Evaluating the spring alfalfa stand

Ice during the winter and record cold temperatures in early April have damaged some alfalfa fields. Now is the time to check alfalfa stands and begin planning for harvest or reseeding. Three kinds of winter injury and winterkill are evident in Iowa this spring.

Ice sheeting has killed localized areas of alfalfa fields across the northern third of the state. Plants killed by long-term ice cover will not green up and the taproots and crown will continue to deteriorate.

Heaving, a mechanical squeezing of alfalfa plants out of the ground during extended periods of early spring freeze/thaw cycles, is being reported in east central Iowa. Heaving can occur in stands of all ages. Plants that have heaved 1 inch or more may have additional cold injury. If they recover and produce good spring growth, they are still vulnerable to cutter bar damage at harvest.

Cold injury occurs on new plant regrowth tissue in temperatures less than about 24-25° F. Like frost damage, many of these plants that broke dormancy and were frozen late in the spring may recover with new crown buds. Frozen root and crown tissue below ground, however, often is the greatest factor in the weakening and loss of alfalfa plants. Plants that had developed full winter dormancy through autumn can tolerate temperatures as low as 5° F without damage, but plants that have begun to recover in the spring are less winter hardy and can be damaged by temperatures in the teens and 20s.

Splitting taproot.

Firm, white taproot.

Crown rot -- alfalfa.

Winter-killed alfalfa.

Winter-injured alfalfa.

Heaved alfalfa plants.

It is difficult to predict cold damage because soil temperatures often are warmer than air temperatures. You cannot determine cold injury and stand condition in early spring without digging plants and assessing crown and taproot condition. Healthy taproots are creamy, white, and firm in texture. If the plants you dig have good taproots and there is still evidence of bud growth from the crown, then the plant may be recovering well, or more slowly than normal, but recovering. A taproot that is spongy in texture, or watery and beginning to take on a tan or yellowish color, is a bad sign. These plants are likely severely cold-injured and deteriorating. If you are seeing signs of this, check fields again in about a week to verify your first assessment.

Stands that are recovering normally can be managed with normal harvest planning. Slowly recovering, winter-injured stands will be weakened and recover under some stress. The best and most conservative management for these weakened stands is to allow the first growth to reach at least half- bloom before first harvest to provide more plant recovery time.

Stands with less than four healthy plants per square foot will likely only produce marginal yields this year. Plan to plant another field this year to start a replacement field. If you need the forage from the current, damaged field, consider taking only a first harvest, destroying the damaged stand, and establishing an emergency forage crop for needed forage (see story on next page).

An Iowa State University Extension publication, Evaluating Hay and Pasture Stands for Winter Injury, Pm-1362, goes into more detail about spring evaluation procedures. The publication also identifies some concerns about reseeding and "thickening up" or "patching in" alfalfa into winter-killed or -injured alfalfa fields.

This article originally appeared on pages 44-45 of the IC-478 (6) -- April 28, 1997 issue.

Updated 04/27/1997 - 1:00pm