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Grazing Can Reduce the Environmental Impact of Dairy Production Systems

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© 2009 Plant Management Network. Accepted for publication 21 July 2009. Published 16 September 2009.


Impact Statement

C. Alan Rotz, Agricultural Engineer, Kathy J. Soder, Animal Scientist, R. Howard Skinner, Plant Physiologist, Curtis J. Dell, Soil Scientist, Peter J. Kleinman, Soil Scientist, John P. Schmidt, Soil Scientist, and Ray B. Bryant, Soil Scientist, USDA-ARS, Pasture Systems and Watershed Management Research Unit, University Park, PA 16802 Corresponding author: C. Alan Rotz. [email protected] Rotz, C. A., Soder, K. J., Skinner, R. H., Dell, C. J., Kleinman, P. J., Schmidt, J. P., and Bryant, R. B. 2009. Grazing can reduce the environmental impact of dairy production systems. Online. Forage and Grazinglands doi:10.1094/FG-2009-0916-01-RS.

Abstract Incorporating managed rotational grazing into a dairy farm can result in an array of environmental consequences. A comprehensive assessment of the environmental impacts of four management scenarios was conducted by simulating a 250-acre dairy farm typical of Pennsylvania with: (i) a confinement fed herd producing 22,000 lbs of milk per cow per year; (ii) a confinement fed herd producing 18,500 lbs; (iii) a confinement fed herd with summer grazing producing 18,500 lbs; and (iv) a seasonal herd maintained outdoors producing 13,000 lbs. Converting 75 acres of cropland to perennial grassland reduced erosion 24% and sediment-bound and soluble P runoff by 23 and 11%, respectively. Conversion to all perennial grassland reduced erosion 87% with sediment-bound and soluble P losses reduced 80 and 23%. Ammonia volatilization was reduced about 30% through grazing, but nitrate leaching loss increased up to 65%. Grazing systems reduced the net greenhouse gas emission by 8 to 14% and the C footprint by 9 to 20%. Including C sequestration further reduced the C footprint of an all grassland farm up to 80% during the transition from cropland. The environmental benefits of grass-based dairy production should be used to encourage greater adoption of managed rotational grazing in regions where this technology is well adapted.

Introduction Understanding the environmental consequences of managed rotational grazing in dairy farming is important to the development of sustainable production systems. Dairy farming, like all livestock production systems, is linked to an array of impacts to water and air quality. Water quality concerns begin with runoff and the erosion of sediment. Well-managed rotational grazing systems provide perennial ground cover, reducing sediment loss compared to continuous grazing or the row crop systems commonly used with confinement feeding strategies (10). With less runoff and erosion, soil P loss to surface waters can be reduced (3,4), although care must be taken to avoid direct deposits of feces and urine into streams (2). Another water quality concern is nitrate leaching to groundwater, which can be significant under urine deposits on pasture (4,8). Use of well-managed rotational grazing helps distribute urine spots, minimizing zones of overlap. Despite the heightened nitrate leaching under these spots, nitrate concentrations in shallow ground water below the root zone of well managed pastures are normally below the 10 ppm limit for drinking water (6,10). Air quality issues are now becoming a challenge for livestock producers as well. Ammonia is an important gaseous emission governed under the Clean Air Act (5). Although agriculture has been exempt from regulation, this may

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change. Since ammonia is primarily emitted during manure handling and storage, the use of grazing can greatly reduce this emission (8). Greenhouse gas emissions (carbon dioxide, methane, and nitrous oxide) and the impact of these gases on global climate is another increasingly relevant concern. There are many sources and sinks of these gases on dairy farms, and little information exists on net farm emissions. The limited information available indicates that grazing systems can provide some reduction in the net farm emission of greenhouse gases (7), but this depends upon the characteristics of the full production system under consideration. An experimental comparison of all environmental impacts of confinement and grazing based production systems is physically and logistically difficult, if not impossible. There are simply too many factors to consider and measure, and these factors are highly interrelated and influenced by soil type, weather, and management decisions. A practical means of comparison is through computer simulation using process level modeling of the entire production system (i.e., the farm). The Integrated Farm System Model (USDA-ARS, University Park, PA) (11) provides a tool for conducting this type of comparative analysis. By simulating various production systems using the same weather and soil conditions, direct comparisons can be made for all predicted environmental impacts. Our objective was to use a comprehensive simulation analysis to compare the predicted environmental impacts of four diverse dairy production systems in Pennsylvania. These systems included: (i) a confinement system with high milk production; (ii) a confinement system with moderate milk production; (iii) a moderate-producing confinement system with rotational grazing during the growing season; and (iv) a low-producing, seasonal-calving herd maintained outdoors all year with rotational grazing during the summer months. Predicted environmental impacts included sediment erosion, runoff of sediment-bound and soluble P, nitrate leaching, ammonia volatilization, greenhouse gas emissions, and the overall C footprint. Farm Simulation The Integrated Farm System Model simulates the major biological and physical processes and interactions of a crop, beef, or dairy farm, providing a tool for comprehensive environmental assessment (9). Crop production, feed use, and the return of manure nutrients back to the land are simulated over each of 25 years of weather. Growth and development of grass, alfalfa, corn, soybean, and small grain crops are predicted based upon daily soil and weather conditions. Tillage, planting, harvest, storage, feeding, and manure handling operations are simulated to predict resource use, timeliness of operations, crop losses, and nutritive changes in feeds. Feed allocation and animal response are related to the nutritive value of available feeds and the nutrient requirements of the animal groups making up the herd. The quantity and nutrient content of the manure produced is a function of the feed consumed. Nutrient flows through the farm are modeled to predict nutrient accumulation in the soil and loss to the environment (9). Environmental impacts include N volatilization (ammonia) from manure sources, soil denitrification and nitrate leaching losses, erosion of sediment, and soluble and sediment-bound P losses in runoff. Carbon dioxide, methane, and nitrous oxide are tracked from crop, animal, and manure sinks and sources to predict the net greenhouse gas emission (Fig. 1). Secondary emissions occurring during the production of resources used on the farm, such as fuel, electricity, machinery, fertilizer, pesticides, and purchased animals, are also included in the prediction of a C footprint of the production system. Whole-farm mass balances of N, P, K, and C are determined as the sum of all imports in feed, fertilizer, deposition, and crop fixation minus the exports in milk, excess feed, animals, manure, and losses leaving the farm.



Fig. 1. Important material flows, gaseous emissions, and nutrient losses for a dairy production system simulated by the Integrated Farm System Model.

Carbon sequestration can also be an important consideration in the evaluation of greenhouse gas emissions from pasture-based systems. Normally, the conversion of cropland to permanent perennial grassland stimulates the sequestering of C for up to 50 years until a new balance is established between C inputs and losses from the soil (1). To represent this process, the Comet-VR model (USDA-NRCS, Fort Collins, CO) (12) was used to estimate C sequestration attained through changes in cropping practices. The predicted sequestration was then used to adjust the net greenhouse gas emission and carbon footprint following the conversion from tilled cropland to perennial grassland. Production Systems The four simulated dairy production systems covered a wide range in production practices found in the mid-Atlantic and northeastern regions of the United States. All production systems were simulated on the same 250-acre land base with a Hagerstown clay loam soil and gently sloping (3 to 8%) terrain. Each system was simulated over 25 years using historical weather data (1982 to 2006) for State College, PA. Reduced tillage practices were used to establish all crops. Across varying milk production levels, animal numbers were adjusted so that each production system produced a similar amount of energy corrected milk (ECM) shipped from the farm. To determine ECM, milk production was corrected to 3.5% milk fat and 3.1% milk protein concentrations. To include carbon sequestration, Comet-VR was used to estimate the annual sequestration for the given soil, crop, and tillage conditions of each production system. Any grassland used on the farm was assumed to be a permanent replacement of rotated cropland. The high-producing confinement herd included 85 large-framed Holstein cows plus 76 replacement heifers housed year-round in free stall barns. Manure was scraped daily, stored as slurry in a bottom-loaded tank, and applied to land in the spring and fall. A random calving strategy was used where 40% of the cows were replaced each year. Animals were fed total-mixed rations of alfalfa hay and silage, corn silage, and high-moisture grain harvested from 150 acres of corn and 100 acres of alfalfa. Annual milk production was 22,000 lb per cow with an average milk fat concentration of 3.4%. The moderate producing confinement herd included 100 average-framed Holstein cows plus 80 replacement heifers housed in free stall barns. Manure was handled as in the first farm. Random calving was used with 35% of the cows replaced each year. Total mixed rations were fed with all of the forage and much of the grain produced from 125 acres of corn, 75 acres of harvested perennial grassland, and 50 acres of alfalfa. Annual milk production was 18,500 lb per cow with a milk fat concentration of 3.5%. The third production system was similar to the second except that the 75 acres of perennial grassland were used for rotational grazing for up to 7 months of the year. Manure produced during grazing was deposited on the grassland



allowing less manure handling and storage during the summer months. Through improved herd health attained by maintaining animals on pasture (13), the cow replacement rate was reduced to 30%. The fourth was a relatively low input production system using 130 smallframed Holstein/Jersey crossbred cows with 95 replacement heifers. Animals were maintained outdoors year-round on 250 acres of perennial grassland with managed rotational grazing during the growing season. A spring calving strategy was used with 30% of the herd replaced each year, which included animals that failed to breed within the appropriate period. Annual milk production was 13,000 lb per cow with a fat concentration of 4.0%. Confinement and Grazing Systems Compared Simulated feed production, feed use, and milk production for the four production systems are given in Table 1. The higher-producing confinement system produced and used about 5% less feed dry matter to produce a similar amount of milk. The lower-producing confinement herd was fed higher-forage diets, so more forage and less grain were produced and used (Table 1, second vs. first data columns). Total feed intake per unit of milk produced was a little greater for the lower-producing herd because more of the nutrient intake was used for animal maintenance. Table 1. Annual feed and milk production for four dairy production systems on a simulated 250-acre farm in central Pennsylvania. Confinement all year

High v Milk Total (lb/cow) production Milk fat concentration (%) ECMz (lb) Feed Harvested forage production (ton DM) and use Grazed forage (ton DM)

Feed intake

Confined, summer Outdoors grazing all year

Moderate w Moderate x

Lowy

22,000

18,500

18,500

13,000

3.4

3.5

3.5

4.0

1,850,000

1,830,000

1,847,000 1,850,000 576

711

503

350

0

0

203

334

Grain produced (ton DM)

264

148

163

0

Purchased feed (ton DM)

82

117

73

265

Total feed intake, DMI (ton DM)

922

976

942

949

DMI / ECM

1.00

1.06

1.02

1.04

v High

production strategy with 85 large-framed Holstein cows and 76 heifers housed year round in free stall barns with feed from 150 acres of corn and 100 acres of alfalfa. Lactating cows are fed a high-concentrate diet. w Moderate production strategy with 100 average-framed Holstein cows plus 80 replacement heifers housed in free stall barns with summer grazing where feed is produced from 125 acres of corn, 75 acres of harvested perennial grassland, and 50 acres of alfalfa. All animals are fed high-forage diets. x Moderate production strategy with 100 average-framed Holstein cows plus 76 replacement heifers housed in free stall barns with summer grazing where feed is produced from 125 acres of corn, 75 acres of rotationally grazed perennial grassland, and 50 acres of alfalfa. All animals are fed high-forage diets. y Low-production strategy with 130 small-framed Holstein/Jersey cows plus 95 replacement heifers maintained outdoors year round on 250 acres of perennial grassland with managed rotational grazing during the growing season. All animals are fed high-forage diets. z ECM is energy corrected milk adjusted to 3.5% milk fat and 3.1% milk protein.

Conversion of the lower-producing confinement system to summer grazing



reduced corn silage use and thus increased grain production 10%. The increased production of grain along with the high-quality pasture allowed a 38% reduction in purchased feed (Table 1, third vs. second data columns). Conversion of all cropland to perennial grassland reduced total feed production by 20% because of lower grass yields relative to corn and alfalfa (Table 1, fourth vs. second data columns). Since grain was not produced on this all-grass farm, a greater amount of feed was purchased to meet the herd’s energy needs. Given the relatively low milk production of this herd, 0 to 12% less grain was used compared to the other herds fed a high-forage diet (Table 1, fourth vs. third and second data columns). Compared to the high-producing herd, this lowinput system used 23% less grain and purchased feed per unit ECM produced (Table 1, fourth vs. first data columns). Among the four production systems, total feed use per unit ECM varied a small amount as influenced by the amount of forage fed and the quality of that forage. Water Quality Impacts Incorporating grassland into dairy production systems, generally improved water quality indicators. Converting 30% of the cropland to perennial grassland reduced sediment erosion loss by 24%, and converting all land to grassland reduced erosion by 87% (Table 2, third and fourth vs. first data columns). Similarly, sediment-bound P loss was reduced 23% with 30% conversion to grassland and 80% with all grassland. Soluble P loss was affected less, but soluble P runoff was reduced 10 and 23% with 30 and 100% use of perennial grassland. Table 2. Annual environmental impacts of four dairy production systems on a simulated 250-acre farm in central Pennsylvania.

Confinement all year

Confined, summer grazing

Outdoors all year

High s

Moderate t

Moderate u

Low v

2,500

1,900

1,900

330

Sediment-bound P runoff (lb)

296

229

232

59

Soluble P runoff (lb)

57

51

44

44

Soil P accumulation (depletion) (lb/acre)

(3.2)

(1.5)

(2.9)

2.2

Nitrate N leaching (lb/acre)

19.5

16.1

21.5

32.3

Nitrate N in shallow groundwater (ppm)

8.3

6.5

8.4

8.1

Ammonia N volatilization (lb/acre)

55.2

53.3

40.4

39.1

Nitrous oxide emission (lb/acre)

6.5

5.6

6.4

5.0

Methane emission (lb/acre)

182

219

173

187

Engine CO 2 emission (lb/acre)

413

444

330

183

6,900

7,588

6,562

6,348

Secondary emissions x (lb/acre)

673

728

456

270

Carbon footprint (lb CO 2 e/lb ECM)y

0.65

0.74

0.59

0.59

0

1,000

1,000

3,400

Erosion sediment loss (lb/acre)

Net farm GHG emissionw (lb CO 2 e/acre)

Potential sequestered carbon z (lb CO 2 /acre/year) Potential sequestered


Grazing Can Reduce the Environmental Impact of Dairy Production Systems carbon z (lb CO2/lb ECM) Carbon footprint with sequestration (lb CO 2 e/lb ECM)

0

0.14

0.14

0.46

0.65

0.60

0.45

0.13

s High-production strategy with 85 large-framed Holstein cows and 76 heifers housed year round in free stall barns with feed from 150 acres of corn and 100 acres of alfalfa. Lactating cows are fed a high-concentrate diet. t Moderate production strategy with 100 average-framed Holstein cows plus 80 replacement heifers housed in free stall barns with summer grazing where feed is produced from 125 acres of corn, 75 acres of harvested perennial grassland, and 50 acres of alfalfa. All animals are fed high-forage diets. u Moderate production strategy with 100 average-framed Holstein cows plus 76 replacement heifers housed in free stall barns with summer grazing where feed is produced from 125 acres of corn, 75 acres of rotationally grazed perennial grassland, and 50 acres of alfalfa. All animals are fed high-forage diets. v Low-production strategy with 130 small-framed Holstein/Jersey cows plus 95 replacement heifers maintained outdoors year round on 250 acres of perennial grassland with managed rotational grazing during the growing season. All animals are fed high-forage diets. w Includes net CO2 emissions from the farm plus methane and nitrous oxide emissions converted to CO 2 e by multiplying by 25 and 298, respectively. x Greenhouse gas emissions during the production of fuel, electricity, machinery, fertilizer, pesticides, and seed used in each production system. y ECM is energy corrected milk adjusted to 3.5% milk fat and 3.1% milk protein. z Determined using the COMET-VR model (12) for a clay loam soil in central Pennsylvania.

The selected number of animals used on this 250-acre land base allowed most of the production systems to maintain a long term P balance with minor use of inorganic fertilizer (Table 2, data row 4). The exception was the lowinput, all grassland system where there was an average annual excess P of 2.2 lb/acre that was not effectively used by growing crops. This occurred because of the lower yield, and thus less removal of P, from perennial grassland relative to the corn and alfalfa crops used in the other production systems. This is a relatively small imbalance, which would not necessarily represent an environmental concern. If we assume that the current soil P level is near optimum, this imbalance may eventually lead to excess soil P. If the soil is deficient in P, the imbalance may help buildup the soil P level over time. In general though, this production system will create a farm imbalance of P more readily than the other production strategies, because less feed dry matter and P is removed from the land and more P is brought onto the farm in purchased feed. A challenge in grazing systems is the control of nitrate leaching. The high N application in urine deposits exceeds the uptake by plants in that area creating greater opportunity for loss. For the confinement system with summer grazing, the simulated nitrate leaching loss was 34% greater than that for the same farm without grazing (Table 2, third vs. second data columns). When animals were maintained outdoors year around, nitrate leaching was 65% greater than that from the high-producing confinement system (Table 2, fourth vs. first data columns). However, the simulation also suggests that greater drainage through pasture soils compensates for the higher concentrations of nitrate N leached under urine deposits. More moisture drains through the soil profile due to less surface runoff and less plant uptake during the summer compared to corn and alfalfa cropland. As a result, the predicted annual average nitrate N concentration in the shallow groundwater below the root zone of the pasture was less than the maximum drinking water standard of 10 ppm and similar to that predicted for the other production systems (Table 2, data row 6). Air Quality Impacts Air quality indicators also improved with greater reliance upon grazing. Volatilization of ammonia from the whole farm was reduced by 24 to 29% with



the use of grazing compared to the two full confinement systems (Table 2, third and fourth vs. first and second data columns). This was primarily due to less manure storage during the summer months. Ammonia emission from manure deposited on pasture was also less than that occurring from free stall barn floors (8). Greenhouse gas emissions were also affected by the use of grazing (Table 2). Methane emission was about 20% greater for the moderate producing confinement system where animals were fed higher-forage diets. With grazing, methane emissions were reduced because less manure was stored during the warm summer months. With the low-production grazing system, the greater animal numbers required to produce the same milk increased methane emission compared to the higher-producing grazing system. Nitrous oxide emission was a little less for the system where animals were maintained outdoors all year due primarily to the elimination of manure storage and the reduction in manure handling. Carbon dioxide emissions were similar across systems except for engine emissions; reduced machinery use with grazing provided less engine exhaust. Across all greenhouse gases, the use of grazing reduced net farm emission by 14% compared to the same farm without grazing (Table 2, third vs. second data columns). The net emission for the lowproduction system where animals were maintained outdoors all year was 8% less than that of the high-producing confinement herd (Table 2, fourth vs. first data columns). Carbon footprint values, which included secondary emissions and C assimilated in animal products, showed a little greater difference across production systems. Use of grazing reduced resource inputs such as machinery, fuel, electricity, and pesticides. With less use of these resources, secondary emissions associated with their production were reduced. The footprint of the moderate-producing herd fed a high-forage diet in confinement was 14% greater than that of the higher-producing confinement herd fed a highconcentrate diet (Table 2, second vs. first data columns). The use of grazing reduced the C footprint by 20% when the same milk production was maintained (Table 2, third vs. second data columns). The low-input system had a C footprint 9% less than that of the high-producing confinement system when expressed per unit ECM. The use of pasture systems may also affect C sequestration, and this can provide a further reduction in net greenhouse gas emission and the C footprint during the transition of cropland to permanent perennial grassland. For this analysis, the cropping strategy used with the high-producing confinement system was assumed to have been used for many years; thus the soil C level was stable and no further long term sequestration or loss occurred with continued use of this strategy. When this cropland was converted to perennial grassland, the predicted annual sequestration rate was as high as 3,400 lb CO 2 e per acre (Table 2). This high sequestration rate offset most of the net greenhouse gas emission from the all grassland farm, reducing the C footprint to 0.13 lb CO 2 e per pound of ECM. For the two moderate producing herds with 75 acres of permanent grassland, C sequestration was less due to the lower proportion of land area in grassland. This provided similar footprints for the two full confinement herds. The use of summer grazing with sequestration reduced the footprint by 25%. Conclusions A comparison of four simulated dairy production systems in Pennsylvania illustrates that the use of grazing can greatly reduce the erosion of sediment and sediment-bound P. Runoff of soluble P, the volatilization of ammonia, and the net emission of greenhouse gases are also reduced with grazing, but nitrate leaching is increased. The C footprint of milk production, expressed per unit ECM, may also be reduced through the use of well managed grazing systems. For approximately 25 years following the conversion of rotated cropland to permanent perennial grassland, C sequestration can greatly reduce net greenhouse gas emission and the C footprint of dairy production systems.



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