Circular Economy on a Small Scale: The Sustainable Use of Olive Tree Biomass Residues as Feed for Lactating Cows in the Sorrento Peninsula

Abstract

To enhance the sustainability of marginal olive and dairy farms in the Sorrento peninsula, two separate crossover trials were conducted on two farms in the area to evaluate olive pruning residue (OlPr) and olive mill leaves (OlLes) as forage sources for lactating cows. Each trial lasted six weeks and consisted of two treatment periods, each including a 15-day adaptation phase followed by a 6-day measurement phase. During the measurement phase, milk production, feed intake, and olive residue consumption were assessed for two homogeneous cow groups: one receiving a ration supplemented with olive by-products and the other receiving a control diet. The olive-supplemented groups exhibited higher dry matter intake and roughage consumption (hay + olive residue) compared to the control groups. The intake of OlLes was about 30% higher than that of OlPr. Compared to the respective control, milk from OlLe-fed cows a had higher fat content and a higher fat-to-protein ratio, a more favorable fatty acid composition in terms of higher monounsaturated and polyunsaturated fatty acids and conjugated linoleic acid contents, a reduced atherogenic index, and a saturated-to-unsaturated ratio. Likely due to the lower level of olive by-product ingestion, only marginal differences were observed in milk fatty acid composition of cows fed OlPr compared to the control. We conclude that the use of OlLes in dairy cow diets may represent a promising strategy for improving milk quality, promoting a more circular agricultural system, reducing reliance on external feed inputs, and mitigating the environmental impact of both olive and milk production.

1. Introduction

Current global challenges, such as international trade competition, climate change and natural resource depletion, require a shift from a linear approach to production, i.e., produce—consume—dispose of, to a circular model that closes material loops through the recovery and reuse of resources [1]. In the agro-industry sector, the recovery and valorization of production residues and by-products is the key factor for progress towards circular economy models and sustainable production [2].

The cultivation of olive (Olea europaea L.) is an important component of the rural economy of Mediterranean countries and plays a significant role in the preservation of culture, environment, and landscape [3]. To date, at least 95% of the world’s olive groves are in the Mediterranean area, which accounts for about 70% of the world’s olive oil production [4]. Italy accounts for about 15% of the total European oil production [4], with about one million hectares of olive groves, 80% of which are located in the southern regions [5]. However, both the agricultural and industrial phases of production of olive oil generates relevant amounts of by-products, including tree pruning waste and leaves, which are often underutilized or improperly disposed of, leading to environmental challenges [6]. Pruning is a vital practice in olive tree cultivation ensuring balanced production, facilitating harvesting, and maintaining tree health. It is usually carried out in winter but may also occur in summer to remove shoots and suckers or to adapt to water availability [7]. Pruning biomass typically includes a large proportion (50%) of branches less than 1 cm thick, of which leaves account for approximately 25% of the dry weight, depending on variations in geography, horticultural practices, and tree lifetime [8]. Pruning generates approximately 6.23 kg of biomass for 1 L of oil, resulting in an estimated 11.8 million tons of biomass on a European scale, which is commonly burned in the field, making pruning alone responsible for over 23% of the total CO₂ emissions associated with olive oil production [9,10]. In addition, biomass residues from olive trees are also produced during mechanical harvesting as leaves, twigs, and other debris are collected with the fruit and must be removed and disposed of prior to olive pressing [10]. As a result, olive leaves represent a relatively large proportion of the milling by-products, accounting for 4–10% of the total weight of processed olives and almost 5% of the total olive oil by-product yield [8]. According to Manzanares et al. [11], in Spain alone, approximately 750,000 tons of olive leaves per year are generated and need to be disposed of. Given the huge volume of olive tree by-products, recycling these materials can help to reduce the environmental impact of oil production while generating economic value.

Olive groves occupy a large part of the territory of the Sorrento peninsula in Southern Italy, shaping the landscape with traditional terraces, placed on hilly slopes, that also protect the soil from erosion and reduce the risk of landslides [12,13]. In this context, optimizing harvest management and pruning residues is crucial [14]. Another traditional activity on the Sorrento peninsula is dairy farming which, despite its long history and a well-established cheese market, has declined significantly in recent decades due to competition from the tourist activity [15,16]. Residual cattle farms are currently situated in suboptimal inland areas with restricted pastures and arable land, which are insufficient to meet the nutritional requirements of even modest herds. Given the structural limitations of farms but the high market potentiality, a strategy for revitalizing the dairy farming in the Sorrento peninsula should focus on practices that reduce production costs and promote environmental sustainability and the link with territory while possibly enhancing the nutritional profile of milk. In this regard, animal feeding is of crucial importance.

One viable strategy involves utilizing biomass residue from olive trees as a locally available feed resource for livestock. Indeed, olive leaves and branches, especially the finer and leafier parts, are suitable for feeding ruminants and are widely available with a long window of use [17,18]. Additionally, their lower water content (approximately 50%) facilitates preservation and handling other by-products of the olive oil chain, such as olive pomace and vegetation water [18].

Previous research has shown the benefits of including olive leaves and leafy twigs in the diets of sheep and goats, as well as the potential to improve the nutritional profile of dairy products from compounds derived from olive leaves [17,19]. However, to the best of our knowledge, the use of olive tree biomass in the diet of dairy cows has not yet been addressed.

Under the hypothesis that olive tree biomass residues can serve as cattle feed, this study aimed to contribute to a more circular and sustainable agricultural system in the Sorrento peninsula by investigating the effects of supplementing lactating cows’ diets with olive tree residues, such as pruning waste, suckers, and oil mill leaves, on milk yield and quality.

2. Materials and Methods

All procedures described in this experiment involving the handling and treatment of animals have been approved by the Institutional Ethics Committee of the University of Naples Federico II (protocol code PG/2020/0079564) and in compliance with the EU requirements concerning the protection of animals used for scientific purposes (Dir. 2010/63/UE) as implemented by the Italian legislation (DL n. 26, 4 March 2014). The experiment was conducted on commercial dairy farms where the supplementation of rations with olive leaves is a common practice. Therefore, no deviations from the standard feeding routine were implemented during the trial.

2.1. Area of Study and Farms

To gain a preliminary understanding of agricultural practices of the inland area of the Sorrento peninsula, Southern Italy, a collaborative survey was conducted involving local technical specialists and utilizing data from the Agricultural Census (Istat, 2010). Following this initial analysis, two representative farms (40°39′ N, 14°26′ E, 330 m a.s.l. and 40°36′ N, 14°23′ E, 276 m a.s.l. for Farms A and B, respectively) were selected for in-depth analysis and to carry out the feeding trials (

Table 1. Characteristics of dairy cows at enrolment and ingredients of experimental diets (mean ± SD).

Item	Farm A		Farm B
	OlPr_FA	Con_FA	OlLs_FB	Con_FB
Animals
Parity, n. lactations	3.40 ± 1.4	3.80 ± 1.3	2.6 ± 1.9	2.8 ± 2.4
Days in milk, d	157.9 ± 36.4	161.5 ± 48.5	72.6 ± 26.5	76.5 ± 31.4
Milk yield, kg/head	18.5 ± 2.1	18.2 ± 2.8	19.2 ± 6.3	19.4 ± 3.1
Diets
Mixed hay, kg/head/d	11.0	11.0	10.0	10.0
Concentrate A, kg/head/d	10.0	10.0	-	-
Concentrate B, kg/head/d	-	-	11.0	11.0
Olive tree residues, kg/head/d	Ad libitum	Ad libitum	Ad libitum	Ad libitum

OlP, olive pruning; OlLs, olive mill leaves; SD, standard deviation.

). Both farms raise Italian Holstein cattle, as well as crossbreeds. Limiting to Farm A, the Agerolese breed is also present, a local dual-purpose (milk and meat) breed developed over several generations through crossbreeding with Breton, Brown Swiss, Jersey, and Friesian breeds [20]. Lactating cows were kept tethered in the barn and milked twice a day by mobile milking equipment. Dry cows and young cattle were kept in open paddocks where they can move freely. Manure was used as the fertilizer of olive orchards. Indeed, due to the moderately sloping terrain, the nearby agricultural area was primarily dedicated to terraced olive cultivation, necessitating the sourcing of the small herd’s feed entirely from external suppliers. This dependence on off-farm feed sources has resulted in diets based solely on commercial concentrates and mixed hay, barely satisfying the fiber, energy, and protein needs of lactating cows. It also contributes to higher feed costs, both in terms of purchase price and transport costs, exacerbated by the sub-optimal location of the farms. However, the economic impact of these increased costs is partially offset by farm-specific factors. Farm A benefits from a price premium for milk supplied to the Provolone del Monaco Consortium (a protected designation of origin cheese), while Farm B mitigates costs by on-farm cheese processing and sales.

2.2. Feeding Trials

The feeding trials on the two farms were conducted at different times and with different types of olive tree residues and are therefore described separately.

2.2.1. Farm A

An initial trial was conducted in January 2023 to investigate the consumption of olive pruning residues by the lactating cow herd (n = 16). Thin olive branches were weighed and offered in each cow’s manger twice a week, and the remaining leaves were weighed the following day to assess intake. Due to a moderate but noticeable level of consumption observed in this initial trial (mean intake: 850 ± 125 g/d as fresh basis), a more comprehensive feeding trial was conducted in February 2023. The study cohort comprised eight cows (6 Holstein Friesian and 2 Agerolese), grouped into two groups of four cows homogeneous in parity, breed, and milk production. Both groups were provided with the same basal diet (Table 1), integrated (olive prune group in Farm A, OlPr_FA) or not (control group in Farm A, Co_FA) with additional olive branches ad libitum.

A crossover design was employed, with each cow group randomly assigned to a sequence of the two dietary treatments. The rations were administered individually twice daily at 08:00 and 15:00 in a predetermined sequence. Freshly cut OlPr (collected no more than two days earlier) was provided first to the cows, in accordance with the experimental design. Approximately 30 min later, hay was offered to all animals, followed by the commercial concentrate used in the farms. This study lasted 6 weeks and included two treatment periods, each consisting of a 15-day adaptation phase and a 6-day measurement phase, which is deemed appropriate for feeding trials in lactating cows [21]. During the measurement phase, milk yield, dry matter intake (DMI) and olive residue consumption were recorded daily for each cow. Milk production was determined at morning milking by measuring the quantity of milk in the mobile equipment with a graduated bucket. Milk samples (200 mL) were also collected and kept refrigerated at 4 °C. On days 2, 3, and 4 of the measurement phases, additional milk samples were collected from each cow and combined in the laboratory in proportion to the quantity produced by each cow. The composited sample was used for the determination of the fatty acid (FA) composition of milk fat.

2.2.2. Farm B

An initial trial to assess the consumption of olive suckers by lactating cows was conducted on this farm in June and July 2023. Freshly cut olive suckers were offered to the cows at weekly intervals, and the remaining suckers were weighed the following day to evaluate intake. However, the minimal consumption of olive suckers was observed. In October 2023, a new intake trial was conducted using residual olive leaves obtained from an oil mill. Once the cows began consuming the olive leaves, a feeding trial was initiated. Two homogeneous groups of multiparous Italian Holstein cows, with four cows each, were formed, based on milk yield and quality (Table 1). The control group received the farmer’s standard diet (control group in farm B, Co_FB), while the olive group received the same diet supplemented with olive leaves collected from a local mill ad libitum (olive group in Farm B, OlLes_FB). The experimental design, sampling procedures, and data recording followed the same methods used on Farm A.

2.3. Feed and Milk Analyses

Feeds and olive by-products (including the thin branches actually consumed by the cows) were dried at 65 °C to determine dry matter (DM) content, ground to pass through a 1 mm screen (Brabender rotary mill; Brabender GmbH & Co., Duisburg, Germany) and analyzed for proximate composition according to the standard methods of AOAC [methods 942.05 976.05, 954.02 for ash, crude protein (CP), and ether extract, respectively] [22]. Neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL) were determined according to the procedures of Van Soest et al. [23] and Robertson and Van Soest [24] adapted to the Ankom²²⁰ Fiber Analyzer unit (Ankom Technology Corporation, Fairport, NY, USA). The Ewers polarimetric method [25] was used to assess the starch content of concentrate samples. The energy contents of the feeds and the diets expressed as the net energy of lactation (NE_L) were estimated according to Nozière et al. [26], while non-fibrous carbohydrates (NFCs) were calculated as 100−(%NDF + %ether extract + %CP + %ash) [27]. Milk samples were assessed on the day after collection for fat, protein, and lactose by infrared spectrophotometry (Milkoscan FT3, Foss Electric, Hillerød, Denmark) and somatic cell count (SCC) (Fossomatic 7, Foss Electric, Hillerød, Denmark). Milk fatty acid (FA) composition was determined according to the methods detailed elsewhere [28]. Briefly, after lipid extraction (Röse–Gottlieb method) and the trans esterification of triglycerides into the FA methyl esters (FAMEs), samples were analyzed by high-resolution gas chromatography (Perkin Elmer Autosystem XL; Perkin Elmer, Shelton, CT, USA) using a fused silica capillary column (100 m 9 0.25 mm i.d.; 0.20 lm film thickness; Supelco, Bellofonte, PA, USA). Fatty acid peaks were identified using the Supelco 37 Component FAME MIX (Supelco, Bellefonte, PA, USA) and standards for conjugated linoleic acids (CLA, C18:2 cis9, trans11 and C18:2 trans9, cis11) and trans-vaccenic acid (C18:1 trans11) (NuChek Prep, Elysian, MN, USA). Fatty acids were expressed as a percentage of the total FAMEs (g/100 g FAs). Values < 0.1 were not quantified. The atherogenic index (AI) was calculated according to Ulbricht and Southgate [29].

2.4. Statistical Analysis

Data from the two trials were separately analyzed using JMP statistical software version 10.0.0 (SAS Institute Inc., Cary, NC, USA). To test for carry-over effects, data were analyzed considering the fixed effects of treatment, sequence, and treatment × sequence interaction, with the cow as a random effect. Since the interaction was not significant, indicating a lack of carryover effect, a repeated measure mixed-effect ANOVA model was used, with diet (control and olive residues), period, and their interaction as fixed factors, and the cow as a random effect. Data for milk FA composition were analyzed by a GLM ANOVA model, using diet (control and olive residues), period, and their interaction as fixed factors. Statistical significance was set at p < 0.05. Data were reported as least squares means (LSM) ± standard error.

3. Results

Table 2. Chemical composition (% DM, unless otherwise stated) and estimated energy content of ingredients used in experimental diet formulations. Values are presented as mean ± SD of 6 samples, except for fatty acid composition of olive residues (n = 2).

Item	Farm A			Farm B
	Concentrate 1	Hay	OlPr	Concentrate 2	Hay	OlLes
Chemical composition
DM, % of fresh matter	87.2 ± 0.11	86.7 ± 0.55	82.5 ± 0.81	87.3 ± 0.09	89.6 ± 0.63	59.9 ± 0.78
Ash	8.0 ± 0.18	8.6 ± 0.39	8.3 ± 0.53	8.6 ± 0.15	7.6 ± 0.25	9.0 ± 0.44
CP	20.0 ± 0.12	10.7 ± 0.32	6.2 ± 0.41	19.5 ± 0.08	9.5 ± 0.27	10.3 ± 0.36
Ether extract	4.5 ± 0.27	2.2 ± 0.36	4.0 ± 0.45	4.6 ± 0.18	1.2 ± 0.32	6.3 ± 0.19
NDF	20.7 ± 0.52	63.6 ± 1.17	47.3 ± 1.74	21.4 ± 0.39	61.7 ± 1.21	42.6 ± 1.17
ADF	17.1 ± 0.28	35.0 ± 1.15	33.9 ± 1.61	10.3 ± 0.21	33.3 ± 1.02	31.6 ± 1.21
ADL	1.1 ± 0.21	7.2 ± 0.49	20.12 ± 0.95	1.5 ± 0.14	7.4 ± 0.36	17.1 ± 1.13
NFC	46.8 ± 0.51	14.9 ± 0.8	34.3 ± 1.4	46.4 ± 0.61	19.9 ± 0.75	32.0 ± 0.69
Starch	35.1 ± 0.68	-	-	40.1 ± 0.77	-	-
NEL, MJ/kg DM	7.3	4.8	2.2	7.5	4.8	4.5
Fatty acid composition, % weight
C14:0 Myristic acid			1.39 ± 0.17			1.37 ± 0.03
C16:0 Palmitic acid			25.24 ± 0.35			24.30 ± 0.79
C18:0 Stearic acid			4.88 ± 0.34			4.54 ± 0.37
C18:1n:9c Oleic acid			20.67 ± 0.34			21.27 ± 0.1
C18:2n:6c Linoleic acid			6.79 ± 0.22			8.26 ± 0.66
C18:3n:3c Linolenic acid			41.04 ± 1.30			40.26 ± 1.11

¹ Based on corn meal, sorghum meal, wheat middling, whole barley meal, wheat bran, soybean extracted meal, whole flaked soybean, dehulled sunflower meal, dried beet pulp, sugar cane molasses, calcium carbonate, sodium chloride, sodium bicarbonate, calcium phosphate, vitamin and mineral supplements. ² Based on corn meal, soybean extracted meal, wheat bran, wheat middling, dehulled sunflower meal, whole barley meal, sugar cane molasses, calcium carbonate, sodium chloride, sodium bicarbonate, calcium phosphate, magnesium oxide, vitamin and mineral supplements. OlPr, olive pruning residues; OlLes, olive mill leaves; DM, dry matter; CP, crude protein; NDF, neutral detergent fiber; ADF, acid detergent fiber; ADL, acid detergent lignin; NFC, non-fibrous carbohydrate; NEL, net energy for lactation; SD, standard deviation; and NS, not significant.

presents the chemical composition of the olive biomass residues and of the feeds used in this study. Both OlPr and OlLes were characterized by significant lignin content, with the lowest level observed in OlLe. While both olive by-products had fiber and CP contents comparable to or lower than the traditional hays, the OlLe was generally higher in nutritional quality compared to OlPr, with lower fiber components (ADF, NDF) and higher fat and protein content. However, both products exhibited a notable ether extract content. The lipid profiles of the two by-products were found to be very similar, with a notable concentration of polyunsaturated fatty acids (PUFAs), particularly 18:2 cis-9,12 (linoleic acid) and 18:3 cis-9,12,15 acid (alfa linolenic acid), as well as a considerable presence of monounsaturated fatty acids (MUFAs), primarily 18:1 cis-9 (oleic acid). Saturated fatty acid levels were moderate, with C16:0 (palmitic acid) as the principal component.

Table 3. Dry matter and nutrient intake, milk production, and composition of cows fed diets supplemented with or without olive residues (LSM ± SEM).

Item	Farm A			Farm B
	Con_FA	OlPr_FA	p	Con_FB	OlLes_FB	p
Dry matter intake, kg/head/d	17.13 ± 0.15	18.18 ± 0.15	<0.0001	17.27 ± 0.14	18.93 ± 0.14	<0.0001
Hay, kg DM/head/d	8.42 ± 0.06	6.50 ± 0.06	<0.0001	7.7 ± 0.06	6.04 ± 0.06	<0.0001
Concentrate, kg DM/head/d	8.70	8.70	-	9.56 ± 0.10	8.50 ± 0.10	<0.0001
Olive residues, kg DM/head/d	-	2.95 ± 2.05		-	4.38 ± 0.09	-
Forage (hay + olive residues) on DMI, %	49.20 ± 0.14	52.03 ± 0.14	<0.0001	44.58 ± 0.28	55.10 ± 0.28	<0.0001
Estimated nutrient intake
NEL, MJ/head/d	105.34 ± 0.29	102.88 ± 0.29	<0.0001	107.00 ± 0.81	110.84 ± 0.81	<0.0001
CP, kg/head/d	2.59 ± 0.008	2.67 ± 0.008	<0.0001	2.58 ± 0.02	2.65 ± 0.02	<0.0001
NDF, kg/head/d	7.31 ± 0.02	7.26 ± 0.02	<0.0001	6.83 ± 0.05	7.42 ± 0.05	<0.0001
Ether extract, kg/head/d	0.59 ± 0.004	0.64 ± 0.004	<0.0001	0.53 ± 0.005	0.74 ± 0.005	<0.0001
Milk
Yield, kg/head/d	17.84 ± 0.70	17.74 ± 0.70	NS	18.51 ± 0.24	18.49 ± 0.24	NS
Fat, %	3.44 ± 0.05	3.49 ± 0.05	NS	3.43 ± 0.04	3.70 ± 0.04	<0.001
Protein, %	3.34 ± 0.04	3.32 ± 0.04	NS	3.33 ± 0.03	3.30 ± 0.03	NS
Fat-to-protein ratio	1.03 ± 0.02	1.05 ± 0.02	NS	1.03 ± 0.01	1.12 ± 0.01	<0.0001
SCC, lg n. cells/mL	5.43 ± 0.04	5.37 ± 0.04	NS	5.36 ± 0.09	5.31 ± 0.09	NS

Con_FA, cows fed the control diet in Farm A; OlPr_FA, cows fed the diet supplemented with olive pruning residues; Con_FB, cows fed the control diet in Farm B; OlLes_FB, cows fed the diet supplemented with olive mill leaves. DM, dry matter; DMI, dry matter intake; CP, crude protein; NDF, neutral detergent fiber; NEL, net energy for lactation; SCC, somatic cell count; LSM, least square means; SEM, standard error of mean; NS, not significant.

presents the intake and milk production of groups that were fed rations with and without olive prune residues in Farm A (Co_FA and OlPr_FA) or olive leaves in Farm B (Co_FB and OlLes_FB). The supplementation of diets with olive residues significantly affected feed intake patterns. Supplementation with both olive by-products reduced hay intake by approximately −20%; (p < 0.001). While OlPr inclusion did not affect concentrate intake, cows supplemented with OlLes slightly reduced concentrate intake by about −11% (p < 0.001). Notably, the consumption of OlLes was approximately 32% higher compared to that of OlPr. These trends resulted in a significant (p < 0.001) increase in DMI and the proportion of forage (hay + olive residues) in the diet of the olive residue-fed groups compared to the control groups, with a greater increase observed in OlLe-fed cows.

In terms of estimated nutrient intake, olive residue supplementation increased (p < 0.001) the supply of CP, NDF, and fat, with greater increments observed in OlLe-fed cows. These changes led to a reduction in energy supply for the OlPr group (p < 0.001), while energy supply increased for the OlLe group (p < 0.001).

The milk yield was relatively low, and the SCC elevated across all groups (Table 3). Moreover, apart from the OlLes_FA group, the fat-to-protein ratio was near to one. The use of OlPr did not significantly affect milk yield, milk macrocomponents, or the SCC. By contrast, the OlLe supplementation increased both the milk fat percentage (p < 0.01) and the fat-to-protein ratio (p < 0.001), while no significant effects were observed on milk yield, the SCC, or milk protein content.

Table 4. Fatty acid composition (% of total FA) of milk produced by cows fed diets with or without olive residue supplementation (LSM ± SEM).

Item	Farm A			Farm B
	Con_FA	OlPr_FA	p	Con_FB	OlLes_FB	p
C4:0	4.66 ± 0.018	4.66 ± 0.018	NS	5.00 ± 0.055	4.81 ± 0.055	NS
C6:0	3.70 ± 0.058	3.74 ± 0.058	NS	3.77 ± 0.048	3.67 ± 0.048	NS
C8:0	2.27 ± 0.069	2.32 ± 0.069	NS	1.85 ± 0.040	1.67 ± 0.040	<0.05
C10:0	1.62 ± 0.040	1.67 ± 0.040	NS	1.76 ± 0.052	1.74 ± 0.052	NS
C11:0	0.17 ± 0.100	0.14 ± 0.100	NS	0.21 ± 0.009	0.22 ± 0.009	NS
C12:0	3.07 ± 0.033	3.09 ± 0.033	NS	2.68 ± 0.070	2.52 ± 0.070	NS
C13:0	0.18 ± 0.011	0.19 ± 0.011	NS	0.16 ± 0.010	0.15 ± 0.010	NS
C14:0	11.50 ± 0.131	11.32 ± 0.131	NS	11.36 ± 0.137	11.02 ± 0.137	NS
C14:1	0.89 ± 0.025	0.87 ± 0.025	NS	0.86 ± 0.022	0.83 ± 0.022	NS
C15:0	1.17 ± 0.066	1.14 ± 0.066	NS	1.22 ± 0.036	1.14 ± 0.036	NS
C16:0	39.69 ± 0.153	39.25 ± 0.153	NS	38.38 ± 0.138	35.85 ± 0.138	<0.001
C16:1	1.65 ± 0.024	1.68 ± 0.024	NS	1.59 ± 0.040	1.46 ± 0.040	NS
C17:0	0.41 ± 0.025	0.42 ± 0.025	NS	0.41 ± 0.025	0.38 ± 0.025	NS
C17:1	0.11 ± 0.060	0.12 ± 0.060	NS	0.15 ± 0.010	0.19 ± 0.010	<0.05
C18:0	7.32 ± 0.090	7.41 ± 0.090	NS	7.87 ± 0.119	8.38 ± 0.119	<0.05
C18:1 t11	0.19 ± 0.080	0.21 ± 0.080	NS	0.20 ± 0.007	0.33 ± 0.007	<0.0001
C18:1 c9	18.78 ± 0.670	19.10 ± 0.670	<0.001	18.52 ± 0.116	20.08 ± 0.116	<0.0001
C18:2 t 9–12	0.29 ± 0.010	0.30 ± 0.010	NS	0.26 ± 0.008	0.46 ± 0.008	<0.0001
C18:2 c 9–12	1.37 ± 0.065	1.45 ± 0.065	NS	2.40 ± 0.026	3.09 ± 0.026	<0.0001
C20:0	0.18 ± 0.014	0.19 ± 0.014	NS	0.21 ± 0.015	0.22 ± 0.015	NS
C18:3 n3	0.16 ± 0.011	0.14 ± 0.011	NS	0.36 ± 0.008	0.69 ± 0.008	<0.0001
Mix CLA 1	0.24 ± 0.018	0.27 ± 0.018	NS	0.41 ± 0.020	0.70 ± 0.020	<0.0001
C22:0	0.19 ± 0.015	0.20 ± 0.015	NS	0.23 ± 0.007	0.22 ± 0.007	NS
C20:3 n3	0.15 ± 0.013	0.12 ± 0.013	NS	0.16 ± 0.008	0.14 ± 0.008	<0.005
Saturated	76.15 ± 0.010	75.75 ± 0.010	<0.05	75.09 ± 0.131	72.02 ± 0.131	<0.0001
MUFA	21.63 ± 0.062	22.00 ± 0.062	<0.001	21.32 ± 0.125	22.89 ± 0.125	<0.0001
PUFA	2.21 ± 0.056	2.28 ± 0.056	NS	3.58 ± 0.037	5.08 ± 0.037	<0.0001
Sat/Unsat	3.19 ± 0.017	3.12 ± 0.017	<0.05	3.10 ± 0.020	2.57 ± 0.020	<0.0001
AI	3.72 ± 0.026	3.61 ± 0.026	<0.005	3.47 ± 0.029	2.95 ± 0.029	<0.0001

¹ (C18:2 9t-11c + C18:2 9c-11t). Con_FA, cows fed the control diet in Farm A; OlPr_FA, cows fed the diet supplemented with olive pruning residues; Con_FB, cows fed the control diet in Farm B; OlLes_FB, cows fed the diet supplemented with olive mill leaves.; FA, fatty acid; MUFA, monounsaturated fatty acid; PUFA, polyunsaturated fatty acid; Sat/Unsat, saturated-to-unsaturated FA ratio; AI, atherogenic index; LSM, least square means; SEM, standard error of mean; and NS, not significant.

presents the FA composition of milk produced in the two farms. Significant differences in milk FA composition were observed between the Co_FA and OlPr_FA groups for only oleic acid (p < 0.01), which resulted in significant higher levels of MUFA, a reduction in saturated FA levels, a reduction in saturated-to unsaturated rations and a decreased AI (p < 0.05). Milk from cows fed OlLes exhibited a more significant shift in the FA profile compared to the respective control group. This included a decrease in C8:0 (caprylic acid, p < 0.05), C20:3 (DGLA, p < 0.01), C16:0 (palmitic acid, p < 0.01), and total saturated FAs (p < 0.0001), alongside an increase in C17:1 (heptadecenoic acid, p < 0.05), C18:0 (stearic acid, p < 0.05), oleic acid, linoleic and trans-linoleic acids, alfa linolenic acid, CLA, PUFA, and MUFA (p < 0.001). These changes resulted in a significant decrease in both the AI (p < 0.001) and the saturated-to-unsaturated ratio.

4. Discussion

The lack of self-sufficiency in feed, and, particularly in forage, is a major constraint for the small dairy farms of the Sorrento peninsula, which face the challenge of procuring, transporting and storing sufficient quantities of bulky feed to meet the nutritional and physiological needs of their animals, often at high cost. This is reflected in the relatively low quantity and quality of forage fed to animals, which likely contributed to the observed low milk production and fat content. Increasing the milk fat content is important for dairy farms producing milk for cheese making, as a decrease in fat content and a fat-to-protein ratio approaching unity has a negative effect on cheese yield due to the reduced availability of fat for incorporation into the casein micelle structure [30]. As a result, the cheese has a lower fat content, leading to changes in both its rheological and sensory properties [31,32]. Another problem highlighted on the farms was the high SCC, which signals impaired mammary gland function and animal health and can affect cheese quality by promoting lipolysis and proteolysis, which in turn leads to rancid or bitter flavors and altered cheese texture [33]. Factors contributing to the increase in SCC include the tethered stall system, which exacerbates mastitis and metabolic disorders, and inadequate dietary fiber, which can induce subacute ruminal acidosis and further increase SCC [34,35]. In this context, it has been hypothesized that the use of olive residues in rations could be a practice that combines fibrous feed supply and waste disposal, thus promoting a circular economy [36].

The chemical composition of the olive tree vegetative residues aligns with previous reports [17,18,37,38,39], notably for their high lignin content. This characteristic is crucial for olive tree survival in Mediterranean climates, as lignin provides structural rigidity and resistance to abiotic stress such as salinity, drought, and extreme temperatures while reducing water loss through transpiration [40]. From an animal nutrition perspective, the complex fiber matrix structure of olive residues, while significantly reducing fiber digestibility, may promote chewing and salivation, potentially mitigating the risk of ruminal acidosis [41]. The increased ether extract content of both olive residues suggests their potential to provide additional energy and essential fatty acids to dairy cows compared to traditional hay [42]. However, it is important to consider that the lipid extraction process may also extract cuticular waxes and liposoluble pigments [43], which may reduce the estimated nutritional profile of the residues. The higher proportion of small twigs in OlPr compared to mill-harvested leaves (OlLes) likely explains the observed differences in NDF, ADF, lignin, and protein content between the two products, as well as the lower DM and energy supply to cows. Moreover, the minimal olive sucker consumption observed in the preliminary trial at Farm B also suggests that pruning residues may be less palatable than mill leaves, possibly due to their less advanced vegetative state. In this regard, the bitter compound oleuropein, the major constituent of the secoiridoid family in the olive leaves, may play a role [44]. On the other hand, suckers and other vegetative organs in the lower part of the olive canopy, can develop biochemical and anatomical defenses in response to abiotic and biotic stresses, which can reduce their palatability by herbivores. The mechanisms include short and branched shoots, reduced and spiny leaf blades, thickening of the cuticle, and a higher cuticular concentration of phenolic compounds and tannins [45]. These defenses, which are ecologically designed to minimize the effects of water deficit and grazing, tend to exacerbate during summer to compensate for the evapotranspiration demand imposed by high temperatures, or because of frequent suckering to remove excess vegetation from the base of the tree [46].

The lack or marginal significant differences in milk yield and quality between the Co_FA and OlPr_FA groups is likely attributable to the lower intake and inferior nutritional profile of OlPr compared to OlLes, which slightly reduced energy supply. By contrast, although OlLe supplementation did not significantly affect milk yield, milk protein, or the SCC, it resulted in an increase in milk fat content, bringing the fat-to-protein ratio closer to the standard value of greater than one [47]. These outcomes, also observed in sheep and goats [48,49], suggest an improvement in the overall nutritional status of the dairy cows, driven by increased energy and protein supply and a higher forage-to-concentrate ratio, although this was insufficient to significantly impact milk yield. The increase in milk fat can be attributed to an increased intake of both fat and fiber in OlLe-supplemented animals compared to those fed OlPr [50]. As mentioned above, this is particularly important given the significant influence of milk fat content on cheese yield and quality.

The high intake of OlLe significantly increased the concentrations of most MUFAs and PUFAs, reflecting the fatty acid composition of the olive by-product and reinforcing the beneficial effects of olive leaf supplementation on milk quality. This finding is consistent with previous observations in small ruminants [48,51]. Notably, the concentration of saturated stearic acid also increased. This finding aligns with previous studies on sheep and cows that were fed diets supplemented with olive oil and pomace [51] and suggests that the increased dietary intake of 18-carbon unsaturated fatty acids promotes the formation of stearic acid in milk fat through microbial biohydrogenation in the rumen. In any case, from a nutritional point of view, the saturated-to-unsaturated ratio and the AI of the milk remained significantly more favorable in the milk from OlLe-fed group. These changes not only enhance the nutritional quality of milk but can also support the production of high-value cheeses, which are central to the Sorrento area’s agricultural economy.

The increase in CLA also aligns with a report from the literature on small ruminants [17]. In addition to its significant role in the development and progression of various human diseases, including certain types of cancer and metabolic disorders, CLA has been shown to exert a protective effect on mammary epithelial functionality, primarily through its antioxidant properties [52,53]. Notably, olive leaves themselves are rich in phenolic compounds and exhibit significant antioxidant activity. However, despite the potential beneficial effects of the antioxidant molecules from olive leaves and CLA, no statistically significant differences in the SCC were observed between the Co_FB and OlLes_FB groups. This finding is likely attributable to the multifaceted etiology underlying the increase in the SCC in milk and the persistence of the causative factors previously described.

Finally, it is well known that feeding diets rich in PUFA and MUFA can reduce milk fat yield under low rumen pH conditions [54]. This reduction is attributed to shifts in microbial populations favoring the production of CLA trans10, cis12, a microbial metabolite originated during biohydrogenation process that can inhibit sterol-CoA desaturase, a key enzyme in mammary fat synthesis [55]. However, in this study, milk fat yield was improved and not adversely affected. This may be explained by the increased chewing and ruminating activity induced by the inclusion of olive leaves in the diet, which could potentially contribute to maintaining an optimal rumen pH, thereby mitigating the negative effects of PUFA and MUFA on milk fat production.

5. Conclusions

Under the actual production conditions tested in this study, the inclusion of olive mill leaves (OlLes) in the diet for lactating cows significantly enhanced milk fat content and improved the fatty acid profile of milk, increasing concentrations of MUFA, PUFA, and CLA. The positive effect on milk fat level also suggests improved rumen fermentation and microbial activity, indicating potential benefits for cow rumen health. However, no significant impact on milk yield or the SCC was observed, highlighting the need for further research to optimize diets incorporating this olive by-product and to address other factors influencing milk hygienic quality and animal health on these farms. Olive pruning residue (OlPr) and, in particular, olive suckers exhibited low voluntary intake, limiting their potential as direct forage sources. Further research is needed to investigate the factors contributing to the low palatability of these olive residues and to improve their potential as animal feed. Of particular interest is evaluating both by-products in the same farms while expanding this study to include more farms with standardized feeding and management practices. In addition, the impact on feeding costs, including any by-product management costs, should be evaluated. Overall, this study demonstrates the potential of OlLe as a valuable feed supplement for lactating cows. Integrating OlLes into the diet is a promising strategy for enhancing milk quality and resource utilization, reducing reliance on external feed inputs, and promoting a more sustainable and circular dairy farming system in the Sorrento peninsula. Further studies are needed to implement this approach on a larger scale and to assess its applicability to other Mediterranean regions facing similar agricultural and economic challenges.