The Impact of Oregano Essential Oil and the Finishing System on Performance, Carcass Characteristics and Meat Quality in Heifers

Simple Summary

The aim of supplementing pasture-finished cattle with high levels of concentrate is to improve performance and the quality of the carcass and meat. Since the high levels of rapidly fermentable carbohydrates in these diets can be challenging, the inclusion of rumen-modulating additives is recommended to prevent metabolic disorders. Oregano essential oil is already used as a rumen modulator; however, its effects in the diet of pasture-finished cattle are still little understood. This study compared the finishing of heifers in confinement or on pasture, both with diets containing monensin or oregano essential oil. Finishing in confinement resulted in superior performance; however, the finishing system had no effect on the quality of the carcass or meat. The use of oregano essential oil afforded similar performance to that of monensin, with no negative effects on meat quality. It was concluded that high levels of supplementation in pasture-finished heifers produce carcasses that are comparable to confinement-finishing, and that oregano essential oil can replace monensin in the diets of cattle finished on pasture or in confinement.

Abstract

The aim of this study was to evaluate the effects of oregano essential oil (OEO) as a replacement for monensin (MON) on performance, carcass characteristics and meat quality in heifers finished either in confinement or on pasture. Thirty-six Nellore heifers (252.44 kg ± 21.80 kg) were distributed in a completely randomised design in a 2 × 2 factorial scheme, with two types of additives and two finishing systems. In both systems, a concentrate at 1.5% of body weight (BW) on a dry matter (DM) basis containing MON (282.2 mg/animal/day) or OEO (300 mg/animal/day) was offered daily. The final BW (FBW) and average daily gain (ADG) were higher in confinement-finished animals than in those finished on pasture (p < 0.01). There was no effect from the finishing system (p ≥ 0.376) or additive (p ≥ 0.057) for hot-carcass weight, hot-carcass yield, subcutaneous fat thickness, or the Longissimus lumborum area. The pH and shear force of the meat did not differ between treatments (p ≥ 0.076). Finishing in confinement resulted in brighter meat than finishing on pasture (p ≤ 0.006). The use of OEO increased the redness of the meat (p ≤ 0.042). Consumer perception (n = 63) of the sensory attributes of aroma, colour, tenderness, flavour and juiciness was not affected by the treatments (p > 0.05). Heifers finished in confinement or on pasture, with the same proportion of concentrate in the diet and the addition of MON or OEO, presented similar characteristics for both the carcass and the meat.

1. Introduction

Brazil is the second largest producer of beef in the world and the largest exporter, accounting for 13.8% of global production and exporting 2.29 million metric tons in 2023 [1]. Despite the significant production, Brazilian meat is below the standards required by international markets [2] for tenderness and marbling. This has been attributed to the predominant use of Bos taurus indicus animals and pasture-based finishing [3,4]. Indica breeds, such as Nellore, have less intramuscular fat in their meat and a lower calpain/calpastatin ratio compared to Bos taurus taurus cattle [4,5,6]. Furthermore, pasture-finishing with low levels of concentrate in the diet can increase the time to slaughter, with a negative impact on the quality of Nellore beef [3,7,8].

Such strategies as pasture supplementation have been adopted worldwide to reduce the effects of forage seasonality on animal performance and increase the quality of the meat [2,7,9,10]. Offering high levels of concentrate in the diet of cattle under a pasture-finishing system can be a suitable option for promoting the production of carcasses and meat with similar characteristics to those seen in animals in confinement, at the same time reducing production costs in terms of the facilities in confinement systems [7,11,12].

To achieve faster and more efficient finishing, energy diets containing a high proportion of ground corn-based concentrate have been adopted, favouring the presence of rapidly fermenting carbohydrates in the rumen [13]. When using these diets, the inclusion of a rumen-regulating additive is recommended to avoid metabolic disorders such as acidosis [14]. Monensin (MON) is the most widely used rumen-regulating additive in the world [15,16,17]. However, the use of traditional antibiotic additives, such as MON, has been restricted by the European Union due to increasing concerns about the development of bacterial resistance in the environment, which can pose a risk to human health [15,16,17]. In this respect, research into essential oils extracted from such plants as oregano (Origanum vulgare) has shown the potential for improving the meat, with antioxidant properties that help to improve the colour and extend the shelf life [18,19,20,21].

Despite being the subject of current research, there are still few studies into the effect of oregano essential oil (OEO) on the performance and quality of meat from cattle finished on pasture, most studies being conducted using animals in confinement that receive a high proportion of concentrate [22,23,24]. In addition, there is no information comparing the use of OEO in different systems. The hypothesis of this study is that using OEO when finishing Nellore heifers, whether on pasture or in confinement, will promote similar results on carcass characteristics and meat quality to those of using MON. We further hypothesise that finishing heifers on pasture with the same amount of concentrate as in confinement promotes similar responses on carcass and meat characteristics, including sensory attributes. The aim of this study, therefore, was to evaluate the carcass quality, physicochemical properties and sensory characteristics of meat from Nellore heifers finished in confinement or on pasture, with either MON or OEO added to the diet.

2. Materials and Methods

2.1. Location of the Experiment

The experiment was conducted at the Centre for Agrarian Sciences (CAC) of the Federal University of Northern Tocantins (UFNT), in the district of Araguaína, in the north of the state (07°11′28″ S, 48°12′26″ W), from July to November 2022, and lasted 115 days. The average temperature, relative humidity and rainfall during the period were 26.5 °C, 64.2% and 91.8 mm, respectively.

2.2. Animals, Treatments and Experimental Design

Thirty-six Nellore heifers were used, with an average age and initial BW (IBW) of 17 ± 0.3 months and 252.4 ± 21.8 kg, respectively. A completely randomised design was used in a 2 × 2 factorial scheme: two finishing systems, confinement or pasture, and two additives in the concentrate, OEO or MON, with nine replications per treatment:

• Finishing in confinement + MON (282.2 mg/animal/day);
• Finishing in confinement + OEO (300 mg/animal/day);
• Finishing on pasture + MON (282.2 mg/animal/day);
• Finishing on pasture + OEO (300 mg/animal/day).

The experimental period was divided into phases, including an initial 10 days of adaptation and five periods, each of 21 days. The proportion of concentrate (DM basis) in the diet was fixed at 1.5% of body weight (BW) in both finishing systems.

All the animals received the same formulated concentrate (

Table 1. Proportion of ingredients and chemical composition of the concentrate, Mombasa grass silage (Panicum maximum) and Mombasa grass.

Item	Concentrate 2		Forage
	MON	OEO	Silage	Grass
Ingredients, %			-	-
Corn meal	89.9	89.9	-	-
Soybean meal	6.47	6.47	-	-
Urea	1.03	1.03	-	-
Mineral mix 1	2.07	2.07	-	-
NaCl	0.52	0.52	-	-
Monensin sodium (mg/day) 3	282.2	-	-	-
Chemical composition, g/kg DM
Dry matter, g/kg as feed	855.5	860.5	292.9	784.8
Crude protein	127.3	127.9	60.4	65.8
Neutral detergent fibre	174.5	160.3	777.6	639.1
Acid detergent fibre	38.3	42.1	407.3	558.8
Ether extract	38.1	38.1	19.7	14.7
Non-fibre carbohydrates	609.0	620.1	57.2	239.9
Total digestible nutrients 4	700.6	704.1	-	-

¹ Fosbovi boi verde^® (Tortuga, DSM-firmenich, São Paulo, Brazil) = Ca(max) 179 g/kg, P(min) 120 g/kg, S(min) 80 g/kg, Co(min) 12.5 mg/kg, Cu(min) 1250.00 mg/kg, Cr(min) 30 mg/kg, I(min) 62.5 mg/kg, Mn(min) 2500.00 mg/kg, Se(min) 25 mg/kg, Zn(min) 5000.00 mg/kg, F(max) 1200.00 mg/kg, Vit A(min) 20000 IU, Vit D(min) 2500 IU, Vit E(min) 350 IU. ² OEO = Oregano essential oil; MON = Monensin. ³ Bovesin 200^®, Phibro. ⁴ Estimated as per Weiss [27].

). The sodium monensin (Bovesin 200^®, Phibro, Teaneck, NJ, USA) was added to the concentrate during preparation, sufficient to allow an intake of 282.2 mg/animal/day, as recommended by earlier studies by Duffied et al. [25]. The OEO used was a commercial product composed of silica sand, calcium carbonate and oregano essential oil (carvacrol and thymol—50 g/kg), which was weighed on a precision balance and offered together with the concentrate when feeding. The OEO dose was chosen based on earlier work by Benetel et al. [26].

The animals in confinement were kept in individual covered pens that had an area of 7.69 m² and concrete floors, and were equipped with shared drinkers and individual feeders. The pasture-finished animals were kept in individual paddocks (0.13 ha per paddock), in a continuous grazing system, on a pasture of Mombasa grass deferred for 60 days. Grazing deferral involves the temporary removal of animals from the area to allow the forage to accumulate; this is then offered to the animals during the dry season, when there is less forage production due to water deficit [28]. Each paddock contained an individual drinker and feeder.

Forage dry matter was determined every 21 days, measuring the average height of the grass at 40 random points in the paddock with the help of a graduated rule [29]. In addition, two representative samples of pasture of medium height were taken from each paddock, cutting 5 cm from the ground all the grass inside a 0.6 m² metal structure (1.0 m × 0.6 m). Each sample was split into two parts for later determination of the chemical composition. The grass samples were dried in a forced air circulation oven at 55 °C for 72 h, and the total DM and percentage of morphological components were determined to estimate forage weight. The average forage availability was 7592.58 kg/ha of the total pre-dried weight at the start of the experiment. The average forage availability during the experiment was 4634.05 kg/ha of the total pre-dried weight.

2.3. Intake and Performance

The diet was formulated to meet the nutritional requirements of female Zebu cattle at the finishing stage, with an ADG of 1.0 kg/day, based on values estimated by BR-CORTE [30]. Intake was determined by subtracting any leftovers from the amount offered in both the confinement and pasture systems. In the confinement system, the diet was offered once a day, at 12:00. Silage of Megathyrsus maximus ‘Mombasa’ was purchased from a local supplier (Table 1) and was offered as a source of roughage, being adjusted daily (5% of the previous day’s consumption) to maintain voluntary intake.

For the pasture-fed animals, the concentrate was offered daily at 10:00. Only the concentrate intake of the animals finished on pasture was assessed; it was not possible to measure pasture intake due to difficulties associated with the aggressive behaviour of the Nellore animals, which might have had a negative effect on their performance and, consequently, on the results of the experiment [31].

Samples of the leftovers and feed were collected weekly for subsequent analysis of their chemical composition. The forage, leftovers and feed samples were pre-dried in a forced air ventilation oven (TMMA035/5, Trammit Medical, Belo Horizonte, Minas Gerais, Brazil) for 72 h at 55 °C. After pre-drying, the samples were ground in a Wiley knife mill (SL-31, Solab, Piracicaba, Brazil) through a 2 mm sieve. The DM (method: 930.15), crude protein (CP—method: 990.03), fat [32], and mineral matter (MM—method: 923.03) content were analysed as per AOAC [33].

The neutral detergent fibre (NDF) and acid detergent fibre (ADF) content were determined following the methodology described by Van Soest et al. [34]. Non-fibrous carbohydrates (NFC) were calculated according to the methods of Mertens [35]. The total digestible nutrient (TDN) content was calculated for each concentrate as per Weiss [27].

The animals were weighed at the start and end of the experimental period with no prior fasting. The ADG was calculated using the following equation:

ADG = Total weight gain/days of the experiment

After 115 days, the animals were slaughtered at a commercial abattoir in Araguaina, in the state of Tocantins (7°11′58″ S; 48°15′02″ W), located 15 km from UFNT. The heifers were fasted for 24 h before slaughter in accordance with the Municipal Inspection Service, as per the slaughtering standards of the Brazilian Ministry of Agriculture, Livestock and Food Supply (MAPA) [36]. According to MAPA [36], a bovine carcass is defined as an animal that has been slaughtered, bled, skinned and eviscerated, and from which the head, hooves, tail, and mammary glands in females have been removed. After being split into half-carcasses, the kidneys, perirenal and inguinal fat, bruises, spinal cord, and the diaphragm and pillars are also removed. The HCY (hot carcass yield) was determined using the following equation:

HCY = (HCW/SW) × 100

where HCW = hot carcass weight; SW = slaughter weight (with no prior fasting).

2.4. Assessment of pH, SFT (Subcutaneous Fat Thickness) and LLA (Longissimus Lumborum Area)

After refrigeration (24 h at 2 °C), a portion between the 10th and 12th ribs, corresponding to the Longissimus lumborum muscle [37], was removed from the right half-carcass, packed in bags for transport, and taken to the Meat Laboratory at CAC for physical, chemical and sensory analysis.

The pH of the meat was determined 24 h after slaughter in the Longissimus lumborum muscle after refrigeration, using a digital potentiometer with a temperature compensation sensor (model Testo 205^®, Testo SE & Co., Lenzkirch, Germany). Standard pH solutions of 4.00 and 7.00 (Testo SE & Co., Lenzkirch, Germany) were used to calibrate the pH meter at 20 °C. The SFT was determined in the Longissimus lumborum close to the 12th rib, using a universal calliper (150 mm/6″, Disma, Curitiba, Brazil) and measuring the thickness at three points to obtain the average. The LLA muscle was obtained using tracing paper to copy the outline of the muscle area, which was then determined using the ImageJ^® v.1.51 software (National Institutes of Health, Bethesda, MD, USA) and expressed in cm², as recommended by Ferreira et al. [38].

The Longissimus lumborum muscle was separated from the ribs and cut into steaks, each 2.54 cm thick, measured using a universal calliper. The steaks were vacuum packed (25 × 14 cm; 18 μm), and frozen at −18 °C (refrigerator—CRM45B, Consul, São Paulo, Brazil), for later analysis. The tissue composition of the muscle, bone and carcass fat (subcutaneous and intermuscular) was assessed as per Hankins & Howe [39], adapted by Muller et al. [37]. The tissue percentages were obtained by dissecting the portion corresponding to the 10th, 11th and 12th ribs, from which the muscle (Longissimus lumborum), adipose and bone tissue were separated and then weighed to determine the percentage of each type of tissue in the carcass [37].

2.5. Assessment of Meat Colour, TL (Thawing Loss), SF (Shear Force) and Chemical Composition

To assess the colour, the samples were removed from their transport packaging and exposed to the air for 30 min, timed using a stopwatch. A portable colourimeter was used to measure the colour (Croma Meter CR-410, Kônica Minolta^®, Tokyo, Japan). The colour was measured employing a 50 mm aperture, illuminant C and an observer angle of 2°. Calibration was carried out using a white tile (Y: 94.2, x: 0.3130, y: 0.3190). For the colourimetric evaluation, the arithmetic mean of three measurements per sample was calculated in the CIELAB colour space [40], which uses the L* (lightness), a* (red to green) and b* (yellow to blue) coordinates. The chroma (C*) and hue angle (H*) were calculated as per MacDougall [41].

The TL was calculated as the percentage difference between the weight of the fresh and thawed steak (after a defrosting period of 24 h at 4 °C). To analyse the SF, 28 h after slaughter, a fresh steak, 2.54 cm thick, was weighed and oven-roasted (9741-79183, Fischer, Brusque, Brazil) until reaching an internal temperature of 70 °C, measured using a kitchen thermometer (TP-101, Weck UD, São Paulo, Brazil). After cooking and cooling to room temperature, the samples were again weighed to calculate the cooking loss (CL). The cooked steaks were stored at 4 °C for 24 h. Five cylindrical samples, each with a diameter of 1.3 cm, were then removed parallel to the muscle fibre of the meat, using a punch (sharpened stainless-steel tube) with a diameter of 1.3 cm. These samples were cut perpendicular to the fibre at an angle of 90° using a texturometer (TA.XT plus, Stable Micro Systems Ltd., Godalming, UK) equipped with a Warner-Bratzler blade, 1.016 mm thick. The texturometer was calibrated at a test speed of 200 mm per minute, post-test speed of 2400 mm per minute, distance of 20 mm and calibration weight of 2 kg. The maximum force for each cylinder was recorded using the Exponent Lite 6.1 software (Stable Micro Systems, Godalming, UK), and the average value per steak was used in the statistical analysis.

To analyse the chemical composition of the meat, the samples were ground in a food processor, weighed and pre-dried in a forced air ventilation oven (TMMA035/5, Trammit Medical, Belo Horizonte, Minas Gerais, Brazil) for 72 h at 55 °C. After pre-drying, the samples were ground in a Wiley knife mill (SL-31, Solab, Piracicaba, Brazil) through a 2 mm sieve and stored in a refrigerator at 4 °C for analysis. The samples were then used to determine the moisture (Method 930.15; [33]), CP (Method 990.03; [33]), MM (Method 923.03; [33]) and fat content [32]. Each component of the chemical composition was corrected and expressed in g per 100 g meat (natural matter). The CP content was analysed using the Kjeldahl method. The sample was subjected to sulphuric acid digestion in a digestion system (TE-040/25, TECNAL, Piracicaba, Brazil), using copper sulphate as a catalyst. Nitrogen was converted to ammonium sulphate, which was distilled using a nitrogen distiller (TE-0363, TECNAL, Piracicaba, Brazil). The total nitrogen content was determined and multiplied by a factor of 6.25.

2.6. Sensory Panel

A total of 63 untrained evaluators were recruited from the CAC campus of UFNT by contacting groups of students and professors through social media. Meat consumption was considered an essential requirement when selecting the participants. Thirty-nine women and twenty-four men, aged between 18 and 53, were recruited. Each of the sensory panellists gave their informed consent prior to their involvement in the study, and agreed to have the data generated from their observations used in this study. The sensory panel met over three days at the Meat Laboratory of UFNT. The evaluators were placed in individual booths with seven participants per session (three sessions per day). The participants were previously instructed on the evaluation and research procedures. Napkins and wooden sticks were provided, as well as crackers and water to cleanse the palate between samples. Each participant received chocolates for their participation.

The steaks used in the analysis, each 2.54 cm thick, with no ageing, were thawed for 24 h at 4 °C (refrigerator—CRM45B, Consul, São Paulo, Brazil). The meat samples were baked in a preheated electric oven (9741-79183, Fischer, Brusque, Brazil) at 180 °C until they reached an internal temperature of 70 °C, measured using a kitchen thermometer (TP-101, Weck UD, São Paulo, Brazil). Each consumer was served a 2.0 cm² cube of meat from one animal from each treatment, and evaluated a total of four samples, which were offered in random order, determined by an earlier draw. The participants received evaluation sheets to assess each sample for the attributes of aroma, colour, tenderness, juiciness and overall liking on a continuous scale from 0 to 100. The fixed value of 0 represented meat that had no aroma, was very dark, extremely tough, lacking in juiciness and was highly disliked, while 100 represented meat with a strong aroma, was very light in colour, extremely tender, extremely juicy and highly liked. The neutral value was fixed at the midpoint of 50, and represented meat that neither lacked nor had a strong aroma, was neither dark nor light, neither tough nor tender, neither dry nor juicy, and was neither liked nor disliked [42,43].

2.7. Statistical Analysis

The analyses were carried out using the SAS^® v.9.4 statistical software (Statistical Analysis System Institute, Cary, NC, USA). The data were submitted to the Shapiro–Wilk and Levene tests of normality and homogeneity of variance, respectively. Once the assumption had been verified, an analysis of variance was carried out using the PROC MIXED procedure of the SAS software [44]. The mean values were compared using Tukey’s test at a significance level of p < 0.05. The following mathematical model was used:

Y_ijk = μ + S_i + A_j + SA_ij + e_ijk

where Y_ijk is the dependent variable; μ is the overall mean of the observations; S_i is the effect of the finishing system (i); A_j is the effect of the additive; SA_ij is the effect of the interaction between the system (i) and additive (j); e_ijk is the experimental error. For the variables, DM intake, CP, NDF, ADG and FBW, the IBW was used as a covariate.

The PROC MIXED procedure was used to analyse the sensory characteristics (juiciness, tenderness, colour, taste and aroma). The fixed terms in the models included the system, additive and its interaction. The tasting round, panellist and session were included as random effects. The mean values of the treatments were determined using the LSMEANS command and were compared by Tukey’s test (p < 0.05).

3. Results

3.1. Performance and Carcass Quality

There was no interaction between the finishing system (confinement or pasture) and the type of additive (MON or OEO) on consumption or performance (p ≥ 0.839;

Table 2. Concentrate dry matter intake and performance in Nellore heifers finished in confinement or on pasture, with a diet containing monensin (MON) or oregano essential oil (OEO).

Variable	Treatment					p-Value
	Confinement		Pasture
	MON	OEO	MON	OEO	SEM 2	System	Additive	S × A 3
Concentrate intake 1
DM, kg/d	4.34	4.46	4.69	4.61	0.056	<0.001	0.233	0.591
CP, kg/d	0.55	0.57	0.60	0.59	0.007	<0.001	0.041	0.593
NDF, kg/d	0.67	0.71	0.72	0.74	0.009	<0.001	<0.001	0.749
Performance
IBW, kg	227.3	231.4	265.6	258.8	4.376	<0.001	0.704	0.839
FBW, kg	331.0	342.0	331.3	330.0	3.870	<0.001	0.208	0.804
ADG, kg/d	1.00	1.06	0.63	0.68	0.038	<0.001	0.208	0.804

¹ DM = dry matter, CP = crude protein, NDF = Neutral detergent fibre, IBW = initial body weight (covariate), FBW = final body weight, ADG = average daily gain. ² SEM = standard error of the mean. ³ S × A = interaction between system and additive.

). DM, CP and NDF intake were higher in the pasture-finished heifers (p < 0.001). CP and NDF intake were higher with the use of OEO compared to MON (p < 0.041; Table 2).

FBW was higher for heifers in confinement than for those on pasture (p < 0.01; Table 2). In addition, animals in the confinement system had higher ADG (p < 0.01; Table 2) compared to those reared on pasture. The type of additive had no effect on FBW or ADG (p > 0.208; Table 2).

There was no interaction between the treatments for HCW, HCY, SFT and LLA or carcass composition (p ≥ 0.178,

Table 3. Carcass characteristics and physical composition in Nellore heifers finished in confinement or on pasture, with a diet containing monensin (MON) or oregano essential oil (OEO).

Variable 1	Treatment					p-Value
	Confinement		Pasture
	MON	OEO	MON	OEO	SEM 2	System	Additive	S × A 3
HCW, kg	175.9	170.3	174.0	175.6	2.092	0.670	0.608	0.436
HCY, %	53.3	50.1	52.6	53.4	0.722	0.376	0.409	0.178
SFT, mm	4.44	4.78	4.52	4.30	0.306	0.751	0.931	0.666
LLA, cm2	54.1	55.1	55.1	52.6	0.929	0.707	0.713	0.372
Muscle, %	61.9	61.3	62.9	60.9	0.391	0.721	0.103	0.407
Carcass fat, %	21.5	22.6	20.8	23.1	0.486	0.898	0.094	0.549
Bone, %	16.6	16.1	16.3	16.0	0.179	0.677	0.326	0.859
EP/B	5.08	5.23	5.15	5.24	0.067	0.735	0.394	0.828
M/F	2.94	2.75	3.12	2.67	0.084	0.768	0.057	0.443

¹ HCW = hot carcass weight, HCY = hot carcass yield, SFT = subcutaneous fat thickness, LLA = longissimus lumborum area, EP/B = edible portion to bone ratio, M/F = muscle to carcass fat ratio. ² SEM = standard error of the mean. ³ S × A = interaction between system and additive.

). The finishing system had no effect on the carcass characteristics (p ≥ 0.376), where finishing on pasture resulted in the same HCW as did confinement (Table 3). Similarly, substituting MON for OEO had no effect on the carcass characteristics under evaluation (p > 0.05; Table 3).

3.2. Meat Quality

There was no interaction between the finishing system and the additive, nor was there any isolated effect from the factors on pH or SF (p ≥ 0.881;

Table 4. Meat quality in Nellore heifers finished in confinement or on pasture, with a diet containing monensin (MON) or oregano essential oil (OEO).

Variable 1	Treatment					p-Value
	Confinement		Pasture
	MON	OEO	MON	OEO	SEM 2	System	Additive	S × A 3
pH	5.39	5.44	5.45	5.51	0.026	0.197	0.328	0.881
SF, N	58.7	66.6	65.9	68.6	3.446	0.524	0.458	0.719
L*	39.9	40.8	37.9	38.4	0.390	0.006	0.331	0.391
a*	18.4	19.1	18.8	19.9	0.226	0.120	0.040	0.641
b*	7.0	7.6	7.0	7.5	0.159	0.821	0.117	0.980
C*	19.7	20.5	20.1	21.3	0.257	0.222	0.042	0.716
H*	21.0	21.6	20.2	20.5	0.295	0.120	0.381	0.817
TL, %	8.36	7.97	6.92	7.76	0.271	0.134	0.676	0.258
CL, %	27.4	26.6	22.9	22.9	0.564	<0.001	0.647	0.679
Moisture, %	74.3 a	73.2 b	74.0 ab	74.9 a	0.197	0.055	0.716	0.005
CP, %	22.0	21.4	21.0	21.9	0.270	0.622	0.784	0.176
Intramuscular fat, %	2.96	3.53	3.58	2.68	0.201	0.785	0.680	0.076
MM, %	1.10 b	1.19 a	1.17 ab	1.12 ab	0.012	0.984	0.375	0.007

¹ pH = hydrogen potential, SF = shear force, L* = lightness, a* = redness, b* = yellowness, C* = chroma, H* = hue angle, TL = thawing loss, CL = cooking loss, CP = crude protein, MM = mineral matter. ² SEM = standard error of the mean. ³ S × A = interaction between system and additive. Mean values followed by different lowercase letters on a line differ statistically between treatments by Tukey’s test (p < 0.05).

). There was also no effect from the treatments on TL, CP or intramuscular fat (p ≥ 0.785; Table 4).

Moisture was lower (p = 0.005), while MM was higher (p ≤ 0.01; Table 4) in meat from animals in confinement fed OEO compared to those fed MON. For the animals finished on pasture, the moisture and MM content of the meat were similar for both additives (p ≥ 0.175; Table 4).

The lightness (L*) of the meat was higher (p = 0.006) in the confined animals than in those on pasture, with no effect from the finishing system on the red (a*; p = 0.12) or yellow (b*; p = 0.08; Table 4) content. There was an effect from the additive on the a* content of the meat (p ≤ 0.040), which was higher in animals fed OEO than in those fed MON (Table 4). Similarly, chroma (C*) was higher (p ≤ 0.042) in the OEO treatment compared to the MON treatment (Table 4).

The interaction between the finishing system and the type of additive had no effect on TL (p > 0.258) or CL (p > 0.679; Table 4). There was also no isolated effect from the factors (p > 0.05) on TL (Table 4). On the other hand, the finishing system had an effect on CL (p < 0.01), where animals finished in confinement showed greater losses than those finished on pasture (Table 4).

3.3. Sensory Panel

There was no interaction (p = 0.610) between the finishing system and the type of additive for the sensory attributes of the meat (

Table 5. Sensory attributes of the meat from Nellore heifers finished in confinement or on pasture, with a diet containing monensin (MON) or oregano essential oil (OEO).

Variable	Treatment 1						p-Value
	Confinement		Pasture
	MON	OEO	MON	OEO	Mean	SEM 2	System	Additive	S × A 3
Aroma	52.8	56.2	56.0	51.4	54.1	3.20	0.800	0.800	0.210
Colour	59.7	57.8	61.3	63.4	60.6	2.55	0.170	0.900	0.430
Tenderness	58.4	66.8	64.2	61.4	62.7	3.04	0.960	0.360	0.070
Juiciness	56.1	59.4	59.9	60.0	58.9	3.12	0.480	0.580	0.610
Overall liking	54.8	59.8	59.8	57.9	58.3	3.00	0.740	0.400	0.190

¹ Scores were assigned on a continuous scale from 0 to 100. The fixed value 0 represented meat with no aroma, very dark, extremely tough, lacking in juiciness and highly disliked; the fixed value of 100 represented meat that had a strong aroma, was very light in colour, extremely tender, extremely juicy and highly liked; 50 was considered neutral, and represented meat that neither lacked nor had a strong aroma, was neither dark nor light, neither tough nor tender, neither dry nor juicy, and was neither liked nor disliked. ² SEM = standard error of the mean. ³ S × A = interaction between system and additive.

). The finishing system (p > 0.170) and the additive (p > 0.360), when evaluated separately, had no influence on consumer perception, with average scores of 58.9, 62.7, 60.6, 58.3 and 54.1 for the sensory attributes juiciness, tenderness, colour, flavour and aroma, respectively (Table 5).

4. Discussion

The higher concentrate DM intake, and consequently of CP and NDF, seen in animals finished on pasture is explained by the higher IBW during the finishing stage, since the offer of concentrate was based on the BW of the animals (1.5% of BW). Although the animals under confinement had an FBW that was 5.8 kg greater, at the start of the finishing stage, animals under the pasture system had an IBW that was 32.9 kg greater. This difference at the start of the finishing stage can be explained by the reduction in BW of the confined animals during the adaptation phase [45,46], which did occur under the pasture system.

The greater ADG in the confined animals can be attributed to a compensatory gain, which occurs after a period of nutritional deprivation (adaptation), where there is a reduction in the metabolic demand of the animal [47]. Subsequently, when the animals return to feeding, a greater amount of energy derived from the feed can be directed to the deposition of muscle and fat on the carcass [48].

Another possible explanation for the increased performance of the confined animals could be the higher energy requirements of pasture-fed animals. According to Marcondes et al. [49], estimates of intake requirements for grazing cattle should consider 10% to 20% higher maintenance requirements than for confined animals, due to the greater expenditure on movement and feed capture [12,49]. It is also important to consider that in deferred pasture finishing systems, there is no plant growth during the dry season. Therefore, not only is forage availability reduced, but also the quality of the available material, compromising forage selection by the animals [7,9,50]. On the other hand, in confinement, the supply of silage was adjusted to maintain the voluntary intake of the animals, with no restriction. Both factors, the greater nutritional requirement and the low forage supply, may have compromised performance in the pasture-finishing system [8].

In general, the literature reports a lower DM intake for animals fed MON, due to the greater intake of glucose from the higher percentage of propionate produced in the rumen in diets containing this additive [25]. Some studies show that essential oils can also improve animal feed efficiency and modify the acetate to propionate ratio through the selection of Gram-negative bacteria [51,52]. However, the results vary depending on the active ingredient in the oil and the type of diet used [20,53,54]. The results of the use of essential oils have shown that these compounds improve animal efficiency and weight gain, and indicate that they can be used as a substitute for MON [20,21,55]. In the present study, variables related to animal performance did not differ between the additives, underlining that OEO had the same effect as MON on animal DM intake. Coelho et al. [56] also found no difference in the performance of cattle raised on pasture during the dry season, and receiving OEO or MON as an additive.

Finishing the heifers in a confinement system or on pasture resulted in similar carcass characteristics, showing that the similar supply and intake of concentrate (1.5% BW of DM) in both systems can afford comparable gains in carcass weight. Diets with a high proportion of ground corn-based concentrate increase the amount of propionate in the rumen, which results in more energy available for tissue deposition on the carcass [57,58]. The high proportion of concentrate in the diet reduced rumen fill, which is often seen in animals finished on diets rich in roughage or pasture [59]; this allowed the animals to consume similar amounts of energy in both finishing systems.

The percentage of carcass muscle was therefore similar under both finishing systems, as was the HCW. Despite the higher FBW and ADG observed in the confined animals, HCW, HCY and physical composition of the carcass were similar to those seen in animals finished on pasture. During the adaptation phase, the confined animals experienced weight loss. This weight loss is the result of the rumen microbiota adapting to the new diet, with more added concentrate and more preserved roughage, in addition to the emptying of the rumen tract [45,46,60]. The greatest weight gain seen in the confined animals during the evaluation phase allowed them to recover the weight lost during adaptation, resulting in a difference of 5.8 kg by the end of finishing. This result did not correspond to an effective increase in tissue deposition on the carcass. Finishing heifers on pasture with high levels of concentrate therefore yielded carcasses comparable in quality and yield to those finished in confinement. The results for animal performance in this study differ in part from those reported by Ferrari et al. [7], who also found greater FBW and ADG in bulls finished in confinement than those finished on pasture. However, these authors found higher HCY in bulls raised on pasture, while the LLA did not differ between the finishing systems. This may be an alternative to finishing heifers in confinement, allowing for reduced facility-related costs [7,12].

The carcass yield obtained in this study (52.34%) is consistent with the category of young heifers, and is within the average HCY for Brazil, from 51% to 54% for zebu carcasses [61]. The HCY of female cattle tends to be lower than that of males, due to the higher percentage of fat deposited as the slaughter weight increases [61,62]. Therefore, removing a greater amount of fat and the mammary glands from the female carcass can reduce the HCY [36,62]. As in the present study, Patino et al. [63] found similar SW, HCW and HCY for cattle finished in confinement (forage to concentrate ratio—50:50) or on pasture supplemented with more than 1.2% BW of DM.

Additives are often used with high-concentrate diets to improve feed efficiency and increase nutrient digestibility by selecting the microbiota and modifying rumen fermentation [14,21,25]. Under the conditions of this study, both MON and OEO allowed the animals to optimise the use of nutrients in the high-concentrate diet and avoid possible metabolic disorders. Both additives can afford an increase in the levels of propionate [52,64,65,66], a volatile fatty acid precursor of glucose that improves energy usage, increases weight gain and, therefore, has a positive impact on carcass yield and tissue deposition [52,64,65,66]. In addition, MON and OEO act to improve the supply of protein in the rumen, promoting the development of fibrolytic microorganisms, responsible for the degradation of forage fibre [51,67], an essential point when finishing animals during the dry season on low-quality forage [9].

Finishing, both in confinement and on pasture, led to a similar fat finish on the carcass, which shows that an adequate supply of concentrates results in well-finished animals, even when finished on pasture. The greater the proportion of grain added to the diet, the higher the rate of fatty acid biosynthesis [11,68]. The results of Valle et al. [69] support this idea: the authors found greater SFT in animals finished on supplemented pasture than in animals with no supplements. In the present study, the SFT obtained in both systems (4.51 mm) is within the minimum range required by the Brazilian meatpacking industry, of 3 to 6 mm [62].

It should be noted that the proportion of fat on the carcass tended to increase when OEO was used (22.84% vs. 21.16%). According to a meta-analysis by Torres et al. [20], essential oils can increase the SFT and LLA of the carcass, which can be attributed to the higher DM intake of the animals fed essential oils compared to those fed MON. This results in an increase in lipogenesis due to the extra energy associated with the higher DM intake and smaller energy loss from methane production, both of which lead to an increase in HCY and SFT [20,68,70]. In the present study, there was no difference between the DM intake of animals fed MON or OEO, nor did the LLA differ between the additives. The increase in the percentage of carcass fat with OEO cannot, therefore, be explained by the higher DM intake. Furthermore, the likely reason for this result may be the effect of OEO of increasing ruminal concentrations of volatile fatty acids such as valerate and butyrate, which enhance lipogenesis [20,26].

The LLA indicates the proportion of muscle on the carcass and has a positive correlation with the edible portion, meaning an increase in meat yield [71]. In the present study, the response in terms of LLA (54.22 cm²) is consistent with the proportion of muscle (61.74%), which also did not differ between the finishing systems (confinement or pasture), or the use of MON or OEO. These results show that when a balanced diet is provided in order to meet the energy requirements for muscle and fat deposition, finishing female cattle on pasture can generate carcasses of similar quality to those obtained with confined animals, so that the effect of physical activity, when the animals explore a wider area, appears to be minimal on these variables for this category [7,72].

The pH of the meat did not differ between the finishing systems or the additives. The average pH of 5.45 is within the correct range for maintaining the microbiological quality and characteristics of the meat, which varies between 5.4 and 5.8 [73]. The final pH is the result of the accumulation of lactic acid from the production of ATP by the use of muscle glycogen reserves during the post-mortem transformation of the muscle into meat [73,74]. It can therefore be inferred that the diet consumed in both systems provided enough energy for glycogen to be stored in the muscles [75]. Essential oils have an antioxidant effect and can alter the pH of the meat [18]; however, in the present study, there was no difference between the pH of the meat of animals fed OEO or MON. Similarly, Monteschio et al. [76] found no effects from the addition of essential oils (rosemary, cloves, or a mixture of thymol, eugenol and vanillin) on the pH of meat from Nellore heifers. In addition to the varying concentration of active compounds in essential oils (e.g., carvacrol, thymol, and eugenol), the composition of the diets can also influence the meat pH [70,76,77].

The average SF value of the meat from all the treatments was 64.9 N, indicating that the meat was very tough (greater than 62.59 N; [78]). According to the literature, one factor that helps to reduce meat tenderness is pasture finishing, which leads to a lower energy intake and less fat deposition, in addition to the grazing animals moving around more [79,80]. In the present study, despite the young animals being finished in confinement or on pasture with a diet that included high levels of concentrate (1.5% BW), the recommended tenderness was not achieved in either system. This is linked to the toughness of zebu meat, which is in turn due to the greater activity of calpastatin, an enzyme that inhibits proteolysis and affects the tenderness of the meat [4,6,73]. Genetic factors possibly contributed more to the toughness of the meat than did physical activity, age or the level of energy in the diet [4]; there was also no difference in tenderness, whether using MON or OEO. According to several studies, essential oils have an antioxidant effect, which can reduce the negative impact of the oxidative process on the proteolysis of muscle fibre, thereby affecting the tenderness of the meat [18,21]; however, this effect was not seen in the present study.

The sensory panel also detected no difference in tenderness in the meat of animals finished in confinement or on pasture, whether with MON or OEO. The meat was considered neither tough nor tender, albeit closer to tender (62.7 points), 0 representing extremely tough meat and 100 meat that is extremely tender. The high energy content of the diet under both finishing systems may have contributed to the medium tenderness detected by the sensory panel [5,81]. Although the meat was classified as very tough, based on its SF, the sensory panel considered it closer to tender. This was probably due to the profile of the Brazilian consumer, where meat is usually consumed in its natural state, with no ageing and higher shear force values [4,82]. The colorimetric variables of the meat remained within the ranges described by Muchenje et al. [83] for the ideal meat colour: L* (33.2–41), a* (11.1–23.6) and b* (6.1–11.3) The meat from the animals finished in confinement was 5.40% lighter than from those finished on pasture, which means that the meat from confinement was lighter in colour. The higher concentration of muscle myoglobin attributed to grazing animals and due to the greater intensity of muscle activity is one of the factors that may have resulted in darker meat on pasture-finished animals [12,84,85]. Muscles with a high concentration of myoglobin exhibit lower glycolytic potential, resulting in a slower fall in pH during the first 24 h after harvesting [81,85]. On the other hand, with a fast fall in pH, the muscle fibres contract more, leading to a greater volume of extracellular space, allowing greater light-scattering in the meat of confined animals [86]. According to Hughes et al. [87], muscle structure has a greater influence on the lightness of the meat than does the quantity and oxidation of myoglobin. The greater fibre contraction and the increase in extracellular space could also explain the greater CL of meat from confined animals [88]. Although the final pH of the meat did not differ between systems, we believe that a combination of the above factors may have resulted in the meat from confined animals being lighter in colour [8,85].

Replacing MON with OEO affected the redness (a*) and chroma (C*) of the meat, which were 4.66% and 4.97% greater with OEO, respectively. The use of OEO in cattle diets prevents lipid oxidation and promotes a stable meat colour due to its antioxidant effect on myoglobin, making the meat lighter in colour [20,70,89]. After cooking, consumers did not detect any difference in the colour of the meat, rating it as neither very light nor very dark.

Losses due to cooking and thawing are related to the water retention capacity, and vary depending on the level of intramuscular fat, being mainly linked to the effect of the final pH [88,90]. Meats with a high percentage of fat lose more liquid during cooking [75,90], while a very low pH can cause protein denaturation of the muscle fibres, affecting the water retention capacity of the meat and possibly resulting in the meat exuding liquid [91,92]. In this study, the EE content of the meat was similar for each of the four treatments, and considered too low to have any effect on CL; similarly, there was no difference in pH between the treatments.

The higher CL values in the meat from confined animals may be explained by the antioxidant effect of the forage consumed by the animals on pasture [93]. The phytochemicals present in the forage can have an antioxidant effect on the muscle, reducing lipid and protein oxidation, which causes the muscle fibres to rupture and increases water loss [90,94]. It is reported that, due to the lower antioxidant effect of MON compared to that of essential oils [18,20,70], monensin can affect the water retention capacity and result in meats with higher CL. As such, results reported by Orzuna-Orzuna et al. [21], in a meta-analysis on the effects of essential oils in beef cattle feed, showed a reduction in the CL of the meat. In the present study, there was no difference in CL between the additives. This may be due to the greater effect of the finishing system [81,85].

Although CL was greater in the meat from the confined animals, the sensory panel detected no difference in juiciness or flavour. Similarly, when employing a sensory panel to evaluate the effects on meat quality of including natural additives in the diet of young bulls, Ornaghi et al. [18] found that the addition of a mixture of essential oils at a dose of 3 g/animal/day had no effect on tenderness, general acceptance, flavour or aroma, which suggests that adding the mixture did not have a negative effect on the acceptability of the meat.

Our results show that using OEO when finishing heifers can produce meat of a similar quality to that obtained when using MON. Furthermore, it should be noted that the effects of additives on the attributes of meat quality may be more clearly seen in studies with a longer period of evaluation, since the finishing period for beef cattle in confinement in Brazil is generally shorter than in other countries [13]. This remains a suggestion for future studies in this area.

5. Conclusions

The data suggests that OEO could be a suitable alternative to MON in high-concentrate finishing systems, whether in confinement or on pasture, as it did not compromise average daily gain, carcass yield, fatness, weight or physical composition. Moreover, OEO enhances the intensity of the redness of the meat. Finishing heifers on pasture with a high-concentrate supplement proved to be a viable alternative to traditional confinement systems, yielding carcasses of comparable weight and carcass fat deposition, despite the higher average daily gain seen with confinement. Furthermore, the pasture-based strategy had no negative impact on the shear force of the meat, a key quality attribute valued by consumers. It should be noted that the lightness of the meat was reduced in cattle finished on pasture. Both pasture-based finishing and the addition of OEO to the diet showed the potential for producing beef with attributes that are desirable to consumers. However, further studies under the same conditions are needed to thoroughly investigate the effects of OEO on rumen fermentation, metabolic parameters and the fatty acid profile of beef. Studying these parameters can help indicate the most appropriate use of OEO in the nutrition of cattle, whether finished on pasture or in confinement, in order to improve animal performance, prevent ruminal diseases, and increase the nutritional characteristics and shelf life of the beef.