Mike Ottman and Ayman Mostafa
“Summer slump” is a decline in growth of alfalfa usually beginning in July in hot summer areas, such as the low elevation deserts of Southwestern U.S (Fig. 1). In more temperate regions, there is a gradual decrease in alfalfa yield in successive harvests throughout the year, but the yield decline in the summer is not as sharp as in hot summer regions. The term summer slump has also been applied to reduction in growth of perennial cool season grasses such as tall fescue during the summer.
Summer slump is associated with high temperatures that occur in the summer. Most likely, the increased humidity and high night temperatures of the summer monsoon season (Fig. 2) contribute to summer slump (Feltner and Massengale, 1965; Robison and Massengale, 1968). Alfalfa is a cool season crop that can be fairly productive in hot, arid areas if the air is dry. Leaf temperature can be almost 20 °F below air temperature under these conditions (Idso et al., 1981). When the humidity in the air increases during monsoon season, the leaf cannot cool itself to... read more