Alfalfa is an important forage crop grown across the world for hay, silage, grazing, green chop, and seed production. As an alfalfa grower, ensuring optimal pollination is crucial to maximize your seed yield and profitability. In this comprehensive guide, we will explore the best practices for pollinating alfalfa to get the highest yields from your crop.
The Importance of Pollination for Alfalfa Seed Production
Alfalfa is an insect-pollinated, cross-pollinated crop. For the plant to produce seeds pollen must be transferred from the anthers of one flower to the stigma of another flower. This cross-pollination leads to viable seed production.
Without adequate pollination, alfalfa plants can experience:
- Poor seed set and reduced yields
- Lower seed quality and viability
- Lack of genetic diversity in the seed produced
Optimizing pollination ensures maximum seed set, higher seed counts per pod, increased seed weight and improved germination rates
Recommended Insect Pollinators for Alfalfa
The most effective pollinators for alfalfa seed production are:
Honey Bees
Honey bees are the traditional managed pollinator used in alfalfa seed fields A strong honey bee colony can have up to 60,000 foraging workers This makes them excellent pollinators that can be easily transported and managed.
Alkali Bees
Native alkali bees are solitary ground nesting bees that form dense aggregations in alkali soils. Each female can visit up to 30 alfalfa flowers per minute, making them highly efficient pollinators.
Leafcutter Bees
Leafcutter bees are also solitary, nesting in pre-established holes and tubes. These bees carry pollen internally, making them excellent alfalfa pollinators. A single leafcutter bee can tripped over 15 flowers per minute.
Best Practices for Pollinating Alfalfa
Follow these recommendations to optimize alfalfa pollination and increase your seed yield:
Time Planting with Pollinator Activity
- Plant alfalfa in early spring to align bloom period with peak pollinator seasons
- Avoid late summer plantings that may not bloom during active pollinator periods
Use Recommended Pollinator Numbers
- Stock fields with 8-10 honey bee colonies per acre
- For leafcutter bees, introduce 6,000-10,000 bees per acre
- Maintain 1,000-2,000 alkali bee cocoons per acre in alkali soil fields
Position Hives for Maximum Effectiveness
- Place honey bee hives on field edges facing into the prevailing wind
- For leafcutter bees, situate nesting shelters throughout the field interior
- Protect wild alkali bee nesting sites and avoid disrupting underground burrows
Monitor Pollination and Bloom Periods
- Scout fields during bloom stage to check for sufficient pollinator activity
- Add supplemental bee colonies if pollination seems inadequate
- Watch for peak bloom duration to optimize bee stocking periods
Avoid Pollinator-Harming Pesticides
- Do not spray insecticides during bloom stage or when bees are actively foraging
- Prioritize non-chemical and bee-safe pest control methods
- Communicate pollination plans with nearby growers to limit pesticide drift
Supplement with Hand Pollination
- Use a soft brush to manually transfer pollen between flowers
- Focus efforts on male-sterile and poorly pollinated plants
- Hand pollination can boost seed yields by up to 30%
Maintaining Healthy Pollinator Populations
To sustain populations of managed alfalfa pollinators:
- Provide adequate carbohydrate resources near nesting sites
- Follow recommended bee handling practices to limit stress and injury
- Monitor for disease and parasite loads in commercial bees
- Avoid moving bees over long distances between pollination sites
Following pollinator-friendly practices tailored to your specific bees, climate and bloom periods is key to maximizing alfalfa seed production. A multi-pronged approach using adequate stocking densities, strategic hive placement, bloom monitoring and pollinator care is necessary. By optimizing alfalfa pollination, growers can reap the rewards of abundant seed yields and profitable harvests for years to come.
Study Species, Field Sites, and Sample Collection
Medicago sativa L., also known as alfalfa or lucerne, is a perennial outcrossing plant. Most alfalfa cultivars are autotetraploid (2n = 4X = 32), and the species exhibits strong inbreeding depression (Li and Brummer, 2009). Bees are required for pollination and seed production (Bohart, 1957). Alfalfa flowers or florets are arranged in racemes, or clusters of flowers. When comparing alfalfa to many other plant species, a flowering stem can be equated to an inflorescence. In this case, racemes, the clusters of flowers, can be equated to flowers on inflorescences, and each flower within a raceme may be referred to as a floret. In botanical terms, a floret is a small flower that is part of a larger flower, a common feature of plant species in the family Asteraceae. Although a flower in alfalfa may not be a true floret, based on the botanical definition of the term, for the purpose of comparison with the structure of many flowering plants, the term “floret” represents a useful terminology. In this manuscript, the term “flower” refers to a floret, raceme to a cluster of florets, and flowering stem to an inflorescence. We use the number of racemes on a stem as a measure of floral display size, which typically describes the number of flowers on an inflorescence (Harder and Barrett, 1995; Karron et al., 2004).
Stems with leaves and mature fruits, called pods, were collected during the 2017 growing season from 32 alfalfa seed production fields located in three major alfalfa seed production areas, namely, the PNW (N = 11 fields), the CEV (N = 9 fields), and the IMP (N = 12 fields). Fifty individual stems were collected throughout each field, at distances large enough to prevent two stems from originating from the same plant, and thus, each stem represents a distinct plant in this study. In some of the PNW fields, only stems with mature pods (no leaves) were collected because these fields were desiccated prior to sample collection. Field-collected samples were shipped to Wisconsin where they were stored in a low-humidity room (20°C at 15–30% relative humidity) until processing.
Of the 50 stems collected per field, 40 were randomly selected for seed threshing. For fields in the CEV and IMP of California, for approximately 20 stems per field (range of 15–25), the number of racemes per stem, a measure of floral display size, was recorded prior to threshing. In addition, on five racemes per stem, the number of mature seed pods per raceme was counted and recorded. A pod is a fruit that developed from a flower (floret) on a raceme. These data could not be obtained for the PNW fields because only partial stems were collected in those fields. The 40 seed-bearing stems per field were individually threshed by hand using a wooden board and block with ridged plastic attached to them. Total seed weight (mg) was obtained for the seeds threshed from each stem, together with the weight of three independent sets of 10 mature seeds. The total number of seeds on a stem was calculated by dividing the total seed weight by the average weight of 10 seeds. We also calculated the average weight of a seed on a stem. The seeds from a stem were placed in an individual paper coin envelope labeled with the region, field number, and stem number, and envelopes were stored in a refrigerator until DNA processing. For samples from CEV and IMP, the number of seeds per stem (seed set), seeds per pod, pods per raceme, seed weight, and the number of racemes per stem were computed for each of the 20 stems per field.
Molly E. Dieterich Mabin
1Vegetable Crops Research Unit, United States Department of Agriculture-Agricultural Research Service, Madison, WI, United StatesFind articles by
1Vegetable Crops Research Unit, United States Department of Agriculture-Agricultural Research Service, Madison, WI, United StatesFind articles by
2US Dairy Forage Research Center, United States Department of Agriculture-Agricultural Research Service, Madison, WI, United StatesFind articles by
2US Dairy Forage Research Center, United States Department of Agriculture-Agricultural Research Service, Madison, WI, United StatesFind articles by
Selfing (self-pollination) is the ultimate form of inbreeding, or mating among close relatives. Selfing can create yield loss when inbreeding depression, defined as a lower survival and reproduction of inbred relative to outbred progeny, is present. To determine the impact of selfing in alfalfa (Medicago sativa L.), we quantified the selfing rate of 32 alfalfa seed production fields located in three regions, namely, the Pacific Northwest (PNW), the Central Valley of California (CEV), and the Imperial Valley of California (IMP). Selfing rates (the proportion of selfed seeds) varied between 5.3 and 30% with an average of 12.2% over the 32 seed production fields. In both the parents and their progeny, we observed an excess of heterozygotes relative to Hardy–Weinberg expectations. We detected notable levels of inbreeding in parents (0.231 ± 0.007 parental inbreeding coefficient) and progeny (0.229 ± 0.005). There were a 15% decrease in the number of seeds per stem (seed set) and a 13% decline in the number of seeds per pod in selfed relative to outcrossed stems, but negligible inbreeding depression for pods per raceme and seed weight. The number of racemes on selfed stems increased significantly in fields with greater selfing rates, supporting the presence of geitonogamous or among flower selfing. Despite the significant level of inbreeding depression, seed set did not decrease in fields with higher selfing rates, where the greater number of racemes on the selfed stems increased the seed set. The effects of the field selfing rate on the seed yield metrics were mostly indirect with direct effects of the number of racemes per stem. Available data indicate that the majority of selfing in alfalfa is pollinator-mediated, and thus, eliminating selfing in alfalfa seed production would require the selection of self-incompatible varieties, which, by eliminating inbreeding depression, would provide a 15% potential increase in seed yield and an increase in future hay yield.
Alfalfa, Medicago sativa L., also known as lucerne, is grown for both seed and hay production in the United States. Alfalfa is valued at $9.3 billion, making it the third most valuable field crop in the United States (NAFA News, 2018). The demand for alfalfa is increasing with a growing global population and a higher demand for beef (Putnam, 2019). Therefore, an important goal for alfalfa producers is to increase hay and seed yields. However, hay yield increases have been slow in alfalfa – forage yield in the Midwest has barely changed over the last two decades (Wiersma, 2001; Brummer and Putnam, 2018). Changes in seed production have been mediated mostly via the acreages planted. For example, in the United States in 2017, 30 million kg of alfalfa seed was produced to meet the needs of alfalfa forage growers; this represents a 12% increase in alfalfa seed production compared to 2012 production (NASS, 2017). While various management practices have been identified to help maintain and improve hay and seed yields (Mueller, 2008; Brummer and Putnam, 2018), the potential impact of self-fertilization on alfalfa seed yield has received less attention.
In the current study, we estimated the selfing rate of 32 alfalfa seed production fields located in three major alfalfa seed-producing regions in the western United States. These regions include the Pacific Northwest (PNW), the Central Valley of California (CEV), and the Imperial Valley of California (IMP). Selfing rates were estimated during the same growing season and using a uniform methodology. We examined the level of genetic diversity within and among fields and among regions. We calculated the coefficient of inbreeding of maternal plants and progeny in each field. We compared various yield metrics, the number of seeds per stem (seed set), seeds per pod, pods per raceme, seed weight and racemes per stem, between selfed and outcrossed stems, and quantified inbreeding depression on the seed yield metrics. We examined the relationships between field selfing rate, number of racemes per plant, and the four seed yield metrics. This study provides the most comprehensive report of selfing rate in alfalfa seed production fields and quantifies its impact on the seed yield metrics. Identifying the mating system of a crop and its prevailing mode of selfing can guide the development of effective strategies to reduce selfing and increase yield.
Making Your Alfalfa Better #791 (Air Date 6/2/13)
FAQ
How do you pollinate alfalfa?
How do pollinators increase crop yield?
How do you grow high quality alfalfa?
How much alfalfa do you plant per acre?
How do bees pollinate alfalfa?
Alfalfa flowers must be tripped and cross-pollinated to produce seed. This involves the use of bees to acquire and move pollen among plants in a field. Alfalfa’s unique flower type with a forceful tripping mechanism makes honeybees reluctant to pollinate the crop.
What factors affect the germination of alfalfa seeds?
Germination is a critical stage in the life cycle of alfalfa plants where seeds sprout and develop into seedlings. Here are key factors that influence the germination of alfalfa plant seeds: Temperature: Alfalfa seeds germinate best at temperatures between 65-80°F (18-27°C). Maintain consistent soil temperatures to promote uniform germination.
How do you grow alfalfa from seed?
Start with growing alfalfa from seed, following a detailed alfalfa growing guide to ensure optimal establishment. After alfalfa establishment, it’s crucial to consider rotation and regrowth intervals. Harvesting alfalfa should ideally occur when the plant reaches roughly 10% bloom stage. This allows for a balance between yield and quality.
Do leafcutter bees pollinate alfalfa?
As a result, leafcutter bees are the primary species for alfalfa seed production these days. Introduced to the U.S. in the late 1930s, leafcutter bees increase seed yield dramatically compared to pollination with honeybees. While great pollinators for alfalfa, the leafcutter bee is solitary and only lives a few weeks during the summer.