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May 28, 2004 Frost or Drought Stressed Alfalfa Drought stress is currently very common in northwestern and the southern two-thirds of North Dakota and much of the state has experienced a hard frost in the last two weeks. Is the management of alfalfa similar or different from these two stresses? Drought initially causes a reduction in alfalfa growth due to a shortage of soil water and severe drought will stop the growth of stem tissue. “When drought-stressed alfalfa receives significant rainfall, growth will re-initiate if the stem is still vegetative,” according to Dwain Meyer, North Dakota State University plant sciences professor. “If the plant has initiated flowering on a couple of nodes, which often occurs with prolonged drought, the plant will continue flowering with only limited stem growth. The plant will set seed if wild bees are in the area to pollinate the flowers; otherwise, the flowers will wither and die. Flowering will occur over a four to six week period, therefore the amount of time and soil water utilized to produce flowers (seed) is the same as that required to produce a second cutting under good moisture conditions.” Drought-stressed alfalfa that receives significant rainfall (1.5 to 2 inches) and has initiated flowering should be clipped or harvested to remove the flowering stems, thus utilizing available soil water for forage growth instead of seed production. Since much of the soil water has been tapped during the drought-stress period, the alfalfa will require additional rainfall to produce a good crop. Five to six inches of rainfall is required to produce a ton of alfalfa. Normal mechanical harvesting equipment is the best method to clip alfalfa. “We tried to ‘clip’ the alfalfa chemically by using Paraquat, but even the high rate did not totally stop stem development and was considered unsuccessful,” Meyer says. Frost damaged alfalfa will vary with stage of growth and duration of freezing temperatures. Alfalfa in the fall of the year will put out a winter rhizome that grows outward and upward, generally penetrating the soil surface and displaying small leaflets. These new stems will frequently overwinter, allowing you to dig down through the snow cover and find the stem anytime during the winter or after the snow melts. Normally these new stems initiate growth in the spring of the year. One form of winter injury is to burn (frost) these new stems off, which forces the plant to initiate new buds elsewhere on the crown. “The new stems that overwinter are very frost tolerant, but once these stems start to grow, their frost tolerance decreases,” Meyer says. “Alfalfa stems that are less than two inches in length have a high osmotic potential, are very tolerant to cold temperatures and can survive a short duration of single-digit temperatures. But frost tolerance decreases as the stem grows because most stem elongation is by uptake of water. Even then, alfalfa is very tolerant of freezing temperatures compared to other crops.” For example, Fargo recorded a 16 degree temperature on April 11when the alfalfa was three to four inches tall and did not kill the growing point; however, the frost caused some leaflets and leaves to die (brown appearing 10 days later). A 23 degree temperature occurred on May 2 when the alfalfa was eight to 12 inches in height and had little effect. Some years ago, a 14 degree temperature on alfalfa about six inches tall, killed an estimated 30 percent of the stems, but 70 percent survived and produced an average yield. An 18 degree temperature at Tioga on May 2 when alfalfa was 10 to 12 inches tall caused some leaf-kill and browning of the leaflets edge, but most stems survived and continued to grow. A 24 degree temperature on June 2, 2000, north of Minot caused the alfalfa to turn white and suffer severe damage with a loss of much of the stand. “Obviously, the alfalfa was more susceptible to cold temperatures as the alfalfa grew taller and approached harvest,” Meyer says. Harvest recommendations differ for frost-damaged compared to drought-stressed alfalfa. If the stem has been killed by frost, there is no need to clip the field since the growing point has been killed. The plants will recover from the crown tissue, although slowly. “The only decision to be made is whether there is a harvestable crop,” Meyer says. “If there is about 8 to 10 inches of growth, there should be enough to justify a harvest. You can tell whether the stem has been killed by waiting for 24 to 48 hours after the frost to see if the stems straighten up from the lax position. If they do, the stem will continue to grow and add to yield potential.” Drought-stressed alfalfa that has bloomed should be clipped following a significant rainfall, but frost-damaged alfalfa does not have to be clipped. ### Source: Dwain
Meyer, (701) 231-8154, dwain.meyer@ndsu.nodak.edu |
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North Dakota State University |
