Drought-stressed alfalfa

For some across the Midwest, recent rain has brought much-needed moisture. For others, the drought continues, combined with periods of above-average heat. Key environmental factors like temperature and soil moisture status greatly impact alfalfa quality and yields. Thus, this year’s conditions have led to difficult harvesting decisions and generated conversations with forage advisory teams on several farms.

Because of its high stem density and dense canopy, alfalfa has a high rate of water use. University of Minnesota agronomy experts share that alfalfa uses 0.1 to 0.3 inches of water per day with a range of 4 to 7 inches per ton of forage, depending on the environment.

Daily water use is influenced by plant growth stage and environmental factors like air temperature and wind speed. For example, on a windy, 90-degree day, alfalfa will likely use 0.3 inches of water. Water use is greatest when alfalfa has a full vegetative canopy before harvest and is greater during summer months when solar energy and air temperatures are greatest. Conversely, water use declines following harvests and in the spring and fall. If irrigating fields, these growth dynamics can be valuable when determining water application to minimize yield-reducing stress while optimizing economic yields.

Alfalfa has an extensive root system that enables it to extract water from deep in the soil. Alfalfa roots are mostly concentrated in the top 4 feet of the soil, but it is not uncommon for roots to reach depths of 15 feet in older stands. Alfalfa increases its root mass and length in response to soil moisture deficits. Alfalfa survives severe drought by going dormant. In the Midwest, alfalfa plants can survive several months in dormancy. In some climates, dormant alfalfa can survive for years. During the onset of drought, alfalfa plants increase carbohydrates stored in the crown to allow the plant to survive dormancy. When water becomes available, alfalfa regrows rapidly from buds on the crown.

Drought stress will impact alfalfa plant growth in a variety of ways. Many changes are due to a dramatic reduction in photosynthesis. Plant cell enlargement is inhibited. The number of basal buds and number of stems are reduced when drought stress occurs in the first two weeks after harvest. In addition, the stem internode length is condensed, resulting in flowering at reduced plant height. Leaf size and growth is also reduced, although to a lesser degree than stem growth. Therefore, the leaf-to-stem ratio is higher under drought conditions.

From a nutrient standpoint, protein levels can see a slight reduction due to reduced biological nitrogen fixation by the alfalfa plant largely through drought’s negative impacts on photosynthesis. Neutral detergent fiber is generally decreased, though the effect varies with severity and timing of moisture stress.

The timing of the drought can be critical to yield reduction. Alfalfa root systems require good soil moisture in the early spring to regrow. Most of the Midwest experienced adequate snow melt and enough early moisture to allow a strong root system to develop this spring. As alfalfa plants broke dormancy, plant health was good, and growth was steady. Though not record-setting, the result was respectable first-cutting yields in many areas.

Dry conditions in June and July have resulted in reduced growth. Severe drought in some areas has affected entire fields while milder drought stress has created inconsistency across fields, impacted greatly by the water-holding capacity of the subsoils.

Normal cutting windows are being evaluated and, in some cases, adjusted. Dr. Dan Undersander from the University of Wisconsin recommends harvesting as normal if a stand is over 10 inches tall and flowering. There is no advantage to raising cutting height; alfalfa can regrow from axillary buds on the stubble, but these shoots are smaller and produce lower yield than stems growing from the crown buds. Plants will maintain quality better under drought conditions, so it may be valuable to go longer between cuttings and let stressed fields approach flowering so the plant can build nonstructural carbohydrate reserves.

New-seeding alfalfa may have poorer stands if a dry period follows seeding. Upon emergence, drought-stressed plants may not develop as extensive of a root system as those in fields seeded under ideal moisture. This can impact current yields as well as future performance. Special caution should be taken in the fall to ensure these new-seeding fields have at least 8 to 10 inches of regrowth before the frost. Dry conditions going into the winter enhance alfalfa survival since dry soils insulate the crown better and result in less disease in alfalfa roots.

Drought-stressed alfalfa fields are often lower yielding but have higher forage quality. Keep in mind, this is not the same for grass fields. It is more important than ever to work with your agronomy team to ensure fertility and insect control are enhanced during periods of drought stress. Strategize with your advisors to optimize forage tonnage and quality in this challenging growing season.

    Barry Visser is a nutritionist for Vita Plus.