The ‘summer slump’ period is a decline in alfalfa growth which usually starts in July in hot summer areas of the West, including the low elevation deserts in Southern California and Arizona.
In more temperate regions, there is a gradual decrease in alfalfa yield in successive harvests throughout the year, yet the yield decline in the summer is not as sharp as in hot summer regions.
The summer slump is associated with high temperatures during the summer. Increased humidity and high nighttime temperatures during the summer monsoon rainy season contribute to the summer growth slump.
Alfalfa is a cool-season crop which can be successfully grown in hot, arid areas if the air is dry. The leaf temperature can be almost 20 degrees Fahrenheit below air temperature under these conditions.
When the humidity in the air increases during the monsoon season, the leaf cannot cool itself to the extent when the air is dry. The plant experiences temperatures higher than optimum for growth.
Also, respiration increases during warm weather which can decrease net photosynthesis and crop yield.
The alfalfa plant uses carbohydrates (sugars) stored in the root to re-grow after cutting. Typically, root carbohydrates are depleted for about two weeks after cutting at which time the plant is large enough to produce enough carbohydrates to replenish the root reserves for the next regrowth cycle.
Root carbohydrate concentration is maximized at full bloom. Cutting at the bud stage causes a decrease in root carbohydrate concentration in successive cutting cycles.
High temperatures during the monsoon season can further reduce root carbohydrate concentration due to increased respiration.
Alfalfa plants can flower at about a foot in height during the summer slump. The yield may be reduced by the lack of height in the plant but also by reduced stem numbers. This response may be due to water stress, or the combined effect of low root carbohydrates and high temperatures.
The hay yield per cutting during the summer slump may be less than 1.0 ton/acre compared to 1.5-2.0 tons per acre during the spring. This yield reduction is caused by reduced plant height, leaflet size, and stem numbers per plant.
Alfalfa grown during the summer slump usually is lower in quality and suited only for dry cows, feedlot animals, and horses. Alfalfa hay produced in the spring is typically suited to lactating dairy animals.
Although eliminating alfalfa’s summer slump is impossible under the severe summer climate conditions, some factors can reduce the impact.
Alfalfa varieties grown in the Arizona low desert are non-dormant in the fall and have smaller crowns and taproots compared to more dormant varieties.
This indeed may be true if the cause of summer slump is reduced root carbohydrates, and the relatively larger roots of these varieties are able to avoid critically low levels of carbohydrates in the roots.
However, growing semi-dormant alfalfa varieties in the Arizona low desert is generally not recommended since yield gain during the summer may be small and variety dependent, and is often offset by a loss of yield during the fall and early spring, compared to non-dormant varieties.
Water stress can worsen the effects of the summer slump, and even cause the plant to flower prematurely as mentioned above. Keeping the crop well watered is among several management practices a grower can utilize to delay or lessen the effects of the slump.
Remember that alfalfa water use slows during the summer slump due to decreased growth. Also, water standing for more than 24 hours when the high air temperature is above 100 degrees can lead to scald injury.
Nutrient deficiencies theoretically contribute to the effects of the slump since anything which reduces crop growth and vigor can intensify the effects of summer slump. However, applications of plant nutrients not needed for crop growth is not expected to reduce the impact of summer slump.
It has been suggested that high soil temperature reduces the effectiveness of the nitrogen-fixing root nodules in alfalfa, and the application of nitrogen fertilizer could reduce the effects of summer slump.
Yet, most of the nodules are found in the 4-12-inch depth where soil temperatures are optimal for nodule functioning and the response to nitrogen during the summer slump has been mixed.
Alfalfa is less competitive with weeds during the summer slump due to the relative growth of the crop compared to weeds. Summer annual grasses, including Bermudagrass and nutsedge, can be particularly competitive with alfalfa.
Herbicides are available to control these weeds, yet are not as effective compared to when alfalfa is more vigorous.
The most effective weed control in alfalfa is a healthy stand. Before the advent of selective herbicides in alfalfa, non irrigation during the summer slump was a weed control strategy.
Insects should usually be controlled during the summer slump to reduce stress on the plant and maintain crop quality. Insects - especially sap feeders including the three-cornered hopper, potato leafhopper, and lygus - reduce crop growth and contribute to low yields.
The sap-feeding potato leafhopper injects toxins in plant tissues which can reduce alfalfa yield for one or two cuttings in the late summer and early fall.
Leaf-chewing insects, including alfalfa caterpillars, can contribute to lower yield, quality, and increase the summer slump by reducing the proportion of leaves in the hay.
Non irrigation during the summer slump and no insect control are an alternative management strategy which may be economical, depending on the costs of water, insect control, and other production inputs, plus the value of the hay produced.
However, the control of multi-host insects (lygus, potato leafhopper, and stink bugs for example) in alfalfa may be necessary to avoid population build-ups and movement to other crops.
Any disease has the potential to contribute to the negative effects of summer slump. Diseases, including Stemphylium and Cercospora, result in leaf loss and lower hay quality. Rhizoctonia and other diseases which impact the root may compromise the ability of the crop to take up water and contribute to reduced yield during the summer slump.
The chemical control of diseases in alfalfa is generally not economical. Avoidance through good sanitation of equipment and the elimination of water standing for more than 24 hours are recommended to reduce the risk of plant pathogens.
A cutting height of one inch is generally recommended for alfalfa on about a four-week harvesting interval which is not under stress or depleted in root carbohydrates. When harvesting frequently in the bud stage, there is some evidence that a cutting height of four inches may have certain advantages.
Cutting alfalfa before the bloom stage is a common practice in Arizona to obtain desired hay quality. This practice may place stress on the alfalfa plant and contribute to summer slump and reduced stand life.
Cutting at full bloom during the summer will replenish the root carbohydrate reserves, reduce the effects of summer slump, and increase the probability of a rebound in yields in the fall.
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