Selecting a sunn hemp cover crop genotype for weed suppression and seed production

Selecting a sunn hemp cover crop genotype for weed suppression and seed production

Summary

At Gainesville (Florida), Griffin (Georgia), and Lajas (Puerto Rico) sixteen sunn hemp accessions were evaluated for a second year for vegetative and reproductive characteristics. Early and intermediate maturing accessions appear to be best for seed production in the southeastern US and Puerto Rico, but tended to produce less biomass than late maturing accessions, which produced little to no seed. Early and intermediate maturing accessions may be day neutral while the late maturing accessions appear to be short day plants. In experiments to evaluate the effect of removal of apical dominance and plant density on weed suppression and seed yield of a commercially available short day sunn hemp cultivar little or no seed yields were obtained. The lowest planting density resulted in the largest number so flowers; however, weed suppression improved as density increased. Broadleaf weeds appeared to be more effectively suppressed by sunn hemp than grasses and the effect of sedges differed with location. Large bees such as the giant resin bee (Megachile sculpturalis) and carpenter bees (Xylocopa spp.) were effective pollinators of sunn hemp, but smaller bee species such as honey bees were not. Potassium (K) content of sunn hemp shoots increased in response to increasing K fertilizer rates.

Objectives/Performance Targets

The objectives covered by this report include:

1) Evaluation of the effect of different geographical locations on biomass accumulation, flowering and seed yield of the USDA’s germplasm collection of Crotalaria juncea.

2) Investigation of how cutting sunn hemp to break apical dominance affects weed suppression and seed yield, and compare the allelopathic potential of the accessions.

3) Identification of bees that are effective pollinators of sunn hemp and to conduct pollination experiments to verify repoerts of self-incompatibility in sunn hemp.

4) Assessment of the effects of potassium fertization on biomass yield and potassium content of sunn hemp.

Accomplishments/Milestones

Evaluation of Phenotypic Characteristics of Sixteen Accessions of Sunn Hemp

Gainesville, Florida

Limited adoption of sunn hemp (Crotalaria juncea) as a cover crop in the U.S. is primarily due to the irregular supply and high cost of imported seed. Seed production in the continental United States would provide a more consistent supply of lower cost seed while also providing growers with a new seed crop. Improved availability and affordability of sunn hemp might encourage its utilization by growers in their cropping systems for weed and pest suppression, soil stability, and nitrogen and biomass accumulation. Sixteen accessions of sunn hemp were evaluated in 2008 and 2009 at Rosie’s Organic Farm in Gainesville, Florida to assess their vegetative and reproductive characteristics and potential for seed production in Florida. The experimental design was a split plot with planting dates (May, June, and July) assigned to the main plots, which were arranged in a randomized complete block design with four replications. The sixteen accessions were randomly assigned to the sub-plots. Data were collected on plant height, leaf area, number of leaves, number of branches, plant weights, days to first open flower, and seed production. Field observations suggested that accessions could be separated into two distinct groups based on size and daylength sensitivity. Analysis of the vegetative and reproductive data provided evidence for one group of short-day accessions and one group of day-neutral accessions. The short-day accessions were taller, with higher shoot biomass, later branching and flowering than the day-neutral accessions, and little or no seed. The day-neutral accessions flowered early and produced viable seed in summer. Of these accessions, PI 314239 and PI 322377 produced the most seed, demonstrating potential for use for seed production in Florida. These accessions had the lowest shoot biomass, making them less desirable as a cover crop. Future work will focus on developing day-neutral cultivars of sunn hemp that retain the cover crop attributes of the commercially available short-day sunn hemp varieties, but are capable of producing seed in Florida.

Griffin, Georgia

There were significant differences among sunn hemp accessions for all traits evaluated. The number of primary lateral branches among accessions ranged from 3 – 7 while the number of secondary branches ranged from 11 – 148 with accessions 2, 10, 13, 14, 15, 16 exceeding 5 primary lateral branches. Leaf number ranged from 255 – 3,129 for all accessions while leaf area ranged from 23 – 66 cm². Sunn hemp accessions 6, 8, 12, and 13 produced the highest number of leaves exceeding 2,400 leaves per accession. However, accessions 2, 3, 7, 14, 15, and 16 produced the greatest amount of leaf area exceeding 20 LAI. Number of nodes ranged from 140 – 946 and number of internodes ranged from 70 – 473. Interestingly, accessions 6, 10, 12 and 13 produced the highest numbers of nodes exceeding 680 nodes and 340 internodes, respectively. Plant height ranged from 95 – 133 cm² while plant width ranged from 26 – 48 cm² among sunn hemp accessions. The tallest accessions included 8, 12, 13, and 14 exceeding 114 cm while accessions 8, 9, 10, and 13 produced the widest plants exceeding 43 cm. Seed numbers ranged from 0 – 4,302 and total seed weight ranged from 0 – 163 g among sunn hemp accessions. However, accessions 1, 4, 8, 9, and 12 produced the greatest number of seeds exceeding 2,000 seeds and the heaviest seed exceeding 160 gm. Apical dominance ranged from 0.13 – 1.00 indicating that early, intermediate, and late maturing sunn hemp accessions were identified. These are similar to 2008 data.

Crotalaria accessions:
1 = 207657, 2 = 234771, 3 = 248491, 4 = 250485, 5 = 250486, 6 = 391567, 7 = 295851, 8 = 314239, 9 = 322377, 10 = 337080, 11 = 346297 , 12 = 391567, 13 = 426626, 14 = 468956, 15 = 561720, and 16 = 652939.

Lajas, Puerto Rico

Biomass
In 2009, the accessions which produced significantly greater biomass were: PI-234771, IAC-1 (PI-561720), and Tropic Sun (PI-468956). The dry weights per plant for these accessions were 0.272, 0.242, and 0.241 kg, respectively. In contrast, the lowest producing accessions ranged from 0.056 to 0.143 kg dry weight per plant. The accessions with the greatest biomass production were also consistently the tallest, with heights averaging between 6.33 and 6.59 ft at four months after planting. Texas 374 (PI-652939) and T’ai-yang-ma (PI-391567) can both be considered moderate in terms of biomass production (0.143 and 0.122 kg, respectively). Data was assessed using Tukey’s Procedure (?=0.05).

Seed Production
The accessions T’ai-yang-ma, K680 (PI-250486), and Sanni (PI-426626) were among the highest seed producers, with yields of 22.6, 24.9, and 25.8 g/plant, respectively. However, it should be noted that these accessions tended to be those with moderate to low biomass production. Furthermore, the top three biomass producers (PI-234771, IAC-1, and Tropic Sun) fell into group with the lowest seed production, ranging from 0-9.3 g of seed per plant. Seed yields were somewhat compromised in 2009, primarily due to an infestation of Utetheisa ornatrix. The majority of the damage from this pest is a result of the insect burrowing into the pods and consuming the seeds within. However, since the pod is left largely intact, it was possible to account for the damage and estimate yields based on the number of pods collected and using data on the number of seeds/pod/accession and seed weight/accession. Greater insect damage to the harvest was observed in the third planting, which may have been a result of reduced vegetation in the field and the migration of U. ornatrix as the first and second plantings were terminated. Nevertheless, T’ai-yang-ma and Sanni remained among the highest yielding accessions from the third planting.

Preliminary Conclusions
Overall, there was a large variation in the morphology and productivity that was expressed amongst the sixteen accessions of C. juncea. Accessions worth investigating further would include those which have a moderate to high biomass production level, as well as a high seed productivity. From the results in 2009, T’ai-yang-ma is the accession which best fits these characteristics. However, in terms of the accessions studied, it would be beneficial to further assess both the highest producers in biomass (PI-234771, IAC-1, and Tropic Sun) and seed production (T’ai-yang-ma, K680, and Sanni), perhaps at different planting times or under different environmental conditions. Additionally, the accession Texas 374 shows promise as both a moderate biomass and seed producer.

Effects of Seeding Rate and Apical Dominance on Flowering and Weed Suppression in Sunn Hemp

Gainesville, Florida

Previous research shows that cutting sunn hemp to break apical dominance leads to more branching and thereby more flowering, suggesting a potential for increased seed yield. The objective of our study was to evaluate how cutting the main stem to break apical dominance and seeding rates affect weed suppression and seed yield. The experimental design was a randomized complete block with a factorial arrangement of the treatments. The study was conducted at a seed production seeding rate of 11 kg/ha, a cover crop seeding rate of 45 kg/ha, and an intermediate seeding rate of 28 kg/ha. Seeds were sown in rows spaced 49 cm apart on May 15, 2008 and May 18, 2009. Main stems were either left uncut or were cut at 3, 4, and 5 weeks after planting (WAP). In 2009, an untreated control plot was added. Weeds within a 91cm x 46cm quadrat were identified and counted, and weed biomass was collected. Leaf area index (LAI) and photosynthetically active radiation (PAR) were measured using an AccuPAR model LP-80 Ceptometer to evaluate canopy closure. In 2008, cutting sunn hemp to break apical dominance had no significant effect on weed biomass, but in 2009 cut plots had significantly less weed biomass than those left uncut. In 2008, the lowest seeding rate of 11 kg/ha had significantly higher weed biomass than the other seeding rates. In 2009, with the added weedy fallow, all seeding rates had significantly less weed biomass than the weedy fallow. By 12 WAP, the highest seeding rate of 45 kg/ha had the lowest weed biomass. Cutting did not significantly impact PAR. Lower seeding rates had higher percentages of PAR penetrating the canopy at 3 and 6 WAP, but by 8 WAP all plots were well-established and there were no significant differences among the treatments. In both 2008 and 2009 cutting induced branching. In 2009 removing the apical meristem at the later dates increased the number of branches per plant. Plants in the lower seeding densities in both years had significantly more branches than plants in the cover crop seeding rate treatment (45 kg/ha). In 2008 and 2009 cutting had no significant effect on flower production. In both years, the lowest seed rate (11 kg/ha) had significantly more flowers than the other two seeding rates. Although flowering was profuse, seed set was minimal which may be due to the lack of an effective naturally-occurring pollinator. The use of commercially available bumblebees was not effective in promoting seed set.

Griffin, Georgia

Both broadleaf weeds and sedges were significantly reduced when compared to grass weeds over the entire season. However, greater weed reduction for all weeds dropped significantly during August per 0.25 m². The only density effects occurred for weed weights after 6, 9, and 12 weeks in sunn hemp plots. After 6 and 9 weeks, weed weights were significantly highest in the non-planted weedy control plots. However, significantly higher weed weights were found at the 40 lb/acre sunn hemp density than at either 10 or 25 lb/acre after six weeks. Sunn hemp density at both 40 and 25 lb/acre significantly reduced weed weights after 9 weeks while only sunn hemp density at 40 lb/acre significantly reduced weed weights after 12 weeks. The only significant effect by cutting date occurred for weed weights as well. Weed weights were significantly lower after 6 weeks of sunn hemp growth for non-cut, and cut after 3, 4, and 5 weeks. However, after 9 weeks the weedy control plot had significantly higher weed weights than all cutting dates and the non-cut control plot. Significantly lower weed weights occurred after 12 weeks; the non-cut, 4 week cut, and 5 week cut sunn hemp. There were no differences in LAI or PAR at all sunn hemp densities and cutting dates. Sunn hemp plants at density 10 produced significantly fewer primary branches than either 25 or 40 lb/acre. Significantly more sunn hemp secondary branches were produced at density 10 and 25 than density 40. Sunn hemp plants growing at both densities 10 and 25 produced significantly more flowers than density 40. Only
sunn hemp plants cut after 5 weeks produced significantly fewer primary and secondary branches than all other cutting dates and the non-cutting control. There were no differences in flower numbers across all cutting dates. No seed production occurred.

Lajas, Puerto Rico

Biomass
An analysis of variance (LSD, ?=0.05) showed that biomass production was significantly lower when C. juncea was planted at a density of 10 lb/acre, with an average of 5251 lb dry weight/acre, as compared to 25 and 40 lb/acre (8098 and 9732 lb dry wt/acre, respectively). Apical cutting treatments showed no effect or interaction with respect to biomass production and planting density.

Seed Production
Yield results showed a high level of variation in seed production and no distinct differences between planting densities and apical cutting treatments. In 2009, the experiment was planted at the end of April as opposed to June in the previous year. In general, while the C. juncea reached the flowering stage as expected, pod set was poor and many plants reached senescence without setting fruit. As sunn hemp is characterized by its sensitivity to day length, the difference in photoperiod may account for such a wide variation in seed yields, as well as for the lack of significant differences amongst the treatments.

Weed Suppression
At 8 weeks after planting, a density of 40 lb/acre showed a reduction in the presence of broadleaf weeds, as compared to a density of 10 lb/acre. Neither planting density nor apical cutting showed any effect on the presence of grasses and sedges in the field. Additionally, there was no interaction between apical cutting treatments and planting density.

Primary Conclusions
Increased planting densities of C. juncea are shown to have some effect on the suppression of certain types of weed populations and may be necessary to produce sufficient biomass for use as a green manure. Conversely, apical cutting treatments do not reflect any significant changes or interaction with respect to weed suppression, biomass, and seed production. Further research should be conducted with different planting densities and under varying environmental conditions, with specific attention to the effect of photoperiod on seed production.

Pollinators and Pollination of Sunn hemp

Observations of bee pollinators of sunn hemp
The sunn hemp flower has a paplionate structure that limits effective pollination by a few types of bees (Westerkamp, 1997), which is a major impediment for seed availability and, hence, for more widespread use of this plant as a cover crop. To pollinate a flower, while imbibing nectar from an opening at the base of the petals, the insect needs to be heavy enough to depress the keel and long enough for the abdomen to reach over the end of the keel to contact pollen pushed out by the style. The pollen is then transferred to the stigma of other plants exposed the same way. We identified bees visiting sunn hemp and observed interesting behavior, but the small number of bees precluded the quantitative studies that had been proposed. Large Megachile and Xylocopa (large carpenter bees) species have been reported to visit sunn hemp and to be the effective pollinators (Purseglove, 1981; Nogueira-Couto et al., 1992; Free, 1993; Westerkamp, 1997; Etcheverry et al., 2003). Our observations agree with these earlier findings. The exotic Megachile sculpturalis, the giant resin bee, has the size, anatomy, and behavior to be a most effective pollinator. It was first recognized in Florida by one of us during this study and was a frequent visitor to our sunn hemp plots during June. At least one native Megachile species was seen, but they visited too infrequently and were too small to be effective pollinators. Honey bees were frequent nectar and pollen robbers. Robbing pollen involved inserting their proboscis into the keel opening of older and wilted flowers to extract pollen but without pollinating.

Artificial Pollination Experiments
Fertilization of sunn hemp has been described to be self-incompatible (Kundu, 1964; Free, 1993). Other reports claim that the plant becomes self-compatible after mechanical stimulation, by brushing pollen on the stigma (Howard, 1919 et al. cf Free, 1993). We conducted experiments to verify these reports. Pollinators were excluded from flowers with mesh bags, and the plants were divided into four groups: 1) Control, no treatment, 2) Keel pulled down so that the style was pushed out and then allowed to retract, 3) Keel pulled down so that the style was pushed out, the flower’s pollen brushed on the stigma, and then the style allowed to retract, and 4) Same as treatment 3, except a mixture of pollen from other plants was brushed on the stigma. Only treatment 4 resulted in seed pod formation. We tested the possibility of cryptic self-incompatibility, where the pollen from the late opening anthers might be compatible but not the pollen from the early opening anthers. Pollen from the oldest flower on a raceme was used to hand-pollinate the remaining more distal flowers. No pods were formed. Crosses among plants within accessions, using mixtures of pollen from several plants, were made, which resulted in seed pod formation in eight (89% average of number of flowers pollinated) out of nine accessions tested.

Soil Potassium Utilization by Sunn Hemp

The capacity of sunn hemp to utilize soil K uptake and the influence of K on shoot biomass yield was assessed using application rates of 0, 45, 90, and 180 kg/ha as potassium magnesium sulfate (22% K2O). Sunn hemp was seeded at 28 kg/ha on 15 May 2008 at the Plant Science Research and Education Unit in Citra, FL. Percent K in the top 15 cm of sunn hemp shoots increased linearly as the rate of K increased and there was a linear decline in K content with time. Total K in aboveground dry plant tissue increased with increasing K rate. Potassium content of shoot biomass at 6 WAP was 68 kg/ha indicating that in addition to nitrogen, sunn hemp can also serve as a source of K for subsequent crops in short rotations.

References

Etcheverry, A.V. Protomastro, J.J. Westerkamp, C. 2003. Delayed autonomous self-pollination in the colonizer Crotalaria micans (Fabaceae: Papilionoideae): structural and functional aspects. Plant Syst. Evol. 239: 15–28.

Free, J.B. 1993. Insect Pollination of Crops. 2nd Ed. Academic Press, San Diego.

Kundu, B.C. 1964. Sunn-hemp in India. Proc. Soil Crop Soc. Florida 24: 396-404.

Nogueira-Couto, R.H. Costa, J.A. Silveira, R.C.M. Couto, L.A. 1992. Polinizacao de Crotalaria juncea por abelhas nativas. Ecossistema 17: 12-16.

Purseglove, J.W. 1981. Leguminosae. p. 250-254. In Tropical crops: Dicotyledons. Longman Group Ltd, Essex, UK.

Westerkamp, C. 1997. Keel blossoms: Bee flowers with adaptations against bees. Flora 192: 125-132.

Impacts and Contributions/Outcomes

Results were presented as a poster at the joint meeting of the Southern Weed Science Society and the Weed Science Society of America in Orlando, Florida in February 2009. An oral presentation of results was also done at the February 2009 meeting of the Florida Weed Science Society. An oral presentation at the Florida State Horticultural Society meeting in June 2009 reported on results that were published in the proceedings of that meeting in January 2010. Citations of the abstracts and the publication are as follows:

Cho, A.H., C.A. Chase, D.D. Treadwell, and R.L. Koenig. 2009. Apical dominance and planting density effects on weed suppression by sunn hemp. WSSA Abstracts, #23, 1 page.

Cho, A.H., C.A. Chase, D.D. Treadwell, and R.L. Koenig. 2009. Investigating sunn hemp seed production to promote cover crop adoption. Proc. Florida Weed Science Society, p. 7.

Treadwell, D.D., C. Chase, M. Alligood, and J. Elsakr. 2010. Potential for sunn hemp (Crotalaria juncea L.) to utilize soil potassium. Proc. Florida State Horticultural Society Meeting 122:243-246.

Collaborators: