By Guojie Wang and Justin Brackenrich
Alfalfa producers must decide to continue or terminate alfalfa stands every spring based on the stand condition and its yield potential. Alfalfa stand can be injured or killed by excessively cold temperatures in winter, ice sheets, or frost heaving in spring. Some alfalfa cultivars developed by alfalfa breeding programs are genetically capable of withstanding adverse winters, widening alfalfa adaptation to colder climates. Through a careful alfalfa cultivar selection, coupled with good soil phosphorus and potassium fertilization practices, alfalfa injury or kill caused by excessively cold temperatures in winter can be significantly decreased if not prevented. Therefore, several cold waves in winter may not cause significant alfalfa stand injury or kill.
Ice Sheets and Frost Heaving
Most alfalfa stand injury or kill happens in late fall and early spring, especially early spring. Ice sheets, enclosing plants in or under a layer of ice, are a common problem in poorly drained soils that cause alfalfa stand injury or kill due to the slowed exchange of oxygen from the atmosphere to alfalfa root systems. The resulting anaerobic alfalfa respiration without oxygen produces alcohol, injuring or killing alfalfa plants. It is much worse when the alfalfa plant breaks its dormancy and grows in early spring when temperatures are high for an extended period and suddenly drop to below 32F causing ice formation. In other words, ice sheets or wet soil conditions in winter months cause limited injury or kill to alfalfa stands. It is well known that alfalfa does not like a wet foot. The more accurate description is that alfalfa does not like a wet foot when actively growing.
Frost heaving is a serious problem when temperatures fluctuate around 32ºF, and alfalfa is grown on poorly drained, fine-textured soils. The alternate freezing and thawing of the surface soil cause soil body vertical expansion and shrinkage; consequentially, this process will highjack alfalfa plant root system above the soil surface. If the alfalfa crown is higher than 1 inch above the soil surface, the alfalfa plant can be killed due to desiccation and severe root breakage. Unfortunately, alfalfa breeding programs made little progress in reducing ice sheets and frost heaving damage to the alfalfa plant. The major mitigation mechanisms are management practices, including high-cutting stubble in the preceding fall, mixed alfalfa with other forage species with branched root systems, and good soil drainage.
Alfalfa Stand Evaluation
A comprehensive field scouting program can be used to assess alfalfa stand in spring. We can estimate the alfalfa ground cover when the stand has 4 to 6 inches of new growth.
|% Alfalfa Ground Cover||Yield Potential|
|>80%, good vigor||Excellent|
|60-80%, good vigor||Fair to good|
|40-60% cover, fair to poor vigor||60% of normal|
|20-40%, poor vigor||<50% of normal|
Alfalfa plant vigor can be assessed by randomly digging out several alfalfa plants, shaking or washing the soil off the root system, and observing the leaf and bud vigor. Cutting open to assess internal root color, an alive and vigorous alfalfa plant root should look white and starchy. Trying to peel the root bark, a vigorous alfalfa plant should resist bark peeling. More lateral roots from the taproot of alfalfa are another sign of a vigorous alfalfa plant.
We can evaluate the alfalfa stand by counting the density of alfalfa individual plants. In order to accurately count the individual alfalfa plants, we may need to dig the plants within a 1-square-foot area in several random locations in the field. Counting the individual vigorous taproots is better than counting alfalfa crowns near the soil surface. Researchers have already established alfalfa plant density standards based on alfalfa stand age.
|Stand Age||Minimum plants/ft2 without yield loss|
|Fall of seeding year||25-30|
|3rd year or older||5-6|
Alfalfa stand can also be assessed by its stem density. Instead of counting the individual alfalfa plants, we examine and count vigorous crown shoots symmetrically distributed around the crown within a 1-square-foot area in several random locations in the field.
|Stem density (stems/ft2)||Action|
|>55||Stem density not limiting yield|
|40-55||Some yield reduction expected|
|<40||Consider replacing the alfalfa stand|
When we have a marginal alfalfa stand, we may deploy multiple assessment methods to conclude with some level of confidence and peace of mind.
Management Considerations for Injured Alfalfa Stands
When we have an injured alfalfa stand but do not want to replace it, several management practices can be considered:
- Delay the first cut of alfalfa from the early bud stage to the 20% blooming stage.
- Based on soil testing results, apply phosphorus and potassium fertilizers in early spring rather than the late fall.
- Employ an integrated weed management protocol based on the weed species, including herbicide applications such as 2,4-DB, Pursuit, Raptor, Post, and Select.
- Interseed bunchgrass species such as orchardgrass and perennial ryegrass into the injured alfalfa stand.
If the injury exceeds the threshold of multiple assessment methods used in the field, a replacement action cannot be avoided without a significant yield loss. Seeding alfalfa in the established alfalfa stand is not a good practice due to its autotoxicity. The chemicals released by the old alfalfa plants prohibit or retard new alfalfa seed germination and growth. It’s better to rotate with another annual crop, such as small grains or annual warm-season grasses, and seed alfalfa again no earlier than the late summer of the same year.Source : psu.edu