Nitrogen and phosphorus excretion on mountain farms of different dairy systems

We developed a procedure to estimate the release of nutrients into the environment on Alpine dairy farms and applied it to a sample of 564 farms in the Province of Trento (north-eastern Italy) as a case study. Farm data (geographical location, herd size, milk production and quality, reproductive events, land use) were gathered from institutional databases and merged. Information on the formulation of the ration was obtained from farm visits. The farms fell into 4 groups: traditional with summer transhumance to highland pastures (T-ALP, 51%), traditional without transhumance (T-noALP, 24%), traditional using silage (T-S, 5%), and modern (MOD, 20%). The model predicted N and P excretion from cows and heifers on a farm basis. The N in manure was computed from total N excreted, assuming a 28% of N loss due to volatilisation. A cow unit was defined as the cow and its share of replacement heifer. The average dietary N content of the lactating cows ranged from 20 to 30 g/kg DM, while on-farm N excretion ranged from 90 to 190 kg/year per cow unit; the modern farms had the highest average value (137 kg), the T-ALP farms the lowest (106 kg). Average P excretion ranged from 10 to 40 kg/year/cow unit. The on-farm N and P in manure per unit of milk decreased asymptotically with increasing cow productivity, from 25 to 19 and from 4.1 to 2.8 g/kg milk, respectively. The modern farms had the greatest amounts of N and P in manure per unit of agricultural land (260 and 51 kg/ha, respectively), the T-ALP farms the lowest (161 and 37 kg/ha, respectively). Within system, there was a huge variation among farms in the N and P load per unit of agricultural land, which was largely explained by the number of cow units per ha and by nutrient excretion per cow unit, but not by herd size or cow productivity. Within dairy system, the N and P contents of the rations for lactating cows were weakly related to the daily milk yield, but strongly related to the annual excretion of the nutrient per cow unit. The farm N loads were below the legal thresholds (340 kg N/ha per year), but the geographical distribution of the loads indicated two critical areas due to farm density.