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.

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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.