A warm weather pattern in late February caused soil temperatures across most of Iowa to rise above 40 degrees F. ISUEO AGRONOMIST BRIAN LANG NOTES THAT THIS is was likely warm enough long enough for alfalfa and some forage grasses (most ryegrass varieties and the less winterhardy orchardgrass and tall fescue varieties) to break dormancy. However, when low temperatures recur, alfalfa plants can reharden to a degree, but only to the extent that it still has stored carbohydrates available. Winter injury occurs either with enough warm-cold cycles to use up the carbohydrates, or if the temperature drops so rapidly that the plant does not have time to sufficiently reharden.
When dormant, alfalfa crown tissue can tolerate soil temperatures down to about 5 to 10 degrees F to where tissue damage could then begin to occur. After breaking dormancy, tissue damage could theoretically start to occur at about 30 degrees F with some plants, but more likely not until soil temperatures get down into the mid-20’s degree F for alfalfa crown tissue. If shoot development occurred with a break in dormancy, and the current air temperatures are cold enough to freeze these shoots, the plant can initiate new shoots as long as sufficient carbohydrates are available. As carbohydrates are depleted, tissue damage will occur. If too much damage occurs before the plant can photosynthesize and produce carbohydrates, the plant will not recover.
Seasonal alfalfa management influences how well plants store carbohydrates entering into the winter. These factors include variety selection (winter survival index, disease resistance, fall dormancy level), age of stand, soil fertility, pest management, soil drainage, soil moisture in fall (higher soil moisture in fall tends to reduce alfalfa hardiness for the winter), last year’s cutting schedule intensity (how much stress was put on the stand), was there a late fall cut or not, and if cut late was there fall stubble left or not. These are all issues dealing with stress management and how healthy the stand enters into the winter, thus winter injury conditions between and within fields can vary considerable.
The current cold front may cause some wide-scale problems with alfalfa, ryegrass, and the less winterhardy varieties of orchardgrass and tall fescue, but we need to take a wait-and-see approach, and scout fields. Scout by digging plants starting about a week after the cold front passes. This will provide some time for the crown tissue to start showing it’s true nature… firm tissue is good, soft tissue is not. If this current weather causes tissue damage, there should not be any visual tissue discoloration yet (first off-yellow, then tan in color) since it’s too soon after the freeze damage, so judge by tissue firmness not tissue color. If by chance the crown tissue is discolored and soft to mushy, damage to the plants likely occurred weeks ago. If crown tissue is dead and drying out, damage to the plants occurred at least a month ago. Most plants in older stands will also exhibit some dark colored crown rot in the center of the crown, which is normal. Ignore this and evaluate the white tissue surrounding this area. Check the illustrations in the resource A3620 mentioned below to assist in your assessment of the stand.
Scouting and stand assessment could end up being completely obvious with nothing greening up. However, also be aware that it is possible for winter-injured alfalfa to initially green-up to some degree with its remaining stored carbohydrates to the extent of a few inches of shoot development, even though the crown tissue is too severely damaged for the plant to survive. Frankly, at this time it’s too pessimistic to discuss replant options. Let’s first scout the fields. But listed below are resources to assist with plans to conduct alfalfa stand assessments, as well as to plan your livestock forage inventory and considerations for forage replant options, if it comes to that.