Alfalfa, or Medicago sativa, is a perennial legume grown globally and highly valued as a high-protein feed source for livestock, particularly dairy cattle. Its deep root system allows it to withstand drought and contribute nitrogen to the soil, making it a valuable crop in agricultural rotations. The appearance of blooms signals a significant phase in the plant’s life cycle, determining its reproductive success and the quality of the final harvested product. The bloom stage directly influences the plant’s ability to produce seed and its nutritional composition as hay.
Physical Appearance of the Flowers
The flowers of alfalfa are small, typically measuring between 5 to 10 millimeters in length, and are clustered in groups called racemes. Each individual flower has a characteristic pea-like structure, known botanically as papilionaceous, consisting of five petals. The two lowest petals are fused to form a boat-shaped structure called the keel, which encloses the reproductive organs.
Modern alfalfa varieties primarily display shades of purple to blue-violet. Flower color can vary significantly, sometimes appearing pale lavender, white, or yellow, depending on the specific cultivar. The color intensity is also influenced by the plant’s age and local growing conditions. These vibrant colors attract the insect species required for successful reproduction.
The Unique Pollination Process
Alfalfa possesses a specialized reproductive mechanism known as “tripping,” which is required for effective pollination. Within the flower’s keel, the pistil and stamens are held under high tension, and this structure must be forcefully opened for the plant to reproduce. Tripping involves an external force, such as a foraging insect landing on the flower, which causes the column of reproductive parts to spring forward and strike the pollinator.
This sudden release is necessary for two reasons: to deposit pollen onto the insect and to break a membrane covering the stigma, which is needed for fertilization. Because of the aggressive nature of this mechanism, common insects, such as the European honey bee, are often reluctant pollinators; they learn to visit the flower from the side to steal nectar without triggering the snap. Specialized insects like the alfalfa leafcutter bee and the alkali bee are far more effective. They are adapted to the tripping process, which ensures the plant’s genetic diversity through cross-pollination.
Impact on Alfalfa Harvest Quality
The timing of the bloom is the largest determinant of the quality of harvested alfalfa hay. As the plant transitions from the vegetative stage to full bloom, a trade-off develops between the total yield and the hay’s nutritional content. Harvesting at the early bud stage, before any flowers appear, results in the highest concentration of crude protein (CP) and maximum digestibility for livestock.
Delaying the harvest until full bloom yields the highest total tonnage of hay but significantly decreases the nutritional value. As the plant matures, the stem mass increases, leading to a rise in undesirable components like Neutral Detergent Fiber (NDF) and lignin. Lignin is an indigestible polymer, and its increase directly reduces the overall digestibility of the forage. Farmers typically aim to harvest when the crop is in the late bud to early one-tenth bloom stage. This provides the best balance between maximizing yield and maintaining the high-quality protein and energy content required for high-producing animals.