Alfalfa, often called the “Queen of Forages,” is an economically important forage crop known for its high nutritional value and productivity. This perennial legume supports dairy and livestock industries globally. Its long growing season and deep root system make it a high-water-use crop compared to many annual alternatives. A consistent and substantial water supply is required for alfalfa to reach its full yield potential, making water the limiting factor in maximizing hay production. Understanding the crop’s precise water needs is foundational for effective management.
Total Water Demand and Efficiency
Alfalfa’s water consumption is measured by Evapotranspiration (ETc), which combines water evaporated from the soil surface and water transpired through the plant’s leaves. Full-season alfalfa stands typically require 30 to over 40 inches of water per growing season, or approximately 3 to 3.5 feet. In hot, arid climates with longer growing seasons, the total water requirement can exceed 50 inches. This high consumption rate maintains the rapid growth cycles between harvests.
During peak summer growth, a mature alfalfa stand can consume between 0.3 and 0.5 inches of water per day. Water Use Efficiency (WUE) measures the amount of water needed to produce a unit of dry matter. Producing one ton of dry hay generally requires 4 to 7 inches of water. This figure is relatively efficient despite the crop’s high overall water demand, stemming from the plant’s ability to maintain high photosynthesis when water is readily available.
Factors That Adjust Water Requirements
Alfalfa’s water demand is highly variable, depending on specific environmental and biological conditions. Climate is a major driver, as high air temperature, low relative humidity, and increased wind speed all contribute to a greater vapor pressure deficit in the air. This deficit causes the plant to lose water through transpiration more rapidly, significantly increasing the daily ETc rate.
Soil texture also influences the frequency and volume of water applications required. Sandy soils have a low water-holding capacity, necessitating frequent, smaller irrigation events to prevent plant stress. In contrast, fine-textured clay soils hold much more water but have slower infiltration and poor drainage, which can lead to waterlogging if applications are too heavy. Optimal production occurs in well-drained loam soils that balance water retention and drainage.
Water demand fluctuates throughout the cutting cycle, following a predictable pattern linked to the plant’s leaf area. Demand is lowest immediately following harvest when the canopy is minimal and transpiration is reduced. Water use increases sharply as the plant regrows, reaching its peak just before the pre-bud or early flower stage when the canopy is fully closed. Managing irrigation to meet this peak demand maximizes yield in each cutting.
Practical Irrigation Strategies
Effective irrigation requires matching the timing and volume of water application to the crop’s fluctuating needs. The goal is to maintain available soil moisture within the root zone, typically between 50% and 70% of the soil’s capacity, ensuring optimum growth without wasteful deep percolation. For the highest yields, soil water depletion should not drop below 40% of the available water holding capacity before irrigation begins. Monitoring soil moisture using probes or the “feel test” is a practical way to schedule applications accurately.
The choice of irrigation method significantly impacts the water delivered to the plant roots.
Irrigation Methods
- Traditional flood or furrow systems are the least efficient, often losing 15% to 50% of applied water to runoff, deep percolation, and evaporation. Their application efficiencies typically range from 50% to 85%.
- Center pivot and other sprinkler systems provide better uniformity and efficiency, commonly rated around 85% with modern low-pressure nozzles.
- Subsurface drip irrigation (SDI) is the most efficient method, delivering water directly to the root zone below the soil surface. This precise delivery minimizes evaporation and runoff losses, achieving application efficiencies of 90% or higher.
Studies have shown that SDI can reduce total seasonal water use by 38% to 47% compared to flood irrigation while maintaining or increasing hay yields. Growers must stop watering several days before cutting to allow the surface to dry and prevent soil compaction from heavy harvest equipment.
Alfalfa’s Response to Water Stress
When the water supply fails to meet the plant’s demand, the immediate consequence is reduced plant growth and yield. The plant’s first defense is to inhibit cell enlargement, decreasing stem length and leaf size. If moisture stress occurs shortly after harvest, the number of new shoots produced will be reduced, resulting in a lower overall yield for that cutting.
Severe water stress can lead to higher forage quality, though at the cost of yield. This happens because the plant prioritizes leaf growth over stems, resulting in a higher leaf-to-stem ratio and lower fiber content. The plant may also fold its leaves to conserve moisture, a visible sign of stress, or shed lower leaves to reduce the transpiring surface area.
Alfalfa is drought-tolerant due to its deep taproot, which accesses water deep in the soil profile. During prolonged dry periods, the plant can enter protective dormancy, where above-ground growth ceases entirely. This survival mechanism conserves resources, preventing the plant from dying and enabling a vigorous recovery once water becomes available.