Effects of Sex and a Diet Containing Bovine Ruminal Content on Performance, Ruminal Fermentation, Digestibility, Carcass Traits, and Meat Quality in Lambs

Simple Summary

This study investigates the effects of different levels of bovine ruminal content inclusion in the diets of crossbred Pelibuey-Dorper lambs. Bovine ruminal content, a by-product of the meat industry, has significant potential as an animal feed ingredient; however, its high moisture content and bulk present certain limitations. The research evaluated the inclusion of ruminal content at 15%, 30%, and 45% in lambs’ diets. The results indicated that these inclusion levels did not affect the lambs’ productive performance, ruminal fermentation parameters, or nutrient digestibility. Moreover, no significant differences were observed in meat or carcass characteristics, including back fat, loin eye area, or meat color. Therefore, it can be concluded that up to 45% bovine ruminal content can be safely incorporated into lamb diets without any adverse effects. The results show that while the diets did not significantly affect the studied variables, sex had a significant impact on final body weight, average daily gain, dry matter intake, and both hot and cold carcass yield. Although specific combinations between treatment and sex significantly affect average daily gain, dry matter intake and propionic acid variables, there is no consistent evidence that bovine rumen content, either alone or in combinations with sex, improves growth or carcass quality in fattening lambs.

Abstract

The objective of this study was to evaluate the productive performance, ruminal fermentation, in vivo digestibility, carcass yield, and physicochemical variables of meat when bovine rumen content (BRC) was included in the diet of lambs. Thirty-six Pelibuey-Dorper crossbred lambs of both sexes, with an average weight of 19.5 ± 1.5 kg, were used in a generalized randomized block design with the sex of the lambs as a blocking factor with four treatments: BRC₀, BRC₁₅, BRC₃₀, and BRC₄₅, corresponding to 0%, 15%, 30%, and 45% BRC inclusion in the diet, respectively. The results indicate that the dietary effect was not significant for any of the evaluated variables, whereas sex showed significant differences in final body weight, average daily gain, dry matter intake, and hot and cold carcass yield. Although specific treatment-sex combinations significantly influenced productive performance variables such as average daily gain, dry matter intake and ruminal fermentation parameters, such as propionic acid, there is no consistent evidence that the inclusion of bovine rumen content promotes superior productive performance or carcass quality in fattening lambs.

1. Introduction

Animal slaughter generates significant amounts of waste, presenting environmental challenges that require effective management and disposal strategies. While certain parts of the animal are suitable for human consumption, many others are not [1]. Animal by-products include all components of a live animal that are not part of the prepared carcass, such as the liver, heart, rumen contents, kidneys, blood, fat, spleen, and meat trimmings [2].

Rumen contents are one of the agro-industrial wastes with substantial potential for environmental impact. The quantity of rumen contents varies widely based on factors such as the age, size, transport conditions, and resting time of the animal. On average, every slaughtered cow produces between 50 and 60 kg of wet rumen contents [3], which consists of undigested feed discarded during the slaughter process. This waste contributes significantly to environmental pollution, particularly in many slaughterhouses in developing countries [4,5,6].

Bovine rumen contents comprise a heterogeneous, fibrous biomass consisting of remnants of undigested plant matter and elongated, porous solid structures, in both fresh and dry forms. Their chemical composition includes approximately 11.6% lignin, 31.4% cellulose, and 20.5% hemicellulose, along with organic compounds, organic acids, and minerals [3]. Additionally, they are a good source of microbial protein, vitamins, and rumen metabolic products such as volatile fatty acids, amino acids, and non-protein nitrogen [7,8], and do not contain harmful compounds [9], making them suitable for animal feed due to their availability [10]. However, the nutrient composition can vary depending on the type of diet and the ruminant’s grazing choices [11,12]. To obtain dry rumen contents, the materials are processed to separate solids from liquids and mechanically separated from the liquid, allowing the solids to be retained as animal feed [13].

Reports indicate that incorporating dry rumen contents into the diets of various animals, including poultry [14,15], rabbits [9,16], pigs [17], sheep [7], and cattle [18,19], does not pose adverse effects on animal health; however, care must be taken in handling rumen contents, and their inclusion in diets must be balanced to meet the nutritional requirements of each species [20] and does not present risks to consumers [21]. We hypothesized that the inclusion of bovine rumen content in diets would promote higher productive performance and adequate carcass quality in lamb fattening. However, limited information is available on the optimal level of bovine rumen content inclusion in lambs’ diets, as well as its effects on production parameters and meat quality. The objective of this research was to evaluate the inclusion of bovine rumen contents in the diets of growing lambs and to assess its impact on production performance, rumen metabolism, in vivo digestibility, and the characteristics of carcass and meat.

2. Materials and Methods

2.1. Location of the Experiment

The research was carried out at the SANDI ranch facilities, located in the municipality of La Trinidad, Zaachila, in the Central Valleys region of Oaxaca, Mexico, at an altitude of 1490 m, with coordinates at 16°55′ north latitude and 96°48′ west longitude.

2.2. Animals, Experimental Design and Treatments

Animal care and handling procedures were conducted according to the technical Specifications for the Production, Care and Use of Laboratory Animals, established in the Official Mexican Standard (NOM-062-ZOO-1999). Thirty-six crossbred Pelibuey-Dorper lambs, with an average age and live weight of 3 months and 19.5 ± 1.5 kg, respectively, were assigned to four dietary treatments (n = 9, comprising 5 intact males and 4 females). The treatments included a diet without bovine rumen contents (BRC) and diets with 15% (BRC₁₅), 30% (BRC₃₀), and 45% (BRC₄₅) BRC inclusion. The research used a generalized randomized block design with the sex of the lambs as a blocking factor. The lambs received VITAMIN-VET^® intramuscularly (2 mL/animal) and were housed in individual pens measuring 1.44 m², with ad libitum access to clean, fresh water. The diets (as detailed in

Table 1. Ingredients and chemical composition of bovine rumen content and diets.

Items		Treatments
	BRC	Control	BRC15	BRC30	BRC45
Ingredients (g/kg dry matter)
Ground corn		431	400	375	340
Soybean meal		181	135	110	105
Alfalfa hay		78	130	100	0
Corn stubble		215	90	20	0
Bovine ruminal content		0	150	300	450
Cane molasses		60	60	60	70
Urea		15	15	15	15
Mineral premix *		20	20	20	20
Chemical composition
Dry matter	882.5	848	864	858	853
As % of dry matter
Crude protein	14.48	18.86	18.86	18.62	18.83
Ether extract	2.45	1.12	1.07	1.21	1.23
Neutral detergent fiber	52.65	54.47	55.72	59.03	57.82
Acid detergent fiber	32.36	39.25	40.95	32.74	41.25
Ash	19.11	9.51	9.34	9.54	9.78
Metabolizable energy ** (MJ/kg DM)	12.04	14.07	14.10	14.19	14.06

BRC: bovine ruminal content, Control: diet without bovine rumen content; BRC₁₅, BRC₃₀, and BRC₄₅: inclusion of 15, 30 and 45% bovine ruminal content in the diet. * Calcium, 27%; phosphorus, 3%; magnesium, 1.75%; sodium, 6.55%; chlorine, 10%; potassium, 0.05%; sulfur, 42 ppm; antioxidant, 0.05%; manganese, 4000 ppm; iron, 978 ppm; zinc, 4000 ppm; iodine, 50 ppm; selenium, 30 ppm; cobalt, 45 ppm; Vit. A: 350,000 IU; Vit. D: 150,000 IU; Vit. E: 600 IU. ** Novotny et al. [25].

) were provided ad libitum twice daily at 08:00 and 14:00 h, and were designed to be isoproteic and isoenergetic, following the recommendations of the NRC [22]. The diets were determined for dry matter (DM, method number 934.01), ether extract (EE, method number 983.23), crude protein (CP, method 992.15) and ash (method 945.18) according to AOAC [23]; neutral detergent fiber (NDF) and acid detergent fiber (ADF) using the technique of Van Soest et al. [24]. The BRC were sourced from feedlot cattle assessed by veterinary staff prior to slaughter in a commercial facility. Afterward, the BRC were sundried for three days.

2.3. Productive Performance, Backfat Thickness, and Loin Eye Area

To assess the productive performance of the lambs, a 60-day fattening period was implemented, which followed a 20-day adaptation period to the diet. The lambs were weighed at the start of the experiment and then every 14 days before feeding (at 8:00 a.m.). The average daily gain (ADG) was calculated by dividing the weight difference by the number of days between weightings. Dry matter intake (DMI) was calculated as the difference between the amount of feed offered and that refused daily. The feed conversion ratio (FCR) was calculated by dividing DMI by ADG for each weighing period.

On day 59 of the experiment, the backfat thickness and loin eye area of the Longissimus dorsi muscle were measured using a signal transducer (Model 5040, Aloka^® SSD-500 V, Tokyo, Japan). The Centralized Ultrasound Processing software (CUP LAB^®, Ames, IA, USA) was used to process ultrasonographic images, specifically at the transverse plane between the 12th and 13th ribs [26]. The readings for the loin eye area were obtained in square millimeters, while backfat thickness was measured in centimeters [27].

2.4. Ruminal Variables

At the end of the productive performance period (day 60), ruminal fluid samples were collected using an esophageal tube 3 h after feeding (at 08:00 h). The pH of the samples was immediately measured with a portable pH meter (Model Orion 3-Star, Thermo Scientific^®, Chelmsford, MA, USA). Following this, the samples were stabilized by adding metaphosphoric acid in a 4:1 ratio and stored under refrigeration until further analysis.

To estimate the concentration of volatile fatty acids, each sample was centrifuged at 1957× g for 15 min, and 1.5 µL of supernatant was collected. The analysis was conducted using a gas chromatograph (Model Clarus 500, PerkinElmer^®, Shelton, CT, USA) equipped with an Elite FFAP column. One µl of the sample was injected at an injector temperature of 200 °C, a detector temperature of 250 °C, and an oven temperature of 140 °C. The run time per sample was 7 min [28].

To quantify ammonia nitrogen (NH₃-N), 20 µL of the sample was placed in test tubes, followed by the addition of 1 mL of phenol and 1 mL of sodium hypochlorite. The mixture was incubated for 30 min at 37 °C, then diluted with 5 mL of distilled water. A standard curve was created, along with a blank sample used as a reference. Readings were taken using a spectrophotometer (Model CARY 1E, Varian^®, Santa Clara, CA, USA) at a wavelength of 630 nm [29].

2.5. In Vivo Digestibility

To evaluate DM, organic matter (OM), CP, EE, NDF, and ADF in vivo digestibility, five male lambs from each treatment group were fitted with fecal collection bags. The animals underwent a three-day adaptation period to the bags, during which their feed intake was adjusted to 90% of ad libitum intake. For the following eight days, samples were collected. Feces were weighed daily, and at the end of the digestibility trial, approximately 10% of the total feces excreted were stored at −4 °C. DM, OM, CP, EE, NDF, and ADF were analyzed according to AOAC methods [23], with NDF and ADF measured using the technique described by Van Soest et al. [24]. Digestibility in vivo was calculated using the formula provided by Harris [30]:

$$\mathrm{i}\mathrm{n} \mathrm{v}\mathrm{i}\mathrm{v}\mathrm{o} \mathrm{d}\mathrm{i}\mathrm{g}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}={\displaystyle \frac{\mathrm{n}\mathrm{u}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{i}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{d}-\mathrm{n}\mathrm{u}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{e}\mathrm{x}\mathrm{c}\mathrm{r}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{d}}{\mathrm{n}\mathrm{u}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{i}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{d}}}\times 100$$

(1)

2.6. Carcass Yield

To assess carcass and meat characteristics, six animals (three females and three males) were slaughtered after a 12 h fast, in accordance with NOM-33-SAG/ZOO/2014 guidelines for the slaughter of domestic and wild animals. The weights of the blood, skin, head, feet, green viscera, red viscera, visceral fat, and hot carcass weight were recorded. At the same time, the weights of the full and empty viscera were noted, and the weight of the gastrointestinal contents was estimated by calculation. The empty body weight was derived from the live weight at slaughter. The carcasses were then stored in a cold room at 4 °C for 24 h, after which the cold carcass weight was recorded. Using the collected data, the hot carcass yield and cold carcass yield were calculated using the following formulas [31]:

$$\mathrm{H}\mathrm{o}\mathrm{t} \mathrm{c}\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}={\displaystyle \frac{\mathrm{H}\mathrm{o}\mathrm{t} \mathrm{c}\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}{\mathrm{L}\mathrm{i}\mathrm{v}\mathrm{e} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{a}\mathrm{t} \mathrm{s}\mathrm{l}\mathrm{a}\mathrm{u}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{e}\mathrm{r}}}\times 100$$

(2)

$$\mathrm{C}\mathrm{o}\mathrm{l}\mathrm{d} \mathrm{c}\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}={\displaystyle \frac{\mathrm{C}\mathrm{o}\mathrm{l}\mathrm{d} \mathrm{c}\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}{\mathrm{L}\mathrm{i}\mathrm{v}\mathrm{e} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{a}\mathrm{t} \mathrm{s}\mathrm{l}\mathrm{a}\mathrm{u}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{e}\mathrm{r}}}\times 100$$

(3)

2.7. pH, Colour, and Water-Holding Capacity of Meat

These determinations were performed on samples of the Longissimus dorsi muscle taken from carcasses stored at 4 °C for 24 h. A potentiometer (Model HI 99161, Meat HANNA^®, Waterproof Tester, Woonsocket, RI, USA) with a glass electrode was used to measure the pH, taking three measurements per sample.

The colour evaluation was conducted using a colorimeter (Model CR-400/410, Konica Minolta^®, Ramsey, NJ, USA) after the surface of the Longissimus dorsi muscle was oxygenated for 5 min. Readings were taken in triplicate from each sample, and the parameters recorded included lightness (CIE L*), redness (CIE a*), and yellowness (CIE b*). The Chroma values and Hue angle (°) were calculated using the following equations, according to AMSA [32]:

$$\mathrm{C}\mathrm{h}\mathrm{r}\mathrm{o}\mathrm{m}\mathrm{a}=\sqrt{({{\mathrm{a}}^{*}}^2+{{\mathrm{b}}^{*}}^2)}$$

(4)

$$\mathrm{H}\mathrm{u}\mathrm{e} \mathrm{a}\mathrm{n}\mathrm{g}\mathrm{l}\mathrm{e}\left(°\right)=\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{t}\mathrm{a}\mathrm{n}\left({\displaystyle \frac{{\mathrm{b}}^{*}}{{\mathrm{a}}^{*}}}\right)$$

(5)

To determine the water holding capacity, a sample of 0.3 to 0.5 g of Longissimus dorsi muscle was taken as the initial sample weight. This sample was placed between two previously dried filter papers (#40 Whatman^®, Dassel, Germany) and set between two glass plates. A constant weight of 5 kg was applied for 5 min. After this process, the sample was weighed again to find the final sample weight, allowing for the calculation of water holding capacity using the formula [33]:

$$\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{h}\mathrm{o}\mathrm{l}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{c}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}={\displaystyle \frac{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}-\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}}\times 100$$

(6)

2.8. Statistical Analysis

For data, an analysis of variance was performed using SAS software version 14.3 [34], under a generalized randomized block design, except for the in vivo digestibility variables where a completely random design was used. Each lamb was considered an experimental unit. The data were subjected to normality tests using the Shapiro-Wilk test (PROC UNIVARIATE). An analysis with PROC LOGISTIC was used for the marbling variable, a binary variable. The means were compared using Tukey’s test (p < 0.05). The data obtained for productive performance, carcass characteristics, and meat quality were analyzed using the PROC GLM. The general structure of the statistical model used was as follows:

$$y_{ij}= \mu +{\mathrm{\alpha }}_i+{\beta }_j+{\left(\mathrm{\alpha }\beta \right)}_{ij}+e_{ij}$$

(7)

where $y_{ij}$ = j-th observation under i-th treatment; µ = overall mean; ${\mathrm{\alpha }}_i$ = treatment effect; ${\beta }_j$ = sex effect; ${\left(\mathrm{\alpha }\beta \right)}_{ij}$ = interaction between treatment effect and sex effect; $e_{ij}$ = random error associated with each observation.

3. Results

3.1. Productive Performance

The addition of BRC to the diet did not significantly affect final body weight (FBW), DMI, ADG, or FCR (p > 0.05;

Table 2. Growth performance, loin eye area and backfat thickness of the Longissimus dorsi muscle of lambs fed different levels of bovine rumen content in the diet.

Items	Treatments					Sex			p-Value
	Control	BRC15	BRC30	BRC45	SEM	Female	Male	SEM	Treat	Sex	Treat × Sex
IBW (kg)	19.25	19.50	19.05	19.16	1.44	18.15	20.12	0.81	0.993	0.098	0.997
FBW (kg)	35.78	37.96	37.84	38.51	1.86	33.82 b	40.49 a	1.33	0.748	0.001	0.691
DMI (kg)	1.52	1.70	1.80	1.85	0.87	1.50 b	1.88 a	0.62	0.123	0.001	0.623
ADG (kg)	0.27	0.30	0.31	0.32	0.01	0.26 b	0.33 a	0.01	0.307	0.001	0.259
FCR	5.57	5.55	5.77	5.78	0.22	5.76	5.59	1.59	0.890	0.460	0.585
LEA (cm2)	17.39	17.48	18.67	18.27	0.69	17.63	18.21	0.49	0.509	0.406	0.940
BT (mm)	0.157	0.172	0.136	0.167	0.02	0.15	0.15	0.01	0.661	0.961	0.957

^a,b Different letters in the line indicate statistical difference (p < 0.05); Control: basal diet; BRC₁₅, BRC₃₀ and BRC₄₅: 15, 30 and 45% of bovine ruminal content in the diet (BRC), respectively; SEM: standard error of the mean; IBW: initial body weight; FBW: final body weight; DMI: dry matter intake; ADG: average daily gain; FCR: feed conversion ratio; LEA: loin eye area; BT: backfat thickness.

). However, sex did influence these variables, with males exhibiting higher values (p < 0.05). Differences in FBW, DMI, ADG, and FCR were not noted for the treatment-by-sex interaction (p > 0.05).

Table 2 presents the variables related to loin eye area and backfat thickness of the Longissimus dorsi muscle. No significant differences (p > 0.05) were found regarding the main effects of BRC inclusion or sex, nor was there any interaction between treatment and sex. However, the post hoc analysis indicated that the combination of BRC₁₅ in males and BRC₃₀ in males significantly outperformed the control treatment in females for average ADG and DMI, respectively.

3.2. Ruminal Fermentation and In Vivo Digestibility

The inclusion of BRC in the diet did not significantly affect pH, NH₃-N, or volatile fatty acid (VFA) concentrations, nor the acetic: propionic ratio, regardless of the sex of the lambs (p > 0.05). Furthermore, no interaction between treatment and sex was found. However, the post hoc analysis indicated that BRC₄₅ in females significantly outperformed both the control treatment, BRC₁₅ and BRC₃₀ in males regarding the production of propionic acid (

Table 3. Ruminal variables in lambs fed with different levels of bovine ruminal content in the diet.

Items	Treatments					p-Value
	Control	BRC15	BRC30	BRC45	SEM	Treat	Sex	Treat × Sex
Ruminal pH	6.06	6.27	6.29	6.14	0.14	0.641	0.716	0.841
NH3-N (mg/dL)	19.74	20.03	18.87	19.05	0.53	0.347	0.219	0.813
VFA (% molar)
Acetic acid (A)	65.32	64.04	67.17	64.98	1.91	0.404	0.242	0.830
Propionic acid (P)	35.78 c	37.96 b	37.84 b	38.51 a	1.86	0.066	0.013	0.178
Butyric acid	13.78	13.88	13.15	12.53	0.90	0.663	0.137	0.223
A:P ratio	1.83	1.69	1.78	1.69	1.3	0.962	0.169	0.236

^a,b,c Different letters in the line indicate statistical difference (p < 0.05); Control: basal diet; basal diet; BRC₁₅, BRC₃₀ and BRC₄₅: 15, 30 and 45% of bovine ruminal content (BRC) in the diet; SEM: standard error of the mean; NH₃-N: ammoniacal nitrogen; VFA: volatile fatty acid.

).

The in vivo digestibility of DM, or OM, CP, NDF, and ADF was not significantly affected (p > 0.05) by the inclusion of bovine rumen contents in the lamb’s diet (

Table 4. In vivo digestibility of nutrients in lambs fed different levels of bovine rumen content in the diet.

Items	Treatments
	Control	BRC15	BRC30	BRC45	SEM	p-Value
Digestibility (%)
Dry matter	76.01	75.32	74.40	74.72	1.22	0.800
Organic matter	73.35	77.38	74.25	79.78	2.67	0.334
Crude protein	66.05	71.82	68.57	66.71	3.14	0.577
Neutral detergent fiber	64.06	64.22	59.97	62.37	1.72	0.302
Acid detergent fiber	57.21	54.39	54.64	51.82	2.23	0.427

Control: basal diet; basal diet; BRC₁₅, BRC₃₀ and BRC₄₅: 15, 30 and 45% of bovine ruminal content (BRC) in the diet; SEM: standard error of the mean.

).

3.3. Carcass Yield and Physicochemical Characteristics of Meat

No significant differences (p > 0.05) were observed in empty carcass yield (whether hot or cold) due to the inclusion of BRC, sex, or the treatment*sex interaction. However, notable differences (p < 0.05) were observed between males and females in hot and cold carcass yields, with females exhibiting higher yields (see

Table 5. Carcass characteristics of lambs (female and male) fed different levels of bovine rumen content in the diet.

Items	Treatments					Sex			p-Value
	Control	BRC15	BRC30	BRC45	SEM	Female	Male	SEM	Treat	Sex	Treat × Sex
Carcass profile
EBW (kg)	33.55	35.55	35.18	35.59	1.28	33.51	36.08	1.33	0.69	0.05	0.18
HCY (%)	57.90	58.12	57.92	56.88	0.60	58.72 a	56.98 b	0.62	0.36	0.01	0.42
CCY (%)	57.45	57.55	57.39	56.45	0.63	57.45 a	56.52 b	0.01	0.45	0.01	0.55

^a,b Different letters in the line indicate statistical difference (p < 0.05); Control: basal diet; BRC₁₅, BRC₃₀ and BRC₄₅: 15, 30 and 45% of bovine ruminal content (BRC) in the diet, respectively; SEM: standard error of the mean; EBW: empty body weight; HCY: hot carcass yield; CCY: cold carcass yield.

).

Meat quality indicators, such as pH and colour (L*, a*, b*), as well as WHC, were similar across all treatments (p > 0.05) (

Table 6. Characteristics of the Longissimus dorsi muscle of lambs (female and male) fed with different levels of bovine ruminal content in the diet.

Items	Treatments					Sex			p-Value
	Control	BRC15	BRC30	BRC45	SEM	Female	Male	SEM	Treat	Sex	Treat × Sex
pH (24 h)	5.74	5.80	5.96	5.93	1.44	5.87	5.89	0.81	0.360	0.574	0.732
WHC (%)	37.07	34.57	34.83	33.38	1.86	33.42	32.12	1.33	0.538	0.370	0.477
L*	45.38	43.28	42.06	43.39	10.87	42.87	42.96	0.62	0.747	0.617	0.054
a*	12.82	13.66	13.50	12.25	0.01	13.69 a	11.37 b	0.01	0.230	0.001	0.728
b*	12.64	12.56	12.21	12.24	0.22	13.06 a	11.27 b	1.59	0.930	0.007	0.229
Chroma	18.00	18.56	18.20	17.32	0.69	18.92	16.01	0.49	0.509	0.406	0.940
Hue angle	44.59	42.60	42.13	44.98	1.02	43.65	44.75	0.01	0.661	0.961	0.957

^a,b Different letters in the line indicate statistical difference (p < 0.05); Control: basal diet; BRC₁₅, BRC₃₀ and BRC₄₅: 15, 30 and 45% of bovine ruminal content (BRC) in the diet, respectively; SEM: standard error of the mean; L*: lightness; a*: red/green intensity; b* yellow/blue intensity; WHC: water holding capacity.

). The sex of the animals did not significantly affect pH, L*, or HCW (p > 0.05). However, the a* and b* values differed (p < 0.05), with higher values observed in the meat from females.

4. Discussion

Al-Wazeer [35] reported a decrease in ADG and an increase in FCR when 30% BRC was included in the diets for Awassi lambs. Conversely, Osman et al. [36] found no differences in FBW, DMI, ADG, or FCR in lambs fed diets containing 0%, 5%, and 10% BRC. In contrast, Agolisi & Ansah [37] observed improvements in ADG and final body weight with the inclusion of 5%, 10%, and 15% BRC in diets for Djallonké sheep. Similar findings were reported by Olafadehan et al. [7], who indicated that a 40% inclusion of BRC in diets for Yankasa sheep resulted in optimal productive performance. In Black Bengal goats, Uddin et al. [38] reported decreases in DMI and FCR; however, FCR was similar to the control when 18% rumen content was included in the diet. Abbator et al. [39] provided BRC and wheat straw to goats fed a peanut-stalk-based diet, suggesting that the 50:50 and 75:25 ratios of BRC to wheat straw improved DMI, but did not affect FBW, ADG, or FCR. Teixeira et al. [40] indicated that dry matter intake decreases due to increased NDF in the diet. In this study, DMI was not affected by the treatments, possibly because the diets had similar NDF content. It is also possible that BRC provides nutrients, such as non-protein nitrogen and energy, that support rumen microorganisms and thereby meet their needs [41].

On the other hand, any decrease in DMI with a 60% inclusion of BRC can be attributed to reduced palatability, which, in turn, affects acceptability and consumption [7,42]. Regarding sex, males exhibited higher DMI, resulting in greater ADG and differences in FBW. The effect of sex is evident in the regulation of fat and muscle mass, which is influenced by sex steroid hormones; for example, testosterone affects fat composition and promotes increased body mass [43,44]. Therefore, the difference in final body weight was not necessarily caused by the experimental diets, which were isoproteic and isoenergetic.

The loin eye area serves as an indicator of muscle development [45] and reflects the proportion of muscle in the carcass [46]. Lambs with larger loin eye areas are generally more efficient and exhibit improved feedlot performance, resulting in greater muscle deposition [47]. Conversely, backfat thickness is indicative of the amount of adipose tissue within the carcass [48]. However, for sheep carcasses, there is currently no established standard for the minimum backfat thickness that signifies an ideal finishing condition [49].

In this experiment, the results may not be linked to the diets’ energy levels but rather to the developmental timeline, as the lambs may not have reached their maximum body growth potential [50], which might be influenced by factors such as breed, delivery type, season of delivery [51], and birth weight [52], among others.

Ruminants rely on a diverse microbiota, primarily composed of bacteria, archaea, fungi, and protozoa, residing in the rumen to ferment and convert feed into VFAs, proteins, and vitamins. These microorganisms must constantly adapt to changes in the composition, form, quantity, and frequency of dietary intake [53,54]. Specifically, ruminal cellulolytic bacteria thrive within an optimal pH range of 6 to 6.5 [55]. A pH lower than 6 results in diminished fibrolytic activity [56].

NH₃-N is generally formed from the degradation of dietary proteins and non-protein nitrogen sources (such as urea and amino acids) in the rumen. It serves as the primary nitrogen source for the synthesis of ruminal microbial proteins [57]. For effective growth and cellulolytic activity, NH₃-N concentrations should range from 15 to 30 mg/dL [58,59]. In this study, the presence of dead rumen microbes and digestive enzymes in BRC likely enhanced the required NH₃-N proportion for microbial cell synthesis without adversely affecting productive variables [37]. The measured NH₃-N values (18.87 and 20.03 mg/dL) fell within the optimal range for satisfactory microbial growth and improved rumen fermentation, aligning with findings by Cherdthong et al. [5], but differing from those reported by Seankamsorn et al. [41], which were 18.5–19.0 mg/dL and 15–16.3 mg/dL, respectively.

VFAs are the primary intermediate products of rumen fermentation, influenced by the characteristics of the substrate and the composition of rumen fluid, particularly its microbiota, which plays a crucial role in production rate [60]. Furthermore, they supply approximately 80% of the metabolizable energy required by the host [61,62]. In this study, incorporating up to 45% BRC into lamb diets did not alter rumen VFAs concentrations, possibly because the diets were isoenergetic and isoprotein. Similar findings were reported by Cherdthong et al. [5], who partially replaced soybean meal with BRC in Thai steers, and by Seankamsorn & Cherdthong [62], who concluded that supplementing Thai cattle with up to 150 g/d of BRC pellets did not alter VFAs concentrations.

The in vivo digestibility of the evaluated nutrients was not affected by the inclusion of BRC in the sheep’s diet, which contrasts with the findings by Olafadehan et al. [7], who observed an increase in vivo digestibility of DM when up to 60% BRC was included in the diets of Yankasa sheep. However, they noted a decrease in CP and OM digestibility, with no effect on NDF and ADF. Similarly, Khattab et al. [63] reported increased digestibility of DM, OM, and CP when 25% and 50% BRC were included in the diets of Rahmani sheep.

In the present study, incorporating up to 45% BRC in the diet did not affect digestibility compared to the control group. This result aligns with Al-Wazeer [35], who found no adverse effects when up to 30% BRC was included in the diets of Awassi sheep. The digestion of DM in the rumen is influenced by the availability of fermentable carbohydrates and by ruminal NH3-N, which is hydrolyzed from non-protein nitrogen and dietary protein by ruminal microbiota [37].

The digestibility of NDF and ADF in this study was consistent with the findings of Seankamsorn & Cherdthong [62], who included 80% rumen content in a supplement for cattle fed a rice straw-based diet. This consistency may be due to a lack of acceptance of cellulolytic bacteria when up to 45% BRC was added to the diet. In contrast, Sadeghi et al. [19] reported that the inclusion of 20% BRC in diets for Holstein calves increased the digestibility of DM and OM. Furthermore, Cherdthong et al. [64] found that including 7.4% and 11% rumen contents increased the population of specific cellulolytic bacteria, such as R. flavefaciens. Mondal et al. [21] demonstrated that including rumen contents in goat diets does not adversely affect nutrient digestibility and can be considered safe for consumption.

However, further studies are needed to investigate the potential increase in the diversity of cellulolytic bacteria with the addition of BRC to diets, which may be related to the amount of semi-digested material or to other unknown factors present in the BRC [64].

Carcass yield is directly influenced by body weight at slaughter and carcass weight [50]. It also depends on factors such as breed, the muscle-to-bone ratio, and the amount of non-carcass tissues [65]. According to Agolisi et al. [42], the lack of differences in carcass characteristics among Djallonke sheep indicates that including up to 12% BRC in their diet did not affect the utilization of dietary nutrients for the growth of various tissues. Similarly, Abouheif et al. [66] reported that a 50% inclusion of BRC in a sheep’s diet did not impact hot carcass weight. Generally, carcass yield ranges from 40% to 52% [67] and can even reach 56% to 58% for fast-growing lambs [68]. The results of this study align with these findings, suggesting that incorporating 45% BRC into the diet of sheep does not adversely affect carcass yield. However, sex did influence carcass yield, which may be explained by the effects of estrogens. While estrogens have a limited impact on muscle and bone development, they enhance fat deposition at an earlier growth stage. This results in females maturing and gaining fat more quickly than males [69].

The postmortem pH at 24 h in the Longissimus dorsi ranged from 5.80 to 5.96 when the diet included BRC, and from 5.87 to 5.89 for females and males, respectively. A pH between 5.5 and 5.6 at 24 h postmortem is associated with lighter meat color. It indicates proper slaughter conditions [70], which are directly related to muscle glycogen levels at the time of slaughter [71]. In contrast, a pH greater than 5.6 is associated with low muscle glycogen reserves at slaughter [72]. Therefore, a high final pH may reflect inadequate handling and stress experienced by the animals prior to slaughter [73]. This finding could explain the results of this study, which were not attributed to BRC inclusion, as the control group also exhibited a pH greater than 5.6. Excessive decreases in postmortem pH can lead to reduced WHC, as pH affects the electrostatic repulsion between filaments, ultimately causing myofibril contraction [71]. These results may account for the observed homogeneity of WHC across the different treatments, as pH was not influenced by the inclusion of BRC.

Meat colour plays a significant role in its appearance, quality, and consumer purchase decisions, influenced by the intensity or alteration of its colour [74,75]. This coloration is influenced by myoglobin concentration and physicochemical state [76], as well as by the pH measured 24 h postmortem [77]. According to Khliji et al. [78], when the values of L* and a* are equal to or greater than 34 and 9.5, respectively, consumers generally consider the meat colour acceptable. Overall, no significant effect of sex on colour was observed when animals were slaughtered at the same age and with the same fattening duration [79,80]. However, females tend to show higher a* values due to increased fat deposition during early growth [69], which aligns with this study’s findings.

5. Conclusions

The results indicate that diets did not significantly affect the evaluated variables, whereas sex significantly influenced final body weight, average daily gain, dry matter intake, and both hot and cold carcass yield. Although some treatment and sex combinations had a significant impact on average dry matter intake and propionic acid, there is no consistent evidence that bovine rumen content, either alone or in combination with sex, improves growth performance or carcass quality in fattening lambs. Future research should focus on optimizing inclusion levels and processing methods for rumen content, as well as standardizing its composition to reduce variability. In addition, studies should evaluate its effects across different production stages and diet formulations, and in combination with feed additives. Investigating rumen microbiota, nutrient digestibility, and animal health parameters may help identify benefits beyond growth performance. Finally, assessing economic feasibility, safety, and environmental impact will be essential to determine its practical applicability in lamb production systems.