Greenhouse-gas emissions of beef finishing systems in the Southern High Plains

Agricultural Systems 176 (2019) 102674

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Greenhouse-gas emissions of beef finishing systems in the Southern High PLAINS

T

K.R. Heflina, , D.B. Parkerb, G.W. Marekb, B.W. Auvermanna, T.H. Mareka ⁎

a b

Texas A&M AgriLife Research, Amarillo, TX, United States of America USDA-ARS CPRL, Bushland, TX, United States of America

ARTICLE INFO

ABSTRACT

Keywords: Life cycle analysis Beef production Greenhouse gas CO2e emissions

Greenhouse gases (GHG) have been implicated in global warming and climate change. While life cycle assessments (LCA) and GHG studies have been conducted for numerous agricultural commodities, there has been little effort to estimate GHG (CO2, N2O, and CH4) from beef finishing systems of the Southern High Plains (SHP) region, which produces approximately 30% of the United States beef. The objective of this research was to quantify the carbon footprint of five beef-finishing systems using a dynamic, systems-based model that calculated CO2e emissions attributable to both animal gain and manure management. The systems consisted of native grass pasture (NGP, System 1); native grass pasture with feedyard finishing (NGP-FY, System 2); wheat pasture with feedyard finishing (WP-FY, System 3); feedyard-only (FY, System 4); and native grass pasture, wheat pasture, and feedyard finishing (NGP-WP-FY, System 5). Although rarely used, the NGP was included as a baseline. Variables in the model and associated management decisions were based on feed type, nutritional content, feed source, and hauling distance. The starting point of the model was a weaned steer (250 kg) and the endpoint was a steer which would grade “choice” (28% body fat) or 30 months in age, whichever came first. Overall CO2e kg−1 gain decreased when cattle were fed high-quality diets and were intensively managed for production in the shortest time possible. The FY produced the desired carcass in the shortest time with the lowest cumulative emissions. The FY also had the highest average daily gain, lowest dry matter and water intake, as well as manure production. Net GHG emissions from FY were 4.84 kg CO2e kg−1 gain (1799 kg CO2e animal−1). Net GHG emissions from NGP-FY, WP-FY, NGP-WP-FY, and NGP were 1.62, 1.81, 2.08, and 3.69 times that of FY, respectively. These results suggest that intensive feeding and management of beef cattle in the FY system result in the lowest overall CO2e emissions to produce a mature steer. Consequently, feeding systems that include native grass and wheat pasture have proportionately larger amounts of CO2e emissions.

1. Introduction The heat trapping capacity of increasing greenhouse gas (GHG) concentrations in the earth's atmosphere is thought be responsible for rising global temperatures and climate change. The three-primary listed GHG's of interest are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The global warming potential (GWP) of these GHG's is not equivalent and therefore not related to concentration alone, e.g., the GWP of N2O and CH4 is 298 and 25 times higher than CO2, respectively (Code of Federal Regulations (CFR), 2015). As such, a multiplier is used in conjunction with mass emissions of GHG to calculate GWP CO2 equivalent values (CO2e). Similarly, carbon footprint (CF) estimates are commonly defined as the total emissions attributable to an entity (e.g., individual, animal, event, product, etc.). However, the ecological

⁎

footprint concept introduced by Wackernagel and Rees (1996) differs fundamentally from simple rates of emission. They defined an ecological footprint as the land area required for resource consumption and waste assimilation of a given population. Conforming to a more recent convention, in this study, “carbon footprint” is a general term used to express GHG emissions via CO2e units that are attributed to the production of beef cattle. The United States Environmental Protection Agency (USEPA) lists the primary anthropogenic sources of GHG emissions and their respective contributions to the total GHG emissions in 2015 as electrical production (29%), transportation (27%), industry (21%), commercial and residential (12%), agriculture (9%), and land use and forestry (United States Environmental Protection Agency, (USEPA), E, 2015). The burning of fossil fuels such as coal, natural gas, and petroleum derivatives are the primary sources of GHG in all sectors

Corresponding author. E-mail address: [email protected] (K.R. Heflin).

https://doi.org/10.1016/j.agsy.2019.102674 Received 22 June 2018; Received in revised form 7 August 2019; Accepted 9 August 2019 Available online 21 August 2019 0308-521X/ © 2019 Elsevier Ltd. All rights reserved.

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K.R. Heflin, et al.

other than agriculture and land use and forestry (United States Environmental Protection Agency, (USEPA), E, 2015). In agriculture, approximately one third of GHG emissions are attributed to CH4 resulting from enteric fermentation of ruminants. Fifteen percent of agricultural emissions are attributed to CH4 and N2O emissions derived from the management and storage of manure (United States Environmental Protection Agency, (USEPA), E, 2015). A report published by the Food and Agriculture Organization of the United Nations. (FAO) (2009), Tackling Climate Change Through Livestock, estimated that the annual GHG emissions attributed to the livestock supply chain account for 14.5% (7.1 GT CO2e yr−1) of all human-induced emissions using the most recent Intergovernmental Panel on Climate Change (IPCC) estimates (Gerber et al., 2013). The Southern High Plains (SHP) is one of the most intensive animal production areas in the United States, accounting for 30% of the nation's fed beef cattle (Texas Cattle Feeders Association (TCFA), 2007). The SHP includes portions of Colorado, Kansas, Oklahoma, New Mexico, and Texas totaling 14.3 million hectares, of which 7 million are classified as grassland (United States Department of Agriculture, Natural Resources Conservation Service (USDA-NRCS), 2006). Approximately 3.3 million head of cattle on feed at any given time in this region, producing > 7 million finished cattle every year (United States Department of Agriculture, National Agricultural Statistics Service (USDA-NASS), 2014). Cattle fed in the SHP originate from all regions of the United States, Canada, and Mexico. Gerber et al. (2013) estimated that beef cattle contribute 41% of the total emissions attributed to animal agriculture with feed production/processing and enteric fermentation as the primary sources of emissions, representing 45 and 39% of the total emissions, respectively. They also reported that manure storage and processing account for 10% of emissions while the remaining 6% is attributed to the processing and transportation of animal products. Feeder cattle, or cattle entering the feedlot system, arrive via several management scenarios resulting in cattle that differ in age and size. Therefore, each of these scenarios have intrinsic CF's associated with the feeding and management from weaning to finishing. Carbon emissions are listed as a regulated GHG pollutant by the USEPA. This same organization has ruled that GHG's are harmful to human health and are a detriment to the environment (USEPA, 2009).The development and implementation of feeding and management strategies that improve feed efficiency can reduce CO2e emissions while enhancing production efficiency (Johnson and Johnson, 1995). Lower GHG emissions may also be achieved with improved genetics, animal health, and nutrition (Mitloehner and Place, 2009). Increasing animal performance may be the most effective strategy to lower emissions (Stackhouse et al., 2012) from the beef cattle sector. A life cycle assessment (LCA) of the beef cattle feeding systems in the SHP is beneficial to evaluate the environmental burdens, defined as resource use per functional unit (FU) produced, to quantify the “footprint” of both the cattle feeding industry as-a-whole and of the smaller FU's within the system. The three FU's used for evaluation in this study were defined as kg CO2e kg−1 gain, kg CO2e d−1 animal−1, and kg CO2e finished animal−1. Currently, agricultural emissions are exempt from regulation under the Clean Air Act. However, should an explicit regulatory framework for agricultural emissions develop, animal agriculture should be prepared with qualitative management-specific data to respond to regulations. The objective of this research was to quantify CF estimates for SHP beef-finishing systems using a dynamic, systems-based model that calculated CO2e emissions attributable to both animal gain and manure management. A model was developed to incorporate regionally distinctive features of the SHP beef industry including waste management practices, fed-cattle sourcing, and concentrate sourcing. The model provided a realistic, industry-relevant, modeling platform for case studies.

2. Materials and methods System productivity was characterized by total days on feed (DOF), dietary gross energy, average daily gain (ADG), water consumption, dry matter intake (DMI), and total manure excreted. 2.1. Modeling approach A spreadsheet-based model was developed to estimate the CF of the animal relative to the production system and calculate CO2e emissions based on inputs used per animal in each of the five feeding systems. All simulations of the beef cattle systems begin with a weaned steer, weighing 250 kg and 205 days of age, prior to entering the feedyard production phase of the model. Modeled animal production system scenarios included a weaned steer finished on native grass pasture (NGP, System 1); a weaned steer placed on a native grass pasture prior to finishing in a feedyard (NGP-FY, System 2); a weaned steer placed on winter wheat pasture prior to being finished at a feedyard (WP-FY, System 3); a weaned steer shipped directly to and finished at a feedyard (FY, System 4); and a weaned steer placed on native grass pasture, then moved to winter wheat pasture prior to being finished at a feedyard (NGP-WP-FY, System 5). Although cattle are rarely raised solely on pasture from weaning to finishing, the NGP system was included in the study as a baseline for comparison of the other systems. The model was used to calculate daily net emissions of GHG's from each production system. Model calculations included system-specific inputs to simulate temporal variability in ambient air temperature, diet composition, animal maintenance requirements, and feeding strategies common to the SHP. Daily cattle gain, including protein and fat deposition, were calculated using National Research Council (NRC) Beef Nutrient Requirements (National Research Council (NRC) and Subcommittee of beef cattle nutrition, 1996). Model simulations of the feeding systems and associated management strategies estimated the CO2e of each system which were then compiled into a composite CF of the SHP feeding industry, assuming relative proportions of each system. The model was designed to quantify accumulated system CO2e emissions for a single steer from the time of weaning until the steer would grade USDA choice or 30 months of age, whichever came first. The USDA grade choice endpoint which correlates to 28% body fat [USDA grade “choice”, United States Department of Agriculture (USDA), 2015a, 2015b], and is the targeted endpoint by feedlots. However, the steer was considered to reach a marketable endpoint at 30 months of age, irrespective of body fat, per industry convention. The accounting process was used to assess relative resource use and CO2e emissions produced per kg of marketed product from each system. Model results from each system were then compared. In addition to animal feeding inputs, this assessment considered GHG emissions from agricultural crop production, waste (manure only) disposal, fossil fuel use, and transportation. This assessment did not include capital goods such as barns, trucks, fencing, roads, tractors, and their associated CF/environmental burden. 2.2. Model assumptions The following model assumptions were applied unequivocally to all production scenarios. The assumptions were:

• Weaned steers entered Sept. 1 at 250 kg and age 205 d (Cook, 2002). • All feed sources were available ad libitum. • Steers were fed diets representative of the individual feeding sys• • 2

tems (National Research Council (NRC) and Subcommittee of beef cattle nutrition, 1996; Galyean, 2015). Daily water consumption averaged 40.9 L d−1 animal−1 (Parker and Brown, 2003). Nutritional quality of native grass pasture diets varied seasonally (National Research Council (NRC) and Subcommittee of beef cattle

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K.R. Heflin, et al.

nutrition, 1996; Galyean, 2015).

weighing 478 kg steer with 28% body fat. This reference weight was derived from comparative slaughter experiments (National Research Council (NRC) and Subcommittee of beef cattle nutrition, 1984). The SBW values (National Research Council (NRC) and Subcommittee of beef cattle nutrition, 1996) were calculated as 4% less than body weight (BW) and used to account for weight loss attributed to environmental stressors, shipping, shipping distance, and handling techniques (Michigan State University (MSU), 2009):

• Literature values were used to estimate crop production CO e. • Cattle and agricultural products were transported by freight trains and semi-trucks. • Corn was shipped into the SHP via unit trains from Midwestern states. • CO e emissions were calculated using the equations detailed in 2

2

• • • • • •

chapters 10 and 11 of the Intergovernmental Panel on Climate Change (IPCC) (2006a). Cattle growth was simulated using National Research Council, Beef Nutrient Requirements guidelines (National Research Council (NRC) and Subcommittee of beef cattle nutrition, 1996). The model endpoint was a steer with estimated 28% body fat, or 30 months of age, whichever occurred first. FU's for the model were kg CO2e kg−1 gain, kg CO2e d−1 animal−1, and kg CO2e finished animal−1. Emissions from birth to weaning and emissions associated with transport and slaughter of the finished animal were not included. A shipping distance of 1262 km was used for transporting the calf to the SHP. Water use CO2e was based on electricity to pump 1135 L min−1 from 91 m depth.

SBW = BW

(1)

0.96

Empty body weight (EBW) and empty body weight gain (EBG) as described in National Research Council (NRC) and Subcommittee of beef cattle nutrition (1996):

EBW = 0.891

SBW

(2)

EBG = 0.956

SWG

(3)

Calculation of SWG required estimates of net energy required for animal gain (NEg Animal) and the equivalent shrunk body weight reference animal:

SWG = 13.91

(NE g Animal 0.9116)

(EQSBW

0.6837 )

(4)

In which EQSBW was calculated as:

EQSBW = SBW

2.3. System boundary and conceptual model

Initial SBW FSBW

(5)

Final shrunk body weight (FSBW) was dependent on the weight of the animal upon entering the feedyard. Animal FSBW in all five systems was estimated as described in Hicks et al. (1989), and were proportional to initial weight, i.e. larger animals had a larger FSBW. The energy requirements for animal maintenance, based on EBW and ambient temperature (NEmTemperature) were calculated using NRC equations (National Research Council (NRC) and Subcommittee of beef cattle nutrition, 1996):

The model simulated relevant processes (i.e., resources consumed, and wastes generated) of a beef steer from weaning to mature endpoint. A conceptual model was developed to illustrate the five production systems and associated system boundaries (Fig. 1). The pathways do not include the inputs and outputs for each of the sub-systems within the boundaries of the system and are shown only to outline the different pathways or management strategies used to produce a mature, marketable animal.

NEmAnimal = (0.077

2.4. Livestock growth model

(6)

EBW 0.75) + NEmtemperature

In which energy NEmTemperature was calculated using the following equations:

Using the National Research Council (NRC) and Subcommittee of beef cattle nutrition (1996) guidelines, cattle growth requirements were calculated using body weight (BW), shrunk weight gain (SWG), empty body weight (EBW), body composition (% fat and protein), and standard reference weight (SRW). Simulated average daily gain (ADG) from the available metabolizable energy and consumed protein was the difference between the energy required for maintenance and energy available for gain. Animal growth was modeled by adjusting the shrunk body weight (SBW) to an equivalent shrunk body weight (EQSBW) reference animal, given a SRW. The SBW value was defined as the mass of the animal after food and water were withheld for a period of time. The National Research Council (NRC) and Subcommittee of beef cattle nutrition (1996) SRW was based on National Research Council (NRC) and Subcommittee of beef cattle nutrition (1984) medium-frame steer

NEmTemperature = (0.00077 AThermo = |Avg tempi °C

SBW 0.75)

(7)

AThermo

20°C|, where i is DOY 1

(8)

365

In which AThermo is the difference between the historical average air temperature for a given day and 20°C. Eq. (7) indicates that the NEmanimal requirement changes by 0.00077 Mcal/(kg SBW0.75) for each degree that the previous ambient temperature differs from 20°C (National Research Council (NRC) and Subcommittee of beef cattle nutrition, 1996). Historical daily average air temperature for each day of the year (DOY) was computed from 22 years of meteorological records from the Texas High Plains Evapotranspiration Network (TXHPET) collected at the USDA Agricultural Research Service Conservation and Production Research Laboratory (CPRL) located in Bushland, Texas (Marek et al., 2005). To determine the NEmAnimal required for animal maintenance and weight gain (also known as retained energy demand (REDemand)), the daily dry matter intake (DMI) was calculated based upon the net energy from the feed composition (NEmfeed) and net energy for maintenance required by the animal (NEmanimal). The DMI (kg d−1) was calculated as:

DMINEmAnimal =

NEmAnimal NEmfeed

Animal net energy for gain based on DMI (Mcal kg lated as:

DMINEg Animal = DMI

Fig. 1. System boundary and management scenarios for the five production systems examined.

DMINEmAnimal

Retained energy demand was calculated as: 3

(9) −1

) was calcu(10)

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K.R. Heflin, et al.

EBW 0.75

REDemand = 0.0635

(11)

EBG1.097

Table 1 Ingredients and nutrient summary (% Diet DM Basis) of Feedyard rations used in the growth model (Galyean, 2015).

Cattle weight gain after maintenance requirements had been met was calculated as:

NEgAnimal = DMINEg

NEgfeed

Feedyard ration concentrate % Ingredient Corn Grain, steam flaked Alfalfa hay, mid bloom Cottonseed, hulls Molasses, cane Tallow Urea Cottonseed, Meal - Sol-41%CP TTU-2.5 mineral supplement DM Total, %

(12)

Cattle weight gain is retained as fat and protein, and the rate at which either is deposited depends on the diet composition and animal weight (National Research Council (NRC) and Subcommittee of beef cattle nutrition, 1996; Tedeschi et al., 2004). Proportioning gain into fat and protein were calculated as:

NEgfat = 0.123

NEgAnimal EBG

NEgprotein = 0.253908

0.0271067

Diet nutrient summary CP, % DIP, % of DM NEm, Mcal kg−1 NEg, Mcal kg−1 eNDF, % of DM P, % K, %

(13)

0.154 NEgAnimal

(14)

EBG

Calculating gain based on fat and protein was required to determine the endpoint for each of the production simulations. Daily shrunk weight gain in fat was calculated as:

SWGFat = NEgfat

SWG

SWGFat SBW

(15)

100

(0.1493

NEmfeed2

47.8 20 15 4.0 2.0 0.5 8.2 2.5 100

59.2 12.5 12.5 4.0 2.5 0.8 6.0 2.5 100

67.2 8.3 8.3 4.0 3.0 0.9 6.0 2.5 100

76.0 4.0 4.0 4.0 3.0 1.0 5.6 2.5 100

14.0 9.0 1.8 1.2 25.0 0.3 1.0

13.5 8.6 1.9 1.3 20.0 0.3 0.9

13.5 8.4 2.1 1.4 14.4 0.3 0.8

13.5 8.3 2.2 1.5 9.2 0.3 0.7

Range june diet

Range july diet

Range august diet

Range sept diet

Range winter diet

Wheat

DM Total, % Diet nutrient summary CP, % DIP, % of DM NEm, Mcal kg−1 NEg, Mcal kg−1 eNDF, % of DM P, % K, %

100

100

100

100

100

100

11.00 1.48

10.50 7.92 1.39

9.70 7.35 1.30

6.90 6.40 1.21

4.70 4.62 0.99

11.00 2.96 1.73

0.88 26.90 0.15 0.00

0.82 27.76 0.15 0.00

0.73 26.12 0.15 0.00

0.64 27.31 0.15 0.00

0.44 27.10 0.15 0.00

1.11 46.20 0.40 3.50

Abbreviations: CP = crude protein, DIP = degradable intake protein, DM = dry matter, Nem = net energy for maintenance, Neg = net energy for gain, eNDF = effective neutral detergent fiber, P = phosphorus, K = potassium.

(17)

0.0196)

92

Range/pasture

(16)

NEmfeed

83.5

Table 2 Range and wheat pasture dietary analysis (% Diet DM Basis) for growth model inputs (Galyean, 2015).

Feedyard data from Hicks et al. (1989) were used to estimate DMI based on initial weight, days on feed (DOF), and seasonal variations in metabolic maintenance. Representative simulation of feed-to-gain conversion was dependent upon accurate estimates of DMI. Hicks et al. reported DMI measurements on a weekly basis, and a simple linear interpolation approach was used to estimate daily DMI. The average measured DMI for the first week was estimated from d 4, and then every 7 d (Fig. 2). Intake calculations for forage-based feeding systems that define DMI as a function of dietary NEmfeed concentration with adjustments for frame size and sex (National Research Council (NRC) and Subcommittee of beef cattle nutrition, 1984, 1996).

DMI = SBW 0.75

75

Abbreviations: TTU-2.5 = Texas Tech University proprietary mineral supplement, CP = crude protein, DIP = degradable intake protein, DM = dry matter, Nem = net energy for maintenance, Neg = net energy for gain, eNDF = effective neutral detergent fiber, P = phosphorus, K = potassium.

The total shrunk weight gain in fat was used to estimate the body fat content on a daily basis until the animal reached the targeted body fat content. The estimated percentage of body fat (%BF) on any given day was calculated as:

%BF =

65

2.5. Livestock emissions model

−1

Net energy feed values for maintenance (NEmfeed Mcal d ) and gain (NEgfeed Mcal d−1) were obtained from literature values based on common feedstuffs (Galyean, 2015). The selected values represented four diets that could be used commercially at feedyards and five range/ pasture forage-based diets with seasonal variations. Ingredients used in the forage and feedyard diets are shown in Tables 1 and 2.

The 2006 IPCC guidelines for GHG inventories (Intergovernmental Panel on Climate Change (IPCC), 2006a, 2006b) were used to estimate GHG emissions from each system. Methane from enteric fermentation along with CH4 and N2O emissions from manure management

Fig. 2. Graphical illustration of estimated daily dry matter intake (DMI) interpolated from average weekly values for feedyard cattle using previously published data from Hicks et al. (1989). 4

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K.R. Heflin, et al.

operations were estimated for each system, along with direct and indirect N2O emissions from soils. Livestock respiration was not included, as it is not a net source of CO2 Steinfeld et al. (2006). Tier One methods for estimating GHG emissions were used when data were insufficient to conduct a more detailed Tier Two assessment. The methods for estimating GHG emissions on a Tier Two level, or higher, require detailed information about feed digestibility, days on feed, type of animal fed, gross energy (GE) of the diet, dietary crude protein (CP), volatile solid (VS) excretion rates, environmental conditions, and the type of manure management system. Tier Two CH4 emission estimates require feed intake and growth data as previously described. Once the feed source and intake were determined, the Tier Two GHG emissions were estimated. Enteric CH4 (Intergovernmental Panel on Climate Change (IPCC), 2006a) was computed as:

EFCH4 =

GE

( ) Ym 100

climate region k, dimensionless (Intergovernmental Panel on Climate Change (IPCC), 2006a). MS(T ,S, k ) = 0.184 feedlot , annual , cool temperature (1.0 if manure is collected daily ) MS(T ,S, k ) = 0.815 Pasture /Range, annual , cool temp (1.0 if manure is collected daily )

Manure-management system default values are based on annual average temperature ≤ 10–14 °C, drylot (feedyard) or range/pasture (Intergovernmental Panel on Climate Change (IPCC), 2006a). Manure management systems are defined as pasture/range/paddock where manure from grazing animals is allowed to lie as deposited and is not managed. A dry lot is a paved or unpaved confinement area without any significant vegetative cover and from which accumulating manure may be periodically removed. Maximum CH4-producing capacity for manure (Intergovernmental Panel on Climate Change (IPCC), 2006a) was calculated as:

DOF

Bo (T ) = 0.19 (18)

55.65

Volatile solids (VS) are the organic material in the livestock manure and consist of both biodegradable and non-biodegradable fractions (Intergovernmental Panel on Climate Change (IPCC), 2006a). The VS value for Eq. (20) was the total VS as excreted by the livestock. Volatile solids excretion rate (Intergovernmental Panel on Climate Change (IPCC), 2006a) were calculated as:

In which: EFCH4 = Emission factor, kg CH4 head−1 d−1. GE = Gross energy intake, MJ head−1 d−1 (GE of the feedstocks used in the model are detailed in Table 3). Ym = % of gross energy in feed converted to CH4, with values of 3.0% and 6.5% for feedyard and grazing cattle, respectively. The energy content of CH4 is 55.65 MJ kg−1. Methane produced from the treatment and storage of manure and manure deposited on pastures was estimated using the detailed Tier Two approach per Intergovernmental Panel on Climate Change (IPCC) (2006a, 2006b) guidelines. CH4 emissions from manure management were estimated using:

EFCH4 M (T ) = (VS(T )

DOF )

Bo(T )

0.67 S,k

MCFS, k 100

VS = GE

(19) In which: EFCH4M(T) = Manure CH4 emission factor for livestock category T, kg CH4 head−1 d−1. Livestock category T = cattle. VS(T) = Daily volatile solid excreted for livestock category T, kg DM animal−1 DOF−1. Bo(T) = Maximum methane-producing capacity for manure produced by livestock category T, m3 CH4 kg−1 of VS excreted. 0.67 kg/m3 = Density of CH4 at standard temperature and pressure of 15 °C and 101.3 kPa (Air Liquide, 2018). MCF(S,k) = Methane conversion factors for each manure-management system S by climate region k, MCF(S,k) = 1.0 % for drylot or range. MS(T,S,k) = Manure-management system where fraction of livestock category T's manure handled using manure-management system S by

N2 OD (mm) = S

Corn Alfalfa Cotton seed hulls Molasses (Cane) Tallow a Urea Cotton seed meal Winter wheat Generic grass

18.7 18.2 19.8 14.7 39.0 0.0 21.2 17.9 18.0

DE % + (UE 100

GE )

1

ASH 18.45

(21)

T

(N(T )

Nex (T )

MS(T , S ) )

EF3(S )

44 28 (22)

N2OD(mm) = Direct N2O emissions from manure management in the country, kg N2O yr−1 N(T) = Number of head of livestock species/category T in the country; set to one for this model Nex(T) = Annual average N excretion per head of species/category T in the country, kg N animal−1 yr−1 MS(T,S) = Fraction of total annual N excretion for each livestock species/category T that is managed in manure management system S in the country, dimensionless EF3(S) = Emission factor for direct N2O emissions from manure management S in the country, kg N2O kg N−1 in manure management system S. (Intergovernmental Panel on Climate Change (IPCC), 2006a), EF3 = 0.02 for dry lot manure management system. 44/28 = Conversion of (N2O-N)(mm) emissions to N2O(mm) emissions Indirect N2O emissions from manure management (feedyard and

Table 3 Dietary gross energy values for model inputs (National Institute of Agricultural Research, (French translation: Institut National de la Recherche Agronomique (INRA)), 2017). GE (MJ kg−1 DM)

1

GE = Gross energy of the feed derived from literature values (MJ kg−1). DE = Digestibility of the feed in percent (Intergovernmental Panel on Climate Change (IPCC), 2006a), DE = 75–85% for feedyard, 55–75% for pasture fed animals, and 45–55% for animals fed low quality forage) UE = Urinary energy expressed as fraction of GE. Typically, 0.04 GE can be considered urinary energy excretion by most ruminants (reduce to 0.02 for ruminants fed with 85% or more grain in the diet) ME = Metabolizable Energy ME = DE – (UE + Gaseous Energy) Ash = The fixed solids content of manure calculated as a fraction of the DMI (0.08 for cattle) measured after combustion at 550–600 °C 18.45 = Conversion factor for dietary GE per kg for DM (MJ kg−1). Direct N2O emissions from manure management occur via combined nitrification and denitrification of nitrogen contained within the manure (Intergovernmental Panel on Climate Change (IPCC), 2006b). The Tier Two methodology for direct N2O emissions from manure management (feedyard) can be written as:

MST , S, k

Feedstock

(20)

VS

a Urea does not contain energy since it is a non-protein nitrogen.

5

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Table 4 Model units and parameters sourced from literature values. Parameter

Units

Diesel fuel use Electricity Natural gas to steam flake Urea production Feed grade molasses Tallow Wheat biomass Corn grain Alfalfa hay Cottonseed meal Cottonseed hull Corn Yield (grain + biomass) Wheat biomass yield Wheat grain yield Alfalfa hay yield TTU supplement Digestible energy Digestible energy Digestible energy Water consumption Diesel semi-truck Diesel train

−1

kg CO2e L Mg CO2e kW hr−1 CO2e kg m3/ kg CO2e L−1 kg CO2e L−1 kg CO2e L−1 kg CO2e ha−1 yr−1 kg CO2e ha−1 yr−1 kg CO2e ha−1 yr−1 kg CO2e kg−1 kg CO2e kg−1 Mg ha−1 Mg ha−1 Mg ha−1 Mg ha−1 kg CO2e kg−1 % FY diet % WP diet % NGP diet L D−1 Mg km L−1 Mg km L−1

Values

Source

2.681 0.0007 0.176 1.13 0.425 0.184 1.81 3.13 0.23 0.508 0.298 11.14 8.35 2.92 8.4 0 (NA) 80 65 50 40.9 51.7 186.4

United States Energy Information Administration (USEIA), 2011 United States Energy Information Administration (USEIA), 2011 Macken et al., 2006 Skowronska and Filipek, 2014 Klenk et al., 2012 Mulvaney, 2014 MSU, 2017 MSU, 2017 Desjardins et al., 2010 Murphy et al., 2010 Murphy et al., 2010 MSU, 2017 Xue et al., 2014 MSU, 2017 United States Department of Agriculture, National Agricultural Statistics Service (USDA-NASS), 2015a, 2015b Galyean, 2015 Intergovernmental Panel on Climate Change (IPCC), 2006a, 2006b Intergovernmental Panel on Climate Change (IPCC), 2006a, 2006b Intergovernmental Panel on Climate Change (IPCC), 2006a, 2006b Parker et al., 2000 CSX Transportation Inc, 2015 CSX Transportation Inc, 2015

pasture-based systems) (Intergovernmental Panel on Climate Change (IPCC), 2006a), were calculated using Tier Two methods. The Tier Two equation for indirect N2O emissions from manure management is:

NVolt

MMS

=

S

T

[(N(T )

MS(T , S ) )

(FracGasMS /100) (T , S ) ]

Direct N2 Oman soils = DMIkg day

MS(T , S, k )

(23)

MMS

EF4 )

44 28

( ) CP % 100

6.25

1

%N retained 100 (25)

0.02

in which DMI = Livestock dry matter intake kg d−1 MS(T,S,k) = Manure-management system by livestock species/category T that is managed in manure management system S, in climatic condition k %N retained = N retained by livestock (proportion of gain in protein*EBG/6.25) 0.02 = Default emission value (Intergovernmental Panel on Climate Change (IPCC), 2006b) Crude Protein = N * 6.25%, where N = Total Kjeldahl Nitrogen Nitrogen intake for cattle was computed as:

in which Nvolt-MMS = Amount of manure N that is lost due to volatilization of NH3 and NOx, kg N yr−1 N(T) = Number of head of livestock species/category T; set to one for this model Nex(T) = Annual average N excretion per head of species/category T, kg N animal−1 yr−1 (This number is converted to daily excretion based upon nitrogen in the diet) MS(T,S) = Fraction of total annual nitrogen excretion for each livestock species/category T that is managed in manure-management system S in the country, dimensionless (This number is converted to daily excretion based upon nitrogen in the diet) FracGasMS = Proportion of managed manure nitrogen for livestock category T that volatilizes as NH3 and NOx in the manure management system S, % (Intergovernmental Panel on Climate Change (IPCC), 2006b), default value = 20% volatilization from all organic N applied or deposited by livestock. Indirect N2O emissions due to volatilization of N from manure management (Intergovernmental Panel on Climate Change (IPCC), 2006b) were computed as:

N2 OG (mm) = (Nvolatilization

1

Nintake (T ) =

DMI %CP 625

(26)

The carbon dioxide equivalent emission conversion factors can be written as:

QCO2 e =

n i=1

[QCO2, i + (298

Q N2 O, i ) + (25

QCH 4,i )]

(27)

in which Q = Cumulative emissions of CO2e per animal (wean to harvest), kg CO2e hd−1 QCO2, i= Daily emission of CO2 attributed to the animal on day i, kg CO2 hd−1d−1 QN2O, i= Daily emission of N2O attributed to the animal on day i, kg N2O hd−1d−1 QCH4, i= Daily emission of CH4 attributed to the animal on day i, kg CH4 hd−1d−1 Coefficients 298 and 25 have units of kg CO2e kg−1 N2O and kg CO2e kg−1 CH4 respectively.

(24)

in which N2OG(mm) = Indirect N2O emissions due to volatilization of N from manure management system (MMS) in the country, kg N2O yr−1 EF4 = Emission factor for N2O emissions from atmospheric deposition of nitrogen on soils and water surfaces, kg N2O (kg NH3-N + NOxN volatilized)−1; default value is 0.01 kg N2O (kg NH3-N + NOx-N volatilized)−1, (Intergovernmental Panel on Climate Change (IPCC), 2006b) Direct N2O (daily) emissions from managed soils (pasture and feedyard), default emission factor of 0.02 kg N2O-N (kg N)−1 of excreted or applied manure.

2.6. Model inputs derived from literature values Model inputs that were not directly calculated using the Intergovernmental Panel on Climate Change (IPCC) (2006a, 2006b) or National Research Council (NRC) and Subcommittee of beef cattle nutrition (1996) methods were sourced from peer-reviewed manuscripts (Table 4). These values were primarily associated with the feedyard and 6

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Table 5 CO2e Emissions summary by production system. Model summary

System 1 (NGP)

System 2 (NGP-FY)

System 3 (WP-FY)

System 4 (FY)

System 5 (NGP-WP-FY)

a

903 30.1 698 250 500 500 5143 4892 20.57 92,583 370 133 28,548 40.9 114 2703 3.9 41 67 0.097 3459 4.95 52 0.543 0.8 346 0.50 6632 56.62 26.49 9.50

649 21.6 444 250 634 601 3238 2854 8.43 42,807 111 96 18,160 40.9 47 1212 2.7 42 20 0.045 1068 2.41 37 0.428 1.0 273 0.61 2918 344.97 7.61 6.57

497 16.6 292 250 676 636 2612 2185 6.13 33,724 79 115 11,943 40.9 28 992 3.4 30 15 0.051 1273 4.36 39 0.408 1.4 259 0.89 3255 715.59 7.64 11.15

452 15.1 247 250 622 568 2104 1731 5.66 39,277 106 159 10,102 40.9 27 529 2.1 29 15 0.061 363 1.47 20 0.363 1.5 231 0.94 1799 660.34 4.84 7.28

541 18.0 336 250 709 683 3023 2562 6.59 55,197 120 164 13,742 40.9 30 1209 3.6 32 29 0.086 1489 4.43 40 0.440 1.3 280 0.83 3737 729.76 8.15 11.12

Total production days Total production months Days on feed Starting SBW (kg) FSBW (kg) Targeted FSBW Avg. DM Consumed (kg) DM Excreted (kg−1 Manure) DM Consumed kg−1 Gain (kg) Total GE Fed (MJ) Gross energy/kg Gain (MJ kg−1 gain) Average GE d−1 (MJ) Total water consumed (liter) Daily water consumption (liter d−1) Water consumption/kg Gain (liter kg−1 gain) Enteric CO2e (kg Total DOF−1) Enteric CO2e (kg d−1) % Emissions from enteric fermentation Manure Mgmt. CH4 (kg CO2e Total−1 DOF) Manure Mgmt. CH4 (kg CO2e d−1) Manure Mgmt. N2O (kg CO2e Total−1 DOF) Manure Mgmt. N2O (kg CO2e d−1) % Emissions from manure Mgmt. N2O Indirect manure Mgmt. N2O (kg CO2e DOF−1) Indirect manure Mgmt. N2O (g CO2e d−1) Managed. soils N2O (kg CO2e Total−1 DOF) Managed. soils N2O (kg CO2e d−1) Total CO2e emissions (kg animal−1) b Indirect CO2e emissions (kg animal−1) CO2e kg kg−1 SBW Gain CO2e kg−1 LCA Day a

Total production days include the 205 days from birth to weaning but the first 205 days are not used in emission calculations. Indirect CO2e emissions account for emissions not directly emitted by the animal. These emissions include: fossil fuels, fertilizer, crop production, electricity, feed, feed additives, and transportation. b

pasture systems. All units derived from the literature values were converted to SI units, CO2e units, and reported on a DM basis where appropriate.

Production time for System 1 was 50% longer than that of System 4, which had the shortest feeding duration of the modeled production systems (Table 5; Fig. 4). The FSBW for System 1 was 122 kg less than the next lowest FSBW and 209 kg less than System 5. The System 1 steer had a relatively small FSBW (< 28% body fat) compared to Systems 2 through 5 when the modeling endpoints, of ≤30 months of age or 28% body fat content were applied. The simulation of production systems produced a finished steer in as few as 247 days following weaning in System 4, and as many as 698 days in System 1 (Table 5; Figs. 3 and 4). The range of simulated feeding days reflects differences in diet quality and temperature effects on animal net energy for maintenance (Fig. 5). The stair-stepped portions of the data plots in Fig. 5 reflect seasonal variation in gain when NEm was met, but NEg was not, due to the increased maintenance needs. This is prominent in Systems 1 through 3, where the NGP portion of the model coincided with cold winter temperatures typical of the Southern High Plains. The lowest 22-year (1991–2012) average daily temperature was −0.78 °C and the lowest single day average temperature during this 22-year period was −18.8 °C (Marek et al., 2005). Moreover, cold temperatures caused the native grass pastures to go dormant, resulting in decreased quality and digestibility of the forage. The low temperatures and decreased digestibility of dormant forages make weight gain negligible during this time of the year (October to March), even if ab libitum conditions are assumed. Conversely, the systems that use highly digestible, concentrated diets and high-quality forages are affected less by colder seasonal temperatures since NEm and NEg are met. This is not to imply that cold weather does not affect beef cattle on high quality forages, but rather there is less of an effect on weight gain/performance. Dry matter intake and overall GE consumed in the five beef production system simulations varied with quality of the diet. Beef cattle modeled in System 1 had greater total DMI, GE consumed, and feed-to-

3. Results and discussion 3.1. Production system comparisons Simulations of each of the five production systems resulted in numerically different values for emission rate, FSBW, cumulative resource consumption, and production time. Cumulative CO2e emissions from System 1 (NGP) were the greatest while those of System 4 (FY) were the lowest (Table 5). Total CO2e emissions from System 1 (NGP) were 56, 51, 73, and 44% greater than Systems 2 through 5, respectively. The reduction in total CO2e emissions in Systems 2 through 5 can be attributed to a nutritionally balanced, highly digestible feedyard rations that decreased production time by increasing ADG with decreased DOF necessary to achieve the desired carcass outcomes. Direct N2O emissions from manure management and CH4 emissions from enteric fermentation were the two largest contributors to the overall CO2e footprint of the five production systems. See (Table 6). The Gerber et al. (2013) report estimated that indirect emissions from production and processing contribute 45% of the total CO2e emissions from beef cattle production. This value is comparable to simulated “indirect” emissions (Table 5) not directly attributed to the steer, such as crop production, fossil fuel use, transportation, electricity, feed, and feed additives. These indirect emissions were greatest in steers finished in the feedyard (Fig. 3) as feedyards rely heavily on these systems for production. These indirect emissions totaled 1, 12, 22, 37, and 20% of total CO2e emissions for Systems 1 through 5 respectively. System 4 had the lowest cumulative CO2e footprint while having the greatest relative percentage of indirect emissions (Fig. 3). 7

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Table 6 Functional unit conversions. Model comparisons to published LCAs

kg CO2e kg−1 Gain

kg CO2e d−1

Total CO2e

a

System System System System System

26.5 7.6 7.6 4.8 8.1

9.5 6.6 11.2 7.3 11.1

6632 2918 3255 1799 3737

8580 4866 5203 3747 5685

a b

1 2 3 4 5

Adjusted total CO2e

b

kg CO2e kg−1 HCW

Enteric emissions (kg CO2e d−1)

Enteric emissions (kg CO2e)

27.5 12.3 12.3 9.6 12.8

3.9 2.7 3.4 2.1 3.6

2703 1212 992 529 1209

Adjusted total CO2e refers to the additional CO2e emissions if emissions for the first 205 days of the steer's life was estimated for Systems 1–5. kg CO2e HCW−1 was estimated based on adjusted total CO2e emissions and 62.5% dressing percentage of the FSBW for Systems 1–5.

gain ratios. Daily average values for GE intake were highest for Systems 4 and 5 as compared to Systems 1 through 3. The larger daily GE values decreased overall production time and increased FSBW for Systems 2 through 5 as compared to System 1. Higher average daily GE values and feed digestibility increased the ADG in Systems 2 through 5. Manure production increased as the efficiency of feed conversion decreased (Table 5). This increased manure production has a direct environmental consequence on the total CO2e emissions produced by feeding systems. Feeding low-quality forage for longer periods of time, as in System 1, increased the total amount of manure excreted. Increased manure production resulted in increased N2O emissions from the manure management and managed soils calculations, thus increasing total CO2e emissions (Table 5; Figs. 3 and 6; Eqs. (22) and (25)).

Fig. 3. Simulated cumulative CO2e emissions and relative contributions of direct and indirect sources to produce a finished steer from five beef production systems in the Southern High Plains.

3.2. Functional unit conversions This modeling effort generated a partial life-cycle carbon footprint estimate for beef-finishing systems. Other modeling efforts report FU based on an entire lifecycle and often report CO2e emissions as a function of hot carcass weight (HCW). Functional unit conversions were calculated to compare different model start and endpoints. Hot carcass weight was a common endpoint in seven peer-reviewed articles, and the five beef production systems modeled in this work were adjusted accordingly. To adjust the FU endpoint from kg CO2e kg−1 gain to kgCO2e kg−1 HCW, two assumptions were made; 1) the steer was raised in a grass-based system for the first 205 days of its life, and a daily emission rate can be estimated based on daily CO2e emissions from System 1 NGP at 9.5 kg of CO2e d−1, and 2) converting SBW to HCW by multiplying SBW by the USDA average of 62.5% dressing percentage. The estimated CO2e emissions per HCW and total emissions for the five modeled beef production systems are detailed in Table 7. The additional 205 days (birth through weaning) added 1947.5 kg CO2e to each of the five productions systems, and the kg CO2e kg−1 HCW was 27.5, 12.28, 12.31, 9.6, and 12.8 for Systems 1 through 5 respectively. These numbers are still lower then the FAO report from

Fig. 4. Production time, and final shrunk body weight (FSBW) for the five beef production systems in the Southern High Plains.

Fig. 5. Simulated days on feed required for a weaned steer to achieve 28% body fat content, or 30 months of age, whichever comes first.

gain ratio as compared to Systems 2 through 5 (Table 5). The high quality, concentrate-based diet used to finish cattle in Systems 2 through 5 decreased the overall DMI and GE while decreasing DMI-to-

Fig. 6. Cumulative CO2e emissions vs shrunk body weight for the five modeled beef production systems in the SHP. 8

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Table 7 CO2e Emissions for different functional units. Functional units

System 1

System 2

System 3

System 4

System 5

FSBW (kg) HCW (kg) kg CO2e d−1 (NGP) Additional model days Additional CO2e emitted Adjusted Total CO2e emitted kg CO2e kg−1 HCW

500 313 9.5 205 1947.5 8580

634 396 9.5 205 1947.5 4866

676 423 9.5 205 1947.5 5203

622 389 9.5 205 1947.5 3747

709 443 9.5 205 1947.5 5685

27.5

12.28

12.31

9.6

12.8

Klenk, I., Landquist, B.L., Ruiz de Imana, O., 2012. The Product Carbon Footprint of EU Beet Sugar. vol. 137. Sugar Industry, Berlin, Germany, pp. 169–177 No. 3. Liquide, Air, 2018. Gas Encyclopedia and Safety Datasheets, Physical Properties of Methane. http://encyclopedia.airliquide.com, Accessed date: 3 January 2018. Macken, C.N., Erickson, G.E., Klopfenstein, T.J., 2006. The cost of corn processing for finishing cattle. Prof. Anim. Sci. (22), 23–32. Marek, T.H., Porter, D., Howell, T., 2005. The Texas High Plains Evapotranspiration Network-an Irrigation Scheduling Technology Transfer Tool. Texas Water Development Board. Irrigation and Water quality-Research Reports, pp. 358–368. Michigan State University (MSU), 2009. Understanding and Managing Cattle Shrink. Mississippi State University Extension Service Publication 2577. Michigan State University (MSU), 2017. US cropland greenhouse gas calculator. In: Michigan Agricultural Experiment Station. Michigan State University. http://surf. kbs.msu.edu, Accessed date: 14 December 2017. Mitloehner, F., Place, S., 2009. Livestock's role in climate change. In: A Closer Look at 'Livestock's Long Shadow'. Cattleman, California, pp. 14–17. Mulvaney, D., 2014. Life Cycle of Greenhouse Gas Emissions from Biosynthetic Base Oil Compared to Poly-Alpha Base Oil. Eco Shift Consulting, Santa Cruz, CA. Murphy, C.F., O'Donnell, M.J., McDonald-Buller, E., Strank, S., Liu, M.H.-P., Webber, M.E., Allen, D.T., Hebner, R.E., 2010. Analysis of Innovative Feedstock Sources and Production Technologies for Renewable Fuels. USEPA (Project Number XA83379501-0, Final Report to USEPA). National Institute of Agricultural Research, (French translation: Institut National de la Recherche Agronomique (INRA)), 2017. Animal Feed Resources Information System. https://feedtables.com, Accessed date: 18 December 2017. National Research Council (NRC). Subcommittee of beef cattle nutrition, 1984. 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Gerber et al. (2013) that estimated an emission rate of 46.2 kg CO2e HCW−1. This global estimate primarily focuses on subsistence beef production systems using low-quality forage over prolonged periods of time. This estimate is not comparable to the five modeled SHP beef production systems. 4. Conclusions The environmental burdens of GHG emissions and resource use per FU (kg CO2e kg−1 gain) increased as the level of system productivity decreased. Cattle fed in grass-based systems having lower quality forage coinciding with seasonal cold temperatures typical of the SHP exhibited lower ADG, FSBW, higher resource consumption, and higher GHG emissions as compared to systems with high-quality forages (wheat pasture) and grain-based feedyard diets. Total CO2e emissions from System 1 were 73% greater than the most efficient System 4, which produced a larger (+122 kg) FSBW grade “Choice” steer in 451 fewer days. System 4 DMI decreased by 60%, while gain was increased by 33% as compared to the least efficient System 1. The GHG emission intensity of the cattle feeding industry, per FU, decreased when cattle were fed high-quality diets and intensively managed. The higher quality diets in Systems 2–5 could reduce overall CO2e emissions attributed to beef livestock production while reducing resource demands per unit of marketable product. References Code of Federal Regulations (CFR), 2015. 40 CFR Part 98 Subpart A, Table A-1. Cook, B., 2002. How Does your Herd Measure Up? Noble News and Views. Noble Research Institute, Ardmore, OK (73401). CSX Transportation Inc, 2015. Freight Train Fuel Efficiency. https://www.csx.com/index. cfm/about-us/the-csx-advantage/fuel-efficiency/, Accessed date: 3 January 2018. Desjardins, R.L., Worth, D.E., Verge, X.P.C., VanderZaag, A., Janzen, H., Kroebel, R., Maxime, W., Smith, D., Grant, B., Pattey, E., Dyer, J.A., 2010. Carbon Footprint of Agricultural Products - a Measure of the Impact of Agricultural Production on Climate Change. Agriculture and Agri-Food Canada. http://www.wamis.org/agm/meetings/ teco14/S5-Desjardins.pdf, Accessed date: 4 January 2019. Food and Agriculture Organization of the United Nations. (FAO), 2009. How to feed the world in 2050. In: Rome: High-Level Expert Forum. Galyean, M., 2015. TTU Diet Formulation Spreadsheet. Texas Tech University College of Agricultural Sciences a nd Natural Resources, Lubbock, Texas. http://www.depts.ttu. edu/afs/home/mgalyean/, Accessed date: 30 December 2017. Gerber, P.J., Steinfeld, H., Henderson, B., Mottet, A., Opio, C., Dijkman, J., Falcucci, A., Tempio, G., 2013. Tackling Climate Change through Livestock: A Global Assessment of Emissions and Mitigation Opportunities. Food and Agriculture Organization of the United Nations (FAO). Hicks, R.B., Owens, F.N., Gill, D.R., Oltjen, J.W., Lake, R.P., 1989. Dry matter intake by feedlot beef steers: influence of initial weight, time on feed, and season of year received in yard. J. Anim. Sci. 68, 254–265. Intergovernmental Panel on Climate Change (IPCC), 2006a. Guidelines for National Greenhouse Gas Inventories. Emissions from Livestock and Manure Management. (Chapter 10). Intergovernmental Panel on Climate Change (IPCC), 2006b. Guidelines for National Greenhouse Gas Inventories. N2O Emissions from Managed Soils, and CO2e Emissions from Lime and Urea Application. (Chapter 11). Johnson, K.A., Johnson, D.E., 1995. Methane emissions from cattle. J. Anim. Sci. 73, 2483–2492.

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