Predicting N excretion in commercial grazing system dairy farms

Sharon R Aarons1, Cameron JP Gourley1, Mark Powell2,

1 Agriculture Research and Development, Department of Economic Development, Jobs, Transport and Resources, Ellinbank Dairy Centre, 1301 Hazeldean Road, Ellinbank, Victoria 3821, Australia website,

2 US Dairy Forage Research Center, USDA Agricultural Research Service, 1925 Linden Drive West, University of Wisconsin, Madison, WI 53706, USA


Improving nitrogen (N) management on dairy farms is best facilitated through management of dairy cow dietary N intakes, due to strong associations between intakes, nutrient use efficiencies and N excretion.  Milk urea N (MUN) has also been used as an indicator of excess N in dairy systems.  While a number of predictive relationships between these parameters have been developed for confinement based systems, less information is available for grazing dairy systems.  Feed intake, N excretion and MUN data were determined from samples collected at five quarterly visits over a year on 43 commercial grazing-based dairy farms representing a range of production systems (n=227).  Relationships were developed between feed N intake, excreted N, feed N use efficiency (NUE) and MUN using these data.  The regression relationships were generally similar to the prediction equations reported in the literature for confinement-based dairy systems.  The coefficient of determination for the relationship between excreted N and N intake (ExcrN = 0.84NIn – 23.6; R2=0.97) was greater than the literature, probably due to the method of estimating excreted N.  Lactating cow N use efficiency declined with N intake (NUE = -0.009NIn + 25.9; R2=0.08), but the relationship to crude protein concentration was stronger (NUE = -0.79CP + 35.9; R2=0.50).  Mean MUN for these grazing system dairy cows (12.7 mg/dL) was similar to levels reported for commercial herd and significant relationships were observed between MUN and crude protein (R2=0.19), N use efficiency (R2=0.10) and excreted N (R2=0.17).  The weaker relationships observed were most likely due to the range of breeds, milk production and feeding systems used by these farmers, in contrast to the experimental herds and confinement systems reported elsewhere.  Despite the lower R2, these relationships suggest that prediction of N intake and excretion could improve nutrient management in grazing systems.