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Journal of Animal Science - Animal Nutrition

Effect of dried distillers grains plus solubles on enteric methane emissions and nitrogen excretion from growing beef cattle1

 

This article in JAS

  1. Vol. 91 No. 6, p. 2846-2857
     
    Received: June 14, 2012
    Accepted: Mar 2, 2013
    Published: November 25, 2014


    2 Corresponding author(s): tim.mcallister@agr.gc.ca
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doi:10.2527/jas.2012-5564
  1. M. Hünerberg*†,
  2. S. M. McGinn,
  3. K. A. Beauchemin,
  4. E. K. Okine*,
  5. O. M. Harstad and
  6. T. A. McAllister 2
  1. Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2P5, Canada
    Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, Alberta T1J 4B1, Canada
    Norwegian University of Life Sciences, Ås, Norway

Abstract

The objectives of this study were to examine the impact of corn- or wheat-based dried distillers grains with solubles (CDDGS or WDDGS) on enteric methane (CH4) emissions from growing beef cattle and determine if the oil in CDDGS was responsible for any response observed. Effects of CDDGS or WDDGS on total N excretion and partitioning between urine and fecal N were also examined in this replicated 4 × 4 Latin square using 16 ruminally cannulated crossbreed heifers (388.5 ± 34.9 kg of initial BW). The control diet contained (DM basis) 55% whole crop barley silage, 35% barley grain, 5% canola meal, and 5% vitamin and mineral supplement. Three dried distillers grains with solubles (DDGS) diets were formulated by replacing barley grain and canola meal (40% of dietary DM) with CDDGS, WDDGS, or WDDGS plus corn oil (WDDGS+oil). For WDDGS+oil, corn oil was added to WDDGS (4.11% fat DM basis) to achieve the same fat level as in CDDGS (9.95% fat DM basis). All total mixed diets were fed once daily ad libitum. Total collection of urine and feces was conducted between d 11 and 14. Enteric CH4 was measured between d 18 and 21 using 4 environmental chambers (2 animals fed the same diet per chamber). Methane emissions per kilogram of DM intake (DMI) and as percent of GE intake (GEI) among heifers fed WDDGS (23.9 g/kg DMI and 7.3% of GEI) and the control (25.3 g/kg DMI and 7.8% of GEI) were similar (P = 0.21 and P = 0.19) whereas heifers fed CDDGS (21.5 g/kg DMI and 6.6% of GEI) and WDDGS+oil (21.1 g/kg DMI and 6.3% of GEI) produced less (P < 0.05) CH4. Total N excretion (g/d) differed (P < 0.001) among treatments with WDDGS resulting in the greatest total N excretion (303 g/d) followed by WDDGS+oil (259 g/d), CDDGS (206 g/d), and the control diet (170 g/d), respectively. Compared with the control diet, heifers offered WDDGS, CDDGS, and WDDGS+oil excreted less fecal N (P < 0.001) but more (P < 0.001) urinary N. Results suggest that high-fat CDDGS or WDDGS+oil can mitigate enteric CH4 emissions in growing beef cattle. However, to completely assess the impact of DDGS on greenhouse gas emissions of growing feedlot cattle, the potential contribution of increased N excretion to heightened NH3 and nitrous oxide emissions requires consideration.



INTRODUCTION

It is estimated that animal agriculture is responsible for approximately 2.9% of total anthropogenic greenhouse gas (GHG) emissions in the United States (Council for Agricultural Science and Technology, 2011). Ruminant livestock have been estimated to account for 17 to 37% of global anthropogenic methane (CH4) emissions (Steinfeld and Wassenaar, 2007; Lassey, 2008). Recent research has shown that enteric CH4 is the largest source of GHG emissions in the Canadian beef production cycle, accounting for 63% of total emissions (Beauchemin et al., 2010).

Byproducts from the ethanol industry, such as dried distillers grains plus solubles (DDGS), are a source of protein and energy for beef cattle diets. In Canada, both corn- and wheat-based DDGS (CDDGS and WDDGS, respectively) are frequently used in cattle diets. Recent shortages on national grain markets have led to an increase of DDGS prices. In addition, reduction of subsidies for production and use of grain-based ethanol may impact ethanol production capacity leading to higher and more volatile DDGS pricing in the future (USDA, 2013).

McGinn et al. (2009) reported that inclusion of 35% dietary DM as CDDGS reduced CH4 emissions (g/kg DM intake) by 16.4% in cattle fed a barley silage-based diet. This response was thought to be due to the high fat level in CDDGS (>12% DM basis). However, WDDGS (<5% fat DM basis) has less than half the fat content of CDDGS and there is no information on the impact of feeding WDDGS on CH4 emission from growing beef cattle. Furthermore, it is unknown if supplementing corn oil to WDDGS has the same effect on CH4 emission as corn oil naturally contained in CDDGS. Despite the potential reduction in CH4, a limitation to using DDGS as an energy source in beef cattle diets is that its high protein results in a dramatic increase in N excretion (McGinn et al., 2009). Excessive N excretion contributes to greater ammonia (NH3) emissions that negatively impact air quality and contribute to emissions of nitrous oxide (N2O), another potent GHG (Todd et al., 2006). Urinary N is more susceptible to leaching and volatilization losses than fecal N (Bussink and Oenema, 1998). Further research is needed to evaluate if N excretion as a result of feeding DDGS offsets gains in reducing GHG emissions through a reduction in CH4. Due to its greater fat content, we hypothesized that feeding CDDGS would be more effective in reducing CH4 emissions from growing beef cattle than WDDGS.

The objective of this study was to examine the impact of CDDGS or WDDGS on CH4 emissions and partitioning of N excretion from growing beef cattle and determine if the oil in CDDGS was responsible for any response observed.


MATERIALS AND METHODS

This study was conducted using the Metabolism Barn and the Controlled Environment Facility at Agriculture and Agri-Food Canada’s Research Centre in Lethbridge, Alberta. The experimental protocol received institutional approval and was conducted in accordance to the guidelines of Canadian Council on Animal Care (CCAC, 1993).

Experimental Design and Animals

Sixteen spayed crossbreed beef heifers (388.5 ± 34.9 kg of initial BW) were used in this experiment, which was designed as a replicated 4 × 4 Latin square with 2 groups of 8 animals, four 21-d periods, and 4 dietary treatments. Heifers were ruminally cannulated before the start of the study and vaccinated with Express 5-PHM (Boehringer Ingelheim Ltd., Burlington, ON, Canada), a modified live vaccine against bovine rhinotracheitis, bovine viral diarrhea, parainfluenza 3, bovine respiratory syncytial virus, Mannheimia haemolytica, and Pasteurella multocida.

Methane emissions were measured using 4 open circuit respiratory chambers with 2 heifers housed in each chamber during each measurement period. Within each group, heifers were paired such that each pair had similar BW. The 4 pairs within each group were randomly allocated to 1 of 4 treatment diets. As only 4 respiratory chambers were available at a time, the 2 groups were offset by 1 wk to facilitate CH4 measurements.

Treatment Diets and Feed Sampling

Treatment diets were formulated as growing (high forage) diets typical of that fed during the first 80 d in western Canadian feedlots. The control diet (control) contained (DM basis) 55% whole crop barley silage, 35% barley grain, 5% canola meal, and 5% vitamin and mineral supplement (Table 1). Three DDGS diets were formulated by replacing barley grain and canola meal (40% of the dietary DM) with CDDGS, WDDGS, or WDDGS plus corn oil (WDDGS+oil). For the WDDGS+oil treatment, corn oil (Great Value; Wal-Mart, ON, Canada) was added to WDDGS (which contained 4.11% fat on DM basis) in a ratio of 6:94 to achieve the same fat level as in CDDGS (9.95% fat on DM basis). Total mixed rations were prepared daily (Data Ranger; American Calan Inc., Northwood, NH). Heifers were fed for ad libitum intake (5% refusal) once daily at 1100 h. Quantities of feed offered and refused were recorded daily.


View Full Table | Close Full ViewTable 1.

Composition of experimental diets

 
Treatment1
Item Control CDDGS2 WDDGS3 WDDGS+oil
Ingredient, % of DM
    Barley silage 55.0 55.0 55.0 55.0
    Barley grain, steam rolled 35.0
    Canola meal 5.0
    CDDGS 40.0
    WDDGS 40.0 37.6
    Corn oil 2.4
    Barley grain, ground 3.4 3.4 3.4 3.4
    Calcium carbonate 1.25 1.25 1.25 1.25
    Salt 0.15 0.15 0.15 0.15
    Molasses, dried 0.13 0.13 0.13 0.13
    Mineral and vitamin premix2 0.06 0.06 0.06 0.06
    Vitamin E (500,000 IU/kg) 0.003 0.003 0.003 0.003
    Flavoring agent3 0.003 0.003 0.003 0.003
Chemical composition4
    DM, % 51.3 ± 2.4 52.0 ± 2.0 52.7 ± 2.1 52.6 ± 1.6
    OM, % 93.3 ± 0.2 92.6 ± 0.1 91.4 ± 0.3 91.3 ± 0.1
    CP, % 13.0 ± 0.5 18.6 ± 0.3 23.5 ± 0.2 22.0 ± 0.6
    NDF, % 32.5 ± 2.7 38.5 ± 1.1 33.9 ± 1.4 33.3 ± 2.3
    ADF, % 18.0 ± 1.1 23.7 ± 0.6 23.6 ± 0.7 23.0 ± 1.6
    Fat, % 3.0 ± 0.1 5.4 ± 0.1 3.7 ± 0.2 5.6 ± 0.2
    Starch, % 35.8 ± 1.3 17.9 ± 0.9 16.8 ± 0.6 17.4 ± 0.7
    GE, Mcal/kg of DM 4.31 ± 0.03 4.42 ± 0.06 4.38 ± 0.06 4.50 ± 0.08
1Treatments were Control = 35% barley grain + 5% canola meal, CDDGS (corn-based dried distillers grains with solubles) = 40% corn dried distillers grains plus solubles, WDDGS (wheat-based dried distillers grains with solubles) = 40% wheat dried distillers grains plus solubles, or WDDGS+oil (WDDGS plus corn oil) = 37.6% wheat dried distillers grains plus solubles + 2.4% corn oil (DM basis).
2Per kilogram of dietary DM: 65 of mg Zn, 28 mg of Mn, 15 mg of Cu, 0.7 mg of I, 0.2 mg of Co, 0.3 mg of Se, 6,000 IU of vitamin A, 600 IU of vitamin D, and 47 IU of vitamin E.
3Anise 422 powder containing ground cumin, fennel, fenugreek, silicon dioxide, and wheat bran (Canadian Bio-Systems Inc., Calgary, AB, Canada).
4Determined using samples pooled by diet within each period; all values except DM are expressed on a DM basis (n = 4; mean ± SD).

Diets and ingredients were sampled once weekly and analyzed for DM by drying at 55°C for 48 h. The forage inclusion level (as-fed basis) was adjusted if the DM concentration of barley silage deviated more than 3% units from the average. Weekly subsamples were composited by period. Orts were sampled daily during the digestibility trial (only group 1) and CH4 measurements (both groups) and pooled by animal at the end of each period. Samples were stored at –20°C until determination of DM and chemical composition.

Nitrogen Excretion and Digestibility

Excretion of N and apparent total tract digestibility of the diets were determined using the 8 animals in group 1 (376.4 ± 29.7 kg of initial BW). From d 1 to 17 heifers were housed in individual tie stalls in a metabolism barn. After they were adapted to the diets over the first 10 d of each period, total urinary and fecal collection were conducted between d 11 and 14. The heifers were fitted with urinary indwelling balloon catheters (Bardex Lubricath Foley catheter, balloon size: 75 cm3, catheters diameter: 8.7 mm; Bard Canada Inc., Oakville, ON, Canada) to ensure separation of urine and feces. Urine was preserved by acidification (pH < 2) with 4 N H2SO4 to prevent volatilization of NH3. Feces were collected using pans placed behind the heifers. Total output of urine and feces was measured every 24 h, and mixed samples were subsampled. Aliquots of the urine (1% of total daily output) were composited by heifer within period, diluted with distilled water at a ratio of 1:5, and stored at –20°C until analyzed. A subsample of the daily feces (∼500 g) was oven dried at 55°C. A representative composite sample was obtained by pooling the dried daily feces based on their respective DM content.

Ruminal Fermentation Measurements

On d 14, composite rumen samples (500 g) were obtained from 3 sites (reticulum, dorsal, and ventral sac) within the rumen of each animal at 0, 2, 6, 12, and 24 h after feeding. Rumen contents were thoroughly mixed and squeezed through 2 layers of polyester monofilament fabric (pore size 355 μm; B. & S. H. Thompson, Ville Mont-Royal, Quebec, Canada) and filtrate (5 mL) was mixed with 1 mL of 25% (wt/vol) metaphosphoric acid for VFA analysis, with an additional 5 mL of filtrate being mixed with 1 mL of 1% (wt/vol) H2SO4 for NH3 N analysis. Samples were stored at –20°C until analyzed. For enumeration of protozoa, filtrate (5 mL) was mixed with 5 mL of methyl green-formalin-saline solution. The samples were stored in the dark at room temperature until analyzed.

Ruminal pH was recorded continuously during the periods of CH4 measurement using the LRCpH data logger system (Dascor, Escondido, CA; Penner et al., 2006). Loggers were standardized in pH 4 and 7 at the start and end of each measurement period with pH being recorded every minute. The pH loggers were placed in the ventral sac of the rumen 2 h before the heifers entered the chambers on d 18 and removed immediately after the heifers were returned to the metabolism barn on d 21.

Methane Emission Measurements

On d 18 of each period, heifers were moved to the Controlled Environment Facility to measure CH4 production over 4 d using 4 large environmental chambers. The chambers measured 4.4 m wide by 3.7 m deep by 3.9 m tall (63.5 m3 volume, C1330; Conviron Inc., Winnipeg, MB, Canada) and housed 2 heifers in individual tie stalls equipped with comfort mats. Heifers were provided with free access to feed and water. The chamber doors were opened once daily for feeding and cleaning. The emission data corresponding to the door opening as well as the time for chambers to return to steady state conditions were omitted from the analysis.

Methane measurements were conducted as described by Beauchemin and McGinn (2006). Briefly, samples from the fresh-air intake and exhaust air duct of each chamber were pumped sequentially at 1 L/min (TD3LS7; Brailsford and Company, Rye, NY) and passed through an infrared gas analyzer (Ultramat 6; Siemens, Karlsruhe, Germany) via a set of solenoids controlled by a data logger (CR23X; Campbell Scientific, Logan, UT). The difference between the incoming and outgoing flow of CH4 was used to calculate the amount generated by the 2 animals inside each chamber. The chambers were ventilated using fans in the fresh-air intakes and exhaust ducts. The air volume of each chamber was exchanged every 5 min. Temperature within the chambers was maintained at 10°C. Air velocity was continuously monitored in each intake and exhaust duct for each chamber (model 8455 air velocity transducer; TSI Inc., Shoreview, MN). Air flow rates in the ducts were adjusted to generate a slight positive pressure (approximately 2 Pa) inside each chamber. Intake and exhaust air stream CH4 concentrations of each chamber were sampled every 30 min using the same analyzer. The gas analyzer was calibrated daily directly after feeding time using N2 as 0 and 405 mg/L of CH4 as standard gases.

Before the start of the experiment the system was calibrated by sequentially releasing 0, 0.2, and 0.4 L/min of CH4 separately into each empty chamber using a mass-flow meter (Omega Engineering, Stamford, CT). A 3-point regression was developed by plotting actual against calculated CH4 emission. The slopes of these best fit linear relationships were used to correct for between-chamber variability.

Blood Sampling

Blood samples for the determination of plasma urea N (PUN) were collected from all 16 heifers by jugular vein puncture on d 21 of each period 22 h after feeding using 10-mL vacuum tubes containing lithium-heparin solution (Vacutainer; Becton Dickinson, Mississauga, Canada). After centrifugation (3,000 × g at 4°C for 20 min) samples were stored at –20°C until analyzed.

Laboratory Analyses

Samples of composited ingredients, diets, orts, and feces were oven dried at 55°C and ground through a 1 mm screen (Standard model 4 Wiley mill; Arthur H. Thomas, Philadelphia, PA). Analytical DM was determined by drying at 135°C for 2 h (AOAC, 2005; method 930.15) followed by hot weighing. The OM content was calculated as the difference between 100 and the percentage of ash (AOAC, 2005; method 942.05). The NDF and ADF concentrations were quantified as described by Van Soest et al. (1991), using amylase and sodium sulfite for the NDF analysis. Fat was determined according to AOAC (2006; method 2003.05) using ether extraction (Extraction Unit E-816 HE; Büchi Labortechnik AG, Flawil, Switzerland). Gross energy in diets, orts, and feces was determined using a bomb calorimeter (model E2k; CAL2k, Johannesburg, South Africa). For the measurement of CP (N × 6.25) and starch, samples were ground using a ball mill (Mixer Mill MM2000; Retsch, Haan, Germany). Nitrogen was quantified by flash combustion with gas chromatography and thermal conductivity detection (Carlo Erba Instruments, Milan, Italy). Total urinary N was analyzed in the same fashion using freeze dried urine. Starch content was determined by enzymatic hydrolysis as described by Rode et al. (1999).

Concentration of NH3 in urine and rumen fluid was determined by the salicylate-nitroprusside-hypochlorite method (Sims et al., 1995) using a flow injection analyzer. Concentrations of VFA in ruminal fluid were analyzed as described by Addah et al. (2012) using gas chromatography (model 5890; Hewlett Parkard, Wilmington, DE) with crotonic acid as an internal standard. Concentration of urea in urine and blood plasma was analyzed using micro-Segmented Flow Analysis (model Astoria2; Astoria Pacific Inc., Clackamas, OR). Ruminal protozoa were enumerated under a light microscope using a counting chamber (Neubauer Improved Bright-Line counting cell, 0.1 mm depth; Hausser Scientific, Horsham, PA) as described by Ogimoto and Imai (1981).

Calculations and Statistical Analyses

Data were analyzed using the mixed model procedure (SAS Inst. Inc., Cary, NC) with animal as the experimental unit for all variables, except for CH4 production, where chamber was considered the experimental unit.

Continuous ruminal pH data were summarized for daily average, minimum, maximum, SD, duration below pH 6.0, and area under the curve (AUC). The AUC was calculated as the sum of the absolute value of pH deviations below pH 6.0 multiplied by the duration below pH 6.0 and reported as pH × h. Intake corrected AUC was calculated as AUC divided by DMI. Durations and AUC below pH 6.0 were considered as critical pH threshold levels below which degradation of fiber was impaired (Weimer, 1996). Protozoa numbers were log10 transformed before statistical analysis. The model for DMI and ruminal fermentation variables included the fixed effect of diet and the random effects of group, heifer nested within group, and period nested within group. For ruminal fermentation variables sampling time (0, 2, 6, 12, and 24 h after feeding) was treated as a repeated measure. Data for N excretion and total tract digestibility trial were analyzed using the same model but without the random effect of group because only group 1 heifers were used in this part of the study. Sampling days (1 to 4) were treated as a repeated measure.

Daily CH4 production (g CH4/d) from each chamber was expressed per unit of combined DMI (g CH4/kg DMI) and proportion of GE (%) and DE (%) intake of the 2 heifers within each chamber on that same day. The model used for CH4 production included the fixed effect of diet and the random effects of group, period nested within group, and chamber nested within group. Day of sampling (d 1 to 4) within each period was treated as repeated measure. Denominator degrees of freedom were estimated using the Kenward-Roger option in the model statement. The PDIFF option adjusted by the Tukey method was included in the lsmeans statement to account for multiple comparisons. The best time series covariance structure was selected based on the lowest Akaike and Bayesian information criteria. Differences among means were tested using a protected (P < 0.05) LSD test. Treatment effects were declared significant at P < 0.05.


Results

Ruminal Fermentation and pH

Feeding CDDGS decreased (P < 0.05) the molar proportion of acetate (Table 2) as compared with all other treatments and increased (P = 0.02) the proportion of propionate compared with WDDGS. Furthermore, CDDGS decreased (P < 0.05) the acetate:propionate ratio as compared with the control and WDDGS diet. Heifers fed WDDGS and WDDGS+oil had greater (P < 0.001) molar proportion of valerate and concentration of NH3 (P < 0.001) compared with the control and CDDGS diets. Furthermore, the proportion of valerate (P = 0.01) and concentration of NH3 (P = 0.006) were greater for WDDGS compared with WDDGS+oil. Numbers of total protozoa were similar among diets that contained DDGS but were lower (P < 0.01) relative to the control.


View Full Table | Close Full ViewTable 2.

Ruminal fermentation variables of ruminally cannulated beef heifers fed a barley silage-based high-forage diet supplemented with barley grain and canola meal, corn or wheat dried distillers grains plus solubles (CDDGS or WDDGS), or WDDGS plus corn oil (WDDGS+oil; n = 16 per treatment)

 
Treatment1
Item Control CDDGS WDDGS WDDGS+oil SEM P-value
Total VFA, mM 151 139 144 144 5.0 0.09
VFA, mol/100 mol
    Acetate 60.3a 57.9b 60.5a 59.8a 0.70 <0.001
    Propionate 22.1ab 23.1a 20.9b 22.7ab 0.76 0.025
    Butyrate 12.4b 14.1a 13.5ab 12.6b 0.57 0.010
    Isovalerate 1.97a 1.83a 1.45b 1.51b 0.114 <0.001
    Valerate 1.69c 1.66c 2.23a 2.02b 0.061 <0.001
    Isobutyrate 1.06 1.01 1.02 0.96 0.049 0.18
    Acetate:propionate 3.42a 3.07b 3.54a 3.24ab 0.139 0.002
    NH3, mM 6.1c 6.3c 15.8a 14.0b 0.68 <0.001
Protozoa
    Total, n × 105/mL 7.9a 4.4b 4.0b 3.1b 1.31 <0.001
    Entodiniomorphs,2 % 99.3 99.8 98.6 98.8 0.37 0.11
    Holotrichs,3 % 0.7 0.2 1.4 1.2 0.40 0.12
a−cWithin a row, means without a common superscript letter differ, P < 0.05.
1Treatments were Control = 35% barley grain + 5% canola meal, CDDGS = 40% corn dried distillers grains plus solubles, WDDGS = 40% wheat dried distillers grains plus solubles, or WDDGS+oil = 37.6% wheat dried distillers grains plus solubles + 2.4% corn oil (DM basis).
2Entodiniomorphs = Entodinium + Diplodinium + Polyplastron + Eudiplodinium + Epidinium + Ophryoscolex.
3Holotrichs = Isotricha + Dasytricha.

The mean and minimum ruminal pH of heifers fed CDDGS and WDDGS+oil was lower (P < 0.05) compared with those fed WDDGS (Table 3). Feeding the control diet resulted in a lower minimum pH as compared with WDDGS (P < 0.001) and WDDGS+oil (P = 0.03) and a greater (P < 0.05) SD of ruminal pH as compared with all other treatments. Ruminal pH in heifers fed CDDGS and WDDGS+oil spent more time (P < 0.05) below pH 6.0 as compared with those fed WDDGS. Feeding WDDGS decreased the AUC expressed as pH × hour per day at pH 6.0 (P = 0.05) as compared with the control. The AUC < pH 6.0 per kg DMI decreased (P < 0.05) for heifers fed WDDGS as compared with those fed control, CDDGS, or WDDGS+oil diets.


View Full Table | Close Full ViewTable 3.

Ruminal pH of beef heifers fed a barley silage-based high-forage diet supplemented with barley grain and canola meal, corn or wheat dried distillers grains plus solubles (CDDGS or WDDGS), or WDDGS plus corn oil (WDDGS+oil; n = 16 per treatment)

 
Treatment1
Item Control CDDGS WDDGS WDDGS+oil SEM P-value
Ruminal pH2
    Mean 6.22ab 6.18b 6.34a 6.18b 0.061 0.025
    Minimum 5.41c 5.47bc 5.76a 5.57b 0.063 <0.001
    Maximum 6.89 6.84 6.85 6.83 0.051 0.67
    SD of mean pH 0.37a 0.32b 0.24c 0.29b 0.019 <0.001
Duration of pH, h/d
    <6.0 6.9ab 8.0a 4.0b 7.6a 1.27 0.021
AUC,3 pH × h/d
    <6.0 3.0a 2.9ab 1.0b 2.0ab 0.60 0.033
AUC/kg DMI, pH × min
    <6.0 20.7a 22.1a 7.6b 19.4a 4.63 0.026
a−cWithin a row, means without a common superscript letter differ, P < 0.05.
1Treatments were Control = 35% barley grain + 5% canola meal, CDDGS = 40% corn dried distillers grains plus solubles, WDDGS = 40% wheat dried distillers grains plus solubles, or WDDGS+oil = 37.6% wheat dried distillers grains plus solubles + 2.4% corn oil (DM basis).
2Ruminal pH determined for 4 d during which the animals were in the chambers.
3AUC = area under the curve.

Digestibility and Nitrogen Excretion

The DMI of heifers fed CDDGS was 9.8% less (P = 0.002) than those fed WDDGS and 12.1% lower (P = 0.014) than the control diet (Table 4). Consequently, feeding CDDGS resulted in lower (P < 0.05) intakes of OM and GE as compared with WDDGS and control diets whereas OM and GE intakes were similar between CDDGS and WDDGS+oil. Intake of CP differed (P < 0.01) among all 4 diets. Heifers offered WDDGS ingested the most CP followed by WDDGS+oil, CDDGS, and those offered the control diet. Heifers fed the control diet ingested less (P < 0.01) ADF as compared with those fed CDDGS, WDDGS, or WDDGS+oil.


View Full Table | Close Full ViewTable 4.

Nutrient intakes and total tract digestibility measured in beef heifers fed a barley silage-based high-forage diet supplemented with barley grain and canola meal, corn or wheat dried distillers grains plus solubles (CDDGS or WDDGS), or WDDGS plus corn oil (WDDGS+oil; n = 8 per treatment)

 
Treatment1
Item2 Control CDDGS WDDGS WDDGS+oil SEM P-value
Intake
    DM, kg/d 9.58a 8.42b 9.39a 8.84ab 0.367 0.001
    OM, kg/d 8.94a 7.78c 8.58ab 8.05bc 0.340 0.001
    CP, kg/d 1.24d 1.58c 2.18a 1.94b 0.079 <0.001
    NDF, kg/d 3.07 3.22 3.18 2.96 0.168 0.11
    ADF, kg/d 1.68c 1.98b 2.22a 2.05b 0.106 <0.001
    GE, Mcal/d 41.3a 37.2b 41.1a 39.7ab 1.66 0.016
Digestibility, %
    DM 70.9a 66.4b 69.0a 66.6b 0.79 <0.001
    OM 71.8a 66.4c 69.3b 66.5c 0.83 <0.001
    CP 64.1b 70.1a 70.8a 69.3a 0.89 <0.001
    NDF 51.3a 46.3b 50.4a 44.1b 2.24 <0.001
    ADF 31.8c 38.5ab 43.0a 37.5b 2.24 <0.001
    GE 69.8a 65.7c 68.4ab 66.8bc 0.96 <0.001
a−dWithin a row, means without a common superscript letter differ, P < 0.05.
1Treatments were Control = 35% barley grain + 5% canola meal, CDDGS = 40% corn dried distillers grains plus solubles, WDDGS = 40% wheat dried distillers grains plus solubles, or WDDGS+oil = 37.6% wheat dried distillers grains plus solubles + 2.4% corn oil (DM basis).
2Nutrient intakes and total tract digestibility determined for 4 d.

Feeding CDDGS and WDDGS+oil reduced apparent total tract digestibility of DM (DMD; P < 0.05), OM (OMD; P < 0.01), and NDF (NDFD; P < 0.05) as compared with heifers fed WDDGS or the control diet. Digestibility of OM in WDDGS was less (P = 0.03) than the control diet. In contrast, apparent total tract digestibility of CP (P < 0.001) and ADF (ADFD; P = 0.02 to <0.001) in heifers fed CDDGS, WDDGS, and WDDGS+oil were greater than for those fed the control diet. In addition, ADFD in heifers fed WDDGS+oil was reduced (P = 0.02) as compared with those fed WDDGS.

Total N excretion (g/d) differed (P < 0.001) among all 4 treatments (Table 5). Feeding WDDGS resulted in the greatest total N excretion (303 g/d) followed by WDDGS+oil (259 g/d), CDDGS (206 g/d), and the control diet (170 g/d). Furthermore, feeding WDDGS, CDDGS, and WDDGS+oil dramatically increased (P < 0.001) urinary N excretion, with diets that contained WDDGS also exhibiting increased (P < 0.001) fecal N excretion as compared with control. Heifers offered WDDGS, CDDGS, and WDDGS+oil compared with the control excreted less N (P < 0.001), expressed as percentage of total N excretion, through feces but more N (P < 0.001) through urine. Additionally, excretion of urea N (g/d) and NH3 N (g/d) as well as PUN concentration of heifers fed CDDGS, WDDGS, and WDDGS+oil were greater (P < 0.001) compared with heifers fed the control diet. Excretion of fecal N (% total N excretion) of heifers fed WDDGS was less (P < 0.05) than those fed CDDGS or WDDGS+oil whereas urinary N excretion (% total N excretion) of heifers fed WDDGS increased (P < 0.05) compared with those fed CDDGS or WDDGS+oil. Although feeding WDDGS+oil reduced (P < 0.001) daily excretion of urea N compared with WDDGS, NH3 N output of heifers fed WDDGS+oil and WDDGS were similar.


View Full Table | Close Full ViewTable 5.

Nitrogen intake and excretion measured in beef heifers fed a barley silage-based high-forage diet supplemented with barley grain and canola meal, corn or wheat dried distillers grains plus solubles (CDDGS or WDDGS), or WDDGS plus corn oil (WDDGS+oil; n = 8 per treatment)

 
Treatment1
Item Control CDDGS WDDGS WDDGS+oil SEM P-value
N intake, g/d 199d 252c 350a 310b 12.7 <0.001
N excretion,2 g/d 170d 206c 303a 259b 10.4 <0.001
Fecal excretion
    Output, kg/d 2.87 2.90 3.04 3.09 0.15 0.096
    Total N, g/d 71.1c 73.3c 101.9a 94.3b 4.17 <0.001
    Total N, % N excretion 42.1a 36.3b 33.8c 36.6b 1.13 <0.001
Urinary excretion
    Output, L/d 6.9c 6.8c 11.2a 9.9b 0.65 <0.001
    Total N, g/d 98.5d 133c 201a 165b 7.7 <0.001
    Total N, % N excretion 57.9c 63.7b 66.2a 63.4b 1.13 <0.001
    Urea N, g/d 53.4d 89.6c 140a 118b 5.8 <0.001
    NH3 N, g/d 0.9c 1.8b 4.6a 4.5a 0.41 <0.001
    Plasma urea N,3 mg/dL 6.6b 11.6a 12.1a 11.2a 0.63 <0.001
a−dWithin a row, means without a common superscript letter differ, P < 0.05.
1Treatments were Control = 35% barley grain + 5% canola meal, CDDGS = 40% corn dried distillers grains plus solubles, WDDGS = 40% wheat dried distillers grains plus solubles, or WDDGS+oil = 37.6% wheat dried distillers grains plus solubles + 2.4% corn oil (DM basis).
2Nitrogen intakes and excretion was measured over 4 d during the total collection period.
3Samples taken on d 21 (n = 16 per treatment).

Methane Emissions

Once in the chamber, DMI of heifers fed WDDGS was 10.2% less (P = 0.015) as compared with those fed the control (Table 6). Compared with the control, feeding CDDGS, WDDGS, or WDDGS+oil reduced (P < 0.01) total CH4 emission (g/d) by 19.5, 16.1, and 23.8%, respectively. The decrease in CH4 emission compared with the control was maintained for CDDGS (P < 0.001) and WDDGS+oil (P < 0.001) when corrected for differences in DMI. However, feeding WDDGS had no effect (P = 0.21) on CH4 emissions when corrected for differences in DMI. This suggests that the decline in total CH4 emissions when feeding WDDGS reflects a decline in feed intake of heifers fed this diet vs. the control. Methane emissions as percentage of GE intake decreased from 7.8% of GE intake for control to 6.6% for CDDGS (P < 0.001) and to 6.3% for WDDGS+oil (P < 0.001). Emissions of CH4 as percentage of DE intake decreased from 11.1% for the control to 10.0% for CDDGS (P < 0.02) and 9.4% of DE for WDDGS+oil (P < 0.001). Feeding WDDGS+oil reduced CH4 emissions per kilogram DMI (P = 0.004) as a percent of GE intake (P = 0.003) and as a percent of DE intake (P = 0.006) as compared with WDDGS alone. There were no differences in CH4 emission between CDDGS and WDDGS+oil (P = 0.35 to 0.93), regardless of how emissions were expressed.


View Full Table | Close Full ViewTable 6.

Daily methane emissions from beef heifers fed a barley silage-based high-forage diet supplemented with barley grain and canola meal, corn or wheat dried distillers grains plus solubles (CDDGS or WDDGS), or WDDGS plus corn oil (WDDGS+oil; n = 8 per treatment)

 
Treatment1
Item2 Control CDDGS WDDGS WDDGS+oil SEM P-value
DMI, kg/d 9.05a 8.57ab 8.13b 8.42ab 0.291 0.024
Methane
    g/d 228a 184b 191b 174b 11.7 <0.001
    g/kg of DMI 25.3a 21.5b 23.9a 21.1b 1.15 <0.001
    % of GE intake 7.8a 6.6b 7.3a 6.3b 0.36 <0.001
    % of DE intake 11.1a 10.0bc 10.7ab 9.4c 0.53 <0.001
a−cWithin a row, means without a common superscript letter differ, P < 0.05.
1Treatments were Control = 35% barley grain + 5% canola meal, CDDGS = 40% corn dried distillers grains plus solubles, WDDGS = 40% wheat dried distillers grains plus solubles, or WDDGS+oil = 37.6% wheat dried distillers grains plus solubles + 2.4% corn oil (DM basis).
2Methane emissions and corresponding DMI determined over 4 d during which the animals were in the chambers. Chamber (data for 2 animals) was the experimental unit.


DISCUSSION

Grain-based ethanol production throughout North America has grown considerably in the last few years due to the mandated inclusion of renewable fuel in gasoline. Whereas most of the ethanol produced in the United States is derived from corn, Canadian ethanol plants ferment wheat as well as corn. As the starch is fermented to ethanol, the remaining nutrients in DDGS (fiber, protein, fat, and minerals) are concentrated about 3-fold (Spiehs et al., 2002). Corn grain is lower in protein but greater in fat (9.8% CP and 4.1% fat DM basis; NRC, 2000) than wheat grain (14.2% CP and 2.3% fat DM basis; NRC, 2000). Consequently, CDDGS is typically lower in protein but greater in fat content than WDDGS as was the case in our study where the DM composition of CDDGS was 31.5 ± 0.5% CP, 10.0 ± 0.3% fat, 37.3 ± 1.3% NDF, 17.9 ± 1.4% ADF, and 4.4 ± 0.6% starch (DM basis) and WDDGS was 45.3 ± 1.0% CP, 4.1 ± 0.1% fat, 23.8 ± 1.0% NDF, 15.3 ± 1.8% ADF, and 8.4 ± 0.9% starch. These values are similar to those previously reported for CDDGS (Spiehs et al., 2002; Klopfenstein et al., 2008) and WDDGS (Beliveau and McKinnon, 2008; Gibb et al., 2008).

Methane Emissions

The control diet fed in our study was a typical high-forage diet fed to growing cattle in western Canadian feedlots with whole-crop barley silage and barley grain as predominant feed components. Because CH4 emissions from feedlot cattle are greater during the growing compared with the finishing phase, effective CH4 mitigation strategies targeting high-forage growing diets are even more desirable to the North American beef industry (Beauchemin et al., 2010). The DDGS inclusion level used in this study is within range of practical feeding strategies as inclusion of up to 40% CDDGS (Klopfenstein et al., 2008) and WDDGS (Gibb et al., 2008) have been shown to have no negative impact on animal performance.

Methane emissions of heifers offered the control diet are in accordance with Beauchemin and McGinn (2006) who reported heifers fed a high-forage diet containing 75% DM of barley silage lost 7.93% of their GE intake as CH4. Beauchemin and McGinn (2005) and McGinn et al. (2009) reported decreased CH4 emissions of 7.3 and 7.1% of GE intake for growing diets containing 70 and 60% barley silage DM. These CH4 emissions tend to be greater than 6.5% (±1.0%) of GE intake as estimated using Intergovernmental Panel on Climate Change (IPCC)tier 2 methodologies for cattle fed a high forage growing diet (IPCC, 2006). The accuracy of IPCC estimates for dairy and beef cattle diets have previously been challenged (Kebreab et al., 2008). Information on CH4 emissions from beef cattle diets containing CDDGS is limited. McGinn et al. (2009) reported that CH4 emissions were reduced by 16.4% (g/kg DMI) or by 23.9% (% for GE intake) when CDDGS (35% of DM) replaced barley grain in a growing diet containing 60% barley silage (DM basis). The response was thought to be due to the high fat level of the CDDGS (12.7% DM basis). In the present study the reduction in CH4 for CDDGS (40% of DM CDDGS) relative to the control diet was similar to that observed by McGinn et al. (2009). Decreased CH4 emissions of heifers fed WDDGS+oil relative to the control diet and WDDGS alone support the hypothesis that the high fat content of CDDGS and WDDGS+oil is responsible for the decrease in CH4. It is unlikely that this reduction was due to other changes in feed composition because all 3 diets supplemented with DDGS contained more NDF and ADF and less starch compared with the barley grain control (Table 1). Methane emissions usually increase rather than decrease with increasing dietary fiber content especially when substituted for starch (Johnson and Johnson, 1995).

Moate et al. (2011) compared CH4 emissions of dairy cows offered diets containing different percentages of byproducts with high residual fat content and concluded that diets supplemented with brewers grain (11.0% fat DM basis), cold-pressed canola meal (12.0% fat DM basis), and hominy meal (16.1% fat DM basis) produced less enteric CH4 emissions than cows fed a control cracked wheat diet. Similarly, Behlke et al. (2007) reported reduced CH4 emissions for lambs fed brome hay-based ration containing 30% DDGS (DM basis) although in that study DDGS replaced corn bran (30% DM basis) instead of grain. In a second experiment, Behlke et al. (2007) observed that a partial replacement of corn grain with DDGS (30% DM basis) increased CH4 production in lambs fed a corn-based high grain diet (71% DM basis), but the fat content of the DDGS used in either study was not reported. The authors conclude that to reduce CH4 emissions from ruminants, DDGS should replace forage rather than grains (Behlke et al., 2007). That recommendation is not supported by our results; CDDGS and WDDGS+oil reduced CH4 emission due to their high fat content even though they replaced grain and lowered dietary starch and increased dietary fiber content. However, CH4 emissions per kilogram of DM intake are generally less for cattle fed a high concentrate diet as compared with a high forage diet (Johnson and Johnson, 1995), with the amount of concentrate used in the second experiment by Behlke et al. (2007) greater than in our study. Therefore, replacement of the forage portion of the diet with high-fat DDGS could have the added benefit of reducing CH4 emissions through increasing the concentrate portion of the diet.

Lipids that are not protected from ruminal digestion decrease CH4 emissions by exerting toxic effects on methanogens and protozoa, which are physically and metabolically associated with methanogens (Martin et al., 2009). Added fat can also enhance propionic acid production or in some cases replace structural carbohydrates that could otherwise contribute to CH4 production (Johnson and Johnson, 1995). Additionally biohydrogenation of unsaturated fatty acids is thought to reduce CH4 formation because both pathways require H2 (Czerkawski et al., 1966). Sources of medium-chain fatty acids, such as coconut oil, reduce CH4 primarily through being directly toxic to methanogens whereas long-chain fatty acids seem to decrease CH4 emissions more through decreased DMI and reduced fiber digestion (Machmüller and Kreuzer, 1999; Beauchemin et al., 2008). Fatty acid profiles of CDDGS and WDDGS are similar and not particularly rich in fatty acids that have specific inhibitory effects on CH4 emissions (e.g., myristic acid). Additionally, comprehensive results from metabolism studies suggest the fat in DDGS may be partially protected from ruminal hydrogenation (Klopfenstein et al., 2008). Therefore, the total level of fat in DDGS as opposed to the fatty acid profile may be the factor responsible for the reduction in enteric CH4 emissions. The reduction in CH4 in our study was relatively high as each 1% addition of supplemental fat reduced CH4 emissions (g/kg DMI) by 6.3% for CDDGS and 6.4% for WDDGS+oil as compared with the control. Based on 17 studies with beef cattle, dairy cows, and lambs over a broad range of conditions, CH4 (g/kg DMI) was calculated to be reduced by 5.6% with each 1% addition of supplemental fat (Beauchemin et al., 2008). Similarly for cattle, Grainger and Beauchemin (2011) calculated that an increase in dietary fat from 5 to 6% (DM basis) decreased CH4 (g/kg DMI) by 5.1%. The fact that WDDGS alone failed to reduce CH4 emission compared with the control is attributable to the relatively low dietary fat level (3.7% DM) as feeding WDDGS+oil with a dietary fat level of 5.6% DM reduced CH4 emissions substantially.

Sulfate (SO2–4) can also act as an alternative electron acceptor in the rumen and in fact the reduction of SO2–4 to sulfite (SO2–3) is thermodynamically more favorable than the reduction of CO2 to CH4 (McAllister et al., 1996). Dietary S is metabolized to form SO2–4 in the rumen, which in turn is reduced to SO2–3 by ruminal bacteria (Burgess, 2008). Consequently, differences in S content among sources of DDGS could also impact ruminal CH4 production. However, previous work using DDGS sourced from the same plants as in the current study showed that the S content of WDDGS (1%) was only slightly greater than CDDGS (0.8%) and that serum SO2–4 concentrations did not differ in feedlot cattle fed WDDGS vs. CDDGS (Amat et al., 2012). Greater S content in DDGS primarily results from the use of sulfuric acid in the cleaning of ethanol fermentation tanks, a practice that has largely ceased in the industry due to concerns that greater S content in DDGS can lead to polioencephalamolacia in cattle (Buckner et al., 2008). Given the similar S content in CDDGS and WDDGS, it seems unlikely that differences in the concentration of this alterative electron acceptor played a role in the observed differences in CH4 emissions between WDDGS and CDDGS in the current study.

Digestibility, Ruminal Fermentation, and pH

Differences in chemical composition of the diets and DM intake caused different nutrient intakes among diets. Treatments containing DDGS supplied more CP and ADF than the control diet. Based on the starch and fat content of the diets it can be assumed that starch intake of heifers offered the control diet and the fat intake of heifers offered CDDGS and WDDGS+oil diet were greater than for other treatments.

The reduction in DMD and OMD in heifers fed CDDGS and WDDGS+oil is consistent with decreased CH4 emissions observed for these diets and was mainly caused by reduced NDFD relative to WDDGS and the control diet. Although CDDGS failed to reduce ADFD compared with heifers fed WDDGS, the reduction in ADFD in heifers fed WDDGS+oil compared with WDDGS alone suggest that the oil reduced overall fiber digestion. High NDFD and ADFD of diets containing WDDGS or CDDGS are likely a reflection of the extensive processing of the grain before ethanol production and possibly the direct impact of ethanol fermentation on the structural integrity of fiber (Ham et al., 1994; Walter et al., 2012).

Increased apparent total tract digestibility of CP in diets containing CDDGS and WDDGS compared with barley grain has been previously described for high grain finishing diets with similar DDGS inclusion levels (Li et al., 2011; Walter et al., 2012). Surprisingly, in the current study ruminal NH3 concentration in the rumen fluid of heifers fed CDDGS was similar to those fed the control diet even though CP intake of heifers fed CDDGS was greater. Unlike ruminal NH3 concentration, PUN and urea N concentration of heifers fed CDDGS were substantially greater than those fed the control diet, suggesting that replacing barley grain in the control diet with CDDGS shifted the site of CP digestion from the rumen to the small intestine. Decreased ruminal concentration of NH3 and valerate in heifers fed CDDGS as compared with those fed WDDGS and WDDGS+oil likely reflect a reduced RDP content in CDDGS compared with WDDGS (Boila and Ingalls, 1994) as DDGS is generally high in RUP (52% of CP; NRC, 2000). Decreased NH3 concentration in the rumen fluid of heifers fed WDDGS+oil compared with WDDGS alone are likely caused by the decline in OM fermentation in response to fat.

Fat feeding has also been shown to reduce CH4 through a reduction in protozoal numbers. Methanogens are metabolically associated with protozoa, and feeding oil can cause substantial decreases in protozoal populations (Ivan et al., 2004). Fewer numbers of protozoa in the rumen fluid of heifers fed DDGS as compared with those fed the control diet were likely caused by factors other than fat, because protozoa numbers were similar across the DDGS diets. Lowering the starch content of the diet by replacing barley grain with fibrous feeds has previously been shown to reduce protozoal populations (i.e., Entodinium spp. in particular) in the rumen (Hristov et al., 2001).

The reason for lower mean and minimal ruminal pH of heifers fed CDDGS and WDDGS+oil as compared with WDDGS alone is unclear. As discussed earlier, the high fat content of CDDGS and WDDGS+oil most likely depressed ruminal digestion causing a reduction in DMD and OMD compared with WDDGS. Consequently, an increase rather than a decrease in mean and minimum ruminal pH for heifers fed CDDGS and WDDGS+oil was anticipated. As cellulolytic microbes are particularly sensitive to low pH (Weimer, 1996), the lower mean and minimum pH of heifers fed CDDGS and WDDGS+oil as compared with WDDGS might have further impaired ruminal fiber digestion, contributing to decreased DMD and OMD. However it is possible that differences in DM intake or intake behavior (e.g., intake frequency and sorting) between these 2 experimental phases may have affected ruminal pH as apparent total tract digestibility was determined between d 11 and 14 and ruminal pH between d 18 and 21. Greater SD of the ruminal pH in heifers fed the control diet compared with those fed diets containing DDGS likely reflect a less stable pH pattern due to highly fermentable starch in barley grain. The ruminal degradability of barley starch is estimated to be 80 to 85% (Huntington, 1997). It has been assumed that substitution of a nonstarch DDGS for the highly fermentable starch in barley grain decreases VFA concentrations and consequently increases ruminal pH, reducing the incidence of subacute ruminal acidosis (SARA; Klopfenstein et al., 2008). Results, mostly obtained in beef cattle fed high concentrate finishing diets, show that substituting DDGS for cereal grains has less impact on ruminal pH than expected and does not reduce incidence or severity of SARA (Beliveau and McKinnon, 2009; Li et al., 2011; Walter et al., 2012). Because the fiber in DDGS is highly fermentable and not effective at stimulating chewing activity and salvia production, adding DDGS to a diet does little to enhance rumen buffering capacity of cattle fed high concentrate diets (Beliveau and McKinnon, 2009). Our study was not designed to compare the effect of DDGS and barley grain under low pH conditions because we fed high forage diets and the ruminal pH of all heifers was above threshold level for SARA. But decreased AUC (pH × hour per day) at pH 6.0 and decreased AUC < pH 6.0 per kg DM intake of heifers fed WDDGS compared with the control is reflected by greater ADFD. Ruminal cellulolytic bacteria prefer pH near neutrality for growth (Weimer, 1996). Consequently, less time spent below threshold levels of pH 6.0 could have resulted in greater fiber digestion in heifers fed WDDGS.

Nitrogen Excretion

Due to its increased supply and competitive price, DDGS is not only used as protein but also as an energy source in beef cattle diets. The concept of using protein rich byproducts as energy sources has the potential to negatively impact the environment. Excess N, largely excreted in the form of urea via urine, is rapidly hydrolyzed to NH3 by bacterial urease. Ammonia is very volatile and disperses easily into the surrounding air (Asman et al., 1998). Once in the atmosphere NH3 is a precursor to the formation of aerosols with a potential negative impact on human health (USEPA, 2009). Furthermore, NH3 is redeposited on the soil surface contributing to eutrophication, soil acidity, and formation of N2O (IPCC, 2006; Hristov et al., 2011). Nitrous oxide is a potent GHG with a global warming potential 298 times (100 yr timeframe) that of CO2 (IPCC, 2007) whereas the global warming potential (100 yr timeframe) of CH4 is only 25 times that of CO2 (IPCC, 2007). Therefore, the observed reduction in CH4 for CDDGS and WDDGS+oil diets could be offset by heightened N2O emissions that could increase the net GHG emission when feeding DDGS. Additionally, excess N can be also lost through runoff and leaching during storage and application, possibly acting as a pollutant of ground and surface water (IPCC, 2006).

Substantial differences in CP content among diets (control: 13.0%, CDDGS: 18.6%, WDDGS: 23.5%, and WDDGS+oil: 22.0% CP; DM basis) resulted in dramatic difference in CP intakes (Table 4) among treatments. The high CP content of the WDDGS and WDDGS+oil diet exceeded CP requirements of growing beef heifers by 2-fold (NRC, 2000), leading to a dramatic increase in total daily N excretion (Table 5). A similar increase in the excretion of total N as well as fecal and urinary N was reported for heifers fed 40% CDDGS and WDDGS in place of barley grain in finishing diets (Walter et al., 2012). As expected, urine was the major route of N excretion for all diets and accounting for more than 60% of daily N excretion. Urinary N is rapidly converted to NH3 whereas fecal N is converted to NH3 at a much slower rate. Shifting N excretion from the urine to the feces is recognized as a means of increasing the environmental stability of manure N (Varel et al., 1999). Compared with the control, the increase in urinary N excretion relative to N excreted in feces observed for all 3 DDGS diets would likely increase N losses in the form of NH3 as well as direct and indirect N2O emissions and leachate. The reduction of urinary N relative to fecal N excretion in heifers fed CDDGS and WDDGS+oil compared with WDDGS was likely caused by reduced OMD in response to fat. Consequently, feeding high fat DDGS not only decreases CH4 emission of diets containing DDGS but could also help reduce volatile N losses. Other strategies to decrease N losses from beef feedlots are reducing N intake, increasing pen cleaning frequency, increasing OM on the pen surface, and acidification of the manure (Erickson and Klopfenstein, 2010). However, CP intake of heifers fed WDDGS was greater compared with those fed CDDGS and WDDGS+oil. Therefore, greater urinary N excretion relative to N excreted in feces of heifers fed WDDGS could also reflect increased N intake.

Concentration of PUN is an indicator of N status in ruminants and concentrations greater than 8 mg/dL are indicative of excessive N intake and wastage (Cole et al., 2003). In our study, feeding CDDGS, WDDGS, and WDDGS+oil resulted in PUN concentrations greater than 11 mg/dL, clearly indicating intake of digestible N in excess of requirements. Using a meta-analysis approach, Kohn et al. (2005) reported that PUN (mg/dL) is linearly related to urinary N excretion rate (g/d) and concluded that blood urea N concentration can be used to predict relative differences in urinary N excretion rate for animals of a similar stage of production within a study but is less reliable across animal types or studies. Our data support the general relationship between PUN concentration and urinary N excretion but indicates that the accuracy of predicting urinary N excretion from PUN concentration is low. For example, in our study, PUN concentrations among heifers fed diets containing DDGS were similar despite differences in total N excretion.

In conclusion, adding CDDGS or fat supplemented WDDGS to the diet of growing beef cattle reduces enteric CH4 production. This response is dependent on the fat content of DDGS as WDDGS (low in fat content) alone had no effect on CH4 emissions. However, feeding DDGS, especially WDDGS, increases N excretion. Therefore, the environmental effect of feeding DDGS to growing beef cattle needs to be measured using a life cycle assessment that accounts for both enteric CH4 and N excretion. An appreciation for the potential environmental consequences of feeding high levels of CDDGS is critical as many ethanol plants lower the oil levels in this byproduct thereby negating its ability to reduce enteric CH4 emissions.

 

References

Footnotes


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