Effect of water management on forage production and on carbohydrate and nitrogen constituents of alfalfa (Medicago sativa L.).

Abstract

Experiments were conducted at Tucson, Arizona during a 3-year period to determine the effect of water management on several characteristics in 'Mesa-Sirsa' alfalfa (Medicaqo sativa L.) grown under field conditions. Three water management treatments were imposed: (a) irrigating to field capacity when 50% of available soil moisture had been utilized to a depth of 1.2 m (high), (b) irrigating to field capacity when 100% of the available soil moisture had been utilized to the same depth (medium), and (o) irrigating similar to the high treatment on first and last harvests of the growing season, but withholding irrigation water during the balance of the growing season (low). Plants receiving medium and high water treatments were harvested at 25% bloom through the entire growing season while those receiving the low water treatment were harvested twice--at the first and last harvest for plants receiving medium and high water treatments. In plants grown under both the high and medium water treatments it was found that dry matter production, plant height at harvest, efficiency of water use, and total water utilization per growth period decreased during the summer months while the daily rate of water utilization increased. Treatments did not have an effect on any of these variables. When irrigation water was returned to plants grown under the low water treatments, dry matter production and plant height at the subsequent harvest were not significantly different from plants grown under the medium and high water treatments. Plant population per unit land area declined steadily during the three years of production. Neither stand decline nor root distribution was significantly influenced by water management over the 3-year period. Variation in root morphology among genotypes was observed. The majority of the variability was attributed to genetic expression. Water stress resulted in significant increased proline and decreased arginine content of root tissue. Sucrose content increased significantly in roots of plants subjected to water stress while fructose and glucose levels were not affected. Water insoluble N and water soluble protein levels were lower at midsummer when compared with the beginning and completion of the growing season. Total available carbohydrates (TAC) and water soluble protein levels decreased following forage harvest, while levels of water insoluble N, water soluble NI amino acid composition of proteins, and free sugar levels did not change during regrowth. TAC levels under the medium and high water treatments decreased during midsummer each of the 3 years of the study. The trend was not as evident during the initial production year as in subsequent years. Net carbohydrate utilization during the 2 weeks following harvest varied among years and harvests. Differences among harvests were attributed to climatic conditions as no treatment effect was observed. Variation among years may have resulted from root maturation and genotypic changes in the plant population. TAC levels of nonharvested summer-stressed plants in the low water treatment remained relatively constant unless rainfall occurred in sufficient magnitude to stimulate top growth, in which case a decrease in TAC levels was observed.