The cold early April has taken a toll on some alfalfa stands.  Across southern MN and WI, stands that looked good initially (after breaking dormancy in late March) need another closer look. The untimely cold snap appears to have caused “winter” injury and even kill in some stands that looked OK just two weeks earlier. 

• In some areas (southern MN and WI, lighter soils, south-facing slopes, late-fall cut), alfalfa had begun to grow and shoot tips were frosted. If the entire shoot tip including the terminal bud was killed by the frost, that stem is done growing. This is usually evidenced by the tip turning brown and bending over like a “shepherd’s crook.” With less severe frost injury, terminal leaves are damaged, but not the bud. In this case, the frozen leaves wilt and turn brown, but the shoot keeps growing and puts out new leaves above the frozen area. Where terminal buds were killed, the alfalfa crown will put out new shoots if the plant is healthy. First cutting will be delayed and reduced, but later cuttings should be near normal.

• One of the most evident results of winter injury is that stands are slow to green-up. If other fields in the area are starting to grow and yours are still brown or only slightly green, check those stands for injury or death. The best way to diagnose winter injury or kill is to dig up random plants (4 to 6 inches deep) and examine their roots. Healthy roots should be firm and white with little evidence of root rot.

• Winter killed roots become gray and water-soaked initially after soils thaw. Then after water leaves the root, the tissue becomes brown, dehydrated, and stringy (see Figures 1 and 2). If the root is soft and water can be easily squeezed from it, or if it is brown, dry, and stringy; it is likely dead.


Figure 1. Frost injury to alfalfa taproot

• If 50% or more of the root interior is blackened from root rot, the plant will likely die during spring green-up or later in the year. See UW Extension Publication A3620 (pdf) for more details on evaluating root health.

• If the root is rotted off, the plant may put out shoots, but will likely die before first cutting, so the stand should be replaced.


Figure 2. Rotting alfalfa taproots

• It is important to walk fields and see if stands are good enough to keep.  Some plants were damaged but had enough reserves to fuel some early growth before dieing. The kill and injury is being reported primarily in 3 and 4 year-old stands that were late-fall harvested. Determine if, on average, at least 5 healthy plants are present per square foot. More importantly, at least 55 stems per square foot are needed to be in an optimal yield range. Fewer stems per square foot mean lower yields (See graph).

More information about diagnosing and managing winter- and frost-damaged alfalfa stands is available at:
www.uwex.edu/ces/crops/uwforage/alfalfa.htm

Managing winter-injured alfalfa stands that are worth keeping

• Allow injured alfalfa plants to mature longer before cutting.  Allowing alfalfa to mature to early or mid bloom will help the plants restore needed carbohydrates for subsequent production. How long and during which cutting depends on the extent of winter injury. For severely injured stands, allow alfalfa to reach nearly full bloom before first cutting, and early flower in subsequent cuttings. This will gives these stands the best chance at survival. Stands with less injury can be harvested at somewhat earlier stages depending on the extent of the injury. Stands with only mild injury should be allowed to reach 10 to 25% bloom for one cutting during the season. It’s often best to choose second or third cutting for these stands, as the first cutting is usually the largest.

• Increase cutting height when alfalfa is harvested at or later than 25% flower. This is particularly important for the first cutting. New shoots that will form the second crop begin to emerge from the crown as alfalfa begins to flower. Close cutting of well-flowered alfalfa will remove the next crop’s shoots. Removing these shoots will further weaken the plants as they attempt to produce new shoots.

• Inter-seed thin or patchy haylage stands with 10-15 lb/ac of non-heading Italian ryegrass, and plan on terminating these stands after this growing season. Including an additional ½ to ¾ bu/ac of oats may improve weed control and boost first cutting yields. Annual ryegrass or Italian ryegrass that heads in the seeding year may be a better choice on hay ground since they wilt a little more rapidly than non-heading types, but quality will be somewhat lower. Legume credits from alfalfa should be adequate to meet nitrogen needs for at least the first two cuttings of these alfalfa-ryegrass mixtures.

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Originally published in Forage Focus (Midwest Forage Association)