Alfalfa: Irrigation, Harvest Scheduling and Rotation Timing

Scheduling Irrigation in Alfalfa

According to Gordon Johnson, Extension soils specialist with the Oklahoma Cooperative Extension Service, alfalfa needs about 6 inches of water available to its roots to produce 1 ton of dry matter. Unfortunately, it is nearly impossible to catch up and put on enough water during the growing season. Hence, it is important to fill the soil profile in late winter while alfalfa is dormant and before soil temperatures warm to above 60°F.

Johnson says watering for three weeks between cuttings for a 2-ton-per-acre yield requires 3-plus inches per week, which may result in standing water. That in itself, however, can be hazardous during the summer, as saturating soils when the soil temperature is above 60°F sets up conditions favorable for the development of Phytophthora root rot, which hastens stand decline. If Phytophthora root rot poses a risk, plant a resistant alfalfa variety, avoid poorly drained fields, maintain fertility and/or irrigate less often.

Following are some additional tips from the University of Wisconsin-Extension and the Alfalfa Management Guide.
1. Begin with a full soil water profile.
2. Monitor the soil profile weekly for moisture content.
3. Soil water should be depleted to about 50% of the available water in the top 2 feet before harvest. The interval between irrigation and harvest varies from two days in lighter-textured sandy soil at high-summer evapotranspiration (ET) rate to 13 days in heavier clay soils at low ET rates (see Chart 1, above left). This soil water reserve can be used for alfalfa growth when irrigation is halted for harvest or when the application rate cannot keep up with ET.
4. Begin irrigating again as soon as harvest is removed to refill the soil profile. Stress during early regrowth will severely limit the next crop yield.
5. For irrigation scheduling, use estimated water consumption provided by services like the U.S. Department of Interior’s AgriMet (www.usbr.gov). Use a soil probe or shovel to check soil moisture and verify the actual field conditions. The root zone should be filled with moisture just before the period of peak crop water use.

Scheduling Alfalfa Harvest

Basically, the question of when to harvest alfalfa can be explained with the following simple rule: As cutting interval increases, or as plants are harvested at later stages of maturity, the yield per cutting increases, but quality of the forage harvested decreases.

Alfalfa forage quality is greatest in early vegetative stages when the leaf weight is greater than stem weight. That’s because forage growth is most rapid until early flowering (see Chart 3 right), yet growth continues until full-flower stage.

When to Rotate from Alfalfa

Alfalfa has a tremendous ability to produce maximum yield over a wide range of stand densities. However, stands gradually thin and the crop eventually needs to be rotated, no matter where you live. In the Midwest, the Northeast and in many areas where fields are irrigated, yields often begin to decline in the third year of production. Depending upon the climate and harvest schedule, fields may go longer.

To decide when to rotate from alfalfa, you’ll first need to evaluate stand density and yield relative to your needs. You should also factor in rotation requirements, such as farm plan, total acreage of forage needed and ability to reseed.

The decision to reseed fields of alfalfa or rotate out of alfalfa should be based on the yield potential of the stand, ideally using actual yields from the field. The next best method is to count stems when the alfalfa is 4 to 6 inches tall and use the data from Chart 4 (right) to estimate yield potential (assuming drought, soil fertility or other conditions are not limiting yield). New seedings, meanwhile, should have at least 25 to 30 plants per square foot during the seeding year.

For more on rotation and how it can help eradicate weeds, see “Caring for Hay Fields.” Also, for tips on preventing field compaction, see “Reducing Compaction” and for information on the latest grasses and hay, see “New Forage Varieties.”