Forage accumulation and radiation use of alfalfa under deficit irrigation

The supply of high-quality forage is fundamental to the development of ruminant animal production in drought areas where irrigation provides strong support to the forage production. The objective was to explore the response of alfalfa (Medicago sativa L.) growth and radiation use to deficit irrigation in a 2-yr field study. Two deficit-irrigation treatments and a full-irrigation control (CK) were established under flood irrigation. Growth-related indicators and radiation use were measured regularly. Results showed that deficit irrigation reduced forage accumulation, leaf area index (LAI), plant relative water content (RWC), and radiation use efficiency (RUE) of alfalfa. Compared with CK, slight deficit irrigation (SD) and moderate deficit irrigation (MD) decreased forage accumulation rate by 20 and 30%, respectively. Logistic model as a function of growing degree days (GDD) fitted well LAI change (R-2 > .9). Deficit irrigation reduced the average rate of quick growth of LAI from 5 to 36%. Water deficit did not significantly change the GDD corresponding to the rapid growth phase but reduced the growth rate. Under deficit irrigation, decreased LAI did not always lead to significant cumulative intercepted photosynthetically active radiation (CIPAR) decrease, but resulted in significantly lowered RUE. In conclusion, the decrease of forage accumulation of alfalfa under deficit irrigation would be caused mainly by weakened photosynthetic ability (representing RUE).