diets that farmers feed their lactating cows (Bos taurus). On confinement-based dairy farms, determinations of diet composition are relatively straightforward because the types, amounts and nutrients contained in stored feeds are often well known. However, on grazing-based dairy farms, diet composition is more difficult to determine because forage intake during grazing must be estimated. The objectives of this study were to determine relationships between (1) feed N intake (NI), milk production, milk urea N (MUN), feed N use efficiency (FNUE) and excreted manure N (ExN); and (2) between feed P intake (PI), dung P concentrations (g/kg dry matter (DM)) and excreted manure P (ExP) for grazing-based lactating cows having a very wide range of diets and milk production. An additional objective was to evaluate how well these relationships compare with similar relationships based on more direct measurement of feed–milk–manure on confinement-based dairy farms. Four dairy farms located in southeastern Australia were visited during autumn and spring, and data were collected on feed, milk and dung of 18 cows on each farm. Estimated dry matter intake (DMI) from pasture comprised 12% to 75% of total diet DMI, and the crude protein (CP) concentrations in the total diets ranged from 167 to 248 g/kg. During spring, as diet CP increased FNUE declined. Total diet DMI and NI provided the best predictors of ExN, and PI provided the most accurate prediction of ExP. These results indicated accuracy in the study's indirect estimates of pasture DMI. Likely due to high levels and great variability in dietary CP and P concentrations associated with use of diet supplements, MUN did not appear to be a good indicator of dietary CP, and P in dung was not a good indicator of dietary P.
