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Alfalfa

Irrigation and Wheel Traffic Considerations

Irrigation can have a direct effect on pest management. Providing adequate moisture for optimum yields is important for maintaining the health of stands and competition with weeds. During a growth period, irrigations should be timed to meet the evapotranspiration (ET) requirement of the crop to maximize yield, but also allow sufficient time for drydown before harvest. Harvesting on moist soils causes compaction, damage to the crowns, and inadequate oxygen for roots.

Damage from a single field operation on wet soils can be long lasting, and encourage weed buildup, poor drainage and root diseases.  In UC Davis studies, wheel traffic (two passes) has been shown to reduce yield an average of 25%, and the effects on yield are much greater when traffic operations are conducted on wet soils. Any method which avoids excessive wheel traffic, especially on wet soils, will improve yield and regrowth potential of alfalfa, and reduce weed infestation.
 
Irrigations after harvest should be delayed until approximately 2 to 3 inches of growth, thus reducing the threat of root rot or scald. Dry soils under an open canopy shortly after harvest to minimize weed germination, particularly summer grassy weeds. This is more difficult to accomplish on sandy soil than on heavier soils, since more frequent irrigations are necessary.

Standing water is a major cause for both root diseases and weed intrusion in alfalfa. Irrigation systems should be designed to allow good uniformity and good surface drainage, especially the tail ends of fields where damage is most common. Utilizing good land-leveling techniques during stand establishment will prevent standing water, thus lowering disease and weed infestation. Corrugation or ‘bedded alfalfa’, and subsurface drains are good methods to prevent standing water on alfalfa.

Scald occurs when excessive standing water occurs along with high temperatures, causing severe lack of oxygen and resulting plant death. This is a common occurrence when temperatures are above 100° F.  Thus under high temperatures, irrigations should be done primarily at night, and fields drained quickly to avoid scald.