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Evaluating winter-damaged alfalfa stands

Bruce Anderson Published on 31 December 2009

Winter can be hard on alfalfa.

Bitter cold, desiccating winds, little snow cover, rapid temperature changes pulling plants in and out of dormancy, ice sheets and heaving soils all can potentially damage or even devastate alfalfa stands. Combine these with stressful management like inadequate fertilization, frequent harvests, harvest during winterization, and older stands already showing signs of deterioration due to root and crown diseases – and you have a recipe for trouble.

Early detection and decision- making are needed when alfalfa stands suffer winter damage. Determining whether or not a damaged alfalfa stand can be salvaged is not an easy task. But here are some tips to help you get started.

Estimate yield potential
Begin by estimating stem density since its relationship to yield potential is quite reliable. In winter-damaged fields, stem development often is delayed from some plants. Allow early plant stems to get 5 to 6 inches tall before estimating stem density so later-developing stems can be included in the count.

Select at least four representative sites in a field. Mark off a section of known size at each site. For example, a square measuring 12 by 12 inches is 1 square foot, 17 by 17 inches is about 2 square feet, 21 by 21 is about 3 square feet, 24 by 24 inches is 4 square feet; a circular ring that is 19 inches in diameter is about 2 square feet. Inside this section, count the number of usable plants (crowns) and the total number of stems growing from these crowns. Then convert both numbers to the number per square foot by dividing by the number of square feet used in the count. When all sites within a field have been counted, calculate the average number of plants and stems per square foot.

Fields with more than 55 stems per square foot should be able to produce up to their maximum potential. Yield tends to decline about one-tenth of a ton per acre for each stem below 55. For example, a field with 40 shoots per square foot can be expected to produce 1.5 tons less than its full potential.

Older, dryland stands with fewer than 25 shoots coming from two or three plants per square foot are unlikely to produce satisfactory yields, especially if alfalfa roots have extracted all the available subsoil moisture. Dryland stands only one or two years old should have 35 or more shoots coming from at least three or four plants per square foot. Highly productive sites that receive abundant moisture should have at least 40 shoots per square foot coming from four to six plants.

Dead or alive
Next identify which plants might live and which might die. Start by examining top growth in early spring immediately after completing stem counts. Just because a plant initially has green growth does not mean it is healthy. Injured alfalfa plants often have uneven spring growth. Some plants may reach a 6-inch height rapidly while other plants lag behind.

Healthy plants should have a uniform, symmetrical appearance with all new shoots about equal in length (see Figure 1*). Winter-damaged plants typically have fewer shoots and their growth often is quite variable in length. Most shoots are shorter than those from healthy plants. Symmetry or distribution of shoots around the crown is poor, often with either poor growth from the center (see Figure 2*) or most shoots coming from just one side. Sometimes you can kick these plants at ground level and the top will readily break off from the root. These plants usually will not survive beyond first cutting.

Winter-damaged plants, like those in Figure 2, usually can be saved if they have healthy roots. To evaluate root health, dig roots to a depth of 6 to 8 inches and cut open the tap root lengthwise. Healthy roots are solid and white to cream-colored. Darker brown damaged areas may occur near the center of the crown but should not penetrate more than about 1-inch deep (see Figure 3*). Adjacent areas may be tannish or slightly reddish-brown. A spongy or stringy texture suggests injury due to cold temperatures or other weather-related conditions. A mushy texture indicates the presence of diseases.

As roots show more discoloration deeper into the root, chances of survival decline. If over 50 percent of the root is damaged, long-term survival is unlikely (see Figure 4*). If more than 50 percent of the root is healthy, plants may survive with extra management care but production will be lower and remaining stand life shortened.

Several steps can help winter-damaged plants recover.

• Topdress with phosphorus, potash, and other nutrients as suggested by soil tests so damaged roots can readily obtain needed nutrients.

• Topdress with 25 pounds of nitrogen per acre since damaged roots usually have few or no nodules and new nodules won’t form until plant recovery has been underway for several weeks.

• Control heavy weed infestations if possible but do not use herbicides that have potential for crop injury.

• Irrigate dry soils, if possible, but avoid waterlogged soils.

• Delay first harvest until at least 25 percent of plants are blooming and use conservative harvest frequency for subsequent harvests.

Boosting yields from thin stands
Besides reseeding or interseeding alfalfa in the existing field (generally not recommended) or another field, there are many options to increase production. They include the following:

Interseed oats immediately
Drill two to three bushels of oats per acre directly into injured stands. Harvest before oats reach boot stage to get oat contribution in both first and second cut or wait until oats are fully headed for one large harvest of oat/alfalfa hay. Hay yield typically increases by 1 to 2 tons per acre compared to thin alfalfa alone.

Interseed a summer annual grass.
Drill seed of adapted grasses around early June, immediately following a late first harvest of alfalfa. Forage crops like sudangrass, sorghum-sudan hybrids, forage sorghum, pearl millet, foxtail millet, teff, and even corn can add over 2 tons of hay per acre to thin alfalfa if summer moisture is available. Heavy weed competition, if present, can be a problem establishing seedlings, however.

Double-crop annual forages.
As described above, use oats to increase yield from a late first harvest followed by a summer annual grass for subsequent cuttings.

Interseed perennial grasses or legumes.
This is a good alternative for multiple-year retention of existing field but current-year yield will be lower. Orchardgrass, tall fescue, festulolium, perennial ryegrass and red clover seedlings develop faster than most other perennial forages. If remaining alfalfa plants grow fast enough to form a canopy, harvest alfalfa early (may weaken winter-damaged plants further) to permit light to reach the seedlings.

Interseed Italian ryegrass.
Drill 15 to 20 pounds per acre directly into injured stands. Early growth may be slow so removal of alfalfa canopy by early harvest may be needed. This can produce very high- quality forage throughout the growing season and sometimes survives winter to provide forage again the following year.  FG

*Figures omitted but are available upon request to

 

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