Nitrogen and phosphorus excretion on mountain farms of different dairy systems

Agricultural Systems 168 (2019) 36–47

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Nitrogen and phosphorus excretion on mountain farms of different dairy systems

T

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Schiavon Stefano, Sturaro Enrico, Tagliapietra Franco , Ramanzin Maurizio, Bittante Giovanni Department of Agronomy, Food, Natural Resources, Animals and Environment (DAFNAE), Viale dell’Università, 16 Legnaro, Padua, Italy

A R T I C LE I N FO

A B S T R A C T

Keywords: Dairy cow Ecological footprint Manure Environmental impact Nitrogen balance

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.

1. Introduction There has been a fall in the number of traditional farms in Alpine areas and new models of dairy farming producing more milk per cow have been introduced (Mack and Huber, 2017; Battaglini et al., 2014). The number of farms adopting the intensive dairy system of the lowland has gradually increased, particularly in the lower valleys, where the cultivation of cereals, the purchase of production inputs and the selling of products are facilitated. The higher milk productivity of modern farms is supported by feed components imported from the flat land (mainly from the Pò Valley) with higher contents of energy and protein concentrates, and less use is made of local forages and pastures to meet animal requirements. These practices may increase N and P intake and excretion of them in the manure (Penati et al., 2011). When used in excess of crop or grassland requirements, N and P become pollutants

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(Wang et al., 2004). Excessive soil application of N and P results in leaching of these elements, eutrophication of water bodies (Anzai et al., 2016), and alterations to the plant and animal biodiversity of grasslands (Humbert et al., 2016). The emission of ammonia or other N compounds from manure can also lead to environmental acidification (OECD, 2013). In the European Union, water bodies are protected from nitrate pollution by fixing a maximum threshold for the amount of N that may be disposed of per unit of agricultural land (EEC, 1991). Standards for the excretion of N in manure per animal are commonly used to calculate the land needed for manure disposal (Schiavon et al., 2012). Nitrogen in manure is calculated as N excreted minus N lost in the atmosphere. These standards are often defined on a national basis with little or no attention paid to the particular characteristics of mountain and marginal areas and different dairy farming systems. Public

Corresponding author. E-mail address: [email protected] (T. Franco).

https://doi.org/10.1016/j.agsy.2018.10.006 Received 10 April 2018; Received in revised form 27 September 2018; Accepted 23 October 2018 0308-521X/ © 2018 Published by Elsevier Ltd.

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farms without ALP (T-noALP), characterized by tie stalls but without summer transhumance of the cows; traditional farms using silages (TS), characterized by tie stalls and modern feed management; and modern farms (MOD), characterized by loose housing and modern feed management. Almost all the farms move replacement heifers to summer highland pastures. Data from three institutional databases were merged and some descriptive statistics are given in Table 1 in Schiavon et al. (Submitted). Information on herd size and composition, expected lactations, calving interval, length of dry periods, and age at first calving from each farm was provided by the Province of Trento’s veterinary service, which provides data for the Italian Ministry of Health’s register of the national cattle population. Body weight (BW) of mature cows of different breeds (Holstein-Friesian, Brown Swiss, Rendena, Alpine Grey, and crossbreds) was estimated from a survey carried out by Gallo et al. (2017) in the same area. CONCAST, the consortium of dairy cooperatives of the Province of Trento, provided from their database information on the milk yield and composition of each farm obtained from bulk milk samples collected every two weeks from each farm for the milk payment system. The lactose and milk protein contents were determined with a MilkoScan FT6000 (Foss, Hillerød, Denmark). Data concerning the UAA of each farm were extracted from the georeferenced cadastral map database of the Province of Trento using the GIS software ArcGis 10® (ESRI Italia, Roma). The UAA of each farm was categorized as either crops, meadows or pastures. The summer Alpine pasture areas were considered separately as they are frequently used by associations of farmers. Technicians registered with FPA-Trento’s milk recording program visited each farm once over the periods April to September and October to March. They recorded the ingredients and their proportions in the rations given to lactating cows, the numbers of dry cows and heifers, and the numbers of animals moved to Alpine pastures during the summer and the length of time spent there. A farm was excluded if information on the formulation of the rations for lactating cows (see below) was missing or unreliable. The final database consisted of 564 farms, housing 18,455 dairy cows (32.7 dairy cows/farm on average), representing 74% of all cows in the Province of Trento. The list of feed ingredients, methods of analysis, sources of information, and estimated or measured N and P contents are given in Schiavon et al. Submitted. The N and P contents of the rations for lactating cows, dry cows and heifers (Table 2) were calculated from the ingredient composition of the rations [see Table 3 in Schiavon et al. Submitted].

administrators and operators need a better definition of excretion that takes local conditions into account (Velthof et al., 2015; Carof and Godinot, 2018). Tools and indicators to discriminate farms according to their impacts are needed in order to identify the best action strategies (Miller et al., 2017). A useful approach would be to build models able to estimate: a) the annual excretion of N and P of the average cow (including its share of replacement heifer), to serve as a unit of measure for comparison; b) the excretion of nutrient per unit of milk produced, as an index of nutrient utilization inefficiency; c) the annual quantity of N and P present in the manure per unit of agricultural land available for manure spreading, as an indicator of potential environmental impact. Sturaro et al. (2013) combined data drawn from institutional databases on all the dairy farms in the Province of Trento, and classified them as: traditional farms with or without transhumance to summer highland pastures (ALP); traditional farms using silage; and modern farms. We hypothesized that a model developed from these databases would be able to predict farm nutrient excretions from data from current databases and farm visits in order to shed more light on the differences among and within dairy systems. In this paper, we describe a model to predict N and P excretion on Alpine dairy farms that accounts for their structural traits, type of rations and use or not of highland pastures. The model was used to examine the causes of variation among and within farming systems in the annual excretions of cows and heifers, in the amounts of N and P in the manure per unit of milk produced and per hectare of farm utilized agricultural area (UAA), and to investigate the geographical distribution of the N and P loads in the study area. 2. Materials and methods 2.1. Sources of information The study was carried out in the Province of Trento (north-eastern Italy), which covers an area of 6,200 km2 divided into 217 local authority districts (ISTAT, 2010). Of the 1,071 dairy farms in the entire province, 610 were selected from the milk recording system of the Federation of Breeders of the Province of Trento (FPA-Trento). Farms with unreliable information were excluded. A cluster analysis carried out by Sturaro et al. (2013) classified the farms into four groups (Table 1) according to barn type (tie stall vs. loose housing), whether or not they used total mixed rations, grass or corn silages, and whether or not they made use of summer Alpine pastures (ALP). These four groups were: traditional with ALP (T-ALP), characterized by tie stalls and movement of heifers and cows to summer Alpine pastures; traditional

Table 1 Profiles of the four dairy farm systems identified by cluster analysis (% within dairy system).

Farms (n) with tie stall, % use of total mixed ration, % use of silages, % with cows on ALPb, % with heifers on ALPc, % Utilized Agricultural Area, ha Cows, n Cows, n/ha Livestock Unitd, n/ha Milk yield per cow, kg/year Farms with solid manure: from cows, % from replacement cattle, % a b c d

All farms

Traditional with ALPa (T-ALP)

Traditional without ALP (T-noALP)

Traditional with silages (T-S)

Modern (MOD)

564 72 19 18 54 85 17.4 ± 13.9 32.5 ± 33.2 2.2 ± 2.6 2.9 ± 3.2 5992 ± 1524

285 86 1 0 100 90 12.1 ± 9.0 20.2 ± 19.5 2.1 ± 2.5 2.7 ± 2.9 5386 ± 1265

138 100 4 21 0 80 15.3 ± 9.6 24.7 ± 17.4 2.1 ± 2.7 2.7 ± 3.5 5749 ± 1175

31 78 81 100 45 71 25.7 ± 14.2 50.1 ± 30.7 2.3 ± 1.3 3.1 ± 1.8 6545 ± 1404

110 0 67 39 7 85 30.9 ± 17.8 67.5 ± 47.7 2.7 ± 3.1 3.5 ± 3.6 7624 ± 1364

91.9 84.6

95.7 88.4

90.3 67.7

58.6 56.8

ALP = summer alpine pasture. Percentage of farms practicing seasonal transhumance of cows and heifers from valley farms to alpine pastures. Percentage of farms practicing seasonal transhumance only of heifers from valley farms to alpine pastures. The livestock unit was computed considering a coefficient of 1.0 and 0.6 for cow and heifers, respectively. 37

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Table 2 N and P contents (g/kg DM) of the lowland permanent farm rations for lactating cows, dry cows and heifers in 564 farms operating different dairy systems.a Traditional with ALP1 (T-ALP)

Traditional without ALPb (T-noALP)

Traditional with silage (T-S)

Modern (MOD)

Mean

SD

Mean

SD

Mean

SD

Mean

SD

NDietCow PDietCow

22.1 3.7

1.8 0.8

22.4 3.9

1.9 0.9

23.2 4.2

2.6 0.8

24.5 3.8

2.4 0.9

NDietDry PDietDry

21.0 3.4

0.6 0.3

21.0 3.4

0.8 0.3

20.3 3.3

1.0 0.3

20.8 3.2

0.8 0.4

NDietHeif PDietHeif

18.7 3.4

0.5 0.3

18.7 3.4

0.5 0.2

18.4 3.1

0.5 0.3

18.7 3.5

0.5 0.4

Acronym

Lactating cows N P Dry cows N P Heifers N P

a Variations in N and P contents comes from differences in N and P contents in the rations. Tabular values were used to assign a N and a P content only to cereals, proteins feeds and by products with a low intrinsic variability in chemical composition. Only data collected in the farms were considered. Pasture in the lowland permanent farm is not practiced because it is excluded for the destination of milk to traditional hard cheese. Pasture is instead the base of feeding on highland temporary summer farms where milk is destined to different types of cheeses. Information about feeds and rations consumed during the permanence at the ALP was not available. Specification are given in the supplementary material (see Table 2 in Schiavon et al. Submitted). b ALP = summer transhumance to alpine pasture.

because their dry matter intakes and ration compositions differ. A mechanistic model was developed from information obtained from the various databases, as described in the model structure (Fig. 1). The number of dry and lactating cows was calculated from the total number of cows in the herd and from the lengths of the dry and calving interval periods (Table 4 in Schiavon et al. Submitted). We estimated the

2.2. The model to estimate N and P excretions The core of the model was the mass balance criterion, where excretion is calculated as the difference between the intake of each nutrient and its retention in the body and secretion with the milk. This criterion was applied separately to lactating cows, dry cows and heifers

Table 3 Dry matter intake and estimated annual N and P balances of the average dairy cow and average heifer in different Alpine farming systemsa. Farms

N balance, kg/year: Standardized dairy cow DM intake, kg/d N intake N secretion + retention N excretion N in manurec Milk N/N intake Heiferd DM intake, kg/d N intake N retention N excretion N in manure Retained N/N intake P balance, kg/year: Standardized dairy cow P intake P secretion + retention P in manure (excreted) Milk P/P intake Heifere P intake P retention P excretion Retained P/P intake

RMSE3

P-values of contrasts

Traditional with ALP (T-ALP)2

Traditional without ALP (T-noALP)b

Traditional with silage (T-S)

Modern (MOD)

Traditional vs Modern

T-ALP + TnoALP vs. T-S

T-ALP vs. TnoALP

17.2 138 31

17.0 139 33

17.8 149 37

18.9 166 44

< 0.001 < 0.001 < 0.001

0.020 0.002 < 0.001

0.19 0.62 0.003

1.52 19.2 7.5

107 77 0.222

106 76 0.238

112 81 0.249

123 89 0.261

< 0.001 < 0.001 < 0.001

0.031 0.031 0.002

0.36 0.36 < 0.001

14.0 10.0 0.036

6.7 46 5 41 30 0.103

6.3 44 5 39 28 0.107

6.2 42 5 37 27 0.110

6.3 43 5 38 27 0.111

0.002 0.009 0.17 0.002 0.002 0.002

< 0.001 < 0.001 0.42 < 0.001 < 0.001 0.044

< 0.001 < 0.001 0.06 < 0.001 < 0.001 0.002

0.32 2.8 0.5 2.6 1.9 0.011

23.3 6.1

24.1 6.6

26.9 7.4

26.1 8.6

0.031 < 0.001

0.001 < 0.001

0.21 0.004

5.07 1.44

17.2

17.5

19.5

17.5

0.26

0.016

0.63

4.51

0.267

0.280

0.282

0.338

< 0.001

0.45

0.042

0.063

8.3 1.1 7.2 0.134

7.9 1.1 6.8 0.139

7.1 1.1 6.0 0.154

8.0 1.1 6.9 0.139

< 0.001 0.17 < 0.001 0.08

< 0.001 0.42 < 0.001 < 0.001

< 0.001 0.055 < 0.001 < 0.001

0.39 0.13 0.36 0.014

a Data presented in this table assumes that the cow and the heifer are kept in the permanent farm for all the year duration. Pasture in the permanent farms is not practiced. Information about feeds and rations consumed during the permanence at the summer alpine pasture was not available. b ALP = summer transhumance to alpine pasture. c Root Mean Square Error. d Computed from N excreted considering a 28% of N volatilization. e The N and P balance of heifers were significantly different between farm systems notwithstanding the low quantitative differences among groups. This resulted because of the low variability of parameters used to estimate the nutrients intake, retention and excretion.

38

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Fig. 1. Structure of the model to estimate on-farm N and P excretions from information gathered from institutional databases and farm interviews.

BW at first calving and the average BW of the cows of each herd using an equation developed from data reported by Gallo et al. (2017) obtained from a survey of 555 cull cows reared on 182 farms, all of which were included in the current study. The equation included age at first calving, BW at maturity by breed, and the farm’s barn system as independent variables. The newborn calf’s BW was related to the cow’s BW (NRC, 2001). The heifer’s BW and average daily gain were calculated from the age and BW at first calving and the estimated BW of the calf. The DM intake of lactating and dry cows and heifers was estimated using the following inputs for each farm: the average milk yield, the BW of the average cow or heifer, the length of the dry period, and the ration content of net energy (NEm) for maintenance (NRC, 2001). The N and P intakes were estimated from the DM intakes and the N and P contents of the rations calculated for each farm (Table 5 in Schiavon et al. Submitted). The amount of N secreted with the milk was determined for each farm from the milk protein content using a MilkoScan, while milk P secretion was estimated from milk protein and lactose contents in accordance with Klop et al. (2014). N and P retention in the bodies of lactating cows, dry cows, calves and heifers was calculated from the changes in BW, assuming these to be 25.0 g/kg for N and 5.9 g/kg for P (NRC, 2001). N and P retention in the newborn calf was entirely accounted to the dry cow. The items of balance for the lactating and dry cows were combined by weighting the relative incidences over the year in order to calculate a unique balance for each cow. The total herd excretions of N and P were calculated by multiplying the cow’s and heifer’s daily excretion of each nutrient by the number of animals in the herd and 365, the number of days in a year (Table 5 in Schiavon et al. Submitted). The herd N and P excretion on ALP or on farm was assumed to be a proportion of the total annual excretion of the herd, based only on the days of permanence of the cows and the heifers on the lowland farm or on ALP, respectively (Table 5 in Schiavon et al. Submitted). Thus, feed or rations consumed during the period of

permanence at the ALP were not considered. The amount of N in the manure was calculated by assuming that 28% of N excreted is lost in the atmosphere, in accordance with current legislation in the investigated area (MIPAAF, 2016). The excreted P was taken as remaining entirely in the manure. The total amount of N and P excreted on each farm was expressed per cow unit, defined as the cow and its related share of replacement heifer either per kilogram of milk produced on the farm or per unit of farm UAA. 2.3. Statistical analysis The data were subjected to a one-way ANOVA in SAS (SAS Inst. Inc., Cary, NC) using dairy system as the source of variation. Orthogonal contrasts were run to test: (1) the effect of traditional vs modern farms (T vs. MOD); (2) the effect of the traditional farms with and without ALP vs those using silages [(T-ALP + T-noALP) vs T-S]; (3) the difference between the traditional farms with ALP and those without (T-ALP vs T-noALP). Before running the statistical analysis, the data were examined for normality and variance equality. If the variance was unequal, the transformed (logarithmic) data were analysed to corroborate the conclusions. We carried out a non-parametric Kruskal-Wallis test to assess differences in the UAA between dairy systems because it was impossible to obtain a normal distribution for this parameter. A posthoc Dunn test was used to compare means with the level of significance (α) set at 0.01. Linear or exponential relationships between variables within each dairy system were assessed using the function statements of the Excel spreadsheet (Microsoft Office, 15.41). 2.4. Cartographic analysis: kernel density estimation of N and P loads in manure The spatial distribution of the amounts of N and P in manure produced by each farm in the Province of Trento in one year was determined with a GIS analysis using a kernel density estimation function 39

Agricultural Systems 168 (2019) 36–47

< 0.001 < 0.001 0.147

4.42 1.037 47.5

(ArcMap, 10.1). This function calculates the density of features in a neighbourhood around those features, using their locations (in our case the spatial coordinates of each farm centre) combined with their value (in our case the total excretions of N and P of the farm), and has been used in other studies on N and P excretion, although with different scales of analysis (Rodriguez-Galiano et al., 2018; Jewell et al., 2007). The density surface obtained is dependent on the bandwidth (i.e. the neighbourhood) set around each feature. We decided to set the bandwidth at a radius of 5 km around each farm. We chose this distance because it is a threshold over which the cost of disposing of N in manure becomes twice that for disposing of the same amount of chemical N (0.7 €/kg; Provolo, 2000). Therefore, it is unlikely that farmers dispose manure farther than 5 km from their farms. 3. Results There were many differences between farming systems with respect to the items of N- and P-balance (Table 3). The cows on the modern farms had greater estimated DM and N intake, greater amounts of N retained, N secreted, N excreted and N in manure, and more efficient N utilization than the cows on the traditional farms. The values for cows on the T-S farms were intermediate. The differences between T-ALP and T-noALP were of lesser relevance. In contrast, the heifers on the modern farms had lower estimated DM and N intakes, lower amounts of N excreted and N in manure, and more efficient N utilization than the heifers on the traditional farms. The heifers on the T-S farms had intermediate values. The differences in P resembled the trends observed for N-balance for both cows and heifers. Within system, the N and P contents of the rations for lactating cows was not related to daily milk yield (Fig. 2). The modern farms had greater total N excretion per cow unit, total N excretion on the farm, and lower amounts of N excreted on the summer pastures than the traditional farms (Table 4). The quantitative differences between T-ALP + T-noALP and T-S with respect to these variables were of minor importance. The major differences between TALP and T-noALP were, in particular, that the former excreted greater amounts of N on the summer pasture, and lower amounts of N on the farm. Within system, the variation in the annual excretion of nutrients per cow unit on the farm was much greater than among systems, largely due (r2 ranging from 0.42 to 0.90) to the nutrient content of the rations for lactating cows (Fig. 3). The relationships between the on-farm N and P excretions per cow unit were positive, but weak, especially for T-S and modern farms (Fig. 4). The amount of N excreted per kg of milk yield was greater on traditional farms compared with modern farms, on T-ALP+T-noALP compared with T-S, and on T-ALP compared with T-noALP. The variations among systems were lower than within system, and the values of this ratio decreased asymptotically with increasing milk yield (Fig. 5). The patterns and relationships obtained for P excretion per kg of milk were similar, except there was no significant difference between the T-S and the other traditional farms. N excretion and N in the manure per unit of UAA was much greater on modern farms than traditional farms, but no significant differences were observed among the traditional farming systems because of the high residual variation (Table 4). No differences between dairy systems were observed for P excretion per unit of UAA, and the residual variability was also high. There were no within-system relationships between farm N excretion per kg of milk and the N in manure per ha of UAA (Fig. 6). The farm N in manure per ha of UAA was weakly or not related to the cows’ average milk yield, nor to herd size. The factor having the greatest influence was the number of cows per ha of farm land, and the slopes of the regressions indicated annual amounts of N in manure produced per cow unit of 106 kg/ha for T-ALP, 118 kg/ha for T-noALP, 111 kg/ha for T-S, and 136 kg/ha for MOD farms. There was a high correlation between the loads of N and P in manure per hectare of land (r2 = 0.94).

ALP = summer transhumance to alpine pasture. Root Mean Square Error b

a

19.9 3.6 43.7 17.1 4.1 36.6

20.7 3.5 48.2

20.2 2.8 51.1

0.089 < 0.001 0.138

0.008 0.159 0.368

4.74 1.60 0.508 < 0.001 20.8 0.9 21.1 4.0

22.7 2.0

21.4 1.2

0.767 < 0.001

0.048 0.128

17.0 5.14 298 215 < 0.001 < 0.001 0.295 0.295 0.012 0.002 0.569 0.569 120 22 256 184 106 25 224 161

121 20 272 196

137 19 362 260

< 0.001 < 0.001 < 0.001 < 0.001

0.011 < 0.001 0.086 0.148 144 7 132 11 125 5 129 23

N excretion: total, kg/year/cow unit total on summer highlands, kg/year/ cow unit total on farm, kg/year/cow unit on farm per kg of milk yield, g/kg on farm per unit of farm UAA, kg/ha N in manure, per unit of farm UAA, kg/ ha P excretion total, kg/year/cow unit total on summer highlands, kg/year/ cow unit total on farm, kg/year/cow unit per kg of milk yield, g/kg per unit of UAA, kg/ha

Traditional with silages (TS) Traditional without ALPa (TnoALP)

< 0.001 < 0.001

T-ALP vs. T-noALP Traditional vs. Modern Traditional with ALPa (TALP)

Table 4 Annual N and P farm excretions per cow unit, per unit of milk yield, and per unit of usable agricultural land (UAA).

Modern (MOD)

P-values of contrasts Farms

T-ALP + T-noALP vs. T-S

17.0 9.8

RMSEb

S. Stefano et al.

40

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Fig. 2. Relationships between the N and P contents of the rations for lactating cows and the average milk yield of 4 types of dairy farm in the Alps (Province of Trento, Italy): traditional with Alpine pasturing (T-ALP) and without (T-noALP), traditional using silages (T-S), and modern.

seasons. This is not a great problem in the study area, however, because the large majority of dairy farms produce milk to make traditional cheeses and use an indoor feeding regime throughout the entire year (Bittante et al., 2011). Pasture and fresh forage are almost never used on the permanent farms, whereas it is the basic diet on all temporary summer farms on highland pastures (Zendri et al., 2016). Nonetheless, this shortcoming could be overcome by using informative platforms with the help of FPA technicians. The characteristics of the farms are discussed in detail in Sturaro et al. (2013). The farm sample size and the quality of the information gathered were sufficient for the purposes of the current study. Previous studies conducted on the same farms found a strong effect of dairy system on milk yield and composition, and the milk fatty acid profile (Mele et al., 2016), milk coagulation and curd firming aptitude (Zendri et al., 2017; Stocco et al., 2017), cheese-making efficiency and yield (Stocco et al., 2018), and the flavour of the ripened cheese (Bergamaschi et al., 2015a, 2015b). In the future, model estimates could be used to study the relationship between environmental indicators and product quality.

The cartographic analysis (Fig. 7) indicated that farms are distributed with a highly uneven spatial pattern in the study area. This pattern coincided with the low elevation areas at the bottom of the main valleys, where the main rivers flow (Sturaro, unpublished). The kernel density function highlighted two hot spots where the concentration of farms and their N excretions were highest (the red areas in Fig. 7). As the geographical distributions of the P and N loads were almost identical, the corresponding figure for P has been omitted. 4. Discussion We developed and applied a mechanistic model to quantify nutrient excretions at the farm level using information anchored to farm characteristics. According to Black (1995), having tested the model for logical, mathematical and numerical correctness, the pattern, magnitude and consistency of response were in line with expectations. Although a further validation of the model, for example measuring the nutrient excretion in experiments and comparing the result with model prediction, would be desirable, we confirm results from previous works that the N or P content of the rations is fundamental information for reliably predicting excretions (Børsting et al., 2003; Schiavon et al., 2015). The fact that the farms were visited only once is a possible shortcoming of our study, as different rations may be used in different

4.1. N excretion per animal category The release of N into the environment is frequently quantified using 41

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Fig. 3. N and P excreted per cow unit (the cow and its share of replacement heifer) on 4 types of dairy farm in the Alps (Province of Trento, Italy): traditional with Alpine pasturing (T-ALP) and without (T-noALP), traditional using silages (T-S), and modern.

Fig. 4. Relationship between the N and P excreted per cow unit on 4 types of dairy farm in the Alps (Province of Trento, Italy): traditional with Alpine pasturing (TALP) and without (T-noALP), traditional using silages (T-S), and modern. 42

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Fig. 5. Milk productivity and on-farm N excretion per kg of milk produced per year on 4 types of dairy farm in the Alps (Province of Trento, Italy): traditional with Alpine pasturing (T-ALP) and without (T-noALP), traditional using silages (T-S), and modern.

the USA. The differences observed between our findings and those of literature were mainly due to different P contents of the rations. The estimated annual excretion of P in manure of the average heifer in our study averaged 6.7 kg/year, with significant but not relevant differences among groups of farms.

standards for N in manure per animal category, making the total farm production exclusively dependent on herd size and composition. These standards are simple to apply, but they are static and ignore the impact of different feeding and management strategies, and the cows’ breed, body weight and production (Xiccato et al., 2005). An alternative to the use of standards is to adopt mass balance approaches, as in the current work (ERM-ABDLO, 2001; Powers, 2004). In the European Union, the reference value for a dairy cow’s N excretion is 128 kg/year, with an excretion of N in manure of 114 kg/year, although there is a wide range of variation according to local conditions (ERM-ABDLO, 2001; Rots, 2004) as it is primarily influenced by N intake (Huhtanen and Hristov, 2009; Powell et al., 2013; Velthof et al., 2015). In Italy, the standard for N excreted is 115 kg/year: 83 kg/year in manure and 32 kg/year lost in the atmosphere (28% of total N excreted, considering the Italy’s climatic conditions). We found the average N in manure produced per cow ranged from 76 to 89 kg/year, according to the farming system. The N in manure produced by the average heifer was in the range of 27-30 kg/year, lower than the 36 kg/year of the standard (MIPAAF, 2016), a difference probably due to differences in the estimated DM intakes. The DM intake of current investigation have been estimated with a mechanist approach using the NRC (2001) equations, where the MIPAAF (2016) values were achieved using the equations proposed by ERM-ABDLO (2001).

4.3. Differences between dairy systems There is a trend toward abandonment of summer Alpine pastures as the labour costs per kg of milk produced are high (Penati et al., 2011). The decrease in highland grazing of cattle puts European mountain areas at risk because livestock play a central role in the conservation of the natural landscape and biodiversity (Mc Donald et al., 2000; Casasùs et al., 2007). Farmers tend to boost their net farm income by extending their areas of lowland in order to increase milk production and/or by adding more concentrates to the diet to increase milk yield per ha (Penati et al., 2011; Sturaro et al., 2013). However, it is difficult to acquire any additional lowland area because it is increasingly used for other purposes (Penati et al., 2011). Extending farms and increasing animal densities in low-lying areas encourages nutrient overloading. Our study shows that the N in manure produced on T-ALP and TnoALP by a cow and a heifer is on average 8 and 10% lower than the cited standards for N excretion (MIPAAF, 2016), respectively. In addition, an increased permanence of the animals at the highland pastures lowered the estimated nutrient excretion in the low-lying land of the TALP (by 18%), T-noALP (by 4%), T-S (by 9%) and MOD farms (by 4%). Average N in manure loads per hectare of UAA on the modern farms (260 kg/ha) were 44% greater than on traditional farms (180 kg/ha), and the same was found for P. The European Union’s Nitrate Directive (91/676/EEC) specifies a maximum N in manure load of 340 kg/ha in non-vulnerable areas and 170 kg/ha in vulnerable areas. All the agricultural land in the Trentino region is classified as non-vulnerable (MIPAAF, 2016). The greater loads of N per hectare of land on MOD compared with traditional farms was due to a number of factors: the 25% greater load of cows per hectare of UAA (1.7 vs. 2.2 cows/ha), the 29% greater milk yield per cow, only partially due to the 17% greater N intake of the lactating cows, and the 18% greater N excreted on-farm per cow unit. In the current paper we assumed that a legal value (MIPAAF, 2016) of 28% of the N excreted was lost in the atmosphere, irrespectively by the housing conditions and manure treatments. This simplification was

4.2. P excretion per animal category As with N, the major determinant of P excretion was P intake, in agreement with Alvarez-Fuentes et al. (2016). In our analysis, the average P content of the ration for lactating cows was 3.90 g/kg DM, notably lower than the average 4.50 g/kg DM found in England (Sinclair and Atkins, 2015) and in the United States (ASAE, 2005), but close to the range of 3.2-3.8 g/kg DM suggested by the NRC (2001) for cows producing 25-54 kg milk/d. Several studies have shown that cows are frequently fed P in excess of requirements (Kebreab et al., 2013; Wang et al., 2014), and it has been shown that a reduction in P in the diet from 4.1 to 3.5 g/kg DM reduces P excretion and would save up to 10-20 dollars per year per cow (Wu et al., 2001; Kebreab et al., 2008). We found the estimated average annual P excretion of a dairy cow ranged from 17.2 to 19.5 kg/year, according to farming system, less than the 20.9-23.0 kg/year found by Poulsen and Kristensen (1998) in Denmark, and the 18.9-21.2 kg/year calculated by the NRC (2001) in 43

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Fig. 6. Relationships between the N in manure load per hectare of the farms’ utilized agricultural areas (UAA) and: a) the amount of on-farm N in manure per kg of milk yield; b) milk yield per cow; c) herd size; and d) load of cows per hectare on traditional farms with Alpine pasturing (T-ALP) and without (T-noALP), traditional farms using silages (T-S) and modern dairy farms in the Alps (Province of Trento, Italy). The red line indicates the threshold for maximum N load (340 kg/ha) permitted by current legislation in this area.

efficiently (Powell et al., 2013). However, the modern farms are less self-sufficient in producing feed ingredients, because on these farms a higher proportion of N comes from the lowlands and a lower proportion comes from local forages (Table 3 in Schiavon et al. Submitted). In the current paper the milk N/N intake ratio for the average cow and heifer ranged 0.22 to 0.26 and 0.10 to 0.11, respectively, in good agreement with the figures quoted by De Klein et al. (2017). According to Powell et al. (2010) a range 0.20-0.25 indicates that substantial improvement can be made. Powell et al. (2010) also indicated that “the highest N efficiency is usually obtained at the lowest level of N intake. This is due to the cow’s ability to make efficient use of low levels of dietary N because the microbes in the rumen are able to synthesize a large proportion of the animals required N”. In the current survey the MOD farmers supplied cows with more protein, but especially with more concentrates, and energy allowance is known to be a much more limiting factor than protein for milk production by ruminants. This can explain why we found an increased milk N/N intake ratio with increasing the N intake. The use of the N in milk/N intake indicator, often showed as nitrogen use efficiency (NUE), is widely used in literature as

justified by the difficult to assign proper emission coefficients for the various farm conditions of manure collection and treatment. Bussink and Oenema (1998) indicated that in housing systems that use straw and produce solid manure the proportion of N released in the atmosphere would be lower than that released in the case of slurry production. In the present paper the MOD farms were those with the lower incidence of solid manure compared to the other kind of farms (Table 1). This suggest that in the MOD farms the N emission in the air of would have been proportionally greater and the N in the manure proportionally lower than the figures assumed for the other kind of farms. When excretions were allocated to milk production rather than farm land, the ranking of the farming system was reversed, the MOD farms having the lowest values, the T-ALP farms the highest. The value of this indicator is higher with increasing dietary nutrient density, and considerably lower with increasing milk production (Fig. 5). This asymptotic trend is due to progressive dilution of the nutrient excreted with increasing milk yields (Clemens and Ahlgrim, 2001). Farms with high milk-yielding cows have been verified as using resources more 44

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Fig. 7. Geographical distribution of the relative index for the concentration of N in manure originating from dairy farms in the Province of Trento. White lines represent the municipal borders. Each black dot represents a farm (barns with animals). The geographical distribution of P was correlated with that of N (r2 = 0.94).

systems in the study area and would be an improvement over the national standards.

an index to measure the efficiency in the use of the N resources. However, in mountain areas, where there are little available lowland areas but a large availability of pastures and forages, this indicator has a minor importance from an ecological and territorial point of view (Powell et al., 2013; De Klein et al., 2017).

4.5. Geographical distribution of the N and P density It appears from Fig. 6 that a number of farms (13%) exceed the threshold limits of 340 kg/ha of N in manure. However, this is not always so, because the farm’s agricultural land does not always coincide with the land available for manure disposal. This is particularly true in mountain areas, where farmers can make use of meadows and pastures surrounding their farms that do not belong to them. In some cases, the nutrient loads may be over- or under-estimated because the manure could be spread on the land of surrounding farms with low animal densities, or on other available areas selected for technical or economic reasons. Thus, the additional area at which the manure is applied is of relevance, because unless there is an appropriate removal of nutrients, or a frequent harvest of plant material, the problem of nutrient overload might be shifted from land which the farms owns to this other category of land. At the scale of our analysis, we did not have the information for addressing this aspect. Therefore, we decided to model the spatial distribution of the total farm excretions as a proxy for the risk of environmental impact. The results suggested that the nutrient load in the agricultural land of the Province of Trento is generally low. However, greater attention needs to be paid to areas where there is a high density of medium-large dairy farms or where farms are moving towards intensification. We found two hot spots, clearly indicated by the two red areas in Fig. 7, where high farm and animal densities, and consequently nutrients excretions, are high. Care is needed to prevent nutrient overloading in these areas and to reduce the risks of water contamination by farms located close together along the creeks and rivers.

4.4. Variations within dairy systems Mu et al. (2017) stated that clustering farms by main characteristics is vital for benchmarking nutrient losses. However, there was a much larger variation in the annual N excretion of the cow unit within farming system (90 to 180 kg/year) than among systems. This was primarily due to the large variation in the N content of the rations for lactating cows. Similarly, P excretion per average cow unit ranged from 10 to 40 kg/year on different farms, and the P content in the diet for lactating cows explained over 0.80 of the total variation. The weak correlation between N and P excretion per cow unit reflected the low degree of consistency in the variation in N and P dietary contents among farms. The variations in both the N and P contents of the rations were consistently unrelated to the productive responses of the cows, and no relationships were found between N excreted per kg of milk and the N load per hectare of land. This suggests that the ration formulation could be optimised and an extension service offered to farmers. Similarly, there were considerable differences between systems in the nutrient loads per hectare of UAA, but there was a much larger variability within system. The N in manure load per unit of land was not greatly affected by herd size and cow productivity, with a partial exception for the modern farms, suggesting that these variables are poor indicators of the environmental pressure exerted by farms. In contrast, the N in manure load of the different types of farm was primarily related to the load of cows per unit of land, and was related to the N content of the rations for lactating cows as a secondary factor. The high coefficient of correlation of the equations in Fig. 6d suggests that these equations could be used to predict the N loads in different farming

5. Conclusions A mechanistic model for predicting the excretion of N and P from 45

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dairy cows and heifers was developed and applied to a case study on the basis of information obtained from existing databases and the characteristics of the rations administered on the farms. The model allowed us to quantify N and P excretion from the average cow and replacement heifer from each farm. Excretion of nutrients per unit of milk produced was greater on traditional farms than on modern farms, but nutrient excretion per cow unit and per hectare of farm land was lower. The N and the P contents of the rations for lactating cows had a large effect on nutrient excretion, but were either weakly or not related to cow productivity, suggesting the possibility of ration optimisation. In some farming systems, the use of summer Alpine pastures reduced the nutrient load in manure for disposal. The estimated loads of N and the P in manure on farm land were primarily related to the load of cows per hectare, and subsequently to the nutrient excretion of the cow unit, which largely depends on the nutrient density of the rations. In these areas, the greater load of animals per hectare and productivity probably reflects an attempt to increase the income per ha of farm land, as there is little available land in the valleys for expansion. The geographical analysis identified two potentially critical areas for manure disposal, due to the high density of farms and animals. In any case, to reduce the risks of environmental impact, strategies encouraging a better distribution of the farms over the land, thereby avoiding excesses in the animal and the nutrient loads, need to be studied and implemented. For this purpose, our model provides a detailed estimate of the total nutrient excretions per unit of farmland, which, in order to estimate the true farmland nutrient balance should be integrated with data on the additional land available for manure spreading and on crop management practices.

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