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Karen Strickler

Bees visit more flowers than set seed.

What factors reduce seed yield?


  Wilting alfalfa flowers;  Photo by Karen Strickler

New!  Counting flowers 

An upper limit on the yield of seed and fruit is set by the number of flowers that bloom and the number of ovules within the flower that produce seed. Yields rarely reach this upper limit, either in natural or agricultural populations, however. The extent to which the reproductive potential of a plant is reduced depends on the number of pollinated flowers, the number of fertilized ovules, fruit and seed predation, weather conditions, and availability of sufficient resources to set seed.

Bees visit more flowers than set seed

Alfalfa produces more flowers than contribute to seed yield. In research in commercial fields, research fields and in the greenhouse, we usually find that only about 50% of the flowers develop pods, even if all flowers are pollinated! Alfalfa leafcutting bees are probably getting pollen and nectar from many more flowers than set seed, too. This can be demonstrated with some simple calculations. In Canada, about 20,000 adult M. rotundata are usually introduced into an acre of crop. In 1995 in Manitoba, an average of 1.75 times the original number of cells introduced into the field were taken out as dormant larvae at the end of the growing season, according to Gerald Huebner of the Manitoba Forage Seed Association. The Canadian Cocoon Testing Centre reports that an average of 75% of the cells produced by bees contain live dormant bee larvae in Canada. Thus 20,000 adult bees produce approximately 20,000 ´ 1.75/0.75 = 46,700 reproductive cells. Each of these cells is made from the pollen and nectar gathered from approximately 2000 flowers which are pollinated as the bee trips the flower. Thus, about 9.33 ´ 107 alfalfa flowers must have been pollinated to create the bees that were harvested in Manitoba. The average number of seeds per flower is about 4 and the average weight per seed is about 2.2 mg. If all of the pollinated flowers produced harvestable seed, then seed yields would be approximately 1,760 lbs. per acre. However, average yields in 1995 in Manitoba were 250 lbs. per acre. Maximum yields in excellent production years can reach about 1000 lbs. per acre. The potential seed yield is about 1.8 times greater than the actual maximum yield, and about 7 times greater than the average yield.

In the Pacific Northwest of the USA, the bee stocking rate for an established stand of alfalfa is about 40,000 live bees per acre of crop. While this number of cells can sometimes be taken out as dormant larva at the end of the season, in most cases 50% or less is recovered. If we estimate conservatively that the same number of total cells (dormant larvae + dead larvae) were created as number of bees were introduced, then 8.0 ´ 107 flowers must have been pollinated to produce them. These should yield approximately 1550 lbs. per acre of seed in the US. Some fields obtain this seed yield; the maximum in the Northwest is about 2000 lbs. per acre. Average seed yield, however, is only 500 lbs. per acre. Thus, potential seed yield is about 3 times the average.

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What factors reduce seed yield?

Numerous factors can reduce potential seed yield in alfalfa. Lygus bugs, the main flower and seed predator, are controlled with pesticides so that their impact normally accounts for a small percentage of crop loss. Other important factors include weather conditions (e.g. temperatures above 90oF cause decreases in the percentage of seed that matures), and resource availability such as soil moisture. Studies of perennial legumes suggest that the plant can respond rapidly to changes in resource levels by increasing or decreasing the rate of development of new racemes and curls. While water is an important resource in this regard, nitrogen has not been demonstrated to limit alfalfa seed yields.

Another factor that can reduce seed yields is self-incompatibility, a physiological adaptation that prevents pollen from the same plant from successfully developing into a seed. Self-pollinated flowers are less likely to produce seed, and produce fewer seeds per curl, than cross-pollinated flowers, whose pollen comes from other alfalfa plants. If spring is cool, flowers may accumulate on racemes before bees are introduced. Once bees are introduced, they are more likely to move between flowers on a raceme or within the same plant than they would if heavy pollination prevents flowers from accumulating. Studies have shown that nectar foraging bumble bees and honeybees turn more frequently and take shorter flights between flowers in profitable patches than in unprofitable patches of flowers. If this behavior applies to leafcutting bees, then early flowers should often be pollinated by incompatible pollen from the same plant when bees are first introduced, and seed yields should be reduced for those flowers.

Finally, it has been shown that some plants produce extra flowers whose function is primarily male, i.e. they provide pollen to the flowers that produce seeds, but donít develop seeds themselves. Similarly, some plants produce excess fruits but only develop those fruits whose seed has the greatest probability of growing into successful plants: the largest seed, the earliest seed, or the seed with the best genetic makeup. This is true of the perennial legume, birdís foot trefoil. It may also be true for alfalfa.

Research in the Pollination Ecology Program at UI was aimed at better understanding the factors that reduce potential seed yields in alfalfa. This understanding should help us to manage the crop and the bees to maximize yields.


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Revised Aug. 6, 2000.
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2000, Karen Strickler. All rights reserved.