Montana State University |
Montana State University |
By Jim Bauder
MSU Extension Soil and Water Quality Specialist
4/4//2000 - BOZEMAN -- A question by Bear Whitmer of Glendive helped me learn why a lot of alfalfa seed is produced "by accident." That's because managing water for alfalfa seed production is probably easier under accidental dryland conditions than under controlled irrigation. Alfalfa seed water management is unique, but the premium for a good supply of good quality seed is attractive.
Until I started looking, I thought that good alfalfa seed production came from stressing alfalfa at flowering -- assuming that stress would cause the plant to produce more seed. Well, the information I found suggested that what's important to good alfalfa seed production is:
The basic guideline I came away with was to fill or insure that the soil is full of moisture early in the season. Keep good water (but not too much) for the plant up until a couple weeks before flowering begins, then let the crop use the subsoil moisture through the completion of flowering. After flowering is complete, make sure the crop has moisture again.
Apparently one of the difficulties in getting good seed production is that of keeping the plant environment ideal for pollinating bees during the flowering period and 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 the water out there in the field when they are trying to do their work and flowering and pollination can be pretty well shut down by too much water.
Here are a couple of web pages I found that have some good information about alfalfa seed irrigation water management from California.
http://agronomy.ucdavis.edu/alfalfa.wg/subpages/CASeed.htm
http://www.agric.gov.ab.ca/irrigate/alfalfa/main.html
An essential component of alfalfa seed production is timely irrigation scheduling. In
general, highest seed yields are obtained 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
providing a buffer to avoid detrimental effects of severe moisture stress. Enough water is
required to mature the seed, but soil moisture must be depleted prior to desiccation or
the plant will not dry down adequately to prepare for harvest.
Alfalfa seed has proven to be one of the more difficult crops to irrigate. This is mainly because proper irrigation timing, and not the amount of water applied, is often the critical factor in obtaining consistently high yields. Growers sometimes resort to several adaptations: the crop is seeded at a lighter rate (as low as 0.56 kg/ha or 0.25 lb/ac) to ensure a low plant density, and moisture stress is introduced to control growth and reduce moisture consumption by the plant.
There is no perfect irrigation management strategy for alfalfa seed. Unless the field to be irrigated 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 be irrigating only once. May or early June irrigation is most effective during warm, dry years as it allows the crop access to more moisture. June irrigation usually is applied about June 10, when vegetative growth is nearly over and flowering is about to begin. June irrigation is recommended if it is decided that the field does not have enough moisture to make it through July. At this point, the moisture use pattern of the crop has largely been determined, so that 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 inches. July or August irrigation is applied after the pollination season is over (usually mid July and about August 10). It usually results in some regrowth, which may interfere with harvest operations. Late September or October irrigation is supplied after harvest. It provides the greatest yield potential, but may contribute to winter kill in some years, if delayed too long. The alfalfa crop needs at least six weeks of conditioning to prevent winter kill.
What conditions lead to consistent yields. Initial plant size should be adequate to ensure good potential for seed production. The first flower should be about 60 cm (2 feet) above the ground. Moisture stress should appear just before flowering commences as the moisture in the upper half of the root zone becomes limiting. This encourages smaller leaves and less growth. However, the moisture remaining in the lower half of the root zone will be adequate to maintain flower production. The smaller leaves will cool the canopy less, allowing the bees to pollinate the flowers at lower temperatures; and less stem growth ensures that the racemes will remain visible longer. The seed should be set before the field is irrigated. Also, it has been reported that irrigation during the ripening stage can contribute to seed discoloration and even seed abortion.
A different irrigation management strategy is required where a producer encounters extremely short runs, sandy soils, salinity, or any condition that limits water uptake by the plant. These situations may lead to extreme moisture stress, resulting in loss of flowers before adequate seed production has occurred. There is normally enough moisture in the first production year to see the plant through pollination, 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 2 m (6 feet) in early spring. If the average of all the readings is less than 50 percent of available moisture, the grower should irrigate as soon as water is available. Otherwise, wait until bud stage (about June 10) to irrigate.
If the field was irrigated in May, it will require a summer irrigation. Summer irrigation is suggested when stem internode lengths shorten to 3/4 inch and the flowers take on a "raspberry" look. The 50 percent available moisture suggestion is a guide and may have to be adjusted for individual fields.
Density problems should not occur during the first production year, but the field may have to be thinned 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.
For additional information or to receive a regular email distribution of agronomy notes such as the one above, Jim Bauder can be contacted by email at "jbauder@montana.edu" or by calling 406-994-5685 at Montana State University.
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Send questions or comments to Carol Flaherty, MSU Communications Services, Bozeman, MT 59717 or email Bauder and Flaherty at carolf@montana.edu.
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