Olive leaves as alternative feed for finishing lambs: evaluation of feed intake, nutrients digestibility, growth performance, and carcase quality

Abstract

This research aims to evaluate how incorporating sun-dried olive leaves (OL) in the diets of Awassi lambs affects production costs, intake and nutrients digestibility, growth performance, and carcase quality. Thirty-six newly weaned male lambs were randomly allocated to one of three diet groups as dry matter (DM) in diets: 1) the control diet (CON), 2) 50 OL g/kg (OL50), and 3) 100 OL g/kg (OL100). On the 50th day, four lambs were randomly selected from each treatment and transported to metabolic cages to evaluate digestibility and N balance. The DM intake was higher (p = .003) in the OL50 and OL100 diets than those on the CON diet. At the same time, the ether extract and metabolisable energy intakes were the highest (p < .0001) in OL100, followed by OL50 and the CON diet. Intake of neutral detergent fibre and acid detergent fibre for lambs fed the OL50 diet surpassed (p < .0001) the other diets. The growth performance parameters were comparable among diets. However, the cost of gain was lower (p = .03) in diets containing OL. The OL incorporation had no treatment effect (p > .05) on the digestibility, N retention, carcase characteristics and meat quality. In conclusion, while the addition of OL at 50 g/kg and 100 g/kg did not affect the growth performance and characteristics of carcase, it did decrease the cost of production, making it a good candidate to replace part of the wheat straw in diets of growing lambs.

Highlights

• Olive leaves percentage substitution of wheat straw improved the nutrients intake in Awassi lambs fed commercial diets.

• Olive leaves incorporation decreased both the cost of the diet and the cost per kg of gain in Awassi lambs.

• Olive leaves inclusion did not have a negative effect on digestibility and N Balance, carcase characteristics and meat quality.

Keywords:

• Cost of diets
• growth parameters
• meat quality
• olive leaves

Introduction

According to the most recent United Nation population estimate, there are 7.3 billion human beings on earth, and this estimate may grow to 9.7 billion by 2050. This in-crease global animal products demands, mainly protein and meat (Harvard Business Review 2016). In the Mediterranean, especially in Jordan, most of these demands are fulfilled using domestic sheep (Ovis aries), which are extensively dispersed, are small ruminant animals raised for wool, meat, milk, and skin. Awassi sheep are the largest breed of sheep in the Mediterranean and the only indigenous breed in Jordan (Haddad and Ata 2009). Besides having a fat tail, this breed is well-suited to semi-arid environments (Jaber et al. 2004). However, the availability and cost of feedstuff in dry and semi-arid regions (such as Jordan) are critical obstacles facing farmers and sheep breeders. In the past, grasslands were the main source of nutrients for sheep. However, because of the absence of precipitation, pastures were adversely affected and currently do not meet animals’ dietary needs. Therefore, a large portion of animal feed is sourced externally, which raises the cost of the diet. To resolve these is-sues, farmers have started to utilise alternative feeds as a component of the ruminants’ rations (Obeidat 2022). Considering the growing costs of feed, animal nutritionists have begun to focus on improving the use of available feed sources by livestock (Adjolohoun et al. 2008).

Previous research has suggested examining alternative feeds will hopefully diminish the cost of production and eventually enhance profits (Obeidat 2017). Hence, any decrease in feed costs without altering the animal’s wellbeing is valuable. Consequently, this research focussed on utilising olive leaves (OL) to improve the efficiency of raising lambs for protein utilisation, it also thoroughly involved the discarding process of plant or industrial debris that are not suitable for human consumption. This practice has a positive impact on the cost of feed and at the same time aids in disposing of plant or industrial residues, which in turn will reduce the potential pollution if remained without using. Also, it will sustain the sheep industry by providing cheap feedstuff. Olive fruit is typically passes through a multi-stage process that produces 200 g/kg oil and 800 g/kg DM basis of by-product divided into, semi-solid and aqueous liquor, which is called olive cake (Niaounakis and Halvadakis 2006). There is a lack of valuable applications for OL that are olive harvests or olive oil manufacturing. Hukerdi et al. (2020) recently observed that completely replacing alfalfa hay with OL (0, 75 or 150 g/kg of dietary dry matter (DM)) had no effect on DM intake, average daily gain (ADG), or final body weight. Similarly, Mattioli et al. (2020) found that growing rabbits fed OL at 100 g/kg of dietary DM in place of bran and part of alfalfa hay had similar live body weight, ADG, and feed intake. The hypothesis of this study is that the utilisation of sun-dried OL will reduce the production cost without adverse consequences on growth performance and carcase characteristics. The objective of the study was to evaluate the effect of feeding growing lambs’ diets containing OL as a partial replacement of wheat straw.

Material and methods

Animals and housing

Thirty-six Awassi lambs with body weights of 16.5 ± 0.9 kg were acquired from a local rancher and transferred into the Jordan University of Science and Technology’s (JUST) animal enclosure facility at Irbid, Jordan. The animals were weighed, ear-tagged, checked for health issues and treated for gastrointestinal parasites. During the study, the lambs were distributed to 12 open-sided pens (4 m × 4 m), each treatment diet was assigned to a group of four pens and each pen contained three lambs. At 8:00am daily, diets were served ad libitum while providing water at all times. The first ten days of the 70 d study were used for adaptation, while the remaining 60 d were used for data collection. The Institutional Animal Care and Use Committee at JUST permit-ted the methodologies used in this study.

Ingredients, diets, and experimental design

Throughout the trial, diets were blended every two weeks or when needed, and samples were taken for quality control immediately after mixing to ensure their chemical composition remained consistent. Olive leaves, an alternative feed that was obtained from the olive oil press, were sun-dried for 3–4 d to make it dry enough to be grinded (i.e. to reach 90% dry matter) and then incorporated as part of a balanced diet. The lambs were randomly categorised into three dietary treatments using a complete randomised design: (1) the control diet (CON), (2) 50 g/kg OL (OL50), and (3) 100 g/kg OL (OL100) of dietary DM in partial replacement of wheat straw as shown in Table 1. The diets were designed to satisfy the nutritional needs of growing lambs set by the National Research Council (2007). In order to keep the diets isonitrogenous, slight changes in the level of barley grain and soybean meal were applied as shown in Table 1. When the study was conducted, the cost of each diet was computed based on the price of the diet’s constituents in the local market. Other costs, such as labour, electricity, and water, were factored into the price of each diet. Most of the differences in the cost came from the replacement of wheat straw because only 1 and 2% of SBM is replaced in diets OL50 and OL100, respectively.

Table 1. Ingredients and chemical composition of diets fed to Awassi lambs.

Item	Diet^a
	CON	OL50	OL100
Ingredients (g/kg DM)
Barley	540	548	560
Soybean meal	190	182	10
Wheat straw	250	200	150
Olive leaves^b	0	50	100
Salt	10	10	10
Limestone	09	09	09
Vitamin–mineral premix^c	01	01	01
Cost (US$/1000 kg)^d	405	381	359
Nutrients
Dry matter, g/kg	918	914	906
Crude protein, g/kg DM	160	161	160
Neutral detergent fibre, g/kg DM	312	293	275
Acid detergent fibre, g/kg DM	133	125	117
Ether extract, g/kg DM	10	13	16
Metabolisable energy, Mcal/kg	2.3500	2.4200	2.4800

^aDiets were (1) no olive leaves (CON), (2) 50 g/kg olive leaves (OL50), and (3) 100 g/kg olive leaves (OL100).

^bContained: 887, 88, 294, 219, 61 dry matter, crude protein, neutral detergent fibre, acid detergent fibre and ether extract, respectively.

^cComposition per kg contained: vitamin A, 600,000 IU; vitamin D3, 200,000 IU; vitamin E, 75 mg; vitamin K3, 200 mg; vitamin B1, 100 mg; vitamin B5, 500 mg; lysine, 0.5%; DL-methionine, 0.15%; manganese oxide, 4000 mg; ferrous sulphate, 15,000 mg; zinc oxide, 7000 mg; magnesium oxide, 4000 mg; potassium iodide, 80 mg; sodium selenite, 150 mg; copper sulphate, 100 mg; cobalt phosphate, 50 mg; dicalcium phosphate, 10,000 mg.

^dCalculated based on ingredients prices during 2022.

DM: Dry matter.

Throughout the research, each group’s leftover feed was collected and weighed daily, then kept in nylon bags at a temperature of −20 °C while waiting for the DM and other nutrients to be examined with the aim of determining the daily nutritional in-take. The animals were weighed at the start of the trial and once every two weeks thereafter until the completion of the study. The lambs were weighed just before morning feeding time to decrease weighing variation and estimate the feed conversion ratio (FCR) and the average daily gain (ADG; FCR = DMI ÷ weight gain).

Nutrient digestibility and nitrogen (N) balance trial

On the 50th day of the experiment, 12 lambs (four from each treatment) were randomly chosen and enclosed in individual-sized metabolic cages (1.05 m × 0.80 m) to assess nutritional digestibility and N balance. After 5 d of acclimating to the metabolic cages, the animals were given a 5 d data-collecting period. Daily faeces were collected, weighed, and documented, with 10% of the manure being conserved for further analysis. Urine was collected, weighed, and kept in plastic containers, with 5% preserved at −20 °C to assess N balance (N intake, N lost in faeces and urine, retained N (g/d), and N retention (%)). In an effort to avoid ammonia losses, each urine sample container included 50 mL of 6 N HCL. To obtain a uniform weight, composited diets (Table 1) and refuse samples were left to dry in a forced-air oven at 55 °C, then filtered through a 1 mm sieve after being air equilibrated using Brabender OHG equipment (Kulturstrase, Duisburg, Germany), then stored for future examination. The AOAC (1990) guide-lines for DM were used to assess the diets and refuse (24 h at 100 °C in an air-forced oven). Crude protein (CP) was obtained using the Kjeldahl procedure. Ether Extract (EE) was determined using the Soxtec method (SXTEC SYSTEM HT 1043 Extraction Unit, TECATOR, Box 70, Hoganas, Sweden). In addition, neutral detergent fibre (NDF) and acid detergent fibre (ADF) parameters were analysed in the samples using the ANKOM2000 fibre analyser device and the technique published by Van Soest et al. (1991) (ANKOM Technology Cooperation, Fairport, NY, USA). The NDF and ADF tests were performed with a heat-stable alpha-amylase and sodium sulphite. The residual ash con-tent was used to represent the results. Metabolisable energy (Mcal/kg) was calculated based on tubular values of the National Research Council (2007).

Slaughtering procedure and meat quality assessment

All lambs were butchered at JUST's Centre for Training and Research Unit facilities at the end of the experiment to assess carcase characteristics. Obeidat et al. (2016) detailed all of the procedures utilised in this study. Trained personnel slaughtered the lambs at 9:00am, roughly 18 h after the final feeding. Right before slaughter, the lambs’ fasted live weights were measured, and the hot carcase weights were recorded shortly thereafter. After cooling the carcases for 24 h at 4 °C, their weights were measured. The dressing percentage was estimated by dividing the cold carcase weight by the live fasting weight times 100%. Non-edible components of carcase were re-moved, weighed, and recorded after slaughter.

According to Obeidat et al. (2016) the linear dimensions of leg fat depth (L3), tissue depth (GR), rib fat depth (J), eye muscle width (A), eye muscle depth (B), and fat depth (C) were measured on the cooled carcases and the longissimus dorsi muscles one day after slaughter. Afterwards, the carcases were split into four portions (loin, rack, leg, and shoulder cuts). The longissimus dorsi muscle was taken from the loin cut and kept vacuum-packed inside a −20 °C ultra-low lab freezer for two weeks until it was time to evaluate meat quality.

Water holding capacity (WHC), cooking loss (CL), shear force, pH, and colour coordinate (CIE L*a*b*) parameters were all measured as discussed by Obeidat et al. (2016). While remaining sealed in plastic bags, the frozen longissimus dorsi muscles were de-frosted overnight in a refrigerator at 4 °C. Muscles were sliced into slices of varying thickness, and the meat quality of each piece was assessed in a different test. Colour was observed in 15-mm-thick slices with a colorimeter (12MM Aperture U 59730-30, Cole-Parameter International, Accuracy Microsensors Inc., Pittsford, NY, USA). All of the slices were placed on a polystyrene tray, covered with a porous film, and oxygenated for two h at 4 °C. Using 25-mm-thick slices of meat, CL was evaluated. The slices were weighed pre-cooking, placed in plastic bags, cooked in a water bath at 75 °C for 90 min using the sous vide cooking method (Baldwin 2012), and re-weighed after cooking. Then, CL was calculated as follow: ((initial weight − final weight)/initial weight)/100%. The cooked slices were chilled at 4 °C overnight before being sliced into six smaller samples (cores) of 1 cm³ each to obtain shear force values. Additionally, to determine the maximum force (kg) required to shear cooked meat cores, on a Salter Model 235, researchers used a Warner-Bratzler (WB) shear blade with a triangular slot sharp end (Warner-Bratzler meat shear, GR Manufacturing Co., Manhattan, Kansas, USA). Grau and Hamm’s (1953) method was used to determine the WHC. Five grams of raw meat were cut into small bits and placed between two filter papers and two quartz plates, then crushed with a 2,500 g weight for 5 min, and finally removed and weighed. The WHC percentage formula = ((initial weight − final weight)/initial weight) × 100%.

Statistical methods

The MIXED procedure in SAS was used to analyse the data (Version 8.1, 2000, SAS Inst. Inc., Cary, NC, USA). Treatment was the only effect included in the fixed effects for all data as shown below. The least-square means were separated using appropriate pair-wise t-tests (i.e. LSD) when the fixed effects were significant (p ≤ .05). Since the lambs were housed in the groups, the data of nutrient intake, feed conversion ratio and cost of gain were analysed using the pen as the experimental unit. However, the rest of the data were analysed using the lambs as the experimental unit. For carcase cuts weight and meat quality traits, cold carcase weight was incorporated as a covariate. $$\text{Yijk}=\mathrm{\mu }+\text{Bi}+b\text{CCW}+\mathrm{\epsilon }\text{ij}$$ where: Yij the dependent variable µ is the overall mean Bi is the treatment effect εij is the random error bCCW linear covariate of cold carcase weight

Results

The cost of the diets was listed in Table 1 as $US/ton. Diet costs were reduced by 6 and 11% for the OL50 and OL100 diets, respectively, when compared to the CON diet. Therefore, when the OL were incorporated into the diet, the total cost of the diet de-creased. Moreover, the diets remained isonitrogenous. Most of the differences in the cost came from the replacement of wheat straw because only 1 and 2% of SBM is replaced in diets OL50 and OL100, respectively.

The effects on nutrient intake and growth performance are reported in Table 2. The DM intake was greater (p = .003) in lambs fed the OL50 and OL100 diets compared with the CON diet. However, lambs fed the OL50 diet had the most significant CP intake (p = .002). At the same time, EE and metabolisable energy (ME) intakes were the highest (p < .0001) in OL100, followed by OL50 and CON. On the other hand, the intake of NDF and ADF for lambs fed the CON diet was greater (p < .0001) than the other diets. The three diet categories did not differ significantly (p > .05) in terms of the growth performance parameters, initial BW, final BW, total gain, ADG, and FCR. Economically speaking, as shown in Table 2, there was a decrease (p = .03) in the cost per kilogram of gain for the OL50 and OL100 diets than the CON diet.

Table 2. Effects of olive leaves (OL) feed on nutrient intakes and growth performance of Awassi lambs.

Item	Diet^a
	CON	OL50	OL100	SE^b	p value
Nutrient intake, g/d
Dry matter	957b	976a	985a	2.2400	.0030
Crude protein	153c	158a	156b	0.4140	.0020
Neutral detergent fibre	299a	288b	268c	0.6610	<.0001
Acid detergent fibre	127a	123b	114c	0.2820	<.0001
Ether extract	9.7000c	12.900b	15.8000a	0.0290	<.0001
Metabolisable energy, Mcal/d	2.2500c	2.3800b	2.4200a	0.0054	<.0001
Initial weight, kg	17.8000	17.7000	17.3000	0.9400	.9300
Final weight, kg	31.0000	31.8000	30.4000	1.2000	.7000
Total gain, kg	13.2000	14.1000	13.1000	0.6800	.4800
Average daily gain, g/d	220	235	218	11.4000	.4800
Feed conversion ratio	4.3600	4.1900	4.5200	0.1870	.5200
Cost ($US/kg gain)	1.7700a	1.5900b	1.6200b	0.0310	.0300

a, b, c within a row, means without a common letter differ (p ≤ .05).

^aDiets were (1) no olive leaves (CON), (2) 50 g/kg olive leaves (OL50), and (3) 100 g/kg olive leaves (OL100).

^bSE: standard error.

Results of the in vivo digestibility and N balance trial are recorded in Table 3. The digestibility coefficients and N balances of the three diets showed no significant treatment effect (p > .05).

Table 3. Effects of olive leaves (OL) feed on nutrient in vivo digestibility and N balance of Awassi lambs.

Item	Diets^a
	CON (n = 4)	OL50 (n = 4)	OL100 (n = 4)	SE^b	p value
Digestibility coefficients
Dry matter	77.000	75.700	77.100	2.4300	.900
Crude protein	76.000	75.000	78.200	3.2700	.780
Neutral detergent fibre	61.500	59.200	58.700	4.7800	.910
Acid detergent fibre	51.800	47.300	46.000	6.1800	.800
Ether extract	66.600	66.500	72.000	9.2900	.880
N balance
N intake, g/d	23.100	25.900	26.600	0.8700	.090
N in faeces, g/d	5.550	6.330	5.830	0.759	.630
N in urine, g/d	4.530	3.880	4.840	0.419	.310
N retained, g/d	13.020	15.670	15.970	1.334	.330
Retention, %	56.200	60.300	59.900	3.930	.740

^aDiets were (1) no olive leaves (CON), (2) 50 g/kg olive leaves (OL50), and (3) 100 g/kg olive leaves (OL100).

^bSE: standard error.

When the lambs were fed the three diet treatments, there was no treatment effect in the carcase and non-carcase components, carcase cut weights, or loin cut tissues percentages (p = .28), as indicated in Table 4.

Table 4. Effects of olive leaves (OL) feed on the carcase, non-carcase components, carcase cut weights, and loin cut tissues percentages of Awassi lambs.

Item	Diets^a
	CON n = 12	OL50 n = 12	OL100 n = 12	SEb	p value
Fasting live weight (kg)	28.200	30.400	29.200	1.260	.470
Hot carcase weight (kg)	13.600	14.300	14.200	0.640	.710
Cold carcase weight (kg)	12.400	13.400	13.300	0.620	.460
Dressing percentage	43.600	44.000	45.300	0.810	.330
Non-carcase components (kg)^c	1.200	1.300	1.200	0.052	.280
Carcase cut weights (kg)^d	11.340	12.020	11.350	0.520	.570
Loin weight, g	299	351	311	27.800	.400
Intermuscular fat (g)	6.800	8.500	7.300	1.240	.610
Subcutaneous fat (g)	24.600	26.000	24.600	4.240	.960
Total fat (%)	10.100	9.200	10.100	0.840	.690
Total lean (%)	50.400	52.000	53.000	2.120	.520
Total bone (%)	30.100	28.500	30.200	2.280	.800
Meat to bone ratio	2.670	1.910	1.830	0.582	.510
Meat to fat ratio	5.470	6.060	5.750	0.529	.730

^aDiets were (1) no olive leaves (CON), (2) 50 g/kg olive leaves (OL50), and (3) 100 g/kg olive leaves (OL100).

^bSE: standard error.

^cNon-carcase components (Heart, liver, spleen, kidney, lungs, and trachea).

^dCarcass cut (shoulder, racks, loins, and legs).

As depicted in Table 5, no treatment effect (p = .20) was detected in the linear dimensions of the longissimus dorsi muscle across the diet groups.

Table 5. Effects of olive leaves (OL) feed on linear dimensions of the longissimus dorsi muscle in Awassi lambs.

Item	Diets^a
	CON (n = 12)	OL50 (n = 12)	OL100 (n = 12)	SE^b	p value
Leg fat depth (L3) (mm)	1.630	1.920	1.810	0.286	.780
Tissue depth (GR) (mm)	8.030	8.370	7.630	0.504	.550
Rib fat depth (J) (mm)	1.690	1.910	1.540	0.215	.450
Eye muscle width (A) (mm)	48.890	49.980	48.420	1.198	.640
Eye muscle depth (B) (mm)	20.060	21.780	20.020	0.755	.200
Fat depth (C) (mm)	1.110	1.090	1.080	0.075	.970
Shoulder fat depth (S2) (mm)	1.370	1.280	1.250	0.149	.780

^aDiets were (1) no olive leaves (CON), (2) 50 g/kg olive leaves (OL50), and (3) 100 g/kg olive leaves (OL100).

^bSE: standard error.

In terms of the pH, CL, WHC, shear force, and colour coordinate properties of the longissimus dorsi muscle in the Awassi lambs, no treatment difference was found (p = .08) as shown in Table 6.

Table 6. Effects of olive leaves (OL) feed on the physicochemical properties of the longissimus dorsi muscle in Awassi lambs.

Item	Dietsa
	CON (n = 12)	OL50 (n = 12)	OL100 (n = 12)	SE^b	p value
pH^c	5.800	5.810	5.790	0.006	.080
Cooking loss (%)	40.450	39.640	39.570	0.529	.440
Water holding capacity (%)	20.380	23.730	21.850	0.994	.080
Shear force (kg/cm^2)	5.580	6.020	4.740	0.439	.130
Colour coordinates
L* (whiteness)	28.200	28.110	28.800	0.433	.490
a* (redness)	1.780	1.870	1.820	0.066	.630
b* (yellowness)	19.790	19.920	20.030	0.205	.670

^aDiets were (1) no olive leaves (CON), (2) 50 g/kg olive leaves (OL50), and (3) 100 g/kg olive leaves (OL100).

^bSE: standard error.

^cpH measured after thawing.

Discussion

There is a positive impact between the incorporation rate of OL and feed cost in Awassi lamb diets, as shown in our study (Table 1). This result agrees with the previous scientific literature that focussed on alternative feeds as a way to reduce the cost of conventional animal diets (Hatamleh and Obeidat 2019; Aljamal et al. 2021). These results might be attributable to the fact that alternative feeds are less expensive than conventional feeds (Atsbeha et al. 2020). Furthermore, due to the incorporation of OL as an alternative feed, assuming that it has no negative impact on the animals’ health and performance as we observed under the conditions of the present study, the diet’s cost decreased, thus positively affecting production costs. Therefore, the use of such by-product will help the livestock producer to improve their profitability.

In this study, the DM, EE, and ME intakes were higher in diets incorporated with the OL (OL50 and OL100) (Table 2). However, the NDF intake decreased in the OL50 and OL100 diets. This is mainly because OL are rich in EE and low in tannins (i.e. it was in the level that does not impact the nutrients intakes), as mentioned by Ruiz et al. (2004). Likewise, the ME intake increased in lambs fed the OL50 and OL100 diets, which agrees with Mateos and Sell (1980). The ME increases together with the EE in the OL-containing diets that reflected the increment in their ME and EE intake compared with the CON diet (Table 2). Also, the NDF and ADF intake values were lower in OL50 and OL100 diets. This phenomenon could be attributed to the fact the NDF and ADF content in the OL is lower compared with the wheat straw. Overall, the intake results indicated that the inclusion of the OL at 50 and 100 g/kg of dietary DM did not negatively impact the nutrient intake and c is considered palatable.

In general, alternative feedstuffs are lower in cost in comparison with conventional feeds (Aloueedat et al. 2019). So, the percentage decrease in cost per kg of gain was attributed to the drop of the total diet cost due to the incorporation of OL. This reduction was similar to what Omar (2002) found when they fed Awassi lambs sesame oil cake as an alternative feed. Similarly, when Obeidat et al. (2012) partly replaced barley grains with carob pods, the cost of production was reduced. In general, the use of such product reduced the cost of the diets (Table 1) and the cost of the gain in the lambs fed the OL diets, which is turn will improve the beneficial use of alternative feeds such as OL.

In agreement with our study (Table 3), there were no differences in the digestibility and N balance parameters when Awassi lambs were fed diets containing sun-dried olive cakes and acid-treated sun-dried olive cakes (Awawdeh and Obeidat 2013). The OL's in vivo apparent digestibility is affected by age of the olive tree, OL is more digestible when it came from a young tree compared with older ones (Molina-Alcaide and Yáñez-Ruiz 2008). Delgado-Pertı́ñez et al. (2000) observed that when OL were dried, DM and CP digestibility decreased. Fegeros et al. (1995) found that when fresh OL are treated with a solution of ammonia, the apparent digestibility of the EE, CP, and NDF increases. The total condensed tannins are relatively higher in OL (9.49 mg/g DM) than in wheat straw (0.04 g/mg DM) (Silanikove et al. 1996; Delgado-Pertı́ñez et al. 2000). The digestibility and N balance were unaffected by the slight increase in condensed tannins (Delgado-Pertı́ñez et al. 2000). The current study herein approved that the use of OL in the diets of growing lambs did not impact the nutrient digestibility and N balance.

Regarding growth performance, previous studies agree with our study when raising lambs supplemented with OL (Mioc et al. 2007; Hukerdi et al. 2019). The fact that the lambs grew at similar rates, resulting in matching carcase weights and characteristics after the trial, is reflected in the uniformity in DM throughout the three diets with isonitrogenous and isocaloric properties. Moreover, lamb diets can incorporate olive cakes at 150 g/kg of maize and still have no effect on ADG, carcase weight, and dressing percentage (Silanikove et al. 1996). Even when Christodoulou et al. (2008) used an olive by-product (fermented olive waste), which is higher in NDF and ADF values than the OL that was used in this study, it showed no adverse effects on intake, ADG, final BW, FCR, carcase weights, or components for lambs.

As a recent study depicted by Obeidat et al. (2021), similar dimensions of the longissimus dorsi were found when Awassi lambs were fed diets containing different alternative feeds (e.g. sweet lupin). Additionally, the findings of Taheri et al. (2013) regarding the longissimus dorsi muscle measurements agree with our findings, as they did not find any differences in the length, depth, and back-fat depth of the longissimus dorsi muscle in lambs when they were fed diets containing olive-pulp silage.

The CL and tenderness of the longissimus dorsi muscle were not found to be different when lambs were fed diets containing olive cake (Ozdogan et al. 2017) which agreed with our results. Nevertheless, the amount of olive cake enhanced the meat’s water-holding capacity, which contrasts with our findings. This might be due to a variety of factors, such as slaughtering time, carcase weight, and meat ageing time, all of which can affect the muscle’s ability to retain water Abdullah and Qudsieh 2009). Similar to our results, Ozdogan et al. (2017) reported that colour parameters of the longissimus dorsi were not different when lambs were fed olive cakes. Moreover, Obeidat et al. (2021) found that when fed sweet lupin, the colour characteristics of the longissimus dorsi muscle in Awassi lambs were comparable to what we found in this study. This is most likely due to similarities in the pH of the longissimus dorsi muscle. The similarity in the growth variables had a similar reflection on the characteristics of the carcases and the quality of the meat, and this in itself is a positive result when alternative feeds are used in feeding the animals. Therefore, it is possible to recommend that this feed be used to feed growing animals.

Conclusion

The current study showed that olive leaves can be used to produce lambs without generating any health issues or impacting growth performance and carcase characteristics and meat quality under the conditions of the present study. As a result, it appears that incorporating olive leaves in the diets of Awassi lambs is beneficial to livestock farmers. More research studies are needed to study the effect of feeding OL at higher levels.

Ethical approval

All procedures were performed after obtaining the approval of the Institutional Animal Care and Use Committee of Jordan University of Science and Technology.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The original data of the paper are available upon request from the corresponding author.

Additional information

Funding

The authors thank the Deanship of Scientific Research at Jordan University of Science and Technology for funding this study [168/2021].