Innovative Technology and Sustainable Development of Organic - 1.

Indicators were expressed per dairy cow, to be able to compare values among farms. Economic data
were derived directly from the farmer’s economic accounts, authorized by a registered public
accountant.
3.2.5 Eutrophication
On-farm eutrophication can be quantified by input–output accounting of nitrogen (N) and phosphorus
(P), which is reported to be relevant and reliable (Mollenhorst et al., 2006). By input–output accounting
of nutrients, one can determine the nutrient surplus, which is assumed to be lost to the environment. In
this research, we computed both a farm and field nutrient balance. The farm balance determines the
average N and P surplus per hectare of farmed land (Thomassen and De Boer, 2005; Halberg et al.,
2005). In order to quantify the effect of overstocking on eutrophication of individual fields, we also
computed a nitrogen balance for specific individual fields, representing grazing pasture and pasture for
mowing, determining the N surplus in kg per hectare of field per year. The P balance of individual
fields was not determined, because the estimated P surplus at farm level appeared to be low, which
corresponds to other registrations in Denmark (Hvid et al., 2004).
The NP input at farm level was determined by quantifying input through imported feed (i.e.,
roughage, concentrates, and other feeds), mineral supplements, manure, and livestock. In addition, the
N fixation of fields through pulses and grass–clover was computed using empirical data from. We
assumed N fixation in grass–clover ley of 18 kg/ha per year for a clover percentage between 1% and
9%, of 78 kg/ha per year for a percentage between 10% and 29%, of 156 kg/ha per year for a
percentage between 30% and 49%, and of 248 kg/ha per year for a percentage >49% (Kristensen and
Kristensen, 1992). The percentage of clover was estimated visually by one observer. N fixation for
pulses was assumed to be 80 kg N/ha per year. Atmospheric N deposition was assumed to be the same
for AMS and CMS farms, 16 kg N/ha per year (Ellermann et al., 2005). The NP output was determined
by quantifying the amounts of sales of milk and livestock, manure, feedstuffs, and crops. Standard
concentrations of N and P were used (Poulsen et al., 2001). The N content of milk, as well as the
amount of milk, was taken from the milk delivery registration. A standard P content of milk was
assumed. The difference between the NP input and NP output determined the NP surplus, and was
expressed per hectare of farmed land.
The field balance was determined only for fields that were fully used for mowing or grazing. The N
surplus per hectare of field balance was computed as the difference between the N input and N output.
The N input for each hectare was estimated by computing the amount of N applied by manure, the
amount of N deposited via feces and urine during grazing, and the N fixation and deposition. The
amount of manure applied was registered in the farmers’ obligatory mineral account for each hectare of
land. In order to estimate the amount of N deposited via feces and urine during grazing, information
about the grazing time (hours per day and days per year) and stocking rate was acquired from the
farmer. Using empirical data for excretion of N per cow based on measured milk yield on yearly basis
and milk urea content (Jonker et al., 1998) and deposition related to grazing time (Oudshoorn et al.,
2008), the amounts were calculated as shown in Equation 1.
Used timeoutside(h/day)
� Totalexcretion
in kg N/cowday
�stockingrate�
daysgrazing
[1]
24 h
N-fixation and N deposition were computed in a similar way as for the farm gate balance.
Thesis Frank W. Oudshoorn 43