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Seasonal Forage Growth Rate and Solar Energy Conversion of Irrigated Vernal Alfalfa¹

Alfalfa (Medicago sativa L.) yields decrease from early to midseason harvests in all major alfalfa production areas of North America. Warm temperatures typical of midsummer accelerate alfalfa development. The resulting shorter period of growth to harvest maturity thus contributes to midseason alfalfa yield decline. It is not clear whether a reduced rate of dry matter accumulation in midseason also consistently contributes to yield decline. The present study examined successive growth periods of winter-dormant ‘Vernal’ alfalfa under irrigation in a mid-latitude, low-rainfall growing region. Over 2 years, forage growth rates during active growth averaged 18.8 g m^–2 day^–1 for the second and third (midseason) growth periods within a year. This was higher than in the first and fourth growth periods. During active growth of the first three cuttings each year 1.5% of incident solar energy was accumulated as forage. This declined to 1.2% during the fourth growth period. Results suggest that the portion of the decreased midseason growth resulting from decreased growth rates in temperate alfalfa-growing regions reflects moisture stress rather than a direct temperature effect.

Key Words: Medicago sativa L. • Photosynthetically active radiation • Growth duration • Seasonal yield

² Agronomist, Washington State Univ., and research agronomist USD-ARS, respectively, Irrigated Agric. Res. and Ext. Ctr., Prosser, WA 99350, USA.

Received for publication October 31, 1983.