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* ARS-USDA, Pasture Systems and Watershed Management Research Unit, University Park, PA 16802
ARS-USDA, US Dairy Forage Research Center, 1925 Linden Drive West, Madison, WI 53706
Corresponding Author:
C. Alan Rotz; e-mail:
alrotz{at}psu.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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Key Words: farm system • nutrient management • simulation • economics
Abbreviation key: BMP = best management practices, DAFOSYM = Dairy Forage System Model, NLEAP = Nitrate Leaching and Economic Analysis Package
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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Because cropland can provide a natural filter to supply clean water, agriculture is an acceptable land use for these watersheds (NRC, 2000). However, agriculture must do its part to maintain low pollutant loading. A Watershed Agricultural Program has been established to encourage the voluntary participation of farmers in developing whole-farm management plans that reduce pollutant loading. The Cannonsville Reservoir, part of the New York City water supply, has been designated as P restricted, thus management practices are being implemented or considered to reduce P loss from farms to the waterways in this area.
Many factors affect P loss from dairy farms. These include the amount of feed and fertilizer brought onto a farm, animal density, feeding strategies, manure application procedures, crops grown, and topography of the landscape. Over the last 20 years, intensification and expansion of dairy farms has increased the tendency for farm-scale surpluses of P, because inputs in feed and fertilizer are often greater than exports of P in product (Haygarth et al., 1998; Lander et al., 1998; Withers et al., 1999). These surpluses increase the potential for P enrichment of runoff (Sims et al., 1998; Kellogg and Lander, 1999; McFarland and Hauck, 1999).
Target implementation of best management practices (BMP) on critical areas for P loss can decrease watershed export of P (Sharpley and Rekolainen, 1997). In the LaPlatte River Basin of Vermont, implementation of BMP (control of barnyard runoff, milkhouse waste treatment, and use of waste storage facilities) decreased P export to Lake Champlain when more than 90% of the animals on the watershed were on farms where BMP were implemented (Meals, 1990). Studies have also demonstrated that changes in crop rotation (Sugiharto et al., 1994), manure-handling procedures (VanDyke et al., 1999), and feeding practices (Withers et al., 1999; Satter, 2001) can reduce surplus P on dairy farms.
Comprehensive analyses are needed to evaluate the environmental and economic impacts of various management practices that can be used to reduce P loading on dairy farms. Such long-term effects are best evaluated through computer simulation. By simulating farms over many weather years, the effects of management changes can be predicted quickly with little cost or risk to the producer. The Dairy Forage System Model (DAFOSYM) provides a tool for this type of assessment (Rotz et al., 1999b). This model has been widely used to evaluate forage conservation (Rotz et al., 1993), manure handling (Harrigan et al., 1996), cropping system (Rotz et al., 2001), grazing (Soder and Rotz, 2001), and feeding (Rotz et al., 1999b) options for various sizes and types of dairy farms in various locations.
The objective of this work was to determine effective management changes that dairy producers in the Catskill region of southeastern New York can make to reduce the potential loss of P to the watershed while maintaining or improving farm profit. Although these farms are in a specific climatic region, they generally represent dairy farms throughout the northeastern and mid Atlantic regions.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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Nutrient flows through the farm are modeled to predict potential nutrient accumulation in soil and loss to the environment (Rotz et al., 1999b). The quantity and nutrient content of the manure produced is a function of the quantity and nutrient content of the feeds consumed (Rotz et al., 1999a). Nitrogen (N) volatilization occurs in the barn, during storage, and between field application and incorporation into the soil (Borton et al., 1995). Denitrification and leaching losses from the soil are related to the rate of moisture movement and drainage from the soil profile as influenced by soil properties, rainfall, and the amount and timing of manure and fertilizer applications using functions from the Nitrate Leaching and Economic Analysis Package (NLEAP; Schaffer et al., 1991).
A whole-farm balance of N, P, and potassium (K) considers the import of nutrients in feed and fertilizer and the export in milk and animals (Rotz et al., 1999b). Supplemental P and K fed, if needed, is the difference between the requirement of the animal group and the sum of that contained in other feeds consumed. Concentrations of the three nutrients contained in milk and live body weight were set as 0.53% N, 0.09% P, and 0.15% K for milk and 2.75% N, 0.79% P, and 0.20% K for live weight (Morrison, 1956; NRC, 1989). Nutrient removals in each crop and that contained in feeds were determined using simulated crop yields and assigned or predicted nutrient concentrations (Table 1
). Phosphorus and K losses from the farm were set at 5% of that applied in manure and fertilizer to represent normal losses through runoff (Sharpley and Rekolainen, 1997). Excess P and K accumulation in the soil was the difference between the total export and import of each mineral divided by the total farm area. This whole-farm balance assumed that, over the long term, these nutrients were uniformly distributed over all available land.
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Farm Descriptions
Two farms were modeled representing actual farms in southeastern New York. The first was an average-sized farm for the region, having about 100 cows, while the other modeled one of the larger farms with 800 cows. Farms were simulated for 25 weather years using historical weather data (1976 to 2000) from Cooperstown, New York. The typical soil in this region was a Lewbeach (shallow silt loam) with an available water holding capacity of about 10 cm. This soil and the rolling terrain of the region was better suited to grass production than deep-rooted annual crops such as corn.
The costs of production and net returns predicted for these farms were long-term annual values determined under the price and economic factors assumed. These values represented the costs and returns for these production systems, but they were not costs and profit of the actual farms.
Average farm.
The average-sized farm consisted of 141 ha of owned land and 40 ha of rented land. Crops included 40 ha of alfalfa, 109 ha of grass, and 32 ha of corn. Alfalfa was seeded with an oat cover crop and maintained with a 4-yr stand life. Sixty-five percent of the manure was applied to the corn crop with the remainder applied to grassland. Additional fertilizer applied to the corn land included 105, 22, and 45 kg/ha of N, phosphate, and potash, respectively. Grass received 100 kg/ha of N and 135 kg/ha of potash. Alfalfa received 135 kg/ha of potash, but when established with an oat cover crop, the rates were 34, 17, and 100 kg/ha for N, phosphate, and potash fertilizers. Simulated crop yields are summarized in Table 2. Yields for rotationally grazed and less intensively managed pastures were adjusted to provide pasture consumption representative of that reported for this region (Fox et al., 1992).
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). Most of the corn and oat silages were stored in the bunker silo along with about half of the alfalfa silage. The largest tower silo was used for corn silage, the mid-sized tower was used for alfalfa silage, and the smallest was used for high moisture ear corn or corn silage.
The herd consisted of 105 large-framed Holstein cows, 60 heifers over one year old, and 45 younger heifers. Annual milk production was 9500 kg/cow with 30% of the milking herd replaced each year. Remaining heifers were sold as registered bred animals at an average price of $1700 each. Cows were housed, fed, and milked in a stanchion barn (Table 3). Heifers were on pasture during the growing season and housed in a barn during the remainder of the year. Cows were fed a high-forage diet supplemented with a grain mix. The mix primarily included corn meal, distiller’s grain, rolled corn, wheat bran, whole cottonseed, and roasted soybeans along with minerals and vitamins.
Manure was handled in a solid form (20% DM) using gutter cleaners, a front-end loader, and box spreader. There was some short-term storage with associated volatile losses, but essentially the manure was spread on a daily basis. The manure remained on the field surface following spreading where any remaining N in an ammonia form was assumed to volatilize into the atmosphere. The primary bedding material was old or weather-damaged hay that was not suitable for feed.
Large farm.
The large farm consisted of 2025 ha of crop and grassland on the Lewbeach soil. Crops included 162 ha each of alfalfa and corn and 1700 ha of grassland. Alfalfa was established with a cover crop of triticale. The grassland included 373 ha of seeded grasses with 117 ha being Reed Canary grass. About 150 ha of the pasture grass was rotationally grazed with lactating cows, and the remainder was used less intensively by heifers and nonlactating cows.
All alfalfa and about 360 ha of grass were each harvested with a two-cutting strategy. First cutting alfalfa was predominantly harvested as dry hay and the second cutting of alfalfa and both cuttings of grass were predominantly harvested and stored as wilted silage. Following these harvests, this land was available for grazing. All corn and the triticale were harvested as silage. All silage was packed and stored on a large concrete pad where movable walls were used to separate silage types. Silage was packed with two large tractors to a depth of as much as 12 m. All hay was stored outdoors.
There were 800 cows and 600 heifers on the farm, with 30% of the cows replaced each year. Milking animals were assigned to three herds for housing and milking. Two of these herds were housed in free-stall barns and milked in double-eight herringbone parlors. The third group, which was primarily older animals, was milked in a tie stall barn with a pipeline milking system. The average annual milk production was 8170 kg/cow. Rations were mixed and fed using mobile mixing wagons. Forages produced on the farm were supplemented with purchased corn meal, soybean meal, whole cottonseed, distiller’s grain, and minerals. Most of the manure was handled as slurry stored in lagoons. This manure was normally applied to cropland in the spring and fall where it was incorporated within a week of application. Sawdust bedding was imported onto the farm at a rate of about one tonne per day.
The economic analysis of the farm systems assumed new equipment and facilities depreciated over an appropriate amount of time (Tables 3 and 4). Prices on feeds, milk, and other farm inputs and outputs were set to reflect long-term average values in current dollars. Prices were held constant across simulated years so that economic differences among years were solely due to weather effects on farm performance. A real interest rate (approximately nominal rate minus inflation) of 6%/yr was assumed on investments. For the average farm, a labor cost was not included because the family management unit supplied all labor. On the large farm, the labor cost was fixed at the current actual cost of $600,000/yr.
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Next, total feed production and use on the farm was verified by comparing predicted feed sales to that reported by the producers. On the average-sized farm, small amounts of corn or hay were sold in only a few of the high-yielding weather years. On the larger farm, no feed was sold from the farm and no forage was purchased. Simulation results showed small amounts of hay or corn grain sold from the smaller farm in half of the weather years, and no forage purchased or sold from the larger farm.
The final step was to compare predicted and actual purchased feed requirements for an average or typical year. On both farms, all forage was produced on the farm, but most of the grain and all other concentrate feeds were purchased and imported to the farm. The predicted requirement of purchased concentrate was found to be within 10% of that reported for the farm. The supplemental P fed on each farm was set in the model to equal that determined from the amount of concentrate and minerals imported to the farm and the P concentration in those feeds.
Management Options
After the modeled farms were found to adequately portray the actual farms, alternative management strategies were evaluated to determine their potential impact on the environment and farm profit. These management alternatives were selected in consultation with the farm owners. Thus, strategies studied were those of interest to the farmer that would potentially alter the whole-farm P balance.
Average farm.
Five alternative strategies were evaluated for the average-sized farm. The first was an adjustment of the P imported as fertilizer and supplemental minerals. On the current farm, small amounts of P were applied as starter fertilizer for the corn and oat crops (about 9 kg P/ha). Because there is excess P on the farm, this added P might be unnecessary. These applications were removed to determine their impact. Like most farms, the amount of P fed to the animals was somewhat higher than that recommended over the past decade (NRC, 1989), and this recommendation has now been reduced (NRC, 2001). The new requirement, based upon absorbable P, was implemented in the model for each animal group to determine the whole-farm impact of reducing this minimum P in diets that was primarily being met through purchased mineral P.
The next strategy evaluated greater use of pasture. Cows were fed during the growing season using intensively managed rotational grazing. This required a conversion of all land near the milking barn to grass, providing 72 ha of pasture. An additional investment of $23,000 in fence and watering equipment was assumed for a total investment of $34,000. This included a high tensile wire fence around the perimeter of the pasture area and electric fence to maintain up to 30 paddocks. This change allowed better use of existing grassland and converted some existing corn and alfalfa land to grass. More intensive use of pasture increased the available yield by 50% (Table 2). Other changes made to represent a grazing farm (Conneman et al., 2000) included: 1) a 5% reduction in milk production, 2) a 15% reduction in the culling rate of cows with increased heifer sales, 3) a 30% reduction in veterinary expenses to reflect improved health, and 4) a 20% increase in the life of all field and feeding equipment due to reduced annual use.
The third strategy evaluated removal of corn and alfalfa from the farm. This included changes made for the second strategy, plus all remaining corn and alfalfa land was converted to grass harvested as silage. To better utilize high quality grass, a three-cutting strategy was used where first and third cuttings were harvested as silage and second was harvested as hay. Considering the lower energy and higher fiber contents of grass silage relative to alfalfa and corn silages, milk production was decreased an additional 5% (Broderick et al., 2002). This change in forage fed also affected the concentrate feeds required to meet animal energy and protein requirements (Rotz et al., 1999a).
The fourth strategy evaluated the conversion of the dairy farm to a heifer-raising enterprise. All mature animals were removed, and heifer numbers were increased to 180 head 
12 mo of age and 180 < 12 mo of age for a total of 360. Animals were purchased as calves ($400/calf) and sold as bred heifers ($1700/heifer) near the time of calving. Feed production and land use were very similar to that of the current farm. Facility changes were made to accommodate the increased number of heifers and to remove milking equipment, but the overall investment required for animal facilities was the same as that of the current dairy. Livestock expenses, consisting of veterinary, breeding, utility, supply, and related costs, were set at $106/heifer produced (Penn State, 2000).
The fifth strategy evaluated expanding the farm business to include other family members using a modern confinement facility. Animal numbers were increased to 250 cows with 200 replacement heifers. A double-ten milking parlor and free-stall facility were used where all cows were housed and fed in confinement. A manure storage tank was added to provide 6 mo of storage. Manure was applied through a custom or contracted operation in the spring and fall where it was tilled into the soil soon after application. The land area was increased to 304 ha, which was enough land to provide all the forage needed for the herd. The added land was used to produce 80 ha of alfalfa (with 20 ha of oat cover crop) and 43 ha of corn. Two large bunker silos were used to store the annual requirement of silage from the corn, alfalfa, and oat crops. A large forage harvester was used, but all other equipment remained the same as that on the current farm. Animals were fed to meet the latest P requirements (NRC, 2001), and no starter fertilizer was used beyond that needed to maintain a long-term nutrient balance.
Large farm.
Five different strategies were evaluated for the 800-cow farm. The first was a reduction in the feeding of mineral P to not exceed currently recommended dietary amounts (NRC, 2001). The second strategy included the same adjustment in the P feeding requirement along with an increase in milk production. Production was increased by 11% to an annual level of 9080 kg/cow. This proposed increase may be achieved through changes in feed and animal management. Although this production is high for the grazing-based management system used on this farm, it provides an achievable goal.
The third strategy involved an intensification of grass use. The current farm has a large land base with 2.5 ha/cow in total land and 2.1 ha/cow in grassland. Much of the grassland is underutilized. Little to no fertilization or pest control is used, so grass yields are relatively low. To simulate greater utilization of the grass, grassland was reduced to 560 ha. This provided 884 ha of total farmed land, or a little over 1 ha per milking animal. Other management changes included application of N fertilizer at a rate of 75 kg/ha and an annual charge of $15/ha of grassland for weed control and perhaps overseeding of improved species. As a result, simulated annual grass yield was increased to 4.7 tonne DM/ha. Wilted silage harvests were taken from 145 ha of the grass in the spring and again in early summer. For the remainder of the year and for the remaining 415 ha of grassland, grass was efficiently utilized through rotational grazing. This strategy, including the existing corn, alfalfa and triticale crops, provided all the forage needed to maintain the herd. A fourth strategy combined the two previous options; grass production and use was intensified, milk production was increased, and dietary P requirement was decreased.
A fifth strategy included the changes assumed for the fourth plus an additional 390 ha of grassland was used to produce bred heifers. Six hundred heifers were added with 300 sold each year. This required an additional investment in heifer barns ($325,000), pasture fence ($30,000), and manure storage ($25,000). Calves were purchased for $400 and bred heifers were sold for $1700. Annual livestock expenses were increased by $106 per heifer produced and annual labor cost was increased by $40,000.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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Average Farm
Simulation results for the 100-cow farm with the current production strategy are listed in the first column of Table 5. Long-term predictions of feed use compared closely to those of the actual farm. A small amount of grass hay was occasionally sold from the farm, and the average purchase of concentrate feed mix was similar (334 tonne DM/yr). This verified that the model was adequately predicting crop production and feed use for the herd with an annual milk production of 9530 kg/cow. Based upon the simulated feed production, purchased fertilizer, purchased concentrate feeds, and export of milk, animals and hay sold, the farm had an annual excess of P. The annual accumulation of soil P averaged over the farm area was 5.1 kg/ha considering the loss to the watershed of 1.0 kg/ha. An economic analysis of this production system indicated an annual net return to labor and management of $988/cow.
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Use of management-intensive rotational grazing reduced the concentrate feed purchased, reduced soil P accumulation, and increased farm profit (Table 5, column 2). Rotational grazing increased the productivity of grassland, which increased the amount of forage produced on the farm. Use of high-quality pasture, along with the assumed 5% reduction in milk production, reduced the purchase of concentrate feed by 16%. High protein contents in pasture forage led to excess N excretion, which increased N volatilization loss 6%. Reducing the amount of manure applied to fallow corn land in the fall though, reduced N leaching loss 22%. The reduction in the import of fertilizer and concentrate feed reduced the import of P to the farm by 3 kg/ha, which reduced the accumulation of excess P in the soil by about the same amount. A reduction in harvest, feeding, and manure handling operations, along with a reduction in purchased feed costs, increased farm profitability by $10,000 or $93/cow. Use of grazing also reduced the economic risk due to annual weather variations. This occurred because grass yields were more consistent than corn yields across weather years on this shallow soil. Also, there was excess pasture forage during most years, so drought years did not have as much effect on production costs.
Replacing the remaining corn and alfalfa with grassland had a small effect on forage use and concentrate feed requirements (Table 5, column 3). Excess P dropped to 1.8 kg/ha; including the newly recommended reduction in dietary P allowed a long-term P balance (data not shown). The major change was a large reduction in N leaching loss to groundwater. Compared to fall application of manure on fallow corn land, the model predicted that much more of the soil nitrate from manure would be captured and retained by the grass, thus reducing the potential for leaching loss in the spring. Production costs were slightly lower than those of the previous grazing strategy due to the reduction in annual costs of tillage, planting, seed, fertilizer, and chemicals for corn production. Due to the assumed decrease in milk production, farm income decreased providing an annual net return similar to that of the current farm. As in the previous strategy, the risk or variation in net return across weather years was reduced due to more consistent crop yields and an abundance of pasture forage during most weather years.
Changing the dairy farm to a heifer raising enterprise may also provide a long-term P balance for the farm, but this is dependent upon the number of animals raised. A production level of 180 bred heifers per year was selected because this provided similar feed utilization as the current dairy strategy (Table 5, columns 1 and 4). Heifers used more forage, so a small amount of forage was purchased during some weather years. The need for concentrate feeds was reduced by over 50%, and the need for supplemental P in heifer diets was low compared to that of lactating animals (NRC, 2001). Both the import and export of P were greatly reduced providing a long-term P balance.
The heifer raising strategy reduced the demand on family labor, but it was less profitable. Production costs were similar to those of the current dairy facility where the purchased calf cost was offset by the reduction in purchased feed and the elimination of milking facility costs. The net return to labor and management was half that of the current dairy farm, but the labor requirement was also less. With a labor requirement of 3800 h/yr for the current dairy and a requirement of 2000 h/yr for raising heifers, the net return per labor hour for raising heifers ($23/h) was just 15% less than that of the dairy ($27/h).
Expansion to 250 cows can also be done in a manner that leads to less accumulation of P on the farm. However, the stocking rate (animals per unit land) and the amount of supplemental P in animal rations are important considerations in planning this expansion. In this analysis, land areas in corn and alfalfa were increased (along with current grass area) to provide nearly all of the forage needed and some high-moisture corn during good corn growing years (similar to the current dairy enterprise). This required a 70% increase in land area with soil similar to that currently used for corn and alfalfa production. Phosphorus requirements in animal rations were reduced to current NRC recommended amounts.
Nutrient flows through the farm were generally improved in the expanded farm. With 6 mo of manure storage and more rapid incorporation of that manure, N volatilization loss was reduced 17% (Table 5, columns 1 and 5). However, with less volatile loss and greater N application per unit of land, N leaching loss increased 35% (16 kg/ha). A long-term P balance was maintained under the land area, animal numbers, and other farm characteristics assumed.
The expanded dairy enterprise also provided a good economic return. Most production costs doubled with the increase in animal numbers and crop area, but the income also more than doubled. The difference between income and production costs (excluding labor), or the annual net return to labor and management, increased by more than three-fold to $326,000 or $1300/cow. Given that this net income must support three families, the net return per family was similar to that of the current farm that supports one family. Although the standard deviation in net return increased considerably, the coefficient of variation (8.6%) was similar to that of the current farm.
Large Farm
Production and economic results of the current production system on the 800-cow farm are listed in column 1 of Table 6. Grazed forage yields were reduced considerably to match simulated and actual feed production and utilization records. With this change, though, predicted forage production, grazed forage consumed, and concentrate feeds purchased were all similar to that experienced on the actual farm.
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, column 1). Nitrogen volatilization loss is quite low compared to that of the smaller farm (Table 5, column 1). This is partially due to more rapid incorporation of manure, but primarily this number is small because the total loss is spread over a large land base. When the loss is determined on an animal unit basis, N volatile loss is only 34% less on this farm compared to the smaller farm. Nitrogen leaching loss is also less when spread over the entire farm area, but the loss per animal unit is similar between farms. Excess P accumulation in the soil is less per unit of farmland, but considering the large land base, this accumulation represents a substantial build up of P on the watershed. Again, an easy and economical method to reduce P import and accumulation on the farm was to reduce the purchased mineral P fed. Implementing the new NRC (2001) P requirements reduced the import of P by 6,300 kg/yr, which brought the farm to a long-term balance (data not shown). Thus, P imported in feed essentially equaled that exported in milk and animals sold. Reducing the mineral P added to feed decreased the annual feed cost, and thus improved farm net return, by $18,400 ($23/cow).
Increasing the milk production level of the herd increased feed consumption and purchased feed costs. This increased cost was more than offset by the increase in milk income, increasing net return by $160,000 per year. Using the current P feeding strategy, this change created a small increase in the accumulation of excess P on the farm (data not shown). By combining this increase in production with a reduction in the dietary P requirement, a P balance for the farm was approached and net return was increased further (Table 6, column 2).
Farming less grassland and farming it more intensively showed some economic benefit with mixed effects on nutrient balance and loss (Table 6, column 3). Grazed forage consumption was increased along with a reduction in harvested grass silage providing a reduction in machinery and fuel costs. Purchased feed cost also decreased. All together, these changes increased the annual net return by $64,400 or $80/cow. Nitrogen leaching loss decreased as N was recycled more efficiently in the higher yielding, higher quality forage. The P balance over the whole land base (2025 ha) was affected little by this change. Because the excess P would be confined to less land, the annual accumulation on the 884 ha farmed would increase to 6.2 kg/ha.
Combining more intensive use of grass with greater milk production and reduced feeding of mineral P improved nutrient use and increased profitability (Table 6, column 4). With the combined changes, a reduction in N leaching loss was obtained along with a long-term P balance. The predicted profitability of the farm more than doubled with the increased milk sales and reduced production costs.
Adding the additional annual production and marketing of 300 bred heifers, increased forage use with a small increase in purchased concentrate (Table 6, column 5). Nitrogen losses increased, but a long-term P balance was maintained. Increased production costs were more than offset by heifer sales providing an additional $194,000 in annual net return.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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One of the easiest ways to reduce P import and the resulting accumulation in soil may be to reduce the amount of P fed. The NRC (2001) P requirements for dairy animals are lower than past recommendations, and recent experimental work has shown that these reduced amounts can be maintained without adverse effects on animals (Wu et al., 2000; Satter, 2001). In many diets, this lower amount of P can be met by removing mineral P added to supplemental feed at a cost of about $3.00/kg of P. As found in these simulations, removing this added mineral can reduce the annual feed cost and thus improve farm profit by about $22/cow. When byproduct feeds high in P content are fed, these supplemental feeds may need to be replaced with feeds lower in P content to achieve the same reduction in imported P. If the substitute feeds can be purchased at a similar or lower cost, an improvement in net return may again be achieved.
Changes in cropping strategies may also affect P loading, but these effects are normally small. A cropping change of replacing corn and alfalfa with grass reduced the excess P on the farm by 0.6 kg/ha. In a previous study, added corn, barley, soybean, or pastureland all had similar effects on the P balance of a dairy farm (Rotz et al., 2000). Cropping changes will only affect the whole-farm P balance if the crop change greatly affects the import of supplemental feed or fertilizer or the export in sold feed. On dairy farms where manure provides most of the P requirement for crops and those crops are used on the farm, cropping changes have little effect on the import and export of P.
Better utilization of a crop such as grass may provide some reduction in the excess P on a farm. This was illustrated on the average-sized farm where the use of rotational grazing of the milking animals provided a substantial reduction in purchased feed. On the large farm where more intensive use of grass had little effect on purchased feed, there was also little difference in the accumulation of excess soil P.
Use of more grass on farms in this region may provide an added benefit by reducing P runoff into streams and ultimately the reservoirs. Topography in this region is very hilly, and the soils are erodable. A sod cover of perennial grass will reduce water movement and help hold the soil and nutrients on the landscape (Sharpley and Rekolainen, 1997). As differences in P runoff among crops were not modeled in this study, this should be considered as an added benefit for grass production. Ideally the accumulation of soil P should be prevented, but if high soil P concentrations occur, runoff loss should be less when a perennial ground cover is maintained.
Intensive use of grazing potentially reduces the whole farm accumulation of P, but this strategy may alter the distribution of P within the farm. When cows fed a high-concentrate diet are on pasture for a major portion of the day, there is a potential for more P to be applied through manure than is removed through grazing causing a long-term accumulation. Although this is a potential concern, this study illustrates that with proper grazing management (i.e., controlling the frequency, intensity and duration of grazing) a P balance can be maintained in pastures. For the farms studied, pastures grazed by lactating cows were near a long-term P balance as long as additional manure was not spread over these fields.
A synopsis of all simulation results indicates that dairy farms in this region can maintain a long-term P balance if: 1) animals are fed to meet NRC (2001) P requirements, 2) the cropping strategy and land base used supplies all of the forage needed, 3) all animals are fed a high forage diet, and 4) replacement heifers are produced on the farm. Here, high forage diets are defined as those designed to feed the maximum amount of forage to each animal group while meeting their energy and protein requirements with supplemental feeds (Rotz et al., 1999a). The current 100-cow farm, the expanded 250-cow farm, and the current 800-cow farm were all able to maintain a P balance when these conditions were met. Raising replacement heifers helps maintain a nutrient balance because these animals consume more homegrown forage and less purchased concentrate relative to lactating cows. Conversion of the 100-cow dairy farm to a heifer-raising facility also provided a long-term balance when these conditions were met. Maintaining a higher milk production through greater import of purchased concentrates increases the amount of excess P, but as illustrated on the 800-cow farm, excess P is minimal when these conditions on dietary P and forage production and use are followed.
The goal for P management on dairy farms should be to maintain a long-term balance by reducing P inputs from feed and fertilizer and maximizing the use of farm-grown forage. When combined with conservation practices that minimize runoff and erosion, P loss to the watershed will be reduced through better recycling of nutrients on the farm. When excess nutrients occur, it is essential that this excess P in manure be applied to parts of the farm that are least vulnerable to loss by runoff and erosion (Gburek et al., 2000). Thus, with consideration of site topography and hydrology, as well as P needs of the crops, carefully managed application of excess P can reduce losses. This may provide short-term help, but it does not offer a long-term solution. A sustainable farm requires a long-term balance. Whole farm simulation provides an effective tool that can assist in the evaluation and selection of sustainable production systems that reduce or eliminate excess P while maintaining or improving farm profit.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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APPENDIX |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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Received for publication January 31, 2002. Accepted for publication May 8, 2002.
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