Annex D5 Principles and methods used for calculating and evaluating nitrogen balances on dairy farm Exemplified with data from 3 Lithuania mixed dairy farms in 2000-2001 Ib Sillebak Kristensen, Danish Institute of Agricultural Sciences, Department of Agricultural Systems, Research Centre Foulum, P.O. Box 50, 8830 DK-Tjele, E-mail: ibs.kristensen@agrsci.dk. Introduction On dairy farms crop fertilisation and feeding practice inadequately describe nitrogen (N) loss related with the production. This is primarily because of (1) interaction between the animal and the crop production system and the manager, and (2) uncertainties of herd N production and crop utilisation. On the other hand farm gate N balances constitute an evaluation of the environmental impact related to N losses which occur in variable extend in the agricultural production process. In Netherlands farm gate N-surplus is used in legislation to limit N-loss, but without including N-input via biological N-fixation. In Denmark legislation is based on regulations of manure storage capacity and N-use in individual crops. If soil-n is assumed in steady state, then the farm gate N-surplus expresses the total losses to air (denitrifikation and ammonium volatilisation) and for N-leaching. In that sense the farm gate balance expresses the potential N-loss to the environment. In Figure 1 results from Danish pilot farm studies are illustrated. The figure shows an increasing N- surplus with increasing stocking rate, however, still with a variation of 100 kg N-surplus ha -1 between lowest and highest N-surplus at the same stocking rate. This underlines the prospects in focussing at farm gate balance even at a fixed stocking rate. 350 300 250 N- kg sur N/ 200 plu ha s 150 100 50 0 0 50 100 150 200 250 300 350 Animal manure, kg N/ha - - Figure 1. The environmental effect from nitrogen per unit of area (expressed as kg N-surplus per ha) as a function of the animal density (expressed as kg N/ha in animal manure). Data from Danish farms 1997-1999. 1
The aim of this paper is to quantify methods for calculating nitrogen balances on dairy farms. The framework for farm gate N-balance In Figure 2 the principle for calculating the farm gate balance is shown. N atmosphere Feed imported (M1) Organic manure imported (M3) Milk and meat (M5) Seed (M3) Crops for sale (M6) Mineral fertilizer (M4) N surplus soil N net feed = M1 + M3 M6 (1) N surplus = N net feed + M2 + M4 + N atmosphere M5 (2) N efficiency animals = M5/(N net feed + M2 + M4 + N atmosphere ) (3) Figure 2. The relevant elements in calculating the farm nitrogen turnover. Estimation of the individual accounts Imported feed and milk/meat sales are calculated on the basis of recorded feed intake and animal production. Milk N sale is equal to milk protein divided by 6.38 (N-content in milk protein) and meat sale calculated as kg live weight sale with N-content of 26.1 g N/kg, {Sibbesen 1990 ID: 3406}. Atmospheric nitrogen (N atmosphere ) includes nitrogen transported with precipitation (N rainfall ), and fixed by legumes (N fixation ). N rainfall is often fixed at 15 kilograms N ha -1. N fixation in grass/clover can be calculated from acreage in legume crops, taking into consideration the soil cover of clover and age of sward, Kristensen et al (1995) Table 1. 2
Table 1. Total input of atmospherically derived nitrogen (total fixed kg N ha -1 ) at varying content of clover and different cropping year, Kristensen et al (1995). Visual clover content (soil cover) 10-29 30-49 Above 49 (dry matter clover content) (13-16) (17-29) Above 30 1 st and 2 nd year sward 78 156 248 3 rd and older sward 47 84 128 In crops where legume DM-yield and legume N-content are measured the N2-fixation is calculated in accordance with harvested legume dry matter and its nitrogen content, following Høgh-Jensen et al (1998). Nfix = DMlegume* N% * Ndfa* (1+Nroot+stubble+Ntrans soil +Ntrans-animal+Nrhizodeposition) where DMlegume = Harvested mass of dry matter in legumes (kg DM ha -1 ) N% = Per cent of N in legume dry matter Ndfa = N derived from atmosphere in harvested legume material Nroot+stubble = N derived from atmosphere in legume stubble and root Ntrans-soil = Proportion of total N-fix located in grass and transferred underground from legume plants to grasses Ntrans-animal = Proportion of total N-fix located in grass and transferred via grazing animal urine to grass = Proportion of total N-fix located in immobilised organic matter in soil Nrhizodeposition In Table 2 is given the parameters for calculating N-fixation: 3
Table 2. Parameters for calculating N2-fixation when legume DM-yield and N-content are measured, after Høgh-Jensen et al (1998) and Kristensen et al (2002). Crop % N in legume DM N% N dfa N root+stubble N trans-soil N trans-animal N rhizo. 9) Kg N-fix per % ton legume-dm of gross of net losses harvested Peas for harvest 3,8 0,70 4) 0,31 37 5) 41 5) 10 Horse bean 49 5) 54 5) 10 Cut for conservation Red clover 2,7 0,75 4) 0,25 0,25-,4 9) 31-34 38-42 20 Lucerne 3,0 0,75 4) 0,25 0,25-,4 9) 34-37 42-46 20 Peas+cereal for silage 2,67 0,82 0,12 24 30 20 1-2. years grass/white clover 4,3 0,90 0,25 0,20 10) 0,25-,4 9) 66-72 82-90 20 1-2. years grass/red clover 2,72 0,90 0,25 0,10 10) 0,25-,4 9) 39-42 49-54 20 >3. years old grass/white clover 4,3 0,90 0,25 0,30 10) 60 75 20 Grazed 1-2. years grass/white clover 4,7 11) 0,78 12) 0,25 0,20 10) 0,20 0,25-,4 9) 70-75 116-125 40 >3. years old grass/white clover 4,7 11) 0,78 12) 0,25 0,30 10) 0,20 64 107 40 2) Losses of grass production used for calculating kg N-fix per ton legume DM of net harvested. 3) N dfa is 10% higher on sandy soils with less than 10% clay and less than 2.5% soil-c. 4) If N-fertiliser is applied then Ndfa = Ndfa 0.00267 * N (Høgh-Jensen and Schjørring (1997) N = fertiliser N. N from organic manure must be recalculated to fertiliser N equivalents. 5) Per ton legume grain DM 9) N rhizodeposition = 0.25 on sandy soils (% clay <10%). = 0.4 on clayey soils (% clay > 10%). Høgh-Jensen and Schjørring (2001) 10) Høgh-Jensen and Schjørring (2000). 11) Søegaard (2001). 12) Vinther (2001). Expansion/detailing the N-balance model into herd and field balances In order to evaluate possibilities for reducing N-losses, the farm gate balance is divided into internal accounts for N-balances for livestock, field and storage, respectively, which in total accumulates to the farm gate balance Sveinsson et al (1998). See Figure 3. Nitrogen turnovers on a farm are divided into the two categories, herd and fields, as shown in Figure 3. Animal feed is converted to milk, meat and manure. Animal manure can be subdivided into three types: faeces excreted in the stable, urine excreted in the stable and faeces + urine excreted during grazing. To this is added any purchased animal manure and bedding to form the total amount of manure. As shown in Figure 3, part of the nitrogen in animal manure may be lost to the atmosphere (N loss atmosphere), mainly as ammonia during manure transfer in stables, storage, field application and grazing. The stubble losses are set to Danish standards (Poulsen & Kristensen, 1998). Seeds, purchased mineral fertiliser, nitrogen from the atmosphere, and the animal manure comprise the input of nitrogen to the fields. The difference between input and yield of nitrogen from the fields can be described as N surplus, soil. This quantity describes the net supply to the soil for an individual year, but conveys nothing about the nitrogen s further turnover in the soil. If changes in soil-n are assumed zero the N surplus, soil = leaching, calculated by difference. 4
The efficiency (output/input) can be evaluated on herd and field level, see Figure 3 for equations. Feed (I1) Farm, N-surplus N-eff anim N-eff Herd, N-surplus N-eff Manure on grazing Milk & meat (O1) Amm. loss grazing (L2) Amm. loss stall (L1) Straw (I2) Roughage (I3) Amm. for straw (I8) HI1 Feed straw storage Feeding losses(m2) Straw(M3) M1 Manure storage Organic manure (O3) Amm. loss storage (L3) Amm. loss feed storage (L4) Organic manure (I4) FO1 Amm. loss spreading (L5) FI1 FI2 Fixation (I5) Field, N-efficiency Denitrification (L6) Precipitation (I6) Fertilizer (I7) Seed (I8) Organic soil-n Change in soil-n (P1) N Cash crop (O2) Leaching, (calculated by difference) (L7) N surplus herd = l1 + Hl1 O1 N efficiency herd = 100 * O1/(l1 + Hl1) N surplus field = Calc. leaching = L7 = N input field + Fl1 + Fl2 L6 O2 + P1 FO1 N efficiency field = 100 * (O2 + FO1)/(N input field + Fl1 + Fl2 P1 L6) Figure 3. Herd and crop nitrogen turnover Examples from Lithuanian Study farms On three Lithuanian mixed dairy farms registrations has been conducted in year 2000-2001. All the farms were characterised by a very scattered field distribution with a roughage crop rotation next to houses and several cash crop fields typically situated further away from farm buildings, making dairy cow grazing impossible and silage production very expensive because of transport cost. N-balances have been calculated on total area (Table 5), and supplemented with N-balances on roughage area and on cash crop area within the farm (Table 6-7). To illustrate the utilisation of nutrient accounting the example of Jasiulis in year 2000 is illustrated in Table 3-8. For calculating the internal balances the required input data is given in Table 3 and 4. Gray colour is input-data fields. In Table 3 the data is given for crops, identity with standard chemical composition, area and input of fertiliser and manure N is given. 5
Table 3. Field-budget, used as input-data for calculating farm gate N-balances System: Mixed dairy Farm: Jasiulis Year= 2000 Manure N-fertiliser from stable Fixation Legume % of N-fix kg N/ton leg. SFUyield Yield Crop group Crops Id Ha Kg N/ha Kg N/ha Kg N/ha DM DM SFU/ha SFU Cash crops Wintercereal, grain 203 35 86 4187 144452 Rape 213 16 93 2848 44144 Beets 351 4 266 96 3960 15840 Flax 212 6 5790 34740 Roughage crops Grass/clover 522 23 170 49 12 90 3696 85378 Whole crop 583 Maize 594 4 203 3974 15896 Accumulated 87 117 4 13 3909 340449 The fixation is calculated from legume content of dry matter in grass/clover, the fixation coefficient from Table 2. From measured yield the N-output from field can be calculated. Table 4. Herd-budget used as input data for calculating farm gate balances. System: Mixed dairy Dairy-cows 17.7 Heifers 6,6 Farm: Jasiulis summer winter summer winter Year= 2000 Days= 180 185 200 165 %SFU Crop 1000 SFU SFU SFU SFU SFU SFU SFU 1000 SFU used of group Crops Id SFU/ha available /cow/day /cow/day /cow/year /heifer/day /heifer/day /heifer/year used total produced Roughage crops Grass/clover 522 3696 85 2,0 6,0 1470 3,0 495 29 73 do. grazed 422 3696 9,0 1620 3,0 600 33 Maize 594 3974 16 4,0 740 0 13 82 Wintercereal, grain 201 4187 144 3,5 3,5 1278 0,7 0,7 256 24 Rape 213 2848 44 0 0 Beets 351 3960 16 0 0 Flax 212 5790 35 0 0 Concentrate rape crushed 145 1,7 312 0 5 Total SFU 14,5 15,2 5419 3,7 3,7 1351 104 Results: 6849 Kg ECM/cow In Table 4 the total produced yield is transferred to input in the herd budget (1,000 SFU available). The feed consumption is calculated from the number of animals, feeding days summer & winter and a daily uptake of feed animal -1 day -1. The feed consumption is adjusted to registered feed used (1,000 SFU used total). And in the right colon the utilisation of produced forage is calculated, showing 27% losses of grass/clover silage and 18% losses of maize silage. The output in milk per cow is given from yield control in the bottom. From Table 4 her herd-balance is calculated with standard N-content of the crops. 6
Table 5. Farm gate N-balances for a mixed dairy farm. Kg N ha -1 year -1. Farm: Jasiulis Year= 2000 Farm account Input Output Ha: 87 LSU/ha: 0.23 Controllable components Kg N Kg N ha -1 Kg N Kg N ha -1 Fertilisers 10273 118 Milk -595-7 N - fixed 1100 13 Meat -89-1 Manure import 0 0 Crops for sale -3726-43 Purchased feed 258 3 Straw NH3-treatment 0 Manure sold 0 0 Random components 0 Precipitation 871 10 Capital components Livestock 0 0 Livestock 0 0 Feed 0 0 Feed -536-6 Manure 0 0 Manure 0 0 Total input 12502 144 Total output -4933-57 Farm balance Farm balance 7569 87 N-efficiency livestock, % 25 N-efficiency field, % 51 % loss of input -components of losses" Amm. loss stable -70-1 5 Straw amm. loss 0 0 0 Amm. loss manure storage -949-11 71 Amm. loss field -209-2 30 Difference (=leaching+denitrification+soil-n changes) -6342-73 48 The farm gate surplus is 87 kg N ha -1. The main reason is a low overall stocking rate of only 0.23 Livestock unit s (LSU) ha -1. If only the roughage area is evaluated the surplus rice s to 177 kg N ha -1 with a stocking rate of 0,73 LSU ha -1. From Danish pilot farm studies a typical N-surplus on farms with 0.8 LSU ha -1 is around 100 kg N-surplus ha -1, Halberg et al (1995). The main reason for this relatively high level of N-surplus is low milk production (22 kg N ha -1 ) in relation to high input level of 236 kg N ha -1 (mainly 176 N-fertiliser ha -1 in the roughage area.). 7
Table 6. Internal livestock N-balances, kg N LSU -1 year -1. Farm: Jasiulis Year= 2000 Livestock account Input Farm Total Part of farm Output Farm Total Part of farm Roug- Cash hage crop Total Total Roug Cash hage crop Number of LSU= 20 20 0 20 20 0 Controllable components Kg N Kg N LSU -1 Kg N Kg N LSU-1 Grain feed from storage 390 20 20 0 Milk -595-30 -30 0 Purchased feed 258 13 13 0 Meat -89-4 -4 0 Conserved feed from storage 1133 57 57 0 Grazed forage 931 47 47 0 Capital components Livestock 0 0 0 0 Livestock 0 0 0 0 Total input 2712 136 136 0 Total output -684-34 -34 0 Livestock balance Balance=Manure produced 2028 102 102 0 Grazing manure -625-31 -31 0 Manure for storage -1333-67 -67 0 -components of "losses" % loss of input Amm. loss stable -70-4 -4 0 5 N-efficiency livestock 25 Table 7. Internal field N-balances for a mixed farm. Kg N ha -1 year -1. Farm: Jasiulis Year= 2000 Field account, farm Input Farm Total Part of farm Output Farm Total Part of farm Roug Cash hage crop Roug hage Cash crop Ha: 87 87 27 60 87 87 27 60 LSU/ha: 0.73 0.73 0.00 0.23 0.73 0.00 Controllable components Kg N Kg N ha -1 Kg N Kg N ha -1 Manure from storage 384 4 0 6 Conserved roughage -1669-19 -62 0 Manure import 0 0 0 0 Grain for feed -390-4 -14 0 Manure under grazing 625 7 23 0 Grazed feed -931-11 -34 0 Fertilisers 10273 118 176 92 Crops for sale -3713-43 0-62 N - fixed 1100 13 41 0 Random components Precipitation 871 10 10 10 Total input 13354 152 250 108 Total output -6703-77 -110-62 Field balance Field balance 6551 75 140 46 %loss of input -components of "losses" Amm. loss field -115-1 0-2 30 Amm.loss grazing -94-1 -3 0 15 Difference (=leaching+denitrification+soil-n changes) -6342-73 -136-44 48 N-efficiency field 51 44 57 8
Table 8. Internal storage N-balances for a mixed farm. Kg N ha -1 year -1. Farm: Jasiulis Year= 2000 Storage account, farm Input Farm Total Part of farm Output Farm Total Part of farm Roug Cash hage crop Total Total Roug hage Cash crop Ha: 87 87 27 60 87 87 27 60 LSU/ha: 0.23 0.23 0.73 0.00 0.23 0.23 0.73 0.00 Controllable components Kg N Kg N ha -1 Kg N Kg N ha -1 Manure from stall 1333 15 49 0 Manure sold 0 0-14 6 Grain feed from field 390 4 14 0 Manure used -384-4 0-6 Conserved feed from field 1768 1669 19 62 0-1232 -1133-13 -42 Capital components Feed 0 0 0 0 Feed -536-6 -20 0 Manure 0 0 0 0 Manure 0 0 0 0 Total input 3391 39 125 0 Total output -2053-24 -76 0 Storage balance Storage balance 1338 15 49 0 % losses of input -components of "losses" Amm. loss manure storage -949-11 -35 0-71 Farm balance from Table 1 7569 87 177 46 Subbalance check (Livestock + Field + Storage balance) 7569 87 177 46 In order to the evaluated possibilities for improvements, the three sub-systems are detailed in Table 6-8. The livestock account shows a high efficiency of milk production with 25% efficiency for dairy, compared to Danish average level of 16% efficiency (Halberg et al., 1995). The reason for this is mainly that the herd in consideration only has a small proportion of young-stock which have a low N-efficiency. The field account N efficiency is 51% on roughage crop and 44% efficiency on cash crop rotation. This is average level compared to Danish pilot farms (Nielsen et al., 2001). If grass/clover yield could be raised from Scandinavian Feed Unit 3696 (SFU=feeding value of 1 kg barley) per ha (Table 3) to the level of 5.500 SFU ha -1 (equal yield in 1. year grass/clover given 143 kg N-fertiliser ha -1 ) the gate efficiencies could be raised 3% units. Another improvement can be made on manure utilisation (Storage accounts). Only 22% of the stall produced animal manure is spread on the fields. The urine is not collected and the faeces is mixed with only limited amount of straw making the evaporating and composting ammonium losses very high especially during summer. If more manure could be used for fertilisation then import of artificial fertiliser could be reduced or yield could be raised. From the N-account it is concluded that improvement can be expected if better storage facilities for manure could be built. Also if higher yield could be achieved with same input, the N-efficiency could be increase, whereas no major improvement can be expected in the herd. All improvements would decrease environmental losses. From the components of losses it seems like N-leaching is the major losses. If soil-n is not in balance some of the calculate losses for leaching and denitrifikation can be accumulated in the soil. Overall this situation is not expected, whereas some soil-n accumulation can be expected in the roughage area, but the cash crop area counteracts this where soil-n typically drops. 9
The key Figures from all 4 Lithuanian cases is shown in Table 9+10. The example Jasiulis, 2000 can be compared with detailed nutrient balances in Table 3-8. The detailed data from the individual farms are shown in farm reports. Table 9. Herd uptake, production and efficiency. Losses in manure and silage production on three Lithuanian mixed farms in 2000-2001. Jasiulis Jasiulis Krapikas Turskiene 2000 2001 2001 2001 Feed uptake, SFU (cow+heifer) -1 - Conserved 2,423 1,847 2,235 2,357 - Grazed 1,855 2,826 3,075 1,585 - Cereal 1,351 1,041 1,495 899 - Concentrate 294 181 0 954 - Total 5,922 5,896 6,805 5,795 ECM, kg cow -1 6,849 6,527 5,806 5,035 Feeding efficiency winter 84 92 91 93 N-production, kg N LSU -1 116 142 124 100 N-efficiency herd, % of N 25 19 18 18 Manure amm. losses 1), % of N 73 78 (100) 2) 85 Silage losses 3), % of SFU 8 33 52 17 1) Produced minus spread on fields 2) No manure used in year 2001 3) Produced minus net uptake in herd From Table 9 it can be seen that homegrown roughage and cereals are the main part of the feed ration, whereas imported concentrate feed is low except at Turskiene where concentrate feed were 16% of the total consumption. All herds had a high winter-feeding efficiency (84-93%), which show an efficient milk production in relation to feeding. The highest N-production from the herd was Krapikiené; the reason was a high intake of grass. The N-efficiency herd varied between 18-23, the same interval as Danish dairy farms, Halberg et al (1995). The registered spread manure indicates big ammonia losses, and better storage facilities could increase manure-n for crops considerable. Danish standard is only 10% ammonia losses in stall and storage, Poulsen & Kristensen (1998). The registered roughage production compared to registered feed in herd also indicate 8-52% losses in field and during storage. Danish standards for silage production are 13% losses. In Table 10 the roughage area per cow is calculated. The roughage area varies from 1.0-2.2 ha LSU -1 lowest at Jasiulis in 2001 with a high crop yield and highest at Krapikiené with only 2,600 SFU ha -1. The quality of the soil can explain some of the difference. However the high yield from Jasiulis were mainly achieved on 1.-2. year old grass/clover high level of efficiently grazed with low proportion of rejected grass and low content of weed. 10
Table 10. Land use, net yield and N-surplus & N-efficiency on three Lithuanian mixed farms in 2000-2001. Jasiulis Jasiulis Krapikas Turskiene 2000 2001 2001 2001 Land use, ha LSU -1 - Grass, wc 1) 1.15 0.95 2.23 1.77 - Maize 0.20 0.15 0.57 0.11 - Cereal 1.73 1.11 1.21 2.00 - Rape, beets, flax 1.28 1.38 0 0 - Total farm 4.35 3.57 4.01 3.88 Net yield, SFU ha -1 - Grass & silage cereals 3.652 4.926 2.711 3.732 - Maize 3.974 4.750 2.160 7.500 - Cereal 4.192 3.497 3.302 3.300 - Rape, kg seed 1.683 1.587 - - - Sucker beets, kg, DM 3.960 8.067 - - - Total 3.898 3.627 2.811 3.394 N-surplus, kg N ha -1 - Total 87 95 108 120 - Roughage 178 156 135 243 - Cash 46 69 47 56 N-efficiency field, % of N - Roughage 47 58 41 31 - Cash 57 43 41 47 1) WC = Whole crop silage of barley At the bottom of Table 10 the farm gate N-balance is shown as well as the N balance for roughage area and cash crop area, respectively. The N-surplus is high compared to Danish standards. However it is difficult to compare these levels, because Danish farms normally include roughage and grazing on the total area, whereas the Lithuanian farms only had grass/clover on 1/3 of the area around buildings. The N-surplus on the roughage area varies from 135-243 kg N ha -1, which is the same level as conventional Danish mixed dairy farms, Halberg et al., (1995) with a high stocking rate. The N-efficiency is typically higher in the cash rotation compared to roughage crop rotation. This is typically and primarily caused by grazing where only milk is produced typically only removing by 30 kg N ha -1, only 50% of the typically N-yield in cash crops. 11
Reference Halberg, N., Kristensen, E. S., and Kristensen, I. S. 1995: Nitrogen turnover on organic and conventional mixed farms. Journal of Agricultural and Environmental Ethics 8, 30-51. Høgh-Jensen, H., Loges, R., Jensen, E. S., Jørgensen, F. V., and Vinther, F. P. 1998: Empirisk model til kvantificering af symbiotisk kvælstoffiksering i bælgplanter. In: Kvælstofudvaskning og -balancer i konventionelle og økologiske produktionssystemer. Eds. Kristensen, E. S. and Olesen, J. E. FØJO-rapport 2, 69-86. Høgh-Jensen, H., Loges, R., Jensen, E. S., Jørgensen, F. V., and Vinther, F. P. 1998: Empirisk model til kvantificering af symbiotisk kvælstoffiksering i bælgplanter. In: Kvælstofudvaskning og -balancer i konventionelle og økologiske produktionssystemer. Eds. Kristensen, E. S. and Olesen, J. E. FØJO-rapport 2, 69-86. Høgh-Jensen, H. and Schjørring, J. K. 1997: Interactions between white clover and ryegrass under contrasting nitrogen availability: N2 fixation, N fertiliser recovery, N transfer, and water use efficiency. Plant and Soil 197, 187-199. Høgh-Jensen, H. and Schjørring, J. K. 2000: Below-ground nitrogen transfer between different grassland species: Direct quantification by 15N leaf feeding compared with indirect dilution of soil 15 N. Plant and Soil 227, 171-183. Høgh-Jensen, H. and Schjørring, J. K. 2001: Rhizodeposition of nitrogen by red clover, white clover and ryegrass leys. Soil Biology and Biochemistry 33, 439-448. Kristensen, E. S., Høgh-Jensen, H., and Kristensen, I. S. 1995: A simple model for estimation of atmospherically-derived nitrogen in grass-clover systems. Biological Agriculture and Horticulture 12[3], 263-276. Kristensen, I. S., Petersen, B. M., Knudsen, L., and Høgh-Jensen, H. 2002: Indirekte beregning af N-fiksering. Danmarks JordbrugsForskning. Rapport. Markbrug. Poulsen, H. D. and Kristensen, V. F. 1998: Standard values for farm manure. A revaluation of the Danish standard values concerning the nitrogen, phosphorus and potassium content of manure. Danish Institute of Agricultural Science report Animal Husbandry. 7, 1-160. Søegaard, K. 2001: Udvikling af afgræsningssystemer. N-gødskning og N-overskud. Studielandbrug. Årsrapport 2001. 39-42. Sveinsson, T, Halberg N., and Kristensen, I. S. 1998: Problems associated with nutrient accounting and budgets in mixed farming systems., 135-140. 25052. Vinther, F. P. 2001. Personal communication. 12