by James W. Bauder

Alot of alfalfa seed is produced "by accident," because managing water for alfalfa seed production is probably easier under accidental dryland conditions than under controlled irrigation. The uniqueness of alfalfa seed water management and the premium that can come from a crop of good quality seed made me think this topic deserved more attention.

Until I started looking, I was under the impression that good alfalfa seed production came from stressing alfalfa at flowering, assuming that stress would cause the plant to produce more seed. However, the information I found listed the following requirements for good alfalfa seed production:

A vigorously growing plant prior to the onset of flowering A deep subsurface soil moisture reserve at the onset of flowering A slowly drying upper soil profile

The basic guidelines in terms of irrigation water management seem to be:

  • Be sure the soil profile is full of moisture early in the season
  • Keep a reasonable amount of water available to the plant up until a couple weeks before flowering.
  • Let the crop begin to use the subsoil moisture through the completion of flowering. After flowering, make sure the crop has good moisture again.

 

It’s All About Management and Timing
As one would expect, an essential component of alfalfa seed production is timely irrigation scheduling. In general, highest seed yields result when irrigation practices prevent severe plant stress and promote slow, continuous growth through the entire production period without excessive stimulation of vegetative growth. When water is available, deep soil moisture applied in the winter and early spring can partially offset summer irrigation requirements and provide a buffer against the detrimental effects of severe moisture stress. Although a certain amount of water is required to mature the seed, soil moisture must be depleted prior to desiccation or the plant will not dry down adequately in preparation for harvest.

When dealing with alfalfa seed, proper timing of irrigations, and not the amount of water applied, is often the critical factor in obtaining consistently high yields. Unless the field has extremely short runs or contains sandy soils, the irrigator must plan the irrigation with a "normal" weather year in mind. If the field holds moisture well, the grower will probably irrigate only once. May or early June irrigation is most effective during warm, dry years as it allows the crop access to more moisture. A June irrigation, recommended if field assessments show there is not enough water to make it through July, is usually applied around June 10 when vegetative growth is nearly over and flowering is about to begin. At this point, the moisture use pattern of the crop has largely been determined, so the plant will not "go rank" unless cool, wet weather encourages it. Irrigation should not be applied after the stand has reached a height of 24".

The next irrigation takes place after pollination season and seed set, usually between mid-July and August 10. Apparently one of the difficulties in getting good seed production is keeping the plant environment ideal for pollinating bees during the flowering period, while keeping the plant environment ideal for seed setting. That means having water available deep in the soil profile but keeping the soil surface dry during the flowering period. Bees don't like water in the field when they are trying to work, and flowering and pollination can be pretty well shut down by too much water. A mid-summer irrigation usually results in some regrowth, which may interfere with harvest operations. Also, irrigation during the ripening stage can contribute to seed discoloration and even seed abortion.

One final irrigation, applied after harvest in late September or early October, provides the greatest yield potential for the following season, but if delayed too long may contribute to winter kill in some years. The alfalfa crop needs at least six weeks of conditioning to prevent winter kill.

 

Planning Ahead
There is normally enough moisture in the soil during the first production year to see the plant through the pollination period, but in the second and subsequent years, it may be necessary to spring irrigate (before the bees go out) and/or summer irrigate (while the bees are out). The grower should sample soil moisture to 6' in early spring. If the average of all the samples is less than 50% of available moisture, the grower should irrigate as soon as possible. Otherwise, wait until bud stage (about June 10) to irrigate. If the field was irrigated in May, it will require a summer irrigation, preferably taking place when stem internode lengths shorten to 3/4" and the flowers take on a "raspberry" look. The 50% available moisture suggestion is merely a guideline and may need adjustment for individual fields.

 

Getting Consistent Yields
What conditions lead to consistent yields? First, initial plant size should be adequate to ensure good potential for seed production. The first flower should be about 2' above the ground. Second, moisture stress should appear just before flowering begins, when moisture in the upper half of the root zone becomes limiting. At this point, the moisture remaining in the lower half of the root zone supplies adequate moisture to maintain flower production. Moisture stress encourages less growth and smaller leaves, which keep the canopy warmer. The alfalfa’s pollinator, the leafcutter bee, is native to Eurasia, where the average summer temperature is 95oF. The bees simply refuse to work if the canopy is too cold. Also, less stem growth ensures that the racemes will remain visible longer. Any condition that limits water uptake by the plant, such as extremely short runs, sandy soils, or salinity, requires a different irrigation management strategy, because these situations may lead to extreme moisture stress, resulting in loss of flowers before adequate seed production has occurred.

Density problems should not occur during the first production year, but the field may need thinning the second year to provide the optimum density. Growers have reported good results by removing every second shank from their cultivator and using wide sweeps to eliminated up to half the plants in the stand. Also, light seeding rates (as low as 1/2 lb/acre) and moisture stress help keep the plant population in check.

Growing alfalfa seed presents some obvious problems, but with proper management and a little luck, it certainly can be done. The following web page provide more information about this topic:
http://www.agric.gov.ab.ca/irrigate/alfalfa/main.html