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TECHNICAL REPORTS

Reducing Crude Protein in Beef Cattle Diet Reduces Ammonia Emissions from Artificial Feedyard Surfaces

USDA-ARS Conservation and Production Research Laboratory, P.O. Drawer 10, Bushland, TX 79012

^* Corresponding author (rtodd{at}cprl.ars.usda.gov)

Received for publication February 7, 2005.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Concentrated animal feeding operations are major sources of ammonia to the atmosphere. Control methods to reduce emissions include acidifying amendments, urease inhibitors, and absorbents. For beef cattle, decreasing crude protein (CP) in diets may be the most practical and cost-effective method to reduce ammonia emissions. Our objective was to quantify the effect of reducing CP in beef cattle diet on ammonia emissions. Two groups of steers were fed diets with either 11.5 or 13.0% CP and all urine and feces were collected. Manures from the two diet treatments were applied in a replicated laboratory chamber experiment, and ammonia emission was quantified using acid gas washing. In four seasonal field trials, manures from the two diet treatments were applied to two 10-m-diameter, circular, artificial feedyard surfaces, and ammonia emission was quantified using the integrated horizontal flux method. Manure from steers fed 11.5% CP diet had less urine, less urinary N, and a lesser fraction of total N in urine, compared with the 13.0% CP diet. Decreasing crude protein in beef cattle diets from 13 to 11.5% significantly decreased ammonia emission by 44% (p < 0.01) in the closed chamber laboratory experiment, and decreased mean daily ammonia flux by 30% (p = 0.10), 52% (p = 0.08), and 29% (p < 0.01) in summer, autumn, and spring field trials, respectively. No difference was observed in winter. On an annual basis, decreasing crude protein reduced daily ammonia flux by 28%. Reducing crude protein in beef cattle diets may provide the most practical and cost-effective way to reduce ammonia emissions from feedyards.

Abbreviations: CP, crude protein • CP11, 11.5% crude protein diet • CP13, 13% crude protein diet • DAT, day after treatment

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
IT IS ESTIMATED that nitrogen cycling through terrestrial ecosystems has doubled, primarily through anthropogenic activities (Smil, 1990; Vitousek et al., 1997). These include the production and use of nitrogen fertilizers, planting of nitrogen-fixing crops, release of nitrogen from storage reservoirs in soils and plant biomass, and the burning of fossil fuels. Many of these sources are agricultural, and estimates of the agricultural contribution to increased N in global ecosystems range from 50 to more than 90%, with animal husbandry comprising the majority of that contribution (Bouwman et al., 1997; Ferm, 1998; Galloway and Cowling, 2002; Howarth et al., 2002).

Ammonia is the primary basic constituent of the atmosphere; it readily combines with and neutralizes oxidized compounds such as SO₂ and NO_x, forming secondary particulates. Ammonia and its compounds can be transported long distances, wash out in precipitation, and return to the earth's surface. Nitrogen enrichment of land and water can have varying effects, from the fertilization of cropland, to excessive enrichment and degradation of sensitive ecosystems (Matson et al., 2002; Rabalais, 2002; Todd et al., 2004).

Animal feeding operations, with abundant masses of manure, are major sources of ammonia. Practices to limit the loss of ammonia via volatilization include decreasing pen surface pH by addition of acidifying amendments (Shi et al., 2001), applying urease inhibitors and/or essential oils (Parker et al., 2004; Varel, 1997; Varel et al., 1999, 2004), managing the pen surface (Adams et al., 2004), absorbing ammonia with zeolites (Eng et al., 2003, 2004; Venglovsky et al., 1999), shifting N from urine to feces (Gueye et al., 2003), or decreasing excreted N by modifying cattle diet (McBride et al., 2003). However, the problems of effectively applying pen surface amendments or the poor cost-effectiveness of some practices present challenges that may prohibit the application of otherwise effective methods to reduce ammonia emission.

Modifying diet by decreasing crude protein while maintaining animal performance may be the most practical method. In feedyards, beef cattle are routinely fed diets that contain 12.5 to 14% crude protein (CP), 0.5 to 1.0% urea, and 0.3% P (Galyean and Gleghorn, 2000). Nutrition and management practices can influence the quantity, form, and route (feces, urine, respiration) of nutrient excretion by the animal (Scott, 1972; Tucker and Watts, 1993; National Research Council, 1996, 2001; James et al., 1999; Gueye et al., 2003; McBride et al., 2003; Kebreab et al., 2004) as well as influence transformations and movements of excreted nutrients (Cole et al., 2003; Greene and Cole, 2003). Because nutrient requirements change with the physiological state of an animal, it may be possible to decrease ammonia emissions and nutrient excretion (Cole et al., 2003) by decreasing CP concentrations of beef cattle finishing diets as time on feed increases (phase feeding) without adversely affecting performance (Galyean, 1998; Erickson et al., 2000; Greene and Cole, 2003; Gleghorn et al., 2004; Vasconcelos et al., 2004). Cole et al. (2005) showed, in vitro, that increasing crude protein in beef cattle diet from 11.5 to 13.0% increased ammonia emission from 60 to 200%, due primarily to increased urinary N.

Our objective was to quantify the effect of reducing crude protein in beef cattle diet on ammonia emission by: (i) collecting urine and feces from two groups of steers fed a diet with either 11.5 or 13.0% CP; (ii) applying the manures to artificial feedyard surfaces under ambient conditions during all seasons throughout the year; and (iii) estimating, and then comparing, ammonia emissions from the two treatments.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Research was conducted at the USDA-ARS Conservation and Production Research Laboratory, Bushland, Texas (35° N, 102° W). In February 2002, two groups of four steers each were fed diets that varied only in CP (11.5 and 13.0%) for 21 d. Supplemental nitrogen in the diets was provided by urea, which comprised 0.52% of the 11.5% CP diet (CP11) and 1.08% of the 13.0% CP diet (CP13). Additional details of diet compositions are given in Cole et al. (2005). Steers were fed ad libitum and individually in feeding stalls. All urine and feces were collected from steers during the last 7 d of the feeding trial. Urine was collected from each steer using a rubber collar attached to a vacuum system that drew urine immediately after excretion into a sealed container, which was emptied twice a day. Feces were collected by hand from the floor of each feeding stall twice a day. After collection, urine and feces from each steer were weighed, subsampled for analysis, and frozen. A subsample equal to 1% of the urine and feces output of each steer was collected at the same time and frozen, for use in an in vitro laboratory experiment. Steers weighed about 500 kg each at the end of the feeding trial.

Laboratory Experiment
The in vitro ammonia emission system was described in detail by Shi et al. (2001) and Cole et al. (2005). Briefly, the system consisted of 18 sealed plastic chambers (20 cm x 20 cm x 12 cm deep) each attached to two ammonia trapping bottles containing 100 mL of 0.9 M sulfuric acid and a vacuum system to pull air through the chambers and ammonia traps at a rate of 3 L min^–1. In each chamber, the feces and urine excreted by one steer (four steers per diet treatment) were added on top of air-dry, 2-mm mesh-screened soil (1550 g of Pullman clay loam: fine, mixed, superactive, thermic Torrertic Paleustolls). There were two chambers per steer. The mass of urine and feces added to each chamber was equal to 1% of the daily excretion by the steer during the feeding trial. Two chambers containing soil but no feces or urine were included to correct for ambient atmospheric ammonia. Acid traps were replaced with fresh traps each day for 3 d then at 2-d intervals until Day 7 of collection.

Field Experiment
We selected a field site where wind flow was unimpeded by barriers and obstructions so that wind, temperature, and ammonia profiles would fully develop, and far enough away from potential ammonia sources so that background ammonia concentrations were minimized. The site was prepared by excavating two circles with a 5-m radius to a depth of 0.2 m. The 80-m² circle represented the area typically allotted to five cattle in southern High Plains feedyards. Fresh pen surface scrapings from a nearby commercial feedyard were then packed in the excavated circles to create an artificial pen surface. The two artificial feedyard circles were 100 m apart, on an east–west axis. A 2.5-m-tall instrument mast was erected in the center of each circle. A third mast was located 40 m west of the western-most circle, where background ammonia concentration was measured. On each mast, 250-mL capacity gas washing bottles were installed at 0.15, 0.25, 0.4, 0.6, 1.2, and 2.4 m.

Ammonia was trapped in the gas washing bottles by first drawing air through a Teflon filter to remove particulates, then bubbling it through an impinger in 80 to 120 mL of 0.05 M sulfuric acid. Air-flow rate of each gas washing bottle was measured with a precision, calibrated flow meter (Dry-Cal DC Lite; Bios International, Butler, NJ) at the beginning and end of each measurement period. Nominal air flow rate was 6 L min^–1; flow rate over four trials from all gas washing bottles averaged 5.98 L min^–1, with a standard deviation of 0.38 L min^–1. At the beginning of a sampling period, gas washing bottles with fresh acid were sealed and transported to the site, exchanged with the bottles there, and then the sealed bottles with samples were transported back to the laboratory. In the laboratory, each sample was decanted into a 100-mL volumetric flask, the gas washing bottle was rinsed with 10 to 30 mL of 0.05 M sulfuric acid, the rinsate decanted into the flask, and then the sample in the flask was diluted to 100 mL by pipetting acid into it. Samples were mixed in the flask, 30 mL were decanted into a sample bottle, and then all samples were refrigerated until analysis. A calibrated flow injection analyzer (QuickChem FIA+ 8000; Lachat Instruments, Milwaukee, WI) was used to quantify ammonium in the samples, with a minimum detection limit of 10 µg L^–1. This corresponded to atmospheric ammonia concentrations of <1 µg m^–3.

Profiles of wind speed and air temperature were defined at the same heights as the ammonia gas washing bottles were positioned. Cup anemometers (12102M; R.M. Young, Traverse City, MI) measured wind speed and aspirated, fine-wire (25.4-µm diameter) thermocouples (ASPTC; Campbell Scientific, Logan, UT) measured air temperature. Other meteorological measurements included incoming solar radiation (LI200X; Licor, Lincoln, NE), net radiation (Q7.1; Radiation and Energy Balance Systems, Seattle, WA), relative humidity and air temperature (HMP45; Vaisala, Helsinki, Finland), manure pack temperature (STP1; Radiation and Energy Balance Systems), wind direction (12005; R.M. Young), and precipitation (TE525; Campbell Scientific). Outputs from meteorological instruments were sampled every 5 s and 5-min means were recorded to a data logger (CR23X; Campbell Scientific).

Ammonia flux was estimated using the integrated horizontal flux (IHF) method (Wilson et al., 1982, 1983; Wilson and Shum, 1992). The method assumes that source strength is uniform from the emitting surface, and that air flow is fully turbulent. The vertical flux density, Q (µg m^–2 s^–1), was calculated by integrating the horizontal flux densities between the limits z₀, the roughness length (m), and z_b, the height (m) at which ammonia concentration over a treated circle equals background concentration:

[1]

where Q(z) (µg m^–2 s^–1), the horizontal flux at height z (m), is the product of wind speed and ammonia concentration at z, and R is the radius of the circle (m). Ammonia concentration of the air at each measurement height was calculated with:

[2]

where c(z) is the acid sample NH₄–N concentration (µg L^–1), v(z) is the acid sample volume (L), M is the mass of NH₄–N in field blanks (µg), f(z) is air flow rate through the gas washing bottle (m³ s^–1), t(z) is the sampling time (s), and K = 17/14 is the ratio of the molecular weights of NH₃ and N. The field blanks were three gas washing bottles that were handled the same as sampling gas washing bottles except no air was drawn through them. They were used to correct for potential sample contamination during field handling and laboratory processing. Ammonia concentration at each height over manure circles was further corrected by subtracting the background ammonia concentration at the same height. Profiles of horizontal flux were integrated using the Trapezoidal Rule. The limits of integration were the roughness length, z₀, and the height at which ammonia concentration was within 20% of background ammonia concentration, z_b. In most cases, ammonia concentration measured over the treated circles was at background concentration at z = 1.2 or 2.4 m. Roughness length was determined from wind speed profiles measured under neutral thermal stability during a preliminary trial; mean z₀ was 6.75 mm.

Four trials, in summer, autumn, winter, and spring were completed, each lasting 29 d, except for the summer trial, which was 22 d. For each trial, the sequence was: (i) thaw out a mass of urine and feces from each diet treatment that was equivalent to the mean single-day output from five steers; (ii) apply urine and feces to treated circles once at the beginning of the trial; and (iii) measure ammonia concentration profiles on DAT (day after treatment) 1, 2, 4, 7, 11, 16, 22, and 29 of the trial. Sample integration times varied from 3 to 6 h during the daytime and from 12 to 16 h during nighttime. The sampling schedule was generally maintained, although inclement weather sometimes forced modification of sampling runs. Feces and urine for each treatment were divided into 80 equal increments and then applied to the center of 1-m² plots gridded on each circle. This application scheme was designed to meet the assumption of the integrated horizontal flux method that source strength is homogenous.

Statistical Analysis
Treatment differences from the in vitro laboratory experiment were tested using a completely random experimental design, with steer within treatment as the error term in the analysis of variance (SAS Institute, 1999). Treatment differences within each seasonal field trial were tested using paired t tests, with each sampling time considered as an observation.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Characteristics of Collected and Applied Manure
Steers fed CP11 produced 28% more feces and 9% less urine than those fed CP13 (Table 1). Nitrogen concentration in collected feces and urine was also less for the CP11 diet. Nitrogen in collected manure was partitioned differently in the two treatments, with urinary N comprising 66.2% of total N in the CP11 treatment, compared with 72.7% in the CP13 treatment. Urinary N typically increases as CP in a diet increases, as was observed for dairy heifers (James et al., 1999) and dairy cows (Frank et al., 2002; Frank and Swensson, 2002; Smits et al., 1995).

View larger version (18K): [in this window] [in a new window]   Fig. 1. The NH4–N concentration of analyte from gas washing bottles over artificial feedyard surfaces treated with manure from steers fed either 11.5 or 13.0% crude protein (CP) diet; summer trial, first day after treatment (DAT 1), 1300–1600 h.

View larger version (16K): [in this window] [in a new window]   Fig. 2. Horizontal ammonia flux over artificial feedyard surfaces treated with manure from steers fed either 11.5 or 13.0% crude protein (CP) diet; summer trial, first day after treatment (DAT 1), 1300–1600 h.

View larger version (24K): [in this window] [in a new window]   Fig. 3. Mean daily ammonia flux density from artificial feedyard surfaces treated with manure from steers fed either 11.5 or 13.0% crude protein (CP) diet. The summer trial began on 22 July 2002; autumn trial on 21 Oct 2002; winter trial on 10 Feb 2003; and spring trial on 14 Apr 2003.

View larger version (22K): [in this window] [in a new window]   Fig. 4. Cumulative measured NH3–N emitted from artificial feedyard surfaces treated with manure from steers fed either 11.5 or 13.0% crude protein (CP) diet. The summer trial began on 22 July 2002; autumn trial on 21 Oct 2002; winter trial on 10 Feb 2003; and spring trial on 14 Apr 2003.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

Decreasing crude protein in beef cattle diets from 13 to 11.5% decreased ammonia flux. The reduction was 44% (p < 0.01) in the closed chamber laboratory experiment. In the field experiment, decreasing dietary crude protein reduced mean daily ammonia flux in summer by 30% (p = 0.10). In autumn and spring, decreasing dietary crude protein reduced mean daily ammonia flux by 53 and 29%, respectively (p 0.08). No difference was observed in winter. Decreased ammonia emissions from the 11.5% CP diet manure were most likely due to less urinary N.

Estimated annual reduction in daily ammonia flux provided by decreasing crude protein was 28%. For a median-sized commercial feedyard located on the southern High Plains, with mean ammonia N emission of 3.2 Mg d^–1 (Todd et al., 2005) reducing dietary crude protein from 13 to 11.5% could potentially reduce ammonia N loss by 327 Mg yr^–1. The magnitude of reduction would depend on many factors, such as the ability to precisely formulate and feed rations, genetic variation in cattle, environmental variability, the maintenance of cattle performance, and manure management. In spite of these uncertainties, reducing crude protein in beef cattle diets may provide the most practical and cost-effective way to reduce ammonia emissions from feedyards.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Mention of trade or manufacturer names is made for information only and does not imply endorsement, recommendation, or exclusion by USDA-ARS.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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