Machine Milking in Small Ruminants: Milking Systems and Association with Milk Quality Produced in the Farms

Abstract

The intensification and continuous evolution of dairy sheep and goat farming have played an essential role in the development and implementation of milking equipment. The increasing demand for time-efficient milking procedures, reduced labour costs, sustained milk production, and optimal mammary health have driven the widespread adoption and optimisation of machine milking technologies. The objectives of this article are (i) the review of milking systems and relevant technological developments in milking equipment and (ii) the evaluation and description of their impact on udder health, as applied on dairy small ruminant farms. Milking systems used on farms depend on the available space and number of animals on the farms. Appropriate settings in milking systems are important for ensuring good milk quality; among them, vacuum level, pulsation rate and ratio are important characteristics that must be monitored regularly. Further, use of appropriate teatcups specific to the animal species to be milked is significant. An important aspect of proper maintenance of the milking system is the cleaning procedure after completion of milking. Points for consideration are quality and temperature of the water used for cleaning, use of detergents and disinfectants, and maintenance schedule and teatcup replacement. Some technological features that are part of milking systems include automatic vacuum shut off, electronic milk recording, electronic identification of animals, automatic flushing of milking clusters and automatic pre-stimulators. Farms will benefit from applying precision technologies, which will use data from tools related to animal genetic background, animal behavioural indicators, environmental conditions and disease-related functions for more holistic and cost-effective farm management. In this context, integration of sensor-based technologies in milking systems will be able to provide real-time information regarding quality of milk produced at individual and farm levels. Moreover, the introduction of automatic system flushing in-between animals during the milking procedure can contribute to breaking chains of potential bacterial transfer and reducing animal infections during milking. Overall, although machine milking has certainly contributed to improved efficiency, milk quality and labour conditions, flaws in system function may adversely affect mammary health.

1. Introduction

Internationally, approximately 20% of sheep and goats are farmed for dairy production. Dairy sheep and goat farming is the primary production type in para-Mediterranean countries in Africa, Asia and Europe [1]. Indeed, dairy sheep farming has been practiced in Southern and Eastern Europe since the ancient Greek and Roman periods [2] and still remains relevant and important in those areas, as dairy products are significant ingredients of local diets. Dairy goat farming is also widely practiced in India and China, where it has played a pivotal role in agricultural development [3,4]; notably, goat farming has recently also expanded in high-income and technologically developed parts of the world, as an alternative to dairy cattle farming [1].

Milking systems constitute essential equipment in dairy farming. They have been designed to simulate the physiological process of sucking. Milking, like sucking, triggers a neuroendocrine reflex, during which hormones play an essential role for milk ejection [5]. The efferent leg of the neuroendocrine milk ejection reflex involves the release of oxytocin, which acts on myoepithelial cells and leads to expulsion of the alveolar milk towards the teat [6]. Milk present within the gland cistern may be ejected at the start of milking, even without pre-stimulation and the subsequent oxytocin release [7]. However, for the complete evacuation of a mammary gland, oxytocin release and milk ejection are necessary.

The development of milking systems began in the late 19th century and was followed by their gradual adoption in dairy cattle farming from 1910 to 1950 [8]. This was associated with an enhancement in the quality of the produced milk [9]. In contrast, the mechanisation of milking in small ruminants occurred later. The first systematic attempts to introduce machine milking in dairy ewes emerged in the 1960s in France [10]. The delayed adoption of this technology can be attributed to several factors, including differences in management systems, the unsuitability for machine milking of the udder morphology in many sheep and goat breeds, and the limited availability of technical knowledge and training among farmers [11]. Technological advances in dairy small ruminant systems have historically followed those achieved in cattle. Notably, robotic milking systems have developed for use on sheep and goat farms significantly later than on dairy cattle farms [12].

The intensification and continuous evolution of dairy sheep and goat farming have played an essential role in the development and implementation of milking equipment. The increasing demand for time-efficient milking procedures, reduced labour costs, sustained milk production, and optimal mammary health have driven the widespread adoption and optimisation of machine milking technologies. In a nationwide investigation in Greece, 78% of sheep farms and 55% of goat farms were found to apply machine milking [13]. In smaller-scale surveys in other para-Mediterranean countries and New Zealand, machine milking was found to be applied on all sheep farms and on 67% to 100% of goat farms [14,15,16].

The objectives of this article are (i) the review of milking systems and relevant technological developments in milking equipment and (ii) the evaluation and description of their impact on udder health, as applied on dairy small ruminant farms.

The literature sources for this review included mainly articles in peer-reviewed journals. Primarily, these were identified during searches performed on repeated occasions in the Web of Science platform (www.webofknowledge.com; Clarivate Analytics, London, United Kingdom). The search terms included ‘dairy’, ‘goat OR caprine OR Capra hircus’, ‘liner’, ‘mastitis’, ‘milking’, ‘milking machine OR machine milking’, ‘milking parlour’, ‘milk production’, ‘milking system’, ‘pipeline’, ‘pulsation rate’, ‘sheep OR ovine OR Ovis aries’, ‘robotic’, ‘teatcup’, and ‘vacuum level’. Subsequently, the articles were retrieved from the relevant journals. Overall, 83.5% of the cited references are articles in journals listed therein.

2. Milking Parlours and Milking Systems

A milking parlour refers to a construction wherein the milking system and equipment are installed and animals are milked. This is usually in an annex to the main farm building or even in a separate building. Often, there is a waiting area before the milking parlour, where animals stand and can be segregated into milking groups.

Milking systems should work as effective management systems, where animals are milked in large numbers in a time-efficient manner. Each farm should adopt and apply a tailored milking routine, from the pre-milking phase until the post-milking actions.

There are various types of milking systems in use on dairy sheep and goat farms (Table 1). Common classification criteria for milking systems include (i) the type of stall wherein animals are restrained during milking, and (ii) the placement of animals at the same level as milkers or above them during milking [17]. The milking system used in a farm would depend on the available space within the milking parlour and the size of the flock or herd.

Table 1. Characteristics of frequently established types of stall in milking systems in use on dairy sheep and goat farms.

Type of Stall	Location of Pit for Milkers	Aligned Milking System	Usual No. of Animal Positions	Main Advantages	Main Disadvantages
Linear one-sided	At the same level as milkers or above them	Mobile milking cart or fixed installation	Up to 48	(a) Easy access to teats, (b) possibility for expansion	(a) Low capacity due to difficulty of use on large farms, (b) time-consuming, due to the necessity for exit of animals after completion of all animals in a milking row
Linear parallel	At the same level as milkers or above them	Mobile milking cart or fixed installation	Up to 60	(a) Easy access to teats, (b) possibility of expansion, (c) time-efficient, as animals on one side of the parlour can be prepared for milking whilst those on the other are milked	(a) Extended time of animals standing within the parlour (i.e., until animals on both sides are milked), (b) need for increased number of milkers
Fishbone (or herringbone)	At the same level as milkers or above them	Mobile milking cart or fixed installation	Up to 48	(a) Time-efficient, due to the possibility for fast entry of animals in the milking parlour	(a) Difficult access to teats, (b) smaller number of animals accommodated in the same space
Circular (or rotary or carrousel)	Above the milkers	Fixed installation	Up to 72	(a) Time-efficient, (b) easy access to teats, (c) high capacity (200–280 ewes per hour)	(a) High cost, (b) difficulty of access for personnel

Overall, of the above types of stalls, the linear types are the ones installed more frequently on dairy small ruminant farms (Greece: Lianou et al. [13]; Spain: Milán et al. [14] and Gonzalo et al. [18]; United Kingdom: Anzuino et al. [19]). In contrast, Chambers and Laven [16] reported that the fishbone type predominated on dairy sheep farms in New Zealand.

Milking systems can involve a mobile milking cart or a fixed pipeline installation. A mobile milking cart consists of (i) one or two milking clusters (i.e., one or two pairs of teatcups), (ii) a compact vacuum pump and (iii) the milk collection bucket. The milker moves the cart from one animal to another, in order to place it behind the animals and use it for machine milking; at the end of the procedure, the collected milk is manually transferred from the buckets to a milk tank by the milker. This system has a low cost and offers flexibility (e.g., it is convenient for farms with transhumant flocks or herds), but requires considerable manual work for moving the cart and emptying milk buckets into the tank. In general, few farms use mobile milking carts [13,14,16,20,21]; due to the significant effort required by the farmer, its use is recommended on farms with fewer than 50 animals. In Greece, only 15.3% and 9.1% of sheep and goat farms, respectively, used mobile milking carts [13], whilst a study in Italy reported that only 6.6% of farms used mobile milking carts [20]. In contrast, relevant studies in Spain and New Zealand did not report use of mobile milking carts on dairy small ruminant farms [14,16,21].

In most cases, farms have a milking system with a fixed pipeline installation. The installation transfers the milk through a milk line directly to cooling tanks, which eliminates manual handling of the collected milk. This set up allows a single milker to simultaneously handle many milking units, thereby improving labour efficiency. In such cases, milk is transported through a closed system; this reduces risk of dirtying by dust and contamination by environmental pathogens, thus preserving milk quality [22,23].

Linear-type stalls can be further classified according to the mode of exit of the animals post-milking. Animals enter into the stall and are restrained by adjustable individual headlocks. After completion of the milking procedure, there are two approaches for the exit of animals from the stall. In the first, animals walk alongside the pit and would exit individually through an opening opposite to the one through which they entered into the stall. Alternatively, the barrier in front of the animals can be lifted, so they would exit at the same time (in a fast-exit type) from the stall, subsequently walking outside the parlour; this latter mode allows quicker turnover of groups and increased parlour throughput [11].

In fishbone-type stalls, both modes of animal exiting can be applied. In circular-type stalls, animal exit is achieved through a single exit gate, where the animals follow a continuous flow of walking away from the platform [24].

All types of milking systems share some basic common components (Table 2), as detailed below.

Table 2. Basic components of milking systems in use on dairy sheep and goat farms.

Component	Role
Vacuum pump	Creates vacuum by extracting air from the system
Interceptor vessel	Prevents liquids and foreign bodies entering into the pump
Regulator	Maintains vacuum stability in the system
Sanitary trap	Prevents milk or liquids entering into the air pipelines
Pipelines	Carry vacuum or milk
Cluster	Supports the two teatcups and a claw joining them
Pulsation chamber	Regulates pulsation rate
Liner/teatcup	Collects milk produced by mammary gland during the milking process

Milk ejection occurs when vacuum is applied onto the teat end, forcing the teat canal to open and milk to flow out. The milk flows from each teatcup to the cluster through milk tubes and pipelines to the milk receiver and onwards to the cooling tank. At the same time, air moves from the cluster via pipelines, through the sanitary trap and distribution tank to the vacuum pump, where it is discharged. Air is intermittently admitted into each pulsator in short, regular pulses, creating the opening and closing action of the liners. The pulsation system is essential for stimulating blood circulation around the teat. This can be achieved with the pulsation chamber (i.e., the space between the liner and the teatcup), by altering the atmospheric air and vacuum, making the liner open and close, leading to a milk-out phase and a massage phase respectively. The liner is open when there is no pressure difference between the inside and outside of the liner, and milk flow takes place during this phase. Then, the massage phase follows, with air getting into the system and causing liner collapse [25,26,27].

One complete liner movement is termed a ‘pulsation cycle’. ‘Pulsation rate’ is the number of pulsation cycles per minute. The pulsation ratio refers to the length of the milk-out phase compared with the massage phase.

The characteristics of a milking system are important for a proper milking routine, in order to ensure high milk yield and milk quality. Various settings have been proposed by different researchers and differ for application in the milking of sheep or goats (Table 3).

Table 3. Proposals for milking systems on dairy small ruminant farms.

Reference	Vacuum Level (kPa)	Pulsation Rate (Cycles min−1)	Pulsation Ratio	Teatcup Material
Sheep Farms
Marnet et al. [28]	36	180	50:50	n/d 1
Sinapis et al. [29]	38	120	50:50	n/d
Kaskous et al. [30]	34–36 2/36–38 3	120	60:40	silicone
Goat Farms
Kaskous et al. [30]	34–36 2/38–40 3	90	60:40	silicone
Carrotte [31]	36–44	70–100	60:40	n/d
Le Du et al. [32]	45–52	90	60:40	n/d
Sinapis et al. [33]	36–44	70–90	65:35	n/d
Zucali et al. [34]	36	90	60:40	n/d
Fernández et al. [35]	44	120	60:40	n/d
Caria et al. [36]	35	90	50:50	n/d
Romero et al. [37]	38	90 or 120	60:40	n/d
Bueso-Ródenas et al. [38]	38	90	60:40	n/d

¹ n/d: not described; ²: for low-line systems; ³: for high-line systems.

Moreover, a recent study showed that in dairy sheep a vacuum level of 46 kPa with concurrent administration of vitamin E and selenium (which would exert an immunostimulatory effect [39]) led to a 12.5% increase in milk yield [40]. Further, comparative studies in goats have revealed that a vacuum level around 35 to 36 kPa resulted in better performance than the lower value of 30 kPa [34] or the higher value of 42 to 44 kPa [34,36].

In a milking system, the location of the pipelines may vary in relation to the position of animals during milking, i.e., low-, mid- or high-level (where pipelines run below, up to 1.26 m above or over 1.26 m above, respectively, the position of standing animals during milking [41]). In general, mid- or high-level systems have lower installation costs and better efficiency, whilst low-level systems require a lower vacuum level to function and also minimise vacuum fluctuations [42]. Díaz et al. [42] and Lianou et al. [43,44] reported that the type of line level did not have an impact on teatcup detachment, milking time, milk yield and milk somatic cell counts and chemical composition, whilst, in contrast, Gonzalo et al. [18] reported that milk produced on farms with low-level systems achieved a better price for bulk-tank milk in relation to somatic cell counts and total bacterial counts. Moreover, high-level systems require a higher vacuum to achieve transfer of milk against gravity, leading to more vacuum fluctuations during the milking procedure and less efficient cleaning of the system after that [45,46].

The use of appropriate teatcups for the animals to be milked is also important, i.e., teatcups specific to the animal species to be milked [47,48]. This can raise issues on mixed sheep and goat farms with only one milking system that is equipped with teatcups specific to only one animal species. Use of inappropriate teatcups leads to their incorrect fitting on the teats of the udder [48]; in turn, this can lead to potential vacuum fluctuation, leading to teatcup detachment, overmilking and stripping, ultimately resulting in reverse milk flows [46,49,50]. In such cases, bacteria present in the milk can invade the teat duct and subsequently colonise the mammary parenchyma [51].

3. Technological Features in Milking Systems

Milking systems can incorporate various technological features that aim to improve the milking routine by reducing overmilking, contributing to maintaining udder health, reducing milking time and collecting information to enable informed decisions during health management of flocks or herds. Usual technological features in milking systems include the following.

• Automatic vacuum shut off: This feature provides automatic detachment of teatcups, without milker intervention. The detachment can be time-based or milk flow-based. The system contributes to maintaining mammary health, as it eliminates overmilking and prevents contact with milkers’ hands. This can lead to time-efficient milking and reducing staffing costs [7,52,53]. Adverse effects can occur when the shut-off parameters are not set properly, leading to overmilking or incomplete milking.
• Electronic milk recording: This feature allows milk recording, which is an essential practice for efficient genetic improvement and application of breeding programmes on farms. The relevant equipment involves a sensor, which is linked to software that allows estimation of the milk yield of individual animals, and subsequently storage of results. Information usually collected during electronic recording includes milk yield and flow rate, milk composition, and conductivity, which are estimated in volume-measuring chambers, by sensors using infrared technology or by conductivity sensors [54]. This equipment can also facilitate the collection of milk samples from individual animals [53,55].
• Electronic identification of animals: On farms where electronic identification of sheep and goats is practiced (e.g., through use of intraruminal bolus or leg tags), this feature supports the association of the identity of animals with data related to the respective milk production (volume and quality), based on data obtained during milking of each animal. This allows the storage of animal-specific results in the farm management software [53,56].
• Automatic flushing of milking clusters: This feature involves the cold-water flushing of milking units in-between the milking of two animals, which can contribute to limiting the dissemination of staphylococci within the milking system. The use of the feature has already been successfully evaluated in cattle farms [57]; although no reports regarding its use on dairy small ruminant farms have appeared, the feature may prove to be efficient, particularly because staphylococci are predominant mammary pathogens in these species.
• Automatic pre-stimulators: An increase in the pulsation rate during the first phase of milking is performed to ensure the release of oxytocin, facilitating milking performance without human intervention [58]. In goats, pre-stimulation was reported to lead to better milk flow and shorter milking duration [59]. However, this has not been confirmed in ewes [60].

4. Errors and Flaws in the Milking Procedure and Potential Association with Mammary Health

The role of milking systems is the removal of milk from the udder with a minimal risk to mammary health. However, flaws in the milking procedure (which may be due to inappropriate functioning of the milking system) can cause adverse effects on mammary health, potentially leading to subclinical or clinical mastitis.

There are conflicting results regarding the possible adverse effects of the milking procedure on the quality of milk produced. Some authors have indicated that various wrong settings in the milking system can adversely affect somatic cell counts and total bacterial counts in the bulk-tank milk produced on farms (Table 4). In contrast, other researchers did not report any differences in the bulk-tank milk somatic cell counts, in accordance with the characteristics of the milking system sheep: [42,43,50]; goats: [44,61].

Table 4. Associations of characteristics of milking procedure and milking systems with quality outcomes in bulk-tank milk on dairy sheep or goat farms.

Reference	Characteristics of Milking Procedure and Milking System	Outcomes
		Somatic Cell Counts 1	Total Bacterial Counts 1
Gonzalo et al. [18]	Longer time between machine milking servicing	higher	higher
	Low-line systems	lower	lower
	Presence of pulsator airline	lower	lower
	Milking ability of equipment < 4.0 L min−1	lower	n/d
	Large pipeline diameters	lower	n/d
	Vacuum changes to >0.8 kPa	higher	higher
	Pulsation rate over 180 min−1	n/d	higher
	Pulse ratios below 55%	n/d	higher
	Animal overmilking	n/d	higher
	Temperature of water post-milking < 65 °C	higher	higher
Sinapis et al. [33]	Vacuum level 36 kPa to 44–52 kPa	higher	n/d 2
Zamiri et al. [62]	Daily injection of oxytocin	lower	n/d
Manzur et al. [63]	Use of teatcups with automatic valves	higher	n/d
Liagka et al. [64]	Separate milking unit for each animal position	n/d	lower
	Congestion of animals waiting to enter into milking parlour	higher	n/d
	Repeat milking during the same milking procedure	higher	n/d

¹ In bulk-tank milk; ² n/d: not described.

Additionally, incorrect or inappropriate practices during the milking routine contribute to bacterial entrance into mammary glands and impairment of teat defence mechanisms, which lead to higher rates of intramammary infections and consequently higher somatic cell counts in bulk-tank milk [65,66]. During the milking procedure, teat health can be compromised: a higher vacuum level or increased pulsation rate can lead to teat compression [67] and increased pressure on the teat end, leading to local oedema [68]; indeed, lesions in teat tissue were found to be more severe in cases of higher vacuum during milking [69]. Animal overmilking (defined as the point when milk inflow into the teat cistern is lower than outflow through the teat canal [70]) has been described more frequently on goat than on sheep farms [70]. This is the result of a longer milking procedure in the former species, which can exert stronger teatcup compression and vacuum load on teats [7], adversely affecting teat condition. Overmilking can also result from machine- or hand-stripping of the mammary glands of ewes or does [71]; this takes place at the end of the milking procedure for each animal to obtain the residual milk that has not been removed by the machine [72,73,74,75]. This can be observed more often in animals with poor udder conformation, where an increased number of manual interventions (e.g., stripping, handling for teatcup plug-in) is required. For example, ewes with lax suspensory ligaments usually develop horizontally orientated teats, which may lead to milk accumulation within the teat cistern [75,76]. Some associations between udder morphological traits and milking-related variables have been reported by various authors (Table 5).

Table 5. Details of associations between udder morphological traits and milking-related variables in sheep and goat.

Udder Morphological Traits	Milking-Related Variables	Type of Association	Reference
Sheep
Udder attachment	Somatic cell counts	negative	[77]
Udder depth	Milk yield	positive	[78]
	Milk yield	negative	[79,80]
	Somatic cell counts	negative	[81]
Udder circumference	Milk yield	positive	[78]
Teat placement	Milk yield	negative	[76]
Teat dimensions	Milk yield	negative	[81,82]
	Somatic cell counts	positive	[81]
	Teatcup detachment	positive	[83,84]
Goats
Udder volume	Milk yield	positive	[72]

The proper milking order to be followed for minimising transmission of mammary pathogens during the milking procedure is as follows: (i) animals in their first lactation period, mastitis-free; (ii) animals in the second and later lactation periods, with low somatic cell counts; (iii) animals with high somatic cell counts; (iv) animals with clinical mastitis [17]. Application of a wrong milking order can be an important contributor to transmission of such pathogens.

Post milking teat disinfection (‘teat dipping’) is not frequently applied on small ruminant farms, despite the protection afforded against bacterial transmission and mammary infection. In Greece, the proportion of small ruminant farms where post-milking teat disinfection is used has been found to range between 14% and 20% [13,85], whilst in Pakistan it was found to be 27% [86]. Omission of this practice has been associated with subclinical mastitis [85] and an increase in total bacterial counts in bulk-tank milk produced on farms [64]. However, the incorrect usage of teat-dipping products post-milking, specifically the application of highly concentrated products, can lead to chapped teats, in which cases the risk of mammary infection increases [66,87], while contaminated dipping solutions may facilitate the occurrence of mastitis by various pathogens (e.g., Pseudomonas aeruginosa [88] or Serratia marcescens [89]).

Nevertheless, overall, machine milking has been associated with lower frequency of mastitis in comparison to hand milking [85]. Marco Melero [90] reported that transmission of staphylococci can occur during milking through the hands of milkers; this aligned well with the findings of Mavrogianni et al. [91], who reported higher staphylococcal colonisation of the teat ducts of ewes on hand milking than on machine milking farms.

5. Cleaning and Maintenance of Milking Systems

An important aspect of proper care of the milking system is the cleaning procedure after the completion of a milking session, which should be performed according to the manufacturer’s guidelines and recommendations. The cleaning procedure includes a combination of chemical, thermal and physical processes that require minimal contact time with the equipment, in order to produce effective results. During the three main phases (i.e., pre-rinse, washing, post-rinse), key cleaning determinants (i.e., water temperature, duration of each phase, detergent concentration and volume, turbulence of circulating water) must be maintained in accordance with appropriate recommendations and settings [92].

5.1. Quality of Water

First, good-quality water should be used for the cleaning [93]. Contaminated water, obtained from improper open water supplies, can contribute to transfer of microbes [94]. Quintana et al. [95] reported the detection of yeasts in milk following contamination from the milking equipment, whilst on dairy cattle farms, coliforms originating from the cleaning water were detected by Perkins et al. [96].

The amount and turbulence of circulating water are important for the physical removal of residues deposited on the equipment [92].

5.2. Temperature of Water

The lowest temperature of water during the pre-rinse phase should be set at 38 °C, which is the melting point of milk fat, aiming to facilitate liquification of milk depositions on the pipelines, as reported in studies performed in dairy cattle farms [97]. Temperatures below 35 °C can lead to bacterial growth in milk (e.g., coliforms and Pseudomonas spp.) and to solidification of milk fat residues on the equipment, as also reported in studies performed in dairy cattle farms [25,98].

The upper limit of the water temperature during the washing phase should be 75 °C, which must not be exceeded, as this can facilitate protein deposition on equipment, leading to irregular findings on teatcup material [25,97,99,100]. In studies conducted in dairy small ruminant farms, the average temperature used for the washing water for milking equipment was found, among all farms in the respective studies, to be 69 °C (Spain) [18] or 70 °C (Greece) [13]. For dairy cattle farms, Reinemann et al. [97] have suggested that the optimal temperature ranges for washing water are 38 °C to 55 °C (pre-rinse phase) and 43 °C to 77 °C (washing phase). In contrast, Keown and Kononoff [101] suggested, also for dairy cattle farms, that the optimal temperature range for washing water is 60 °C to 73 °C (washing phase).

Notably, Lianou et al. [102] reported that when the water temperature was below 70 °C or over 90 °C, there was a significant increase in biofilm-forming staphylococci in the bulk-tank milk; these bacteria may lead to potential problems in dairy processing plants, as they adhere onto stainless steel surfaces [103] and can be further transferred to cheese and other dairy products [104].

5.3. Use of Detergents and Disinfectants

With regard to use of detergents during the cleaning process, alternations between alkaline and acidic detergents should be applied periodically. Alkaline detergents are effective at removing organic residues, including milk proteins and fats; these detergents act better in situations with increased temperature and lower water hardness. A study conducted in dairy cattle farms indicated that acid detergents could be used for the removal of mineral deposits from water and milk and for the elimination of milk stone [97]. Sodium hypochlorite, which is often contained in liquid detergents, can lead to trichloromethane residues in the bulk-tank milk [105]. In view of this, Gleeson et al. [106] suggested the use of peracetic acid as an alternative option.

The frequency of acid rinsing of milking parlours after the completion of the milking session is determined by the quality of water used for cleaning. Notably, the frequent use of disinfectants has been associated with the development of antibiotic resistance [107,108] and teatcup material deterioration [95]. However, the omission of the use of acid solutions, especially in areas with hard water, can lead to milk stone appearance on the pipes of milking systems [25]. Increased water hardness may hinder effectiveness of cleaning chemicals and can favour development of deposits on parts of milking systems on dairy cattle farms [109,110]; for example, an association has been reported with increased water hardness and higher total bacterial counts in the bulk-tank milk [110].

The application of faulty post-milking cleaning that leaves dirty teatcups with milk residues can contribute to bacterial transmission among milked ewes [99], as it is a suitable micro-environment for staphylococcal growth. Indeed, staphylococcal dissemination occurs mainly through contaminated milking equipment, whereon these bacteria can grow, multiply and disseminate quickly [111], and are thereafter transferred to animals milked from these contaminated teatcups [25]. The parts of milking equipment where pipelines join are considered to have higher number of bacteria [112] and thus require particular attention during the cleaning process.

Overall, the strict application of the washing protocol (including the consistent availability of hot water) is considered a challenging point on dairy cattle farms [113].

5.4. Maintenance Schedule and Teatcup Replacement

The timely and appropriate servicing of the milking system is also important. Lianou et al. [44] reported that increased intervals between maintenance visits and the delayed servicing of milking systems have been associated with higher somatic cell counts produced on these farms. In this respect, it is noted that the use of unsuitable or worn-out teatcups can be risk factors for intramammary infections, due to causing lesions in teats (e.g., congestion and oedema) and compromising defence mechanisms. Teats too short or too long for a specific teatcup might not have the proper fit and compression during milking. Use of damaged or overused teatcups, or of teatcups that are improperly positioned or not designed for the animal species to be milked, can also lead to lesions on teats [48].

6. Future Directions

Technological advances in milking equipment can offer opportunities for enhancing animal welfare, milk quality and profitability on dairy small ruminant farms. Future developments are expected to make these more easily accessible and cost-effective for farmers, thus facilitating wider adoption and improved flock or herd management.

Farms will benefit from applying precision technologies, which will use data from tools related to animal genetic background, animal behavioural indicators, environmental conditions and disease-related functions for more holistic and cost-effective farm management [114]. Use of automations in the milking systems of dairy sheep or goat farms will help farmers to improve farm efficiency, to save time and labour costs, to enhance the health status of flocks or herds and to obtain data to be used in farm health management decision making [53].

In this context, integration of sensor-based technologies in milking systems will be able to provide real-time information regarding quality of milk produced at individual and farm levels. This can be extended to include early detection of subclinical mastitis in a farm’s animals. Moreover, the introduction of automatic system flushing in-between animals during the milking procedure [57] can contribute to breaking chains of potential bacterial transfer and reducing animal infections during milking. A device that can improve the efficacy of the milking procedure and reduce the necessary time is the addition of automatic removal of clusters from ewes or does.

These technologies were developed initially for the dairy cattle industry and are being evaluated for potential use on small ruminant farms. Nevertheless, this may not be an easy and straightforward process. Indeed, there are differences between cattle and small ruminant farming. A primary one is that these are different animal species; significantly, there is a large diversity of breeds among dairy sheep and goats, a notable consequence of which is the broad anatomic variance among them at udder and teat level. Other differences include the farmers’ demographic and educational characteristics, and a reluctance has been reported among small ruminant farmers to adopt new technologies [115]. Additionally, there are also differences in the logistics and finances of each industry. Therefore, potential use of these technologies could be a gradual and more conservative process than in the cattle industry.

7. Conclusions

A variety of milking systems are available for use on dairy sheep and goat farms. Proper regulation of the milking system and correct monitoring of milking equipment are essential, as technical flaws or suboptimal function can significantly contribute to reducing the quality of milk produced on farms and also increase the risk of intramammary infections. Although machine milking has certainly contributed to improved efficiency, milk quality and labour conditions, flaws in system function may adversely affect mammary health. Relevant technological advances will be applied in the future on dairy small ruminant farms, although the pace of their integration will be cautious and slower than on cattle farms.