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Seasonal Diet Affects Ammonia Emissions from Tie-Stall Dairy Barns

J. M. Powell^*,1, G. A. Broderick^* and T. H. Misselbrook

^* USDA-ARS US Dairy Forage Research Center, Madison, WI 53706
Institute of Grassland and Environmental Research, North Wyke, Okehampton, EX20 2SB, United Kingdom

¹ Corresponding author: mark.powell{at}ars.usda.gov

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Federal and state regulations are being promulgated under the Clean Air Act to reduce hazardous air emissions from livestock operations. Although much is known about air emissions from livestock operations in Europe, few data are available on emissions from livestock facilities in the United States and the management practices that may minimize these emissions. The objective of this study was to measure seasonal and diet effects on ammonia emissions from experimental tie-stall dairy barns located in central Wisconsin. Four experimental chambers each housed 4 lactating Holstein dairy cows for three 28-d trial periods corresponding to spring, early fall, and winter. A 4 x 4 Latin square statistical design was used to evaluate 4 diets [corn silage (CS)- or alfalfa silage (AS)-based diets at low or high crude protein] in each chamber for a 4-d ammonia monitoring period. Partially due to higher crude protein levels, average ammonia-N emissions during spring (18.8 g/cow per d) were approximately twice the emissions recorded during early fall (8.4 g/cow per d) and 3 times greater than emissions during winter (6.7 g/cow per d). Ammonia-N emissions accounted for approximately 1 to 3% of consumed feed N, 2 to 5% of excreted manure N, and 4 to 11% of manure ammonical N. Nighttime ammonia emissions were on average 30% lower than daytime emissions. Forage type did not affect ammonia emissions during winter or early fall. Only during early spring were ammonia emissions lower from chambers containing cows fed low-CP diets than from cows fed high-CP diets. Of the total chamber N inputs (feed and bedding), 93, 91, and 95% were recovered in N outputs (milk, manure, body weight change, and ammonia N) during spring, early fall, and winter trials, respectively. Confidence in the accuracy of ammonia emission results was gained by the relatively high chamber N balances and favorable comparisons of study data with published relationships among the variables of feed N intake, milk urea N, manure N, and urine N excretion, and ammonia emissions.

Key Words: diet • ammonia emission • manure • tie-stall

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Research, extension, the feed industry, and veterinarians have long advocated dairy cow diets that maximize milk production while assuring good animal health and reproduction. Under practical conditions, only 20 to 30% of the nitrogen (CP) fed to a dairy cow is converted into milk protein (Jonker et al., 2002; Powell et al., 2006). The remaining feed N is excreted about equally in urine and feces, although this can be highly influenced by diet (Castillo et al., 2000; Broderick, 2003). As much as three-fourths of the N in urine is in the form of urea (Bristow et al., 1992). Urease enzymes, which are present in feces and soil, rapidly convert urea to ammonia, which is in equilibrium with ammonium. Depending on pH, ammonium is transformed into ammonia gas and lost to the atmosphere. Loss of N as ammonia is thought to range from 20 to 55% of manure N excretions (MWPS, 2001). The main factors that affect ammonia N loss are housing and manure-handling strategies, diets, bedding type, barn ventilation, and temperature.

After release, ammonia combines with other chemicals in the atmosphere to form fine particulates that can adversely affect human health. Ammonia is redeposited as ammonium containing dust particles, and as acid rain and nitrates, which can be detrimental to natural ecosystems. Excessive nutrients in lakes and streams accelerate eutrophication and impair water quality. The ammonia produced by dairy farms in the Midwest may be a main contributor to the N loading of the Mississippi river and the hypoxia zone in the Gulf of Mexico (Burkhart and James, 1999).

Over the past 15 yr, environmental concerns related to animal agriculture have focused on improvements in manure management to mitigate runoff and pollution of lakes, streams, and other surface waters (Moody and Burns, 2006). Air quality legislation targeted at animal agriculture is now being promulgated by the US Environmental Protection Agency. The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) enacted in 1980 aims to control the release of hazardous substances that might endanger public health. The Clean Air Act amendments of 1990 required the Environmental Protection Agency to establish National Ambient Air Quality Standards for pollutants considered harmful to human health. Of principal concern are fine particles in the atmosphere, referred to as PM 2.5, or particles less than 2.5 µm in diameter. Ammonia is a major precursor for fine particulates (NRC, 2003). The CERCLA requires the reporting of the release of a hazardous substance in excess of threshold levels (e.g., 45.5 kg of ammonia over a 24-h period). Although CERCLA is focused on emissions of hazardous wastes from industrial plants, the increased size and geographic consolidation of animal feeding operations make their ammonia emissions subject to the notification provisions (Aillery et al., 2006a,b).

Much is known about air emissions from livestock operations in Europe (e.g., Hutchings et al., 2001; Webb and Misselbrook, 2004; Pedersen, 2006), and air emission standards are in place. Little information is available, however, on emissions from livestock facilities in the United States and how management practices can be altered to minimize these emissions. A report by the National Academy of Sciences (NRC, 2003) made an urgent call for processed-based research that could assist producers and regulatory agencies in developing strategies to abate harmful air emissions from livestock farms.

Tie-stall barns are the most common housing type on dairy farms with small to medium herds, mostly in the Midwest and Northeast regions of the United States (USDA, 2004). On these farms, cows are confined to stalls, and manure is collected in a gutter behind the cows. The objective of this study was to measure seasonal differences in ammonia emissions from a tie-stall dairy barn containing lactating dairy cows fed different forage types and CP levels. These CP levels displayed a range of ammonia emission rates in a preliminary laboratory study (Misselbrook et al., 2005). An additional objective was to validate these results through mass N balances and by comparing data collected on manure N and urine N excretions, MUN concentrations, and ammonia emission with published values of these parameters.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Tie-Stall Air Emission Chambers
Four chambers (Figure 1) to house 4 dairy cows each were constructed at the end of an existing stanchion barn equipped with a standard manure gutter cleaning system at the research facilities of USDA-Agricultural Research Service’s US Dairy Forage Research Center (Prairie du Sac, Wisconsin; 43°19' N, 89°44' W). Technical aspects of chamber design, operation, and calibration have been described by Powell et al. (2007), and the chambers have been used to evaluate seasonal bedding effects on ammonia emissions (Powell et al., accepted). In brief, a 36.6 m x 18.3 m area was divided to accommodate the 4 chambers, each approximately 6.0 m wide x 9.1 m long x 2.9 m high and containing 165 m³ of air space. Airflow through each chamber was controlled by an intake fan, and maintained within a range of approximately 1.5 to 33 air exchanges/h (Table 1), depending on ambient conditions and associated needs to maintain cow comfort. Airflow rates, temperature, and relative humidity for each chamber were averaged over 2-min intervals, which corresponded to the measurement interval of ammonia concentrations in exhaust air, as described below. Temperature and relative humidity were measured using a CS500-T Platinum Resistance Temperature detector and a Vaisala IN-TERCAP capacitive relative humidity sensor (Campbell Scientific, Logan, UT). Measurements were made approximately at the center, 4.6 m from the end of each exhaust duct. Temperatures were greatest during early fall, followed by spring and winter (Table 1). The relative humidity in chambers was greatest during winter, followed by fall and spring.

View larger version (18K): [in this window] [in a new window]   Figure 1. Configuration of tie-stall ammonia emission chambers (Powell et al., 2007).

A data logger was programmed using Loggernet software (Campbell Scientific, 2003). The data logger opened a solenoid valve through a solid-state relay for 1 min to allow air to flush the sampling line. Over each minute, the data logger averaged temperature, relative humidity, differential pressure (air velocity for inlet and exhaust), and ammonia concentration.

General Chamber Management
Three dietary trials were conducted: a spring trial from April 11 to May 27, 2005; an early fall trial from August 29 to October 14, 2005; and a winter trial from January 2 to February 17, 2006. During each trial day, cows were milked, fed, and chambers were cleaned from approximately 0600 to 0900 h. Unconsumed feed per cow was collected, weighed, and sampled, and cows were offered fresh feed as a TMR at a per-cow rate of between 25 to 30 kg of DM, at approximately 10% in excess of the previous day’s consumption. Cows were bedded on rubber mats with pine shavings used as bedding (approximately 2.5 kg of dry weight/cow per d). At approximately 0900 h, chamber curtains were lowered and curtain wall seams were sealed; emission recordings were made from 1000 to 1500 h. Cows were milked from 1500 to 1700 h, curtains were lowered again at approximately 1800 h, and nighttime emission measurements were made from approximately 1900 until 0500 h the next morning. The daily cycle of cow feeding, chamber cleaning, and ammonia emission recordings was repeated during 4 consecutive days, Tuesday through Friday, this being the measurement period of a replication for each experimental unit.

Diet Treatments and Management
A 4 x 4 Latin square statistical design was used to allocate 4 diets to each of the 4 chambers for the 4-d ammonia monitoring period described above. The intended diets consisted of a 2 x 2 factorial arrangement of 2 forage mixtures: a 66%:34% alfalfa silage:corn silage DM ratio (AS) or a 66%:34% corn silage:alfalfa silage DM ratio (CS), and 2 CP levels: 16.5% (LP) or 18.5% (HP; Table 2). After each 4-d ammonia monitoring period, diets were reallocated to chambers and cows were allowed to adapt to the new diets for 10 d before the next ammonia monitoring period began.

Dairy Cow Selection and Milking
Cows were selected based on lactation number and DIM, with the goal of having 4 cows per chamber (i.e., 16 cows per trial) that resembled the profile of a lactating cow herd on a typical dairy farm. In general, each chamber contained 1 cow in first, 1 cow in second, and 2 cows in third lactation; at the onset of each seasonal trial the cows were from 70 to 365 DIM (Table 3). Cows were weighed at the beginning, middle, and end of each trial to estimate the amount of N in BW gain as a component of the chamber N balance calculations described later.

Manure Management and Sampling
To collect manure, pans were constructed of stainless steel (1.23 m long x 0.38 m wide x 0.076 m deep, with a 0.025-m lip that was flush with back of manure gutter) and placed into a bracket to keep pans high enough so that the manure scraper could function normally to clean the nonchamber part of the barn. To facilitate urine collection, plastic urine deflectors were constructed to direct urine into manure pans. Pans were scraped clean and manure was weighed during each milking period. After each manure collection, approximately 10 kg of the total manure mass per chamber was blended in a cutter mixer (model R60, Robot Coupe, Ridgeland, MS), and a subsample was placed in 120-mL specimen cups and stored frozen (–20°C) until analyzed.

Feed, Bedding, and Manure Analyses
Samples of feed offered, feed refused, and bedding were oven-dried (60°C, 72 h) and ground to pass a 2-mm screen. Ground feed and bedding subsamples were oven-dried (100°C, 24 h) for DM determination, and analyzed for total N content by combustion assay (FP-2000 nitrogen analyzer, Leco, St. Joseph, MI). Manure samples were thawed and subsamples were analyzed immediately for total N using a micro-Kjeldahl assay, ammonium N by distillation (Peters et al., 2003), and oven-dried (100°C, 24 h) for DM determination.

Data Validation
The reliability of chamber ammonia emission data was assessed by determining chamber N balances (the difference between N inputs and N outputs for each chamber) daily, and by comparing data collected on excreted N (feces plus urine), manure ammonium concentrations, overall ammonia emissions, and ammonia emissions as percentages of N inputs and output, with published values.

Chamber N Balances.
Chamber N balances (CNB, %) were the percentage differences between N inputs (feed and bedding) and outputs (milk, manure, ammonia N emission, and cow BW gain) calculated as follows:

[1]

In this equation, milk N was milk volume multiplied by its N concentration; manure N was manure DM (kg) multiplied by its N concentration; ammonia-N emissions were derived by multiplying by 24 the average hourly daytime and nighttime ammonia flux from each chamber assuming 12-h lengths for each period; cow N gain was the difference between cow mass (kg) before and after each trial multiplied by body N concentration of 24.7 g/kg (Marini and Van Amburgh, 2003); feed N was the difference between feed N offered and refused; and bedding N was bedding DM mass (kg) multiplied by its N concentration.

Excreted N.
Excreted N (ExN, g/chamber per d) in feces and urine was calculated by subtracting bedding N input from the sum of manure N and emitted ammonia N as follows:

[2]

Total Ammonium N.
Total ammonium N (TAN, g/chamber per d) in manure was determined by multiplying manure DM (kg) by its ammonium N concentration as removed from each chamber.

Emitted Ammonia N.
Emitted ammonia N (g/chamber per d) was calculated as percentage of nitrogen intake (NI), ExN, and TAN as follows:

[3]

[4]

[5]

Urinary N Excretion.
Urinary N (UN, g/chamber per d) was calculated as the sum of TAN and emitted ammonia N. This calculation assumed that fecal N was not volatilized during this short period (Haynes and Williams, 1993), which implied that all TAN and emitted ammonia N was derived from UN.

Statistical Analysis
Statistical analyses of the 4 x 4 Latin square were performed using the SAS statistical package (SAS Institute, 1990). Seasonal (early fall and winter) and diet differences in response variables were analyzed by generalized least-squares ANOVA, assuming chamber and time periods to be random effects, and seasons, diets, and season x diet interactions to be fixed effects. Where relevant, the protected least significant difference (LSD) test was used to determine significant differences among treatments at P < 0.05.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Seasonal Diet Effects on Milk Production, ExN, and UN
Very few treatment interactions were observed, and those that occurred accounted for only a small proportion of total sums of squares in the least-squares ANOVA (data not shown). Results are therefore presented as seasonal forage type and CP level effects on ammonia emissions and other response variables.

Seasonal differences in feed DMI, NI, milk production, feed N use efficiencies (FNUE), ExN, UN, and ammonia N emissions are given in Table 4. Average NI values were greatest and FNUE lowest during the spring trial, when only AS, and therefore the greatest levels of dietary CP, was fed (Table 2). During both early fall and winter, FNUE by cows fed the LP (32.9%) diets were greater (P < 0.05) than those of cows fed the HP (29.9%) diets. There were no significant differences in FNUE by cows fed diets based on AS or CS during either the early fall or winter trials.

As reported by Nennich et al. (2005), ExN in the present study was closely related to NI (Table 2). Manure N excretions (ExN) were greatest during spring (420 g/cow per d) when only AS (and therefore the highest levels of dietary CP) was fed. During both early fall and winter, there was significantly less ExN (318 g/cow per d) by cows fed LP diets than by cows fed HP diets (354 g/cow per d). Also during early fall and winter, ExN was the same from cows fed AS or CS (336 g/cow per d).

Seasonal Diet and Diurnal Effects on Ammonia Emissions
Greatest ammonia emissions were recorded during spring (18.8 g/cow per d). As mentioned previously, only AS was fed during the spring trial. Forage-type effects on ammonia emissions could be evaluated, therefore, only during early fall and winter. During both seasons, forage type did not significantly impact ammonia emissions, and only during the early fall trial did dietary CP level affect ammonia emissions. During the early fall, ammonia emissions from chambers containing cows fed the LP diet (8.0 g/cow per d) were significantly (P < 0.05) less than from cows fed the HP diet (8.8 g/cow per d). Frank and Swensson (2002) also found that ammonia emissions from cows fed a low CP (13.1 to 13.5%) diet were significantly less than emissions from cows fed a high CP (17%) diet.

Ammonia-N emissions accounted for approximately 1 to 3% of NI, with the greatest percentages occurring during spring (Table 4). On average, ammonia-N emissions accounted for 2 to 5% of ExN, and 4 to 11% of either UN or TAN.

During each of the 3 seasons, temperatures were lower and relative humidity greater during night than day (Table 5). Cooler nighttime temperatures resulted in lower nighttime ammonia emissions during each of the 3 study seasons. Ammonia emissions during the night were approximately 10 to 20% less than during the day.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

CNB
Chamber N balances (Table 6) provided a method to account for N inputs and outputs and therefore an indirect way to evaluate the relative accuracy of the ammonia emission data (NRC, 2003). Feed accounted for 99% of chamber N inputs and manure accounted for approximately 62 to 86% of N outputs. The CNB (% of N inputs recovered in N outputs) were greatest during winter (95%) followed by spring (93%) and early fall (91%).

Our inability to capture all ExN was likely linked to 2 possible explanations: 1) incomplete urine collection, and, to a lesser extent, 2) error in measuring ammonia N loss during manure handling, sampling, and analyses. Each morning and evening approximately 100 to 150 kg of wet manure mass was removed from each chamber. To obtain a representative sample for DM and N analyses, the total wet manure mass was mixed manually, sampled, blended, subsampled, frozen, thawed, and analyzed. Ammonia-N losses during this process may have occurred, but were likely slight. Manure removal, blending, and sampling was accomplished over an approximately 90-min period and N analyses were done immediately after thawing samples, which were stored at –20°C in tightly sealed plastic urine specimen cups.

Urine losses were possible, although visual observations during twice-daily manure collections indicated that these were likely low. The maximum amount of UN that could have been lost through drainage beneath manure pans can be calculated from the amount of N (g/chamber per d) required to achieve 100% chamber N balance (Table 6), assuming all unaccounted-for N was due to uncollected urine. Concentrations of N in dairy cow urine vary considerably (1 to 20 g/L; Bussink and Oenema, 1998). Assuming an average urine N concentration of 10 g of N/L for the present study, the 235, 254, and 186 g of N required to achieve chamber N balances of 100% during spring, early fall, and winter, respectively (Table 6) would translate into possible daily urine losses of approximately 18 to 25 L/chamber or 4 to 6 L/cow per d. This could comprise approximately 20 to 30% of excreted urine volume, assuming an average daily excretion of 20.5 L of urine/cow per d, although urine excretion volumes also vary greatly (Nennich et al., 2006). Even if all unaccounted-for N in the chamber N balances (Table 6) was attributed to uncollected urine, these losses would not necessarily have affected the measured seasonal and dietary effects on ammonia emissions, the principal study objectives.

Seasonal Diet Effects and Diurnal Differences in Ammonia Emissions
Seasonal and diurnal differences in ammonia emissions (Tables 4 and 5) can be attributed to the relationship between temperature, urease enzymatic activity, and subsequent ammonia production and loss. Urease is produced by microorganisms abundantly present in feces and, therefore, barn floors (Ketelaars and Rap, 1994). Muck and Steenhuis (1981) observed occasional 0.5- to 1.0-h lags in urease activity and ammonia emissions from urine deposited on dairy barn floors. In the present study, the data did not indicate any discernible lags in ammonia emissions, either during the initial part of the 6-h daytime measurement period, or the initial part of the 12-h nighttime measurement period. After chamber walls were lowered, we provided a 40- to 60-min stabilization period for the ammonia analyzer. After this period, all ammonia emission recordings were used to determine, for example, seasonal and diet effects (Table 4) and diurnal differences (Table 5) in ammonia emissions.

In the present study, seasonal diet effects on ammonia emissions could be determined by comparing 1) winter and early fall (daytime temperatures of between –5 and 35°C; Table 1) when similar diets were fed, and 2) AS treatments having CP levels of 17% (spring) and 17.2% (early fall and winter). For the first comparison, average ammonia emissions (across all 4 diets, Table 4) during winter (6.7 g/cow per d) were 20% less than during early fall (8.4 g/cow per d). For the second comparison of AS-based diets at approximately 17% CP, average ammonia emissions during winter (7.2 g/cow per d) were 17% less than during early fall (8.7 g/cow per d) and 62% less than during spring (18.2 g/cow per d).

Urease activity is low between 5 and 10°C and increases exponentially above 10°C (Braam et al., 1997). Smits et al. (1995) determined that 46% less ammonia was emitted from free-stall dairy barns in the United Kingdom during winter (10°C) than summer (24°C). In the Netherlands, Kroodsma et al. (1993) determined that ammonia emissions from a free-stall barn during winter (11.8°C) were only 18% less than during summer (18.2°C). Pedersen (2006) determined exponential increases in ammonia emissions from 9 free-stall dairy barns in Denmark within the temperature range of approximately 2 to 22°C.

In the present study across AS treatments, the much greater ammonia emissions during cooler (17.5°C) spring compared with warmer (21.4°C) early fall was likely due to much greater NI during spring than early fall (Table 4). Feed N consumption in excess of animal requirements is excreted in urine (Castillo et al., 2000; Broderick, 2003; Wattiaux and Karg, 2004), which increases ammonia emissions from dairy barn floors (Misselbrook et al., 2005). Estimated UN during spring was approximately twice as high as that during the other 2 trial seasons (Table 4).

The ammonia emissions measured during the present study were somewhat lower than has been measured in other tie-stall studies (Figure 2). This may have been due, in part, to the pine shavings used for bedding. In a preliminary laboratory study (Misselbrook and Powell, 2005) and a study with the same chambers used in the present study (Powell et al., accepted), ammonia emissions from pine shavings were 20 to 25% lower than emissions from other tie-stall bedding materials tested (wheat straw, chopped newspaper, composted manure solids). A calculated average ammonia emission rate based on the chamber floor surface area from the present study (5.3 g/m² per d) was similar, however, to the average (5.8 g/m² per d) of 2 studies in the United Kingdom (Misselbrook et al., 1998, 2001), in which emissions were measured from outdoor concrete yards used by dairy cattle.

View larger version (20K): [in this window] [in a new window]   Figure 2. Ammonia emission rates: Present study compared with published studies. Numbers on the x-axis indicate 1) compiled from literature review of Monteny and Erisman (1998), dairy cow type not provided; 2) measured from cement barn floor and reported for 450-kg dairy cow by Amon et al. (2001); 3) manure, which included straw, and reported for 500-kg dairy cow by Demmers et al. (1998); 4) compiled from literature review by Anderson et al. (2003) and reported as kg/cow per yr; 5) simulations by Rotz and Oenema (2006) and revised to include only lactating cows by C. A. Rotz, USDA-ARS; personal communication; 6) hourly emissions reported by Pedersen (2006) scaled to daily emissions; 7) simulations by Pinder et al. (2004); and 8) monthly emissions reported by Hutchings et al. (2001) scaled to daily emissions.

Average ammonia N emissions (11.3 g/cow per d) by lactating cows in the present study accounted for only 1.6 to 5.0% of ExN (Table 4). These emission rates were lower than a general ammonia N loss value from tie-stall barns of 8% of ExN summarized in a literature review (Rotz, 2004), and less than simulated ammonia N losses of 5.6 and 7.5% of ExN in the Netherlands and Pennsylvania, respectively (Rotz and Oenema, 2006). In Denmark, Pedersen (2006) reported that 5% of ExN was lost from tie-stall barn floors. In the United Kingdom, Webb and Misselbrook (2004) used a mass flow model to estimate ammonia N emissions of 3.5 and 12.5% of ExN for dairy calves and adult cattle, respectively. In the present study, ammonia N emissions accounted for 4.2 to 11.4% of excreted TAN (Table 4). This range was lower than the modeled 6 and 21% of TAN emitted by calves and adult dairy cattle in tie-stall barns in the United Kingdom (Webb and Misselbrook, 2004).

Relationship Among UN, MUN, and Ammonia Emissions
Estimates of UN excretions were calculated by adding manure ammonium N to emitted ammonia N. Average daily UN excretions ranged from 33 to 54% of ExN (Table 4), somewhat lower percentages than would have been expected for lactating Holstein cows consuming similar diets and producing similar amounts of milk (Broderick, 2003; Wattiaux and Karg, 2004). In other studies, UN excretions have been estimated from a combination of NI and milk N (Jonker et al., 1999), a combination of MUN and BW (Kauffman and St-Pierre, 2001), and MUN only (Jonker et al., 1998; Kauffman and St-Pierre, 2001; Kohn et al., 2002). A comparison of actual vs. predicted UN using various algorithms in the literature (Figure 3) indicates that the present study’s estimates of UN (TAN plus emitted ammonia N) are somewhat lower than expected. This perhaps supports the previous hypothesis that unaccounted N in the chamber N balances (Table 6) may have been because of our inability to collect all urine.

View larger version (12K): [in this window] [in a new window]   Figure 3. Actual vs. predicted urine N (UN) excretion (predictions using equations from the following sources: UN = 0.0259 x MUN x BW, Kauffman and St-Pierre (2001); UN = 12.54 x MUN, Jonker et al. (1998); UN = 17.64 x MUN, Kauffman and St-Pierre (2001); UN = 15.1 x MUN + 27.8, Kohn et al. (2002).

View larger version (14K): [in this window] [in a new window]   Figure 4. Relationship between chamber-weighted MUN, total excreted N (ExN), and urinary N (UN). Chamber-weighted MUN was calculated as the sum of each cow’s fractional MUN outputs, which was computed by multiplying each cow’s fractional chamber milk production by its associated MUN value. ExN and UN were calculated by dividing daily ExN and UN per chamber by 4 cows per chamber. • = spring, = early fall, = winter.

View larger version (11K): [in this window] [in a new window]   Figure 5. Relationship between chamber-weighted MUN and ammonia-N emissions. Chamber-weighted MUN was calculated as the sum of each cow’s fractional MUN outputs, which was computed by multiplying each cow’s fractional chamber milk production by its associated MUN value. Ammonia-N emissions were calculated by dividing daily chamber emissions by 4 cows per chamber. •= spring, = early fall, = winter.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

	ACKNOWLEDGEMENTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Much appreciation goes to Paul Cusick and Diego Calderon for their assistance in chamber maintenance, calibrations, and conduct of these experiments; to Jill Davidson and co-workers for their assistance in cattle selection and management; and to Michael Casler for his assistance with the experimental design and statistics.

Received for publication August 7, 2007. Accepted for publication October 11, 2007.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 


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