The effect of lubabegron fumarate feeding duration on the growth performance and carcass characteristics of finishing steers

Abstract

Angus-cross steers (n = 180; 483 kg ± 56.9 [SD] initial BW) were used in a finishing trial to determine the effect of lubabegron fumarate (Experior 10, Elanco; Greenfield, IN, USA) feeding duration on growth performance and carcass outcomes. Steers were stratified by BW into 36 pens (5 steers/pen; 9 pens/treatment) and assigned 1 of 4 treatments: no lubabegron fumarate (LUB) supplementation (CTRL), or LUB supplementation on the last 21 (LUB21), 49 (LUB49), and 77 (LUB77) days prior to harvest. All LUB treatments received 36 mg/steer·d⁻¹ with a 4-day withdrawal prior to harvest. Body weights, blood, and ultrasound measurements were collected throughout the study. Serum glucose, NEFA, and insulin were used to calculate revised quantitative insulin sensitivity check index (RQUICKI) (De Sousa et al. 2022). Estimated ultrasound empty body fat (EBF) % was calculated using equations from Guiroy et al. (2002) and Tatum et al. (2012). Data were analyzed as a randomized complete block design and repeated measures using Proc Mixed SAS 9.4. Steers fed LUB for 49 or 77 days had greater final shrunk body weight (P = 0.01) and HCW (P = 0.02) compared to CTRL steers with LUB21 being intermediate, while DMI was unchanged (P = 0.90). Feeding LUB tended to increase overall ADG (P = 0.09) and enhance G: F (P = 0.01) compared to CTRL. Estimated ultrasound EBF% was the least for LUB77 (P < 0.01). Feeding LUB for 77 days reduced marbling score compared to the other treatments (P < 0.01). Rib fat thickness tended to be lower in LUB77 compared to LUB21 (P = 0.09), while REA tended to be greater in LUB49 and LUB77 compared to CTRL (P = 0.06). No TRT × DAY effect interactions were observed for ultrasound intramuscular fat, rib fat, or ribeye area (P ≥ 0.13), although DAY effect (P < 0.01) indicated these measures increased over time. Serum NEFA concentrations were lower in LUB77 than in CTRL, LUB21, and LUB49 (P = 0.03), whereas insulin concentrations tended to be lower in LUB49 and LUB77 than in CTRL (P = 0.09), with LUB21 intermediate. Overall, LUB supplementation improved G: F without affecting DMI; LUB49 and LUB77 increased FSBW and HCW relative to CTRL, with LUB21 intermediate. Feeding LUB for 77 d reduced marbling score, whereas no statistically detectable reduction relative to CTRL was observed with 21 or 49 d of supplementation.

Lubabegron feeding duration in finishing cattle influenced the balance between growth performance and marbling score.

Introduction

Growth-enhancing technologies (beta-agonist, beta-modulators, implants, and ionophores) are instrumental in supporting cattle productivity and efficiency in the modern cattle feeding industry. A beta-modulator product, lubabegron fumarate (Experior—Elanco Feed Additive; Elanco, Indianapolis, IN USA), is amongst the newest growth-enhancing technologies (GET) available in the cattle feeding industry. Lubabegron fumarate was approved by the U.S. Food and Drug Administration (FDA) to reduce ammonia (NH₃) gas emissions per kg of live body weight (BW) and hot carcass weight (HCW) in steers and heifers fed in confinement for slaughter during the last 14 to 91 days on feed (FDA 2018; FDA-FOI 2018).

Lubabegron fumarate has been shown to enhance relevant growth performance variables (average daily gain [ADG], dry matter intake [DMI], etc.) and carcass characteristics (HCW, dressing percentage [DP%], ribeye area [REA], etc.; Vogel et al. 2023; Kayser et al. 2024; Wendler et al. 2025). Hot carcass weight is a key economic driver in the United States beef production system. Cattle marketed on a grid basis are effectively valued individually at commercial abattoirs based on a combination of HCW, quality grade, and carcass yield after harvest. This system rewards cattle with heavy carcasses, a high degree of marbling, and high red meat yield. Thus, it is of interest to producers to maximize HCW without detriment to marbling score.

Lubabegron fumarate can be fed for the last 14–91 days on feed (DOF) (FDA-FOI 2018). This offers a much longer feeding window compared to ractopamine hydrochloride. Previous studies indicate lubabegron fumarate can negatively influence marbling (Kube et al. 2021; Kayser et al. 2024; Wendler et al. 2025). To optimize growth performance and carcass quality, there must be a clearer understanding of what the optimal duration of lubabegron fumarate supplementation is prior to harvest. Therefore, the objective of this study is to determine the effects of dietary lubabegron fumarate supplementation duration on growth performance, carcass characteristics, and metabolite response. The hypothesis was that increasing the duration of lubabegron fumarate supplementation would improve growth performance and HCW, with concurrent downward shifts in marbling score.

Materials and methods

The experimental phase of the study was initiated on March 14, 2025, and completed on July 14, 2025, in Central Iowa. All study procedures were reviewed by Iowa State University’s Institutional Animal Care and Use Committee (IACUC-24–225).

Experimental design

One hundred and eighty Angus-crossbred steers (Initial BW = 482.7 kg ± 56.90 kg), from a single source, were utilized in a feedlot finishing trial at the Iowa State Beef Nutrition Farm. Steers were allocated to 36 pens (5 hd/pen) in a randomized complete block design. Steers were blocked by weight to target similar initial and final body weights (heavy block = 16 pens and light block = 20 pens). Steers were assigned to 1 of 4 experimental treatments: no lubabegron fumarate (CTRL), lubabegron fumarate for 21 d prior to harvest (LUB21), lubabegron fumarate for 49 d prior to harvest (LUB49), and lubabegron fumarate for 77 d prior to harvest (LUB77). Lubabegron fumarate was supplemented at a rate of 36 mg/steer·d⁻¹ and was removed on d 98, 4 days prior to harvest, to comply with current Elanco Experior User Agreement feeding recommendations. Thus, LUB was removed from the diet on d 98, immediately following blood collection, and steers were harvested on d 101 after completion of the withdrawal period. Specifically, LUB77 began LUB supplementation on d 21, LUB49 began supplementation on d 49, and LUB 21 began supplementation on d 77 of the experiment. Supplementation of LUB ended on d 98 for all treatments, and therefore LUB77 and the other LUB treatments did not receive LUB for the final 4 days prior to harvest. The remaining treatments (LUB 21 and LUB 49) were managed in a similar way to achieve the targeted number of days on LUB while adhering to the aforementioned withdrawal guidance. Due to weighing and sampling logistics, blocks were staggered by 21 days and started on treatments by day 21, while two sampler steers were selected per pen that were close to average pen bodyweight.

At study initiation, bodyweights were measured, blood was collected from sampler steers, and steer mobility was evaluated using North American Meat Institute (NAMI) mobility scoring system (Davis et al. 2024): 1 = normal stride with no apparent lameness or change in gait; 2 = exhibiting minor stiffness, short strides, slight limp, and keeps up with normal cattle; 3 = exhibiting obvious stiffness, discomfort, limping with difficulty taking steps, causing lag behind normal cattle; 4 = extremely reluctant to move even when encouraged, statue-like. This scoring system was also used on d 101 prior to shipping. Additionally, on d 1, all steers were administered a high-potency combination implant (28 mg of estradiol benzoate [EB] and 200 mg of trenbolone acetate [TBA]; Synovex Plus; Zoetis LLC, Madison, NJ). On d 21, 49, and 77, BW were measured for all steers and blood was collected from sampler steers. All BW were adjusted using a 4% pencil shrink (BW × 0.96). Ultrasound images were collected for the heavy block on d 21, 49, 77, and 98. Thus, 4 pen replicates per treatment were included in the ultrasound analysis. The light block was not imaged due to weather logistics prior to harvest. As the light block would not have had d 98 ultrasound images, those pens were excluded from analysis. Ultrasound carcass imaging was performed by one consistent certified ultrasound technician throughout the study using Evo II (IBEX Evo II, E.I. Medical Imaging, 170 Loveland, CO), and images were analyzed by The CUP Lab in Ames, Iowa. Carcass images included ribeye area (REA; cm²), backfat thickness at the 12th rib (RF; cm), and intramuscular fat percentage (IMF; %).

Prior to harvest, BW was measured and gait score was assessed and recorded. Steers were shipped to a commercial abattoir approximately 105 km away. Trained personnel including a USDA grader collected HCW, REA, 12^th rib fat (RF), marbling score, quality grade (QG), and yield grade (YG) from a 48-h chill. All personnel were blind to treatment. The marbling score scale was as follows: 300 = slight, 400 = small, 500 = modest. Due to the meat processing facility removing the KPH at the time of slaughter, an industry average of 2.5% was used for the yield grade calculation. Carcass empty body fat percentage (EBF_c %) was calculated using Guiroy’s (2002) EBF_c% equation: 17.76207 + (4.68142 * FT) + (0.01945 * HCW) + (0.81855 * QG) – (0.06754 * REA), where these carcass measurements were collected by the professional personnel at the commercial abattoir for all steers in the study as previously described. Estimated ultrasound EBF% was calculated using Tatum’s (2012) interim HCW equation: 0.2598 * (initial BW^1.1378), Guiroy’s (2002) EBF_c % equation, and ultrasound measurements for heavy block steers. Specifically, the interim bodyweight for a given interim period served as the “initial BW” term in Tatum’s equation.

Sampling

Total mixed ration (TMR) and individual ingredient samples were collected weekly within treatment. All TMR and individual ingredients were dried in a forced-air oven (70 °C, ≥ 48 hrs) to determine dry matter content. Once each sample’s dry matter content was determined, all samples were ground through a 2 mm screen (Retsch ZM100 grinder; Retsch GmBH, Haan, Germany) and composited to be analyzed for crude protein, amylase neutral detergent fiber, amylase neutral detergent fiber organic matter, ether extract, and ash percentage by a commercial laboratory (Dairyland Laboratories, Inc., Arcadia, WI).

Blood samples were collected from the jugular vein on d 21, 49, 77, and 98 into a serum collection tube without additive and a plasma collection tube containing trace-element K₂-EDTA. The serum and plasma tubes were held at room temperature and on ice, respectively, before centrifugation. Serum and plasma samples were transported to the lab and centrifuged at 2000 × g for a 15-minute duration. Serum and plasma samples were aliquoted into microcentrifuge tubes and stored in a −80 °C freezer. Circulating serum glucose, non-esterified fatty acid (NEFA), and insulin were analyzed on d 21, 49, 77, and 98 using commercially available kits: glucose (FUJIFILM Wako Diagnostics; intra-assay CV = 2.3%, inter-assay CV = 1.7%), NEFA (Wako Pure Chemical Industries LTd., Chuo-Ku Osaka, Japan; intra-assay CV = 2.9% and inter-assay CV = 7.3%), and insulin (Bovine Insulin ELISA assay; Mercodia, Inc., Winston Salem, NC; intra-assay CV = 7.7% and inter-assay CV = 14.4%) concentrations. Non-esterified fatty acids, insulin, and glucose concentrations were utilized to determine the revised quantitative insulin sensitivity check index (RQUICKI), using a published equation (De Sousa et al. 2022): RQUICKI = 1/[log(glucose) + log(insulin) + log(NEFA)]. When the RQUICKI value is higher, this is associated with greater insulin sensitivity; RQUICKI is utilized as a proxy for insulin sensitivity for cattle, and not a true insulin tolerance test.

Feed delivery and animal health

The SDSU 4-point bunk scoring system was used to evaluate bunks every day at approximately 0630 hrs to adjust feed deliveries. Bunks were managed to be slick at 0630 h most mornings. Cattle were fed once/day at 0700 hrs utilizing a Farm Aid 250 Reel Feed Mixer Wagon. Table 1 below describes the diet formulation utilized in this study. Farm personnel and graduate students checked health, bunks, and pen cleanliness using a checklist at the ISU Beef Nutrition Farm. If cattle were pulled and had a rectal temperature of 103.5 °F or higher, they were treated with antibiotics and considered morbid. Initial treatment consisted of subcutaneous administration of tulathromycin (Draxxin Injectable Solution; Zoetis, LLC, Kalamazoo, Michigan). If a second intervention was required, florfenicol (Nuflor Injectable Solution; Merck Animal Health, Friesoythe, Germany) was administered subcutaneously. Subsequent treatments (third or greater) involved subcutaneous injection of a synthetic corticosteroid (Dexamethasone Sterile Injection; MWI Animal Health, Boise, ID). Any lame or injured cattle received an oral anti-inflammatory (Meloxicam; Boehringer Ingelheim Animal Health, Rhein, Germany) and a pour-on flunixin transdermal solution (Banamine Transdermal; Merck Animal Health, Madison, NJ). Following treatment, all cattle were returned to their pen of origin, and their health status was closely monitored. One steer from the LUB77 treatment died as a result of bloat which was determined to be unrelated to the dietary treatment based on postmortem necropsy by a consulting veterinarian.

Table 1.

Diet composition.

	CTRL Diet	LUB Diet1
Diet, % DM basis
Dry-rolled corn	50	50
Sweet Bran	20	20
Corn Silage	15	15
Dried distillers grains	10	5
Basal premix2	5	5
Lubabegron fumarate premix3		5
Analyzed composition4
Crude protein	13.69	13.95
aNDF	19.83	19.62
aNDFom	19.25	19.05
Ether extract	4.18	4.26
Ash %	4.46	4.61
NEm, Mcal/kg5	2.03	2.03
NEg, Mcal/kg5	1.41	1.40

¹All steers in LUB21, LUB49, and LUB77 received this ration.

²% dry matter basis of diet: 1.5% limestone, 0.01% vitamin A and E premix, 0.31% salt, 0.0135% Rumensin 90, 3.122% DDG; Mineral mg/kg of diet: 0.15 mg/kg cobalt, 10 mg/kg copper, 20 mg/kg of manganese, 0.10 mg/kg selenium, 100 mg/kg zinc sulfate, 0.5 mg/kg iodine. Used dried distillers grains as a carrier.

³Targeted 36 mg/steer·d-1 of lubabegron fumarate on 100% dry matter basis. Used dried distillers grains as a carrier.

⁴Based on TMR analysis from Dairyland, Inc., Arcadia, WI.

⁵Calculated through tabular energy values.

Statistical analysis

All statistical analyses were conducted using SAS version 9.4 (SAS Institute Inc., Cary, NC). Significance was declared at P ≤ 0.05, and tendencies were discussed when 0.05 < P ≤ 0.10. Pen served as the experimental unit for all analyses. Pen-level data were analyzed using PROC MIXED with treatment and block included as fixed effects. Least squares means were separated using Tukey-adjusted pairwise comparisons. Because lubabegron supplementation commenced immediately following body weight measurements, ultrasounds, and blood collections on d 21, measurements obtained on d 21 were included as covariates for serum glucose, NEFA, insulin, RQUICKI, ultrasound measurements, and estimated ultrasound empty body fat (EBF) in repeated-measures analyses. Treatment, block, day, and the treatment × day interaction were included as fixed effects. Pen was the experimental unit. The following covariance structures were selected based on the lowest corrected Akaike information criterion (AICC) formation: TOEPH (glucose and NEFA), UN (insulin), and CS (RQUICKI). The denominator degrees of freedom used for all metabolite repeated measures were the Satterthwaite approximation. Least squares means were separated using Tukey-adjusted pairwise comparisons. Area under the curve (AUC) was calculated by (timepoint metabolite data¹ + timepoint metabolite data²)/2 * (timepoint² — timepoint¹) for each serum metabolite and analyzed using PROC MIXED with treatment included as a fixed effect and pen as the experimental unit. PROC UNIVARIATE was used to evaluate distributional assumptions and generate descriptive statistics for AUC, serum metabolites, estimated ultrasound EBF, growth performance, and carcass characteristics. Proportions within each USDA quality- and yield-grade category were analyzed using PROC GLIMMIX of SAS 9.4 with a binomial distribution and logit link. Treatment and block were included as fixed effects, and pen was the experimental unit and was included as a random effect. Mobility-score distributions on d 1 and d 101 were analyzed separately using PROC SURVEYFREQ, with weight block specified as a stratum and pen specified as the cluster. Treatment differences were evaluated using second-order Rao–Scott chi-square tests and their design-adjusted F tests. Data are presented as percentages by treatment. Differences were considered significant at P ≤ 0.05 and tendencies at 0.05 < P ≤ 0.10.

Results

Growth performance

Growth performance data are displayed in Table 2. Initial BW and d 21 SBW were not affected by LUB treatment (P ≥ 0.15). On d 49, LUB77 steers had been supplemented LUB for 28 days, which resulted in increased SBW compared to other treatments (P < 0.01). By d 77, LUB49 and LUB77 had greater SBW than control, with LUB21 being intermediate (P = 0.01). For SBW on d 98, a similar trend was noted where LUB49 and LUB77 had greater SBW than CTRL steers (P < 0.01), with LUB21 being intermediate. The same response was noted for FSBW, where LUB49 and LUB77 were greater than CTRL with LUB21 being intermediate (P = 0.01). Lubabegron fumarate treatment did not influence DMI at any interim time point throughout the experiment or on an overall basis (P ≥ 0.15). There were no differences in ADG among treatments (P = 0.30) from d 0 to 21. From d 22 to 49, LUB77 exhibited greater ADG compared to LUB49 cattle (P = 0.03), while the CTRL and LUB21 groups remained intermediate and did not differ from either treatment (P ≥ 0.22). From d 50 to 77, ADG was greatest in the LUB49 group (P = 0.02) compared to both CTRL and LUB77 (1.68 and 1.66 kg/d, respectively), with LUB21 being intermediate. From d 78 to 101, LUB21 had greater ADG than all other treatments (P < 0.01). For the entire duration of the trial, a tendency (P = 0.09) was observed for lubabegron fumarate supplementation to improve ADG regardless of duration. From d 22–49, LUB77 steers had the greatest G: F (P = 0.02). From d 50–77, LUB49 had greater G: F compared to CTRL (P = 0.02), with LUB77 and LUB21 being intermediate. From d 78–101, LUB21 had greater G: F than CTRL and LUB49 with LUB77 being intermediate (P < 0.01). Overall, feeding LUB, regardless of duration, improved G: F by 7.9, 7.3%, and 9.9% for LUB21, LUB49, and LUB77, respectively compared to CTRL (P = 0.01).

Table 2.

Effects of lubabegron fumarate feeding duration on growth performance.

	Treatments1
Item	CTRL	LUB21	LUB49	LUB77	SEM	P-value
Initial SBW, kg.	463.0	463.3	464.4	465.1	0.88	0.40
SBW, kg.
d 21	507.7	507.1	512.3	509.6	1.90	0.15
d 49	573.1b	572.1b	576.5b	585.0a	2.10	< 0.01
d 77	620.1b	621.0ab	632.0a	631.2a	2.96	0.01
d 98	644.9b	656.8ab	661.7a	662.2a	3.71	< 0.01
FSBW, kg.	652.1b	662.1ab	667.2a	668.2a	3.65	0.01
DMI, kg/d
d 0–21	11.23	11.35	11.55	11.66	0.151	0.15
d 22–49	12.46	12.62	12.58	12.43	0.159	0.77
d 50–77	12.72	12.50	12.44	12.35	0.216	0.60
d 78–101	12.97	12.99	12.39	12.86	0.249	0.21
ADG, kg/d
d 0–21	2.40	2.29	2.51	2.40	0.080	0.30
d 22–49	2.26ab	2.20ab	2.15b	2.47a	0.072	0.03
d 50–77	1.68b	1.74ab	2.05a	1.66b	0.097	0.02
d 78–101	1.34b	1.84a	1.29b	1.53b	0.072	< 0.01
G: F, kg: kg
d 0–21	0.212xy	0.200xy	0.214x	0.196y	0.0053	0.09
d 22–49	0.174b	0.174b	0.173b	0.199a	0.0055	0.02
d 50–77	0.128b	0.138ab	0.159a	0.130ab	0.0073	0.02
d 78–101	0.104b	0.141a	0.101b	0.125ab	0.0056	< 0.01
Overall, kg.
DMI	12.25	12.27	12.15	12.22	0.132	0.90
ADG	1.91y	2.01x	1.99x	2.00x	0.032	0.09
G: F	0.151b	0.163a	0.162a	0.166a	0.0025	0.01

^a,bDifferences of least square means in a row without common superscripts differ (P ≤ 0.05).

^x,yDifferences of least square means in a row without common superscripts differ (0.05 $<$P ≤ 0.10).

¹Treatments: CTRL = no lubabegron fumarate supplementation; LUB21, LUB49, and LUB77 = lubabegron fumarate supplementation during the final 21, 49, and 77 days on feed (DOF), respectively.

Carcass characteristics

Carcass characteristic results are displayed in Table 3. Feeding LUB for 49 or 77 d resulted in improved HCW relative to control steers with LUB21 intermediate (P = 0.02). LUB treatments did not influence DP or YG (P ≥ 0.19). Feeding lubabegron fumarate for 77 d prior to harvest resulted in a 12.5% decrease in marbling score; however, when fed for 49 or fewer days, no statistical evidence of a reduction was detected relative to CTRL (P < 0.01). Supplementing LUB for 77 d tended to decrease RF compared to LUB21, with CTRL and LUB49 being intermediate (P = 0.09). Furthermore, a tendency for increased REA was observed for LUB49 and LUB77 compared to CTRL (P = 0.06). EBF_c %, based on actual carcass measures, decreased by 2.10 percentage points for LUB77 compared to LUB21 with CTRL and LUB49 being intermediate (P < 0.01).

Table 3.

Effects of lubabegron fumarate feeding duration on carcass characteristics.

	Treatments1
Item	CTRL	LUB21	LUB49	LUB77	SEM	P-value
HCW, kg.	415.9b	424.8ab	425.7a	427.9a	2.63	0.02
DP, %	63.82	64.11	64.06	64.11	0.202	0.75
YG	3.40	3.60	3.49	3.32	0.092	0.19
Marbling Score	537a	522a	526a	470b	11.4	< 0.01
RF, cm	1.73xy	1.83x	1.78xy	1.55y	0.087	0.09
REA, cm2	94.77y	96.13xy	96.39x	96.52x	0.482	0.06
EBFc, %	32.66ab	33.63a	32.69ab	31.53b	0.373	< 0.01

^a,bDifferences of least square means in a row without common superscripts differ (P ≤ 0.05).

^x,yDifferences of least square means in a row without common superscripts differ (0.05 $<$P ≤ 0.10).

¹Treatments: CTRL = no lubabegron fumarate supplementation; LUB21, LUB49, and LUB77 = lubabegron fumarate supplementation during the final 21, 49, and 77 days on feed (DOF), respectively. The marbling score scale was as follows: 300 = slight, 400 = small, 500 = modest.

USDA quality grade distributions were not affected (P ≥ 0.52) by lubabegron fumarate feeding duration (Table 4), where the proportions of carcasses grading Prime, Choice, and Select did not differ statistically amongst treatments. Choice was the predominant quality grade across all treatments, ranging from 80.00 to 91.11% of carcasses. Likewise, the proportions of carcasses classified as YG 3, YG 4, and YG 5 did not differ among treatments (P ≥ 0.32). Because YG 2 occurred infrequently, it was evaluated descriptively. Two CTRL carcasses (4.44%) and one LUB49 carcass (2.23%) were classified as YG 2, whereas no YG 2 carcasses occurred in the LUB21 or LUB77 treatments. No carcasses were classified as YG 1.

Table 4.

The effects of lubabegron fumarate on USDA quality grade and yield grade distribution.

	Treatments1
Item	CTRL	LUB21	LUB49	LUB77	SEM	P-value
USDA QG distribution,2 %
Prime	4.44	8.88	11.11	2.26	0.762	0.52
Choice	91.11	84.43	80.00	84.10	0.430	0.59
Select	4.44	6.67	8.89	13.64	0.571	0.54
YG distribution,2,3 %
YG 24	4.44 (2)	0.00 (0)	2.22 (1)	0.00 (0)	–	–
YG 3	62.22	51.11	51.11	70.45	0.309	0.36
YG 4	28.90	35.56	42.22	27.28	0.321	0.52
YG 5	4.44	13.33	4.44	2.27	0.724	0.32

¹Treatments: CTRL = no lubabegron fumarate supplementation; LUB21, LUB49, and LUB77 = lubabegron fumarate supplementation during the final 21, 49, and 77 days on feed (DOF), respectively.

²Values are presented as percentages within treatment.

³No YG 1 carcasses were observed.

⁴YG 2 frequencies are presented descriptively as percentage (n). Because only three carcasses were classified as YG 2 and two treatments contained no YG 2 carcasses, this category was not analyzed inferentially.

The marbling score scale was as follows: 300 = slight, 400 = small, 500 = modest.

Ultrasound performance

Carcass ultrasound data are shown in Table 5. No TRT × DAY effects were noted for ultrasound intramuscular fat (IMF), RF, and REA (P ≥ 0.13). Ultrasound IMF, RF, and REA all increased over time (DAY; P < 0.01). A tendency for a TRT effect was observed for ultrasound IMF, where CTRL cattle tended to have the greatest ultrasound IMF score, with LUB21 being intermediate and not different from the other treatments (P = 0.09). A TRT effect (P = 0.03) was noted for ultrasound RF, where LUB21 and LUB77 steers had lesser overall RF compared to CTRL, with LUB49 intermediate. Additionally, there were no differences shown among treatments in ultrasound REA for TRT effect (P = 0.18).

Table 5.

Effects of lubabegron fumarate feeding duration on ultrasound carcass characteristics.

	Treatments1					P-value
Item	CTRL	LUB21	LUB49	LUB77	SEM	TRT	TRT × DAY	DAY
IMF, %					0.264	–	0.20	< 0.01
Pooled d 21 covariate 2	4.21
d 49	5.37	5.27	4.57	4.71
d 77	6.29	5.92	5.57	5.85
d 98	6.91	6.60	6.12	5.76
RF, cm					0.047	–	0.13	< 0.01
Pooled d 21 covariate 2	1.17
d 49	1.56	1.55	1.50	1.48
d 77	1.98	1.83	1.91	1.76
d 98	2.29	2.07	2.13	2.07
REA, cm2					1.273	–	0.13	< 0.01
Pooled d 21 covariate 2	85.85
d 49	94.76	93.54	93.19	96.70
d 77	94.72	94.50	98.74	97.64
d 98	97.76	98.34	99.52	100.61
Overall
IMF, %	6.19x	5.93xy	5.42y	5.44y	0.223	0.09	–	–
RF, cm	1.94a	1.82b	1.85ab	1.77b	0.036	0.03	–	–
REA, cm2	95.75	95.46	97.15	98.32	0.950	0.18	–	–

¹Treatments: CTRL = no lubabegron fumarate supplementation; LUB21, LUB49, and LUB77 = lubabegron fumarate supplementation during the final 21, 49, and 77 days on feed (DOF), respectively. Pen was the experimental unit (n = 4 pens/treatment; 16 pens total). Values are least-squares means; SEM = standard error of the mean.

²Measurements obtained on d 21 before treatment initiation were pooled across treatments and included as covariates (IMF, P = 0.004; RF and REA, P < 0.0001). Pooled d 21 values are not treatment-specific least-squares means.

Abbreviations: d = day; IMF = intramuscular fat; REA = ribeye area; RF = rib fat thickness; TRT = treatment.

^a,bWithin a row, means without a common superscript differ (P ≤ 0.05).

^x,yWithin a row, means without a common superscript tend to differ (0.05 < P ≤ 0.10).

Estimated ultrasound empty body fat data are shown in Fig. 1. A TRT × DAY effect was observed for estimated ultrasound EBF (P = 0.01), where estimated EBF increased with DOF and all LUB-supplemented treatments exhibited lower estimated ultrasound EBF percentages compared to CTRL by the end of the experiment.

Figure 1.

The effects of TRT × DAY on estimated ultrasound EBF. Data were analyzed as repeated measures with the mixed procedure of SAS. Within a panel, ^a,bunlike superscripts differ (P ≤ 0.05); ^x,yunlike superscripts tend to differ (0.05 < P ≤ 0.10).

Mobility

Mobility score distributions did not differ among treatments on d 1 (P = 0.42) or d 101 (P = 0.29; Table 6). On d 1, most cattle received a mobility score of 1 (77.78 to 93.33%). On d 101, the proportion receiving a score of 1 ranged from 24.44 to 51.11%, whereas the proportion receiving a score of ≥ 2 ranged from 48.89 to 75.55%.

Table 6.

Effects of lubabegron fumarate on mobility score^*.

	Treatments1
Item	CTRL	LUB21	LUB49	LUB77	Chi-Square
Mobility d 1 Score, %					0.42
1	86.67	77.78	80.00	93.33
$\ge \mathit{ }$2	13.33	22.22	20.00	6.67
Mobility d 101 Score, %					0.29
1	24.44	51.11	37.78	39.53
$\ge \mathit{ }$2	75.55	48.89	62.22	60.47

¹Treatments: CTRL = no lubabegron fumarate supplementation; LUB21, LUB49, and LUB77 = lubabegron fumarate supplementation during the final 21, 49, and 77 days on feed (DOF), respectively.

^*Mobility score evaluated cattle mobility North American Meat Institute (NAMI) mobility scoring system (Davis et al. 2024): 1 = normal stride with no apparent lameness or change in gait; 2 = exhibiting minor stiffness, short strides, slight limp, and keeps up with normal cattle; 3 = exhibiting obvious stiffness, discomfort, limping with difficulty taking steps, causing lag behind normal cattle; 4 = extremely reluctant to move even when encouraged, statue-like.

Serum metabolites

Serum glucose AUC data are shown in Table 7, and serum glucose repeated measures data are shown in Figs. 2a, 3, and 4. No TRT or TRT × DAY effects (P ≥ 0.38) were noted for circulating glucose concentrations. However, serum glucose decreased (DAY; P < 0.01) over time. Serum glucose AUC was not affected by LUB treatment (TRT; P = 0.19).

Table 7.

Effects of lubabegron fumarate on metabolite AUC.

	Treatments1
Item	CTRL	LUB21	LUB49	LUB77	SEM	P-value
Glucose, mg·d/dL	6182.91	6640.77	6041.52	6150.69	204.858	0.19
NEFA, µEq·d/L	11386.00	11321.00	11154.00	9748.33	538.738	0.19
Insulin, µg·d/L	117.30	93.18	78.25	84.74	12.804	0.16
RQUICKI,2 AU·d	27.79x	31.27xy	32.37xy	32.78y	0.833	0.08

NEFA = nonesterified fatty acids; RQUICKI = revised quantitative insulin sensitivity check index; AUC = Area under the curve.

AUC was calculated by (timepoint metabolite data¹ + timepoint metabolite data²)/2 * (timepoint^2—timepoint¹) for serum glucose, NEFA, Insulin concentration for each treatment; LUB21 and LUB49 AUC includes sampling days that preceded the start of supplementation.

RQUICKI was derived through the log transformation of glucose, NEFA, and insulin concentrations; LUB21 and LUB49 AUC includes sampling days that preceded the start of supplementation.

^x,yWithin a row, least-squares means without a common superscript tend to differ (0.05 < P ≤ 0.10).

¹Treatments: CTRL = no lubabegron fumarate supplementation; LUB21, LUB49, and LUB77 = lubabegron fumarate supplementation during the final 21, 49, and 77 days on feed (DOF), respectively.

²RQUICKI is a dimensionless index; therefore, its AUC is expressed as arbitrary units multiplied by days (AU·d).

Figure 2.

(a–d) The effects of TRT × DAY on circulating (a) glucose, (b) NEFA, (c) insulin, and (d) RQUICKI. Data were analyzed as repeated measures with the mixed procedure of SAS. Within a panel, ^a,bunlike superscripts differ (P ≤ 0.05); ^x,yunlike superscripts tend to differ (0.05 < P ≤ 0.10).

Figure 3.

The effects of TRT on circulating glucose concentration. Data were analyzed as repeated measures with the mixed procedure of SAS. Within a panel, ^a,bunlike superscripts differ (P ≤ 0.05); ^x,yunlike superscripts tend to differ (0.05 < P ≤ 0.10).

Figure 4.

The effects of DAY on circulating glucose concentration. Data were analyzed as repeated measures with the mixed procedure of SAS. Within a panel, ^a,bunlike superscripts differ (P ≤ 0.05); ^x,yunlike superscripts tend to differ (0.05 < P ≤ 0.10).

Serum NEFA AUC data are shown in Table 7, and serum NEFA repeated measures data are shown in Figs. 2b, 5, and 6. No TRT × DAY effect was noted (P = 0.19) for circulating NEFA. A TRT effect was noted (P = 0.03) where CTRL, LUB21, and LUB49 had greater circulating NEFA concentrations compared to LUB77. In addition, it was noted that serum NEFA concentrations increased over time (DAY; P < 0.01). However, no TRT effects were reported amongst treatments for serum NEFA AUC (P = 0.19).

Figure 5.

The effect of TRT on circulating NEFA concentration. Data were analyzed as repeated measures with the mixed procedure of SAS. Within a panel, ^a,bunlike superscripts differ (P ≤ 0.05); ^x,yunlike superscripts tend to differ (0.05 < P ≤ 0.10).

Figure 6.

The effect of DAY on circulating NEFA concentration. Data were analyzed as repeated measures with the mixed procedure of SAS. Within a panel, ^a,bunlike superscripts differ (P ≤ 0.05); ^x,yunlike superscripts tend to differ (0.05 < P ≤ 0.10).

Serum insulin AUC data are shown in Table 7, and serum insulin repeated measures data are shown in Figs. 2c, 7, and 8. There was no TRT × DAY effect (P = 0.92) for circulating insulin. However, a tendency for a TRT effect (P = 0.09) was noted where LUB49 and LUB77 exhibited lower circulating insulin concentrations compared to CTRL, with LUB21 being intermediate. Additionally, a DAY effect (P < 0.01) was also noted where circulating insulin concentrations decreased over time from d 21 to 77 and then increased to d 98. Conversely, there were no differences among treatments for serum insulin AUC (P = 0.16).

Figure 7.

The effect of TRT on circulating insulin concentration. Data were analyzed as repeated measures with the mixed procedure of SAS. Within a panel, ^a,bunlike superscripts differ (P ≤ 0.05); ^x,yunlike superscripts tend to differ (0.05 < P ≤ 0.10).

Figure 8.

The effect of DAY on circulating insulin concentration. Data were analyzed as repeated measures with the mixed procedure of SAS. Within a panel, ^a,bunlike superscripts differ (P ≤ 0.05); ^x,yunlike superscripts tend to differ (0.05 < P ≤ 0.10).

RQUICKI AUC results are displayed in Table 7, and RQUICKI repeated measures data are shown in Figs. 2d, 9, and 10. There was no TRT × DAY effect (P = 0.42) for RQUICKI. A TRT effect (P = 0.05) was observed where LUB77 had a greater RQUICKI value compared to CTRL and LUB21, with LUB49 being intermediate. The RQUICKI value increased over time (DAY; P < 0.01) from d 21 to 77 but decreased from d 77 to d 98 in the present study. In addition, a tendency for TRT effect was noted for RQUICKI AUC (P = 0.08), where LUB77 had lesser AUC compared to CTRL, with LUB21 and LUB49 being intermediate.

Figure 9.

The effect of TRT on RQUICKI value. Data were analyzed as repeated measures with the mixed procedure of SAS. Within a panel, ^a,bunlike superscripts differ (P ≤ 0.05); ^x,yunlike superscripts tend to differ (0.05 < P ≤ 0.10).

Figure 10.

The effect of DAY on RQUICKI value. Data were analyzed as repeated measures with the mixed procedure of SAS. Within a panel, ^a,bunlike superscripts differ (P ≤ 0.05); ^x,yunlike superscripts tend to differ (0.05 < P ≤ 0.10).

Discussion

Lubabegron fumarate is a beta-modulator approved for use prior to harvest in feedlot finishing cattle. This technology has been shown to reduce ammonia gas emissions (FDA-FOI 2018) and markedly enhance feedlot productivity by improving live growth performance, feed efficiency, and HCW (Kube et al. 2021; Park 2025). However, despite consistent and substantial improvements in growth performance and HCW, previous research indicates trade-offs may exist in certain carcass quality attributes, specifically marbling score (Teeter et al. 2021; Vogel et al. 2023). Thus, the primary objective of this study was to generate data across varying durations of lubabegron fumarate supplementation prior to harvest to evaluate the resulting impacts on growth performance, carcass characteristics, and longitudinal ultrasound measurements for cattle fed for an equal number of total days.

In the present study, as expected, LUB supplementation improved live growth performance, resulting in greater FSBW, ADG, and improved G: F in agreement with various previous reports (Kube et al. 2021; Ortner et al. 2024). In addition to consistent improvements in FSBW and other key growth variables, such as ADG and G: F (Vogel et al. 2023; Wendler et al. 2025), lubabegron fumarate consistently elicits sizeable improvements in HCW compared to cattle not supplemented with LUB (Teeter et al. 2021; Park 2025). Regardless of duration, overall ADG and G: F were enhanced by LUB supplementation. Data from interim weigh dates reveal that when LUB supplementation was initiated for each respective LUB treatment, ADG and G: F increased significantly during the following 28-d interim period for that particular treatment. This response was particularly evident later in the experiment, when growth rate had begun to decline, possibly because of changes in the composition of gain or declining implant payout. Higher seasonal temperatures may also have contributed, although environmental conditions were not measured. During this same period, LUB21 exhibited improved ADG and G: F compared to CTRL and other LUB treatments. More research is needed to corroborate and replicate interim growth performance findings reported here. It should be noted, that in addition to cattle being heavier and having a higher degree of adiposity, during the later portion of this experiment cattle experienced greater ambient temperatures, although not abnormally high for that time of year. More research may be warranted to better understand the effects of lubabegron fumarate on growth performance and circulating metabolite response during times of higher seasonal ambient temperature. Dry matter intake response to lubabegron fumarate supplementation can be aptly described as variable (Edmonds et al. 2024; Eekhoff et al. 2025). In the present study, LUB treatment did not affect DMI.

In alignment with prior work, our data suggest LUB supplementation increased HCW. In the present study, steers fed lubabegron during the final 49 or 77 d on feed had greater HCW than CTRL steers, whereas LUB21 was intermediate and did not differ from either CTRL or the longer-duration treatments. Interestingly, no differences were observed for dressing percentage or yield grade. While this finding aligns with some previous data (Ortner et al. 2024; Park 2025), the majority of previous reports suggest feeding LUB enhances dressing percentage and USDA yield grade (Kube et al. 2021; Kayser et al. 2024).

The impact of LUB feeding duration on marbling score and quality grade was of primary interest in the present study. Here we observed that when LUB was fed for the longest duration (77 days prior to harvest), marbling score was reduced by ∼12.5%, which aligns with prior reports (Edmonds et al. 2024; Kayser et al. 2024). Yet, interestingly, in the present study, when LUB was fed for 49 or fewer days prior to harvest, no statistical evidence of a reduction in marbling score was detected compared to control. The contrasting marbling responses of LUB49 and LUB77 are noteworthy because both treatments increased final shrunk body weight and HCW relative to CTRL, whereas reduced marbling was observed only with LUB77. In agreement with results reported here, some prior reports suggest that shorter LUB feeding durations (<56 d) result in minor or no measurable reduction in marbling score (Edmonds et al. 2024; Henderson et al. 2026). Additionally, in this experiment, LUB feeding duration did not statistically influence USDA quality grade and YG distributions. Thus, the present findings, together with previous reports, suggest that feeding LUB for 49 days may improve growth performance, with no statistically detectable reduction in marbling score or shift in USDA quality-grade distribution in the present study.

The causative factor related to marbling reductions in the LUB77 treatment remains unclear. Indeed, in this experimental setting, this effect may be attributed simply to the greater number of days on LUB and thus a greater number of days exposed to a potent anabolic stimulus, which resulted in a lower marbling score. However, it is interesting to note the relative differences in body composition, as indicated by estimated ultrasound-based EBF% at the time of LUB initiation. In practical feeding scenarios, if cattle are consuming calories above maintenance, it is known and accepted that as body weight and DOF progress, degree of adiposity will increase. Subcutaneous fat deposition increases and accelerates later in the finishing period, reflecting the biological shift toward increased adipose tissue accretion as cattle approach physiological maturity (Van Koevering et al. 1995; Pillmore et al. 2024). While subcutaneous fat deposition increases at an increasing rate over time, marbling develops in a distinct manner, accruing over time in a steady, linear fashion (Bruns et al. 2004). In this study, as a consequence of the study design, LUB supplementation was initiated at varying degrees of not just subcutaneous fat deposition and EBF% but also at varying degrees of marbling development. We quantified the relative differences in body composition and other carcass characteristics within treatment groups across days on feed. Ultrasound-based empty body fat percentage was approximated at each of the ultrasound timepoints by using ultrasound carcass measures, an estimated interim HCW (Tatum et al. 2012), as well as a previously published equation for EBF% estimation (Guiroy et al. 2002). As expected, calculated estimated ultrasound EBF% and IMF increased with DOF, and LUB77 steers were the leanest treatment group at their respective time of LUB initiation based on estimated ultrasound EBF (∼27.4% EBF when LUB feeding began) and were the leanest by the end of the experiment. Conversely, when LUB49 and LUB21 each began LUB supplementation, their estimated EBF was ∼30% and ∼32.5%, respectively. While not measured directly, these estimated EBF% values generated using ultrasound data are useful in demonstrating the relative differences that existed across days on feed and each respective initiation point for LUB feeding. A limitation of this study is that cattle were fed and harvested at equal DOF; thus, the individual influence of days on LUB and estimated EBF% or IMF score at the time of LUB initiation are confounded. Thus, it cannot be confirmed that body composition or IMF score alone are primary contributing factors to the negative effects of LUB on marbling. More work is warranted to better understand the relationship between body composition, LUB feeding duration, and the resulting impact on marbling score and carcass quality measures.

Beyond body composition, another potential explanation for the LUB-induced marbling response is the dilution effect, whereby an outsized increase in REA would result in a proportionally smaller amount of IMF rather than a difference in absolute amount of IMF present in the longissimus, thus decreasing quality grade (Kayser et al. 2024; Wendler et al. 2025). Given the disparate results in marbling score for LUB49 and LUB77 with equivalent REA response, it is unlikely that the dilution effect is the primary factor contributing to the observed differences in marbling score. However, observations reported by Park (2025) indicate downregulation of lipogenic genes and upregulation of myogenic genes in the longissimus and skeletal muscle tissues of steers fed lubabegron.

Lubabegron fumarate enhances insulin responsiveness and decreases circulating insulin concentrations (FDA-FOI 2018). When insulin responsiveness is enhanced, feedlot cattle exhibit improved growth performance through more efficient uptake and use of glucose throughout the body. In the present study, potential evidence of this response may be found in the LUB49 and LUB77 treatments, where steers tended to have lower overall circulating serum insulin concentrations based on repeated measures analysis and also displayed the greatest improvements in HCW relative to CTRL with LUB21 intermediate. However, there was no treatment effect on AUC for serum insulin. Serum glucose decreased with increasing days on feed and no effects of treatment were noted for serum glucose AUC. This is consistent with preliminary research where serum glucose AUC for lubabegron fumarate-fed cattle did not differ from control cattle (FDA 2018). In this experiment, feeding LUB for 77 days resulted in decreased serum NEFA concentrations compared to CTRL. Reports from in vitro studies indicate lubabegron fumarate does not influence lipolytic activity (Hwang et al. 2021, 2022), unlike ractopamine hydrochloride (Peterla and Scanes 1990). Interestingly, previous work where LUB was fed for 37 d reported decreased serum NEFA concentrations compared to control steers (Henderson et al. 2026). This perhaps suggests LUB increases tissue uptake of circulating NEFA as opposed to eliciting a lipolytic response. Conversely, other work reported no influence of LUB on circulating NEFA concentrations compared to control steers (Eekhoff et al. 2025). The effect of LUB on circulating NEFA concentration appears to be variable.

As DOF increases, insulin sensitivity generally decreases as a result of increased adiposity (Eisemann et al. 1997; De Sousa et al. 2022). Insulin sensitivity can be approximated using RQUICK index values as a proxy (De Sousa et al. 2022), where a numerically greater RQUICKI value indicates a greater degree of insulin sensitivity. It should be noted that RQUICKI does not directly measure insulin sensitivity. Here we observed that, as expected, increased duration of LUB supplementation resulted in the greatest overall insulin sensitivity as indicated by RQUICKI. Interestingly, when assessed across days on feed, RQUICKI increased to d 77 and then decreased during the final interim period. The final portion of the experiment occurred during June and July in Central Iowa, when cattle were exposed to higher ambient temperatures, which could have contributed to this change in insulin sensitivity. Classically, cattle experiencing acute heat stress exhibit decreased voluntary feed intake (Baumgard and Rhoads 2013). However, interim DMI for the final 21 d period was not influenced by the higher ambient temperatures, thus, based on DMI data, it is unlikely cattle underwent acute heat stress at this time.

Conclusion

In conclusion, supplementation of lubabegron fumarate to finishing steers improved growth performance and carcass weight without affecting DMI or mobility outcomes. Steers fed LUB had greater feed efficiency while FSBW and HCW were greater for LUB49 and LUB77 than for CTRL, with LUB21 intermediate. Feeding lubabegron for 77 d reduced marbling score compared to all other treatments and resulted in leaner body composition based on estimated ultrasound empty body fat percent compared to LUB21 while LUB49 and CTRL did not differ from either treatment. However, no statistical evidence of reduced marbling score relative to CTRL was detected with 21 or 49 d of LUB supplementation; LUB49 and LUB77 steers had greater FSBW and HCW than CTRL, whereas LUB21 steers were intermediate. Steers suppplemented LUB for 77 d had greater RQUICKI values than CTRL and LUB21, with LUB49 intermediate; however, RQUICKI is an indirect index and should not be interpreted as a direct measurement of insulin sensitivity. Collectively, these results indicate that duration of lubabegron supplementation can influence the balance between growth performance outcomes and carcass quality attributes.

Glossary

Abbreviations

ADG

average daily gain

AICC

corrected Akaike information criterion

AUC

area under the curve

aNDF

amylase-treated neutral detergent fiber

aNDFom

amylase-treated neutral detergent fiber exclusive of residual ash

BW

body weight

CTRL

control

DDG

dried distillers grains

DMI

dry matter intake

DOF

days on feed

DP

dressing percentage

EB

estradiol benzoate

EBF

empty body fat

EBFc

carcass empty body fat

FDA

U.S. Food and Drug Administration

G: F

gain-to-feed ratio

GET

growth-enhancing technology

HCW

hot carcass weight

IMF

intramuscular fat

LUB

lubabegron fumarate

LUB21

lubabegron fumarate supplementation during the final 21 days on feed plus four days of withdrawal

LUB49

lubabegron fumarate supplementation during the final 49 days on feed plus four days of withdrawal

LUB77

lubabegron fumarate supplementation during the final 77 days on feed plus four days of withdrawal

N

nitrogen

NAMI

North American Meat Institute

NEFA

nonesterified fatty acids

NEm

net energy for maintenance

NEg

net energy for gain

NH₃

ammonia

QG

quality grade

REA

ribeye area

RF

rib fat

RQUICKI

revised quantitative insulin sensitivity check index

SBW

shrunk body weight

SEM

standard error of the mean

TBA

trenbolone acetate

TMR

total mixed ration

TRT

treatment

YG

yield grade

Funding

None declared.

Conflicts of interest

The authors Emily Dahlback, Ty Skartvedt, and Dathan Smerchek declare no conflicts of interest.