A Review of Alfalfa Drying Technology and Equipment Throughout the Whole Process

Abstract

Alfalfa, as a high-quality forage crop, undergoes a drying process that is critical to its product quality and commercial value. This paper systematically reviews research progress on alfalfa drying technologies and equipment throughout the entire process. First, it proposes a comprehensive production technology model covering three core stages: drying pretreatment, drying conditioning and optimization, and product quality control. This model emphasizes adaptability to material characteristics, processing methods, product quality, and economic efficiency. Second, it delves into the drying mechanisms of alfalfa, detailing the forms of water presence (free water or bound water), migration pathways, and the three-stage water loss periods: constant rate, first falling rate, and second falling rate. It identifies “asynchronous drying of stems and leaves” as the core issue causing nutrient loss and technical challenges. Subsequently, a comprehensive review was conducted on pre-treatment equipment such as mowing and flattening, as well as various drying methods including natural drying, hot-air drying, solar drying, and microwave drying. The principles, characteristics, and impacts of these methods on alfalfa quality were evaluated. Additionally, a comprehensive quality assessment system for alfalfa hay was summarized, incorporating physical, chemical, and biological methods. Finally, future development directions are proposed: developing domestically produced, intelligent drying equipment; integrating clean energy to reduce energy consumption; and achieving precise control of drying processes through establishing multi-scale heat and mass transfer models. These efforts will advance China’s alfalfa drying industry toward standardization, integration, and intelligence, ensuring a stable supply of high-quality hay.

1. Introduction

Alfalfa (Medicago sativa L.) is a perennial herbaceous legume renowned as the “king of forage” due to its nutrient-rich stems and leaves, excellent palatability, and strong adaptability [1,2]. It is widely cultivated across Asia, Europe, North America, and South America, as shown in Figure 1 [3]. With a history of dietary consumption, alfalfa offers health benefits including antioxidant properties, cardiovascular protection, and cholesterol reduction, making it a valuable health food crop [4,5,6]. In recent years, China’s rapid livestock industry development has driven continuous optimization of agricultural industrial structures; the traditional “grain–cash crop–forage” ternary structure has been restructured into a “grain–cash crop–forage crop–dedicated forage” quaternary structure [7,8]. The traditional feeding methods relying solely on crop straw supplemented with concentrates are now insufficient to meet modern livestock demands. This necessitates high-yielding quality forage to satisfy the growing demand for premium feed [9]. As presented in Figure 2, China’s consumption and import demand for alfalfa have shown a steady increase annually.

The alfalfa industry currently faces several challenges, including insufficient supply of premium alfalfa, uneven product quality, and relatively low industrialization. This necessitates the urgent establishment of an efficient, low-loss, whole-process drying technological model. Drying is an essential post-harvest process for alfalfa [10]. Appropriate drying methods and techniques are crucial for preserving alfalfa’s quality, color, and aroma, yielding high-quality forage products that enhance market competitiveness. Conversely, suboptimal drying techniques and parameters may degrade forage quality, potentially leading to nutrient loss, scorching, mold growth or green loss [11].

Currently, alfalfa drying technology in China remains underdeveloped. To enhance production standards and overcome limitations imposed by inadequate equipment and technical systems, it is imperative to conduct comparative studies of diverse drying equipment, methods, and processes both domestically and internationally, coupled with thorough evaluations of hay product quality. Based on China’s specific industrial conditions, efforts should prioritize independent R&D of localized drying equipment and processes while assimilating advanced foreign technologies. This approach will fill equipment and technical gaps, strengthen weak links, and establish a comprehensive production technology model for alfalfa drying that meets China’s high-quality alfalfa production demands. This will resolve the critical issues of market development lag and the shortage of high-quality forage in China’s alfalfa industry.

2. The Whole-Process Drying Technological Model for Alfalfa

The whole-process drying technological model for alfalfa refers to an integrated system focused on drying alfalfa or alfalfa bales. It prioritizes pre-drying treatment, drying modulation, drying optimization, and product quality control with the objectives of optimizing the drying process, promoting advanced drying equipment, enhancing drying efficiency, improving alfalfa product quality and establishing a comprehensive alfalfa drying production model to meet market demands. While ensuring whole-process coverage across production stages, this technology critically analyzes drying methods, process routes, and parameter control within key steps. It does so by prioritizing regional factors such as natural environmental conditions, production scale, economic constraints, and available drying equipment. Consequently, it enables the development of diversified whole-process drying models characterized by material-specific customization, environmental adaptability, regional suitability, process compatibility and economic scalability. Therefore, to achieve comprehensive coverage of production stages, full compatibility of methods and processes, full adaptability to operational environments, and guaranteed product quality, establishing alfalfa drying production technology also requires meeting corresponding criteria, primarily including:

(1)

Material property adaptability. The alfalfa drying technology must be adapted to the relevant material properties. Significant variations exist in the nutritional content at different growth stages and plant positions, as well as in drying rates. Cutting time significantly affects initial moisture content and nutritional composition, which directly influences subsequent drying. Furthermore, bale density and shape significantly impact drying equipment selection, process parameters, and drying uniformity. Therefore, when selecting alfalfa harvesting and processing methods, appropriate cutting times must be chosen based on these material characteristics. Developing and adopting suitable technologies—such as cutting and flattening, and matching bale density—maximizes nutrient retention and enhances drying rates.

(2)

Adaptability of methods and processes. For alfalfa drying, thin-layer drying methods (including natural drying, hot-air drying, and solar drying) have been relatively well-studied. Drying processes primarily encompass traditional drying, wet-harvest drying, phased drying, and dynamic drying. Different drying methods and processes require distinct equipment. Therefore, it is essential to develop specialized alfalfa drying machinery tailored to the major alfalfa cultivation and production regions in China. A key focus should be overcoming the bottleneck of lacking efficient, large-scale, bale-based drying equipment, thereby enhancing production efficiency to meet market demands.

(3)

Product quality adaptability. The ultimate goal of whole-process drying technology is to produce high-quality products. Alfalfa varieties exhibit differences in nutritional content. Factors such as mowing time, field drying duration, tedding frequency, bale moisture content, and drying processes all impact final product quality. Therefore, all production stages must be comprehensively considered to establish a holistic evaluation system to achieve the objective of efficient production of premium alfalfa.

(4)

Economic Viability. The whole-process drying technological model should integrate multiple factors affecting economic viability, including production scale, drying equipment, processing methods, and cost inputs. This integration enables the provision of tailored technical models and integrated equipment-process solutions aligned with practical production requirements.

Building upon the above requirements and considering the actual production conditions in China’s major alfalfa cultivation regions, this study focuses on key technological breakthroughs in alfalfa harvesting, pre-treatment, drying equipment and processes based on existing production models and research. Through integrated equipment design, process innovation, optimized method configuration, performance testing, and iterative refinement, a whole-process drying technological model adapted to China’s natural conditions is proposed. This model aims to progressively achieve technological autonomy, localized equipment manufacturing, batch-scale production, and controllable product quality. The whole-process drying technological model is categorized into three key stages: pre-drying treatment, alfalfa drying, and product quality control. The specific technical process flow is illustrated in Figure 3.

3. Analysis of Alfalfa Drying Mechanism

Alfalfa drying is not a simple process but involves complex simultaneous heat, mass, and momentum transfer phenomena. Gaining a deep understanding of the moisture, heat, and mass transfer processes and mechanisms occurring during actual alfalfa drying at different scales is crucial for improving product quality and enhancing drying efficiency.

3.1. Forms of Water in Alfalfa

Alfalfa is a non-saturated, moisture-retaining substance with capillary structures. Hydrophilic substances present on their surfaces and internally enable them to adsorb large amounts of water [12,13]. Freshly harvested alfalfa typically contains 75–85% moisture content (wet basis), comprising water, proteins, starch, fats, and sugars. The moisture distribution is heterogeneous, primarily categorized as free water and bound water [14,15]. Free water, residing in macro-pores such as cell lumens, intercellular spaces, and vessels, is most readily removed during drying. Bound water is further categorized into physicochemical bound water and chemical bound water. Physicochemical bound water refers to water molecules bound to dry matter within alfalfa plants, further classified as adsorption-bound water that attaches to particle surfaces and penetrates internal matrices, and osmotic-bound water that diffuses through cell walls driven by concentration gradients. Compared to free water, removing physicochemical bound water requires significantly greater energy expenditure [16]. Chemically bound water stabilized via chemical bonds (e.g., hydrogen bonds) and van der Waals forces with organic compounds. While removing physicochemical-bound water requires significantly greater energy than free water [16]. Chemically bound water refers to water molecules chemically bonded to organic substances within alfalfa particles, such as proteins, starch, and fats, through chemical bonds and intermolecular forces like van der Waals forces. Water molecules in chemically bound water maintain a strict quantitative relationship with dry matter and do not participate in chemical reactions within alfalfa particles [17,18]. Therefore, removing chemically bound water is unnecessary during alfalfa drying. The characteristics and distribution patterns of moisture in alfalfa are summarized in

Table 1. Forms and characteristics of moisture in alfalfa.

Forms	Location	Characteristics	Proportion	Removal Difficulty
Free water	The moisture is primarily found within the cell, including the vacuole (for the most part), the cytoplasm, as well as the vessels of the vascular system and the intercellular spaces.	The most readily removable moisture during drying exhibits vapor pressure essentially equivalent to that of pure water at identical temperatures, with free water constituting the primary evaporative component during the initial drying phase.	85–90%	easy
Physicochemical bound water	Tightly bound to the surfaces of macromolecules within the alfalfa plant.	The water molecules are strongly bound to internal dry matter, capable of adsorbing onto surfaces or penetrating through surfaces into the interior. Its vapor pressure is significantly lower than that of pure water at equivalent temperatures.	10–15%	hard
Chemically bound water	Combined with organic substances such as proteins, starch, and fats within the internal structure of alfalfa.	This restricted-mobility water exhibits limited fluidity and evaporative capacity. Excessive removal may damage plant tissue structure (such as protein denaturation and cell wall rupture), affecting hay quality.		very hard

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3.2. Analysis of Moisture Transfer Processes in Alfalfa

The moisture migration from internal alfalfa tissues to the surface, where it subsequently evaporates into the air, constitutes a complex physical process. The drying process of alfalfa is divided into physiological and biochemical stages. The physiological phase spans from cutting until moisture content decreases to approximately 40% wet basis, during which living cells maintain metabolic activity dominated by catabolism. Respiratory metabolism converts starch to mono- and disaccharides while consuming simple sugars, concurrently degrading proteins into water-soluble nitrogenous compounds. Unable to obtain nutrients from the surrounding environment, the plant relies solely on consuming its own stored nutrients to sustain physiological activities [19]. The biochemical phase commences as moisture declines from 40% to the safe storage levels (<15% w.b.), characterized by cell death and cessation of respiration/transpiration. Enzymatic degradation predominates in this stage, with limited decomposition of macromolecular carbohydrates (starch, cellulose) but significant breakdown of soluble sugars, leading to water loss and reduced enzyme activity. This minimizes nutrient losses.

Free water in alfalfa plants is primarily lost through stomata, the vascular system, and intercellular spaces under the key driving force of the water potential gradient, resulting in a relatively rapid drying rate [20]. Significant moisture differentials between hydrated internal tissues and evaporation sites establish steep water potential gradients from core to surface. Key resistance factors at this stage include cell wall resistance, semipermeability of cell membranes, size and tortuosity of intercellular spaces, the waxy cuticle layer on stem epidermis, and tissue thickness. Liquid water migration pathways are systematically illustrated in

Table 2. Migration pathways and transport characteristics of liquid water in alfalfa.

Migration Pathways	Characteristics
Intracellular Migration	Intracellular water movement occurs through cytoplasmic streaming and diffusion.
Transmembrane Transport	Water traverses cell membranes to migrate between adjacent cells or into intercellular spaces through three principal pathways: the apoplastic route (via cell walls and intercellular spaces, exhibiting lower hydraulic resistance: 0.15–0.35 MPa·s/m), the symplastic pathway (through plasmodesmata connecting cytoplasms, pore diameter 30–60 nm), and the transcellular route.
Vascular Transport	In living tissues or immediately post-harvest, xylem vessels maintain transient water transport driven by capillary forces, though with limited efficacy and duration (typically <30 min).
Intercellular Diffusion	Water diffuses through intercellular spaces in both liquid and vapor phases.

and Figure 4.

Internal gaseous water diffusion occurs under specific conditions. When alfalfa exhibits elevated internal temperatures with a temperature gradient, moisture may vaporize within the plant tissue’s warmer regions (typically >45 °C) and subsequently diffuse as vapor toward cooler or lower-pressure areas. This phenomenon may be more pronounced during high-temperature drying but plays a relatively minor role in natural air-drying.

Surface evaporation, another form of water transfer in alfalfa, which generally occurs on the upper and lower epidermis of plant leaves and on the surface of stems exposed to air, is governed by the vapor pressure deficit (VPD = 1.2–3.5 kPa) between plant surface and ambient air, where greater VPD enhances the evaporative driving force. A relatively stationary aerodynamic boundary layer exists on the alfalfa plant surface, so moisture at the interface undergoes phase change to vapor by absorbing latent heat. This vapor must traverse this boundary layer via diffusion and convective transport before entering the bulk atmosphere.

The overall pathway of water transfer within alfalfa, driven by concentration gradients, is as follows: water migrates in liquid form toward the surface through pathways such as the cytoplasm, across membranes, and through intercellular spaces →upon reaching the surface, it absorbs thermal energy and vaporizes→the resulting water vapor diffuses through minute stomata or cuticle pores on the plant surface→diffuses through the aerodynamic boundary layer→enters the main airflow and is carried away.

3.3. Analysis of Alfalfa Moisture Loss Patterns

Alfalfa exhibits an initial moisture content of 70–80% (wet basis) at harvest, requiring reduction to below 14% for long-term storage [21]. Alfalfa drying, one of the most widely used drying methods, removes moisture through heat application. As a porous medium with interconnected voids/capillaries and a moisture-resistant cuticular layer, alfalfa primarily loses water via stomata, spaces, and the vascular system through conductive and convective heat transfer mechanisms [22]. From the perspective of transport phenomena, this is considered a process of heat and mass transfer both within and outside the material, thus involving two types of resistance: thermal resistance and mass transfer resistance [23].

Alfalfa dehydration is not uniform but typically exhibits distinct phase characteristics [24], primarily divided into three periods: the constant-rate drying period, the first falling-rate period, and the second falling-rate period, as shown in Figure 5.

The constant-rate period primarily involves evaporation of free water, characterized by rapid and relatively constant dehydration rates, with stable moisture loss per unit time. At this stage, evaporation mainly occurs from the free water on the plant surface and the internal free water very close to the surface. Alfalfa surfaces remain continuously saturated due to internal liquid diffusion rates matching or exceeding surface evaporation rates. Surface evaporation dominates this stage, with drying rates primarily governed by external conditions: ambient temperature, relative humidity, air velocity, and ventilation rate. Higher temperatures, lower humidity, greater wind speeds, and more vigorous air exchange accelerate drying rates [25]. Minimal internal transport resistance enables rapid moisture replenishment to surfaces. During this phase, visible wilting occurs while chlorophyll retention and tissue pliability persist.

During the first falling-rate period, alfalfa plants exhibited a marked reduction in water loss rate, which continued to decrease over time. By this stage, surface free water had largely evaporated, and the plant surfaces were no longer completely moist. The receding evaporation front migrates inward, establishing internal moisture diffusion as the rate-limiting factor governed by escalating cellular resistances: cell walls, cell membranes, and intercellular spaces within the tissues [23]. The influence of external drying conditions (temperature/humidity/ventilation volume/wind speed) on drying rate diminishes, while physical characteristics of the alfalfa plant, such as stem and leaf structure, thickness, wax layer, and stomatal state, become the primary factors affecting internal water diffusion. This prolonged phase induces chlorophyll degradation, lignification, and visible phenotypic transitions: color shift (deep green→light green or yellowish-green), stem hardening, and leaf brittleness. This stage typically involves a longer drying time and constitutes the main part of the drying process.

The second falling-rate period exhibits asymptotically declining dehydration rates approaching equilibrium moisture, governed primarily by the material’s inherent moisture-binding properties and ambient equilibrium relative humidity. This stage primarily involves the evaporation of bound water, which is tightly bound to plant tissues, requiring higher energy to break the chemical bonds—specifically hydrogen bonds—between it and the plant structure. Simultaneously, the diffusion pathway lengthens and resistance increases, as water must migrate from deeper, more microscopic, and tighter structures. Internal water diffusion resistance and the binding force between water molecules and the material become the dominant controlling factors. Excessively high temperatures during this stage may cause thermal damage such as Maillard reactions, charring, and nutrient loss. At this point, alfalfa plants exhibit typical hay characteristics: a yellowish-brown or light brown coloration, brittle stems prone to breakage, and leaves that detach easily.

The objective of drying is to reduce the moisture content to the safe storage level (approximately 15–18%). Excessively low moisture content leads to significant losses from leaf drop, while moisture content exceeding the safe level promotes mold growth and disease, compromising preservation and usability. Water activity represents the free water in food, indicating the energy state of water, and serves as a critical parameter for determining appropriate drying processes [26,27]. The nonlinear relationship between water content and water activity, which gives rise to the sigmoidal shape of the isotherm, suggests that water exists in different coupled states in foods at different water content levels. [13], as illustrated in Figure 6. Zone I represents the region up to the first inflection point (commonly referred to as the “knee”) in the sorption curve. This low-humidity range features water molecules strongly bound to functional groups like carboxyl and amino groups within the material, resulting in the lowest water activity. This water can be simply regarded as part of the solid matrix (bound water). Water in this zone cannot dissolve solutes, does not exhibit a plasticizing effect on food solids, and is unavailable to microorganisms. Consequently, dried foods remain relatively stable under low-humidity environmental conditions. When the total water content of a food is close to the boundary of zone II (which also includes zone I water), the water is primarily in the form of a saturated monolayer on food molecules (e.g., proteins and polysaccharides), covering all ionic, polar, and nonpolar surfaces. This primarily includes water bound by hydrogen bonds and water within capillaries <1 μm in diameter, with water activity ranging from 0.2 to 0.85. Water in this zone acts as a swelling agent and partial solvent, accelerating chemical reaction rates.

As the water content moves further into zone III, the molecular mobility (which is inversely proportional to viscosity) of water and food constituents increases by several orders of magnitude. At this stage, the water primarily consists of free water from capillary condensation with water activity values between 0.85 and 0.99. This fraction represents the least tightly bound and most readily mobile water within the material. It serves to dissolve and dilute components, while also facilitating chemical reactions and microbial growth [13]. The desorption isotherms of alfalfa leaves exhibit the zone II isotherm. The desorption isotherms of top stems, middle stems, and root stems on an alfalfa show similar isotherm patterns across different temperatures, with no significant inflection points where the tangent to the curve crosses the curve itself. The temperature significantly influences desorption isotherms, and for alfalfa stems and leaves, the desorption isotherms shift successively to the lower right as temperature increases, while maintaining similar curve shapes [28,29].

In actual drying processes, it is not common to completely remove all bound water, as this is unnecessary and detrimental. Alfalfa exhibits certain unique characteristics. Its leaves are thin with a large surface area, leading to extremely rapid water loss during the constant rate stage. However, its stems are relatively thick and possess a waxy cuticle layer, resulting in very slow water loss during the decreasing rate stage (especially in its later phase). This stem-leaf drying characteristic is the primary cause of excessive leaf drying and subsequent loss during alfalfa drying, representing a key technical challenge. The asynchronous drying of stems and leaves is a critical factor affecting alfalfa hay quality, including protein content and palatability [30].

3.4. Simulation Analysis of Alfalfa Drying

During the drying process of materials such as alfalfa, drying efficiency and product quality are closely related to the internal temperature and moisture distribution within the materials [31]. Investigating drying mechanisms using conventional methods is challenging and lacks guaranteed accuracy. With advances in computer technology, an increasing number of researchers have begun using numerical simulation software to study fluid flow, heat transfer, and mass transfer during food drying processes [32,33,34]. Du utilized COMSOL software to simulate the heat and mass transfer mechanisms in hot-air drying of round hay bales, analyzing how different operating parameters (air duct opening diameter, air velocity, temperature) affect the uniformity of physical field distributions (air velocity, temperature, humidity) within the bales [35]. In a solar-assisted air-source heat pump alfalfa drying system, simulations of temperature and air velocity distributions within the drying chamber provided visual evidence for structural optimization and operational parameter selection [36]. Using a CFD multi-physics simulation system, the air distribution and drying process of four round bale dryer designs were analyzed. This clarified the advantages of radial drying design under ideal conditions and emphasized the decisive influence of bale density uniformity on drying efficiency [37]. The heat and mass transfer process in alfalfa involves expelling moisture through heat flow, influenced by multiple factors including drying temperature, humidity gradient, and drying equipment. Simulation enables a deeper understanding of the temperature and humidity fields within alfalfa under specific process parameters, thereby elucidating the moisture transport mechanism.

4. Alfalfa Drying Technology and Equipment

4.1. Alfalfa Pre-Drying Treatment Technology and Equipment

Alfalfa pre-drying treatments primarily encompass cutting–flattening techniques, as well as mechanized baling technology for alfalfa.

4.1.1. Alfalfa Cutting–Flattening Technology and Equipment

During the alfalfa harvesting process, mechanized cutting–flattening constitutes the primary cutting technology. This technique employs counter-rotating ribbed rollers to mechanically compromise the cuticular wax layer and epidermal integrity of stems, generating micro-fissures that establish accelerated moisture migration pathways and expanded evaporation surfaces. Consequently, the drying rate of the stems approaches that of the leaves, shortening drying time and improving product quality [38,39,40]. This technology artificially creates numerous voids and pathways, exposing previously enclosed vascular bundles (particularly xylem vessels) directly to air. Water can thus evaporate more directly from the vessels without slow diffusion through thick stem wall tissues. Simultaneously, the increased evaporation surface area allows more moisture to come into direct contact with dry air, accelerating evaporation [41].

Prominent alfalfa cutting–flattener manufacturers include New Holland and John Deere, Kuhn, CLAAS, Macdon and ROTEX. Research on mower conditioners in developed countries like Europe and the United States began earlier, resulting in mature product technology, excellent operational performance, and high automation levels. However, prohibitive costs and extended maintenance cycles limit their widespread adoption in China. New Holland’s trailed units employ reciprocating blades with automated headland positioning, enhancing field efficiency [42]. Kuhn’s FC813 model adjusts conditioning intensity based on crop density and moisture content. It features agile turning, excellent visibility, easy header maintenance, long service life, and high reliability. John Deere’s 635 utilizes a gear-driven transmission with high-flotation tires, delivering 3.5 m cutting width and 67-112 kW PTO power. In recent years, researchers have designed cutting–flattener tailored to China’s topography and alfalfa cultivation characteristics, specifically for mountainous and hilly regions, filling a gap in alfalfa harvesting machinery for these areas [43,44,45]. The Weimeng MC840 disk mower employs eight disk-type cutting units with adjustable cutting angles, enabling rapid blade replacement and compact transport width. The Weimeng MC840 disk mower employs eight disk-type cutting blades with adjustable cutting angles, enabling rapid blade replacement and compact transport width. The Jixuan 9GXY-3.0 cutting–flattener utilizes disk blades primarily for alfalfa harvesting. Its towed structure ensures robust construction and high reliability. Typical alfalfa cutting–flattening equipment is shown in

Table 3. Typical alfalfa cutting–flattening equipment.

Model	Cutting–Flattener	Structural Features and Key Performance
New Holland C320R Disk Cutting–Flattener		Featuring a three-dimensional suspension cutting unit, it adapts to various terrain conditions, ensuring uniform mowing results and offering multiple optional compacting rollers. The working width is 3.15 m, with 16 cutting blades.
CLAAS DISCO Series Cutting–Flattener		This suspended mower, equipped with the CLAAS MAX CUT cutting system, delivers validated cutting quality and reliable performance. Designed for medium-sized farms, the high-performance unit offers a selectable working width range of 2.6–3.4 m with 7–8 cutter disks.
Huade 9GBXQ-3.0 Cutting–Flattener		Featuring a traction suspension system for enhanced operational stability, the cutter head allows adjustment of stubble height and angle while delivering excellent ground contouring performance. With a working width of 3.0 m, it incorporates seven cutter disks operating at 2980 rpm, complemented by a flattening roller rotating at 980 rpm.
LOVOL MC290-Y Cutting–Flattener		Equipped with a 4.9 m wide header, it delivers fast operating speeds and high harvesting efficiency, with flexible adjustment of cutter speed and stubble height. Four flattening rollers perform two-stage flattening.

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4.1.2. Mechanized Alfalfa Baling Technology and Equipment

Alfalfa is an ideal forage crop for modern agricultural and livestock development. To prevent rain damage in the field and facilitate transportation and storage, enhancing alfalfa’s marketability [46], it is typically compressed into bales. Baling equipment is classified into rectangular and round balers based on final bale geometry.

Rectangular balers typically comprise a pickup mechanism, feeding system, and baling chamber, enabling the completion of alfalfa pick-up feeding, compression baling, and knot-tying unloading operations in a single pass. Based on power source, they are categorized into trailed square balers and self-propelled square balers, with trailed balers being more widely used. Representative manufacturers include CNH, CLAAS, Massey Ferguson, New Holland, John Deere, and Weston. ROBB et al. developed a rectangular baler design [47], while KENNETH et al. designed an integrated square bale compactor combined with a shredder, available in both trailed and self-propelled configurations [48]. The Massey Ferguson MF1840S (small square, twin-twine) features a 2.2 m pickup width and 450 mm × 360 mm (W × H) bale chamber, noted for minimal tine clearance, efficient pickup, and maneuverability. New Holland BC5070 (small square, twin-twine) offers a 1900 mm pickup width and 460 mm × 360 mm (W × H) chamber, optimized for large fields with its extended pickup profile. China Huade 9YFLD-2.2 model (1200 mm × 900 mm W × H chamber, bale length ≤ 2743 mm, 2200 mm pickup) employs a unique pre-compression system incorporating a rotary cutter rotor, feeding tines, and guide components. The 9YFZ-2.2C square baler produced by Shunbang Company features a compression chamber cross-section of 450 mm × 350 mm (width × height), bale length of 800 mm, and working width of 2200 mm. This machine features an innovative pick-up system, incorporating a cutting and shredding device to cut, crush, and convey crops. This replaces the spring-tine pick-up and feed fork mechanisms traditionally used in balers.

Round balers represent another essential alfalfa harvesting technology, characterized by structural simplicity, low failure rate, and ease of maintenance. The round bales they produce are convenient for storage, silage, and transportation. Based on bale formation technology, round balers can be categorized into external steel roller wrapping [49] and internal belt wrapping types. Internationally renowned manufacturers of round balers include CLAAS, Vermeer, McHale, New Holland, Kverneland, KUHN, KRONE, John Deere, and AGCO. Their balers exhibit distinct characteristics and advantages in mechanical structure, power systems, hydraulic systems, and control systems, maintaining leading global standards. The CLAAS ROLLANT series incorporates reinforced compression rollers, helical cut-feeding mechanisms, hydraulic MPS PLUS pressure systems, operator terminals, and centralized lubrication, delivering high reliability and bale density under demanding conditions. The United States applied “rope-net wrapping” technology to variable-diameter bale chambers, significantly boosting baling efficiency [50]. Vermeer introduced the world’s first self-propelled round baler featuring advanced telematics and a compact turning radius in 2017, while Andy Collings et al. pioneered continuous-operation balers utilizing pre-compression chambers in 2011 [51]. Simmons et al. integrated sensors on the bale chamber sidewalls to monitor density across different cross-sections during compression, enabling feedback-based adjustment of alfalfa feed intake [52]. Dutertre et al. addressed the issue of forage accumulation in chamber sidewalls by increasing the protrusion thickness of the outer grooves on the steel rollers compared to the central grooves. This optimization significantly improved the compression effect on the feed material, greatly alleviating forage buildup [53]. China now boasts relatively mature round baler manufacturers, such as Shanghai Star Modern Agricultural Machinery Co., Ltd. and Inner Mongolia Huade Forage Machinery Co., Ltd. In 2023, Huade launched the 9YGQ-2.2D cutting and picking round baler, featuring fully automated feeding, wrapping, cutting net technology, automatic bale unloading, and automatic lubrication. Equipped with a GPS positioning system, it enables integrated baling operations for straw in the field, producing bales measuring 1.2 m × 1.4 m. Shanghai Star has developed and improved a new generation of products. Through operations including pickup, feeding, compression, baling, net wrapping, and bale discharge, it forms uniformly dense round bales. This machine is primarily suitable for harvesting straw from various crops such as rice, wheat, and corn, as well as forage grass.

Concurrently, researchers globally have conducted studies on various aspects of balers, including knotters [54,55,56,57,58], pickup mechanisms [59], bale blockages [60,61,62], baling mechanisms [63,64,65], and operational parameters [66,67,68,69], achieving notable research outcomes [70,71,72,73,74,75]. Typical alfalfa baling equipment is summarized in

Table 4. Typical alfalfa baling equipment.

Model	Alfalfa Baling Equipment	Structural Features and Key Performance
MF2270XD High-Density Large Rectangular Baler		The system features a 2.26 m pickup width and produces 1.2m in length and 0.9m in width bales, demonstrating uniform feeding, high operational efficiency, high-density bale formation, proven reliability, and intuitive operator control.
John Deere L330 Large Rectangular Baler		The baler incorporates an extended pickup head with a 210 cm intake width, producing bales adjustable from 60 to 300 cm in length with standardized cross-sections of 80 cm (width) × 90 cm (height). It features a pre-compression chamber with a single-tine baler and feed rollers with an exceptionally large processing capacity.
STAR 9YFQ-1.8 (THB3060) Rectangular Balerx		The implement utilizes a low-profile spring-tooth pick-up roller with a 2240 mm working width, producing bales with a 360 mm × 460 mm cross-sectional area and adjustable density ranging from 110 to 180 kg/m3.
John Deere F441R Round baler		The baler features a five-row spring-tooth pick-up unit with a 2.2 m working width. Its baling chamber diameter is adjustable between 1.25 and 1.35 m, which, combined with a fixed chamber width of 1.21 m, enables high-density bale production.
CLAAS ROLLANT 540 RC Round baler		Equipped with a new-type grass-pressing roller and high-strength spiral cutting feed mechanism, it produces bales measuring 1.25 m in diameter and 1.2 m in width. With a pick-up width of 2.1 m, it features fast baling speed, low failure rate, and high reliability, making it suitable for harsh operating conditions.
Kvernel Fast Bale Round baler		The system incorporates wide dual-cam pickup guidance technology, non-stop baling with primary wrapping-chamber-only mode switching, and automatic bale-unloading and flipping technology. It can achieve continuous non-stop operation encompassing pick-up, cutting and feeding, baling, net wrapping, film wrapping, and bale unloading.
Model 9YGQ-2.2D Cutting-Pickup Round Baler		The baling chamber integrates 18 heavy-duty steel rollers to form uniformly dense round bales, operating at a 2.2 m pickup width with standardized bale dimensions of 1.2 m × 1.4 m. This system performs integrated field operations including cutting, tedding, feeding, baling, and net wrapping on both standing and windrowed crop residues, and is equipped with GPS positioning technology.

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4.2. Alfalfa Drying Methodology and Equipment

Drying is one of the most frequently employed methods to prolong the shelf life of alfalfa, as it can reduce the water activity, inhibit the growth of microorganisms and hinder various chemical reactions, and it has been conducted by humankind around the world for thousands of years [76,77,78]. The drying method has a significant impact on the nutritional components, color, and rehydration ratio (RR) of forage [79,80,81]. Current prevalent drying methodologies include air drying [82], heated-air drying [83], thin-layer drying [84], deep-bed drying [85], microwave radiation-assisted drying [86], superheated steam drying [87], solar-energy drying [88], leaf-stem segregated drying [89], and chemical desiccant drying [90].

4.2.1. Air Drying (AD)

AD utilizes solar radiation and ambient airflow to dissipate moisture from alfalfa plants into the atmosphere, categorized as sun-drying or shade-drying based on whether the material is exposed to direct sunlight. After mowing, alfalfa is piled in the field for drying, typically at a thickness of 10–15 cm. The piles are regularly turned, with the frequency determined by the pile thickness. Alfalfa with a thickness of 15 cm requires turning 6–10 h after cutting, while alfalfa with a thickness of 10 cm requires turning 24 h after cutting. When alfalfa moisture content reaches 40–50%, arrange it into loose rows. At 35–40% moisture, form small piles or ridges. Bale and store when the moisture drops below 18%. This method is highly susceptible to environmental and climatic conditions. Alfalfa is prone to moisture regain during nighttime and early morning hours. Raking and turning during drying can cause leaf loss. Rainfall and overcast, humid weather facilitate microbial growth such as mold, leading to spoilage. Additionally, prolonged exposure to direct sunlight in open fields accelerates the degradation of chlorophyll, carotene, and other compounds [91,92], causing color fading and yellowing, which diminishes the nutritional and commercial value of alfalfa hay [93]. Studies indicate that rainfall can result in crude protein losses of approximately 35% [94], while consecutive rainy days may cause over 50% nutrient loss [95]. Drying rate correlates positively with solar irradiance, ambient temperature, and wind velocity, but inversely with air humidity. Shade-drying similarly suffers from leaf loss, mold risk, and climatic vulnerability.

In summary, while requiring minimal capital investment and low operational costs, AD entails extended processing duration, high labor intensity, substantial quality deterioration, uncontrollable degradation, and pronounced environmental constraints.

4.2.2. Hot-Air Drying (HAD) and Equipment

HAD is a commonly used mechanical drying method that employs heated air as the drying medium. Through heat and mass transfer with the forage, it achieves the removal of moisture from the forage. This process promotes the evaporation of water and volatile compounds [96] and creates an environment favorable for oxidation reactions [97]. Compared with AD, HAD exhibits greater controllability and efficiency. Key parameters, namely air temperature, air velocity, forage density, and ventilation rate, influence the drying kinetics and the characteristics of the final product. By adjusting these process parameters, desired product characteristics can be achieved. Research and optimization of drying parameters for numerous herbaceous plants have been conducted to produce higher-quality products [96]. Research has verified positive correlations between the drying rate and both air temperature and air velocity [98]. Higher temperatures enhance drying uniformity and preserve nutrients during industrial-scale processing. However, excessive heat can lead to protein degradation and reduce forage quality, highlighting the necessity of selecting optimal temperature. Common HAD equipment includes rotary drum dryers, tubular dryers, and implantable dryers. Nevertheless, research has primarily concentrated on process parameters rather than drying equipment itself [23,99].

HAD may result in lower total volatile compound content [100], significant deterioration of herbal aroma, and pigment degradation [101]. Additionally, this method suffers from high product shrinkage rates [96] and elevated energy consumption.

4.2.3. Solar-Energy Drying and Equipment

Solar energy represents a clean, renewable resource increasingly applied to alfalfa drying to reduce energy consumption—a critical research focus in contemporary drying technology. Solar drying utilizes solar radiation energy to heat air as the drying medium. The temperature of hot air provided by direct solar heating is limited and subject to significant fluctuations due to weather conditions, typically requiring integration with wet harvesting processes. Alfalfa exhibits maximal pliability and minimal handling loss at 35–45% MC, and solar drying at this stage substantially enhances drying rates. Solar dryers reduced peppermint leaf drying time by 23–25% compared to conventional greenhouse dryers [102]. When coupled with moisture-enhanced harvesting, solar thermal systems heated air to dry thick layers of loose hay and hay bales, achieving 60 h for loose hay and 48 h for bales, with a bale processing capacity of 3.8 t/d [103]. To address temperature limitations, solar-assisted heat pump systems can be integrated to improve efficiency [104], producing mint leaf quality comparable to sun-cured products [105]. Integrated systems combining solar collectors, heat exchangers, reflectors, primary/auxiliary drying chambers, and water heaters reduced chamomile drying time by 50% while increasing volatile oil content compared to direct sun-drying [106].

Solar drying equipment is typically integrated with other heat sources, not only conserving electrical energy consumption but also enhancing renewable energy utilization in the drying process while overcoming the instability inherent in solar drying. Yu et al. designed a heat pump drying system based on a solar-assisted flash dryer with steam injection circulation, investigating the effects of structural and operational conditions on open, bypass, and closed air circulation systems [107]. Naemsai et al. integrated heat recovery units into solar-assisted heat pump dryers, evaluating thermal efficiency and economic viability [108]. Rocha et al. developed a direct-expansion solar-assisted heat pump system, proposing an air-solar dual-source evaporator alternative to mitigate performance degradation under low solar radiation conditions [109].

Researchers have also conducted extensive studies on the energy efficiency and economic analysis of renewable energy drying systems. Gao utilized a TGS-2 SASHP solar-assisted air-source heat pump drying system to analyze energy consumption based on the dehumidification capacity per unit energy input. They concluded that low temperatures, high wind speeds, and moderate densities enhance the specific moisture extraction ratio (SMER), indicating higher energy utilization efficiency. Simultaneously, the solar-assisted system significantly reduces energy consumption, achieving energy savings of up to 25% [36]. Du designed and experimentally tested an IoT-based solar hay bale drying and storage system. Compared to field drying, this system reduced dry matter loss by nearly 30%. Drying alfalfa hay to a safe moisture content (≤17%) consumed an average of 83 kWh/t, comparable to or lower than high-temperature rapid drying systems [110]. Qian utilized a solar-air source heat pump system for alfalfa drying. In combined heating mode, the specific power consumption per temperature rise was 0.069 kWh/°C, representing an 8.0% reduction compared to heat pump heating alone and an 86.2% reduction compared to solar heating alone, demonstrating excellent economic operation potential [111]. By drying various products in a newly designed solar-heat pump dryer, the system achieved overall performance coefficients ranging from 1.96 to 2.28, dehumidification efficiencies between 0.03 and 0.46 kg/kWh, and energy utilization rates from 0.19 to 0.48. This equipment primarily relies on solar energy and heat pumps, with electricity supplied by a photovoltaic system, achieving near energy self-sufficiency [105]. Hamid Mortezapour achieved a maximum drying efficiency of 72% and reduced total energy consumption by 33% when drying saffron using a heat pump–assisted hybrid photovoltaic–thermal solar dryer [112]. Combining solar energy with other heat sources, particularly in regions with abundant solar resources, can decrease reliance on supplementary heat sources, lower overall energy consumption, reduce operating costs, and thereby enhance economic viability.

4.2.4. Microwave Drying (MD) and Equipment

Microwave drying is currently a viable drying method in the drying processing industry [113]. Compared to AD and HAD, it enables rapid moisture evaporation with significantly shorter processing duration, reduced energy consumption [114], and superior product quality characterized by less shrinkage, better color retention, and improved rehydration capacity [115]. Microwave power, drying time, and material dielectric properties (e.g., loss tangent, permittivity) directly influence output quality [116]. When drying parsley, increasing power from 360 W to 900 W reduced drying time by 64% while improving color preservation [117]. Coriander leaves dried at 850 W exhibited higher trans-β-carotene integrity and pigment extractability than 45 °C convective drying [118], with analogous results observed in sage leaves [119]. Microwave-dried products demonstrate superior retention of total phenolic compounds, flavonoids, and antioxidant activity compared to 45 °C convection-dried products. Similar results were observed in Gynura pseudochina leaves [120]. When integrated with other methods, microwave drying serves either as a pre-drying stage to reduce initial moisture or as a terminal dehydration step [96].

Compared to other drying methods, microwave drying offers advantages in speed and efficiency. Its primary drawback is uneven heating, which creates temperature gradients within the product during drying—particularly in larger-sized materials—leading to inconsistent drying, overheating, and quality degradation [121].

4.2.5. Other Drying Methods and Equipment

Beyond the aforementioned drying methods, additional hay drying techniques include stem-leaf separation drying, chemical desiccant drying, and high-voltage dielectric plasma discharge (HVDPD) drying. Chemical desiccants are a key method for achieving off-site alfalfa drying. Common desiccants include potassium carbonate, sodium carbonate, sodium bicarbonate, and potassium hydroxide. Alkali metal ions can modify the hydrophobicity of the plant cuticle or increase the number of micropores in the cuticle [122,123], accelerating moisture permeability. Grncarevic et al. observed that field-spraying 2% K₂CO₃ solution on swathed alfalfa accelerated drying by 43% compared to stem conditioning alone, while pre-harvest application reduced total losses by 14–21% [124]. Uneven alfalfa drying arises from differing drying rates between stems and leaves. Mechanically separating stems and leaves for independent drying prevents leaf waste and enhances product quality. Digman et al. proposed enhancing alfalfa leaf utilization through stem-leaf separation, pressing, and anaerobic storage, conducting corresponding process trials [125]. Wenhao Hu et al. employed HVDPD for alfalfa and compared it with other drying methods. They concluded that HVDPD achieves higher drying rates while preserving the internal structure of dried alfalfa, resulting in superior rehydration properties, nutritional quality, and higher feed value [126]. Dominant alfalfa drying methods and features is summarized in

Table 5. Dominant alfalfa drying methods and features.

Drying Methods	Drying Equipment and Model	Energy Consumption	Color	Protein Loss	Drying Time and Features
AD [82,127]	No equipment	Utilizing natural airflow, temperature, and sunlight to remove moisture from object surfaces	Caused substantial color degradation	Significant protein loss	This approach reduces operational costs; however, it needs a long drying time and entails high labor intensity, significant susceptibility to environmental constraints.
HAD [98,128]	Model: GREG M hay dryer	relatively high energy consumption	Optimal color properties were achieved with 60 °C air-convective drying.	Compared to natural drying, protein retention is higher.	This method features diverse equipment types with enhanced drying rates and comparatively reduced quality degradation. Drying times vary significantly depending on the equipment used, but they are shorter than natural drying times.
Solar-energy Drying [88,129]	Model: AMS-150 solar dryer	Utilizing renewable resources to reduce energy consumption costs.	No data available	Compared to natural drying, protein loss is reduced.	Drying costs are relatively high, but energy-efficient and environmentally friendly. Drying time depends on the intensity of solar radiation and is sensitive to ambient temperature, making it suitable for regions with abundant solar resources.
Solar-Assisted Heat Pump Drying [111]	Model: TGS-2 SASHP drying system	High energy efficiency and complementary advantages, subject to weather and climate conditions.	No data available	Compared to natural drying, protein loss is reduced.	This system necessitates the deployment of solar energy and heat pump equipment, resulting in high drying costs; though compared to solar drying, time is reduced, it incurs significant energy consumption intensity.
Infrared Radiation Drying [86]	Model: DS-2500	Low energy consumption, energy-saving and environmentally friendly	Caused substantially higher color degradation compared to other drying methods	Showed good preservation of protein compounds	This approach offers reduced operational expenditure, enhanced drying efficiency, and superior nutrient retention rates; however, it exhibits impaired drying uniformity.
Chemical Desiccant Drying [90,130]	No equipment	Low energy consumption	Good color retention	Controls protein loss effectively	Chemical desiccants reduced drying time by altering plant epicuticular structures; however, they exhibit differential efficacy on stems and leaves, resulting in a slight reduction in hay color.
Leaf-stem Segregated Drying [89]	Model: DS-2500	High energy consumption	Excellent at preserving color	Minimal protein loss	This technique necessitates investment in specialized separation equipment. Alfalfa leaves dry quickly, while the stems take relatively longer to dry. While enabling segregated stem-leaf drying to significantly reduce leaf loss and enhance nutritional specifications, it faces challenges in maintaining consistent separation efficiency.

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5. Quality of Dried Alfalfa Products

5.1. Factors Affecting Alfalfa Hay Quality

The quality of alfalfa hay is influenced by drying methods, processing parameters, and equipment design. Distinct drying methods yield significantly different product quality profiles. During AD, repeated mechanical operations (tedding and baling) induce substantial leaf shatter, degrading forage quality [127]. Prolonged solar exposure triggers decomposition of heat-sensitive nutrients, while shade-drying or stacking elevates mold incidence [82]. Solar drying with staged protocols reduces nutrient depletion [131]. Experimental studies on variable-temperature drying reveal that ascending-temperature drying achieves optimal results with minimal nutrient loss and low energy consumption, outperforming constant-temperature and descending-temperature modes [132]. To enhance alfalfa quality after HAD, parameter optimization enhances efficiency while preserving nutritional integrity [133]. Studies indicate that spraying chemical agents facilitates moisture dissipation, minimizes nutritional quality loss, and maintains mineral content [132], without affecting the digestibility of dry matter, crude protein, or fiber [134].

Additionally, factors such as alfalfa variety, cutting time, cutting duration, crop rotation, rainfall exposure, and additives all influence hay product quality. Within a single day, earlier cutting times and shorter air-drying periods result in less nutrient loss, with hay quality being influenced by the interaction between cutting time and air-drying duration [19].

5.2. Evaluation Methods for Alfalfa Hay Quality

Alfalfa hay quality is primarily assessed through three aspects: physical, chemical, and biological evaluation. Physical assessment examines judgment characteristics such as color and odor, offering convenience, simplicity, and speed. Chemical evaluation involves measuring nutrient content in alfalfa hay through chemical methods, providing the scientific basis for assessing its feeding value. Biological evaluation determines the conversion outcomes of nutrients in alfalfa hay through animal digestion tests or feeding trials.

5.2.1. Physical Evaluation Methods

Physical evaluation methods determine alfalfa hay quality through direct observation of color, odor, stem-to-leaf ratio, weed proportion, and presence of mold. Premium-grade hay exhibits standard greenish-yellow color, rich aromatic scent without off-odors, and is free from mold or pest/disease infestation. It contains 30–40% leaf matter with tender stems; low weed content; no soil contamination; no insect damage; and no signs of pest/disease infection such as white fluffy substances or brown spots [135]. Inferior alfalfa hay typically exhibits a pale yellow or light white color, accompanied by a putrid odor, brown spots, or white, fluffy substances. Leaf content ranges from 10% to 20%, and it may show signs of pest or disease infestation. Low-quality alfalfa hay has poor nutritional value and fails to meet the demands of modern agricultural and livestock development [136]. Quantitative evaluation can also be conducted through physical measurements based on three indicators: yield per unit area, fresh-to-dry ratio, and stem-to-leaf ratio. Yield per unit area and fresh-to-dry ratio are crucial indicators of alfalfa hay quality. The fresh-to-dry ratio reflects dry matter accumulation, determining hay yield. The stem-to-leaf ratio indicates nutrient content; a high ratio signifies lower crude protein and higher crude fiber content, indicating lower nutritional value.

In summary, forage yield indicates the quantitative value of alfalfa hay, while the stem-to-leaf ratio reflects its qualitative value. For evaluating alfalfa hay quality, physical assessments should be conducted first. To further determine its nutritional value, chemical evaluations are also required.

5.2.2. Chemical Evaluation Methods

The moisture, crude protein (CP), crude fiber (CF), crude fat (EE), crude ash, and non-nitrogenous extractives in alfalfa hay were determined using traditional approximate nutrient analysis methods. While foundational for quality assessment, this method provides an oversimplified and inadequate evaluation of feeding value due to its inability to resolve specific nutrient fractions [137]. Therefore, the Van Soest fiber analysis method was employed to revise and reclassify the two indicators of crude fiber and non-nitrogenous extractives. Neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL) were adopted as indicators for evaluating fibrous substances in forage. This method distinguishes between soluble cellular contents—such as fats, sugars, starches, proteins, and hemicellulose—that are readily utilized by animals, and indigestible cell walls, overcoming the empirical nature of “crude fiber” and enabling accurate nutritional valuation of forage fiber. Component correlations reveal CP positively associates with ruminant digestibility, whereas NDF and ADF exhibit negative correlations. Both proximate and Van Soest analyses remain static compositional assessments disconnected from animal utilization, only reflecting the content of forage nutrients rather than intake, digestibility, or metabolic availability, thus retaining inherent limitations.

Additionally, the Cornell Net Carbohydrate and Protein System (CNCPS), developed at Cornell University in the late 20th century, has gained widespread adoption in the United States and Canada [138]. This system classifies carbohydrates and proteins in forage into degradable and non-degradable fractions based on conventional nutritional value and ruminant digestion patterns [139]. CNCPS accurately models ruminal degradation of protein and carbohydrates, nutrient absorption efficiency, and energy utilization, providing robust theoretical guidance for production practices. Near-infrared reflectance spectroscopy (NIRS) represents another chemical analysis method. It is a spectroscopic technique for rapidly determining the component content in forage, established by exploiting the specific spectral absorption characteristics of substances in the near-infrared region [140,141]. This method enables rapid detection of indicators such as digestible dry matter (DDM), dry matter intake (DMI), and relative feed value (RFV) in alfalfa hay, offering feasibility for field-based hay estimation [139]. Zhou et al. [140] demonstrated that NIRS can not only measure forage nutritional components but also estimate effective energy and utilization rates.

Different countries employ varying grading standards for alfalfa hay. The United States primarily uses CP, NDF, ADF, RFV, and total digestible nutrients (TDN) as evaluation metrics [142], as shown in

Table 6. The classification standard of alfalfa in America.

Index	Level
	Premium	Level 1	Level 2	Level 3	Level 4
CP (%)	>22	20–22	18–20	16–18	<16
ADF (%)	<27	27–29	29–32	32–35	>35
NDF (%)	<34	34–36	36–40	40–44	>44
RFV *	>185	170–185	150–170	130–150	<130
TDN (%)	>62	60.5–62	58–60	56–58	<56
TDN (90%DM)	>55.9	54.5–55.9	52.5–54.5	50.5–52.5	<50.5

* RFV = DDM × DMI ÷ 1.29; DDM(%) = 88.9 − 0.779ADF; DMI(%) = 120/NDF.

. The RFV for alfalfa at full bloom is set at 100. Corresponding RFVs are calculated based on NDF and ADF levels, with RFV > 100 indicating superior forage quality [143]. The evaluation of alfalfa hay quality in China primarily relies on the agricultural industry standard—Alfalfa Hay Grading Standard [144]—established by the Ministry of Agriculture and Rural Affairs, and the group standard—The Hay Quality Grade of Alfalfa [145]—promulgated and implemented in 2018, as shown in

Table 7. The classification standard of alfalfa in China.

Physicochemical Index	Level
	Super Grade	Super Grade	Level 1	Level 2	Level 3
CP (%DM)	≥22.0	≥20.0, <22.0	≥18.0, <20.0	≥16.0, <18.0	<16.0
ADF (% DM)	<27.0	≥27.0, <29.0	≥29.0, <32.0	≥32.0, <35.0	>35.0
NDF (% DM)	<34.0	≥34.0, <36.0	≥36.0, <40.0	≥40.0, <44.0	>44.0
RFV	>185.0	≥170.0, <85.0	≥150.0, <170.0	≥130.0, <150.0	<130.0
Weed Content (%)	<3.0	<3.0	≥3.0, <5.0	≥5.0, <8.0	≥8.0, <12.0
Ash (%)			<12.5
Moisture Content (%)			≤14.0

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5.2.3. Biological Evaluation Methods

Biological evaluation involves determining the conversion outcomes of nutrients in alfalfa hay through animal digestion or feeding trials, primarily focusing on the efficiency of nutrient digestion, absorption, and utilization by animals.

Digestibility tests are among the most commonly used methods for evaluating alfalfa hay quality and feed value, generally categorized into in vivo and in vitro digestibility tests. In vivo approaches include the total fecal collection method and the rumen fistula nylon bag method. The total fecal collection method is the standard technique for determining apparent digestibility. Test animals are placed in metabolic cages, where feed intake and total fecal output are precisely recorded. Specific nutrient contents—including dry matter, organic matter (DM), CP, and crude fiber—are analyzed in both the feed and feces [146]. Apparent digestibility is calculated using the formula: Apparent digestibility (%) = [(Nutrient intake − Nutrient in feces)/Nutrient intake] × 100. A nylon bag containing a quantified alfalfa hay sample is inserted into the rumen of a live ruminant via a rumen fistula. The bag is removed at scheduled intervals, residues are washed, and the remaining dry matter and nutrient content are measured to determine the dynamic degradation rates of DM, protein, and other components within the rumen [147]. In vitro digestion tests simulate the animal digestive environment in a laboratory setting to measure digestibility or gas production. Primary methods include in vitro digestibility, gas production, and enzymatic digestion. The in vitro digestibility method involves incubating alfalfa feed samples with rumen fluid-buffer mixtures for a specified duration, filtering residues, measuring residual dry matter or organic matter, and calculating in vitro dry matter digestibility or organic matter digestibility. Studies indicate that the in vitro digestibility method correlates with the nylon bag method for forage digestibility, with a correlation coefficient of 0.81 [148]. The gas production method proposed by Menke can effectively predict the amount of feed degraded or metabolically fermented by rumen microorganisms to a certain extent [149]. The enzymatic digestion method uses enzyme solutions to replace rumen digestive fluids in determining the degradation rate of forage nutrients. While relatively accurate for evaluating concentrate feeds, it requires further optimization to accurately assess the nutritional value of roughage [150].

Feeding trials involve grouping experimental animals under similar conditions and feeding them a basal diet primarily or exclusively composed of alfalfa hay as roughage (possibly supplemented with different concentrates). These trials compare milk yield and composition, daily weight gain, feed conversion efficiency, and other metrics among animals receiving different feeding levels. However, feeding trials are lengthy and influenced by numerous non-feed factors, making it difficult to precisely quantify the relative value of the forage itself in isolation.

6. Conclusions

To improve alfalfa drying efficiency and ensure product quality, it is essential to rapidly reduce its moisture content through advanced drying technology. This study provides an overview of the whole-process alfalfa drying techniques and establishes a systematic production technology model.

(1)

The whole-process drying technological model for alfalfa must balance material characteristics, process adaptability, energy costs, and economic feasibility. The drying strategy should be optimized based on material properties (such as initial moisture content and stem-leaf differences) and regional conditions to achieve efficient, low-loss drying. Establishing an intensive processing model combining “pre-treatment at production sites + centralized drying” is crucial for scalability.

(2)

Drying mechanisms and moisture migration patterns are key to optimizing drying processes. Alfalfa drying involves complex migration of free and bound water, progressing through constant-rate, first falling-rate, and second falling-rate periods. The primary challenge lies in asynchronous stem-leaf drying due to the waxy stem layer hindering moisture transfer. Cutting–flattening techniques that disrupt stem structure can significantly enhance drying efficiency. Future research should integrate multi-scale heat and mass transfer models to enable precise drying control.

(3)

Domestic drying equipment and intelligent technologies represent the future development direction. Currently, China relies on imported drying machinery, highlighting the urgent need for domestically developed systems tailored to local conditions. Integrating solar, heat pump, and other renewable energy sources can reduce energy consumption. Additionally, incorporating AI and deep learning into alfalfa processing will enable digital platforms for smart forage production, improve drying precision and efficiency. Promoting standardized, integrated, and intelligent drying technology models will effectively address the shortage of high-quality alfalfa hay in the industry.