Sunn hemp, Crotalaria juncea L., is a warm-season legume that is planted before or after a vegetable cash crop to add nutrients and organic matter to the soil (Cherr et al., 2006, 2007; Mansoer et al., 1997; Wang et al., 2005). This cover crop provides other benefits to growers such as suppression of weeds (Adler and Chase, 2007; Cho et al., 2015; Collins et al., 2008; Javaid et al., 2015; Morris et al., 2015; Mosjidis and Wehtje, 2011) and suppression of plant-parasitic nematode populations by not providing resources (Bhan et al., 2010; Braz et al., 2016). Sunn hemp is available to growers in northern Florida, where 10,000 ha of vegetables (potatoes, Solanum tuberosum L., and cabbage, Brassica oleracea L. var. capitata) are annually produced (Elwakil and Mossler, 2016; USDA-NASS, 2014).
A major advantage of sunn hemp over many cover crops, particularly sorghum-sudangrass (SSG) [Sorghum bicolor (L.) Moench], is that it is a poor host for fall armyworm [Spodoptera frugiperda (J. E. Smith)] and other important moth pests (Meagher et al., 2004). Southern Florida is a major overwintering region for fall armyworm populations that annually migrate northward and infest corn acreages along the eastern United States. Sorghum-sudangrass is one of the common cover crop and forage grasses in the United States and therefore is a likely source of fall armyworm populations (Meagher et al., 2004; Pair and Westbrook, 1995). Therefore, large scale replacement of SSG with sunn hemp in Florida could mitigate fall armyworm populations before and during these annual migrations on an area-wide scale.
Another ecological service that cover crops can provide is flowers that provision pollinators and enhance biological control (Campbell et al., 2016). Studies have shown that early season flowering by cover crops can increase pollinator populations, which promote late-season pollination in cash crops (Riedinger et al., 2014). Flower density and the richness of the cover crop plants have the most direct influence on bee visitation and native bee abundance (Ellis and Barbercheck, 2015; Saunders et al., 2013).
The first sunn hemp cultivar commercialized in the United States was ‘Tropic Sun’, which was a direct increase of the original seed purchased in 1958 from the island of Kauai, HI and developed by the USDA Natural Resources Conservation Service (NRCS) and the University of Hawaii College of Tropical Agriculture and Human Resources (CTAHR) as a cooperative release in 1983 (Rotar and Joy, 1983; later identified as developed from PI 468956, Wang et al., 2006). Seed production in the tropics is good; however, because of this cultivar’s flowering response to short days, seed production is generally poor in the continental United States (Mosjidis, 2007; White and Haun, 1965). Therefore, seed availability to growers was limited to importation of ‘Tropic Sun’ from Hawaii or bringing in other germplasm from Brazil, India, South Africa, or other foreign locations. A breeding program to increase flowering and seed production was initiated at Auburn University in 2002. ‘AU Golden’ was developed from accession PI 322377, which was obtained from the National Plant Germplasm System. In a patent application, this germplasm was shown to be able to produce seed in Alabama, with the data extrapolated to indicate flower production, seed production, or both in the region between 40° north and 40° south latitude, and at least 400 lbs seed per acre when grown between 28° north and 28° south latitude (Mosjidis, 2014). These characteristics make it a potentially useful cover crop for much of Florida, which lies between 25° and 31° north latitude. However, it is not clear whether the local environment, including such factors as soil type, disease, herbivores, and appropriate pollinators, is conducive to this line of sunn hemp.
To examine the potential and optimize sunn hemp cover crop usage in north-central Florida, multiple sunn hemp lines were tested in field studies to compare flowering and seed production across different seeding rates and planting dates. One objective was to test whether a low seeding rate would result in the production of higher numbers of flowers and seed pods. A second objective was to test whether planting ‘AU Golden’ earlier in the season would result in both higher numbers of flowers produced and, because we felt that there was more pollinator activity at this time of year, higher seed production. A final goal was to test whether similar sunn hemp seeding could be obtained but with the potential for higher overall biomass per acre if a mixture of sunn hemp and SSG was used as a cover crop.
Abdul-Baki, A.A., Bryan, H.H., Zinati, G.M., Klassen, W., Codallo, M. & Heckert, N. 2001 Biomass yield and flower production in sunn hemp: Effect of cutting the main stem J. Veg. Crop Prod. 7 83 104
Adler, M.J. & Chase, C.A. 2007 Comparison of the allelopathic potential of leguminous summer cover crops: Cowpea, sunn hemp, and velvetbean HortScience 42 289 293
Bhan, M., McSorley, R. & Chase, C.A. 2010 Effect of cropping system complexity on plant-parasitic nematodes associated with organically grown vegetables in Florida Nematropica 40 53 70
Braz, G.B.P., Oliveira, R.S. Jr, Crow, W.T. & Chase, C.A. 2016 Susceptibility of different accessions of Crotalaria juncea to Belonolaimus longicaudatus Nematropica 46 31 37
Campbell, J.W., Irvin, A., Irvin, H., Stanley-Stahr, C. & Ellis, J.D. 2016 Insect visitors to flowering buckwheat, Fagopyrum esculentum (Polygonales: Polygonaceae), in north-central Florida Fla. Entomol. 99 264 268
Cherr, C.M., Scholberg, J.M.S. & McSorley, R. 2006 Green manure as nitrogen source for sweet corn in a warm-temperature environment Agron. J. 98 1173 1180
Cherr, C.M., Scholberg, J.M.S., McSorley, R. & Mbuya, O.S. 2007 Growth and yield of sweet corn following green manure in a warm temperature environment on sandy soil J. Agron. Crop Sci. 193 1 9
Cho, A.H., Chase, C.A., Treadwell, D.D., Koenig, R.L., Morris, J.B. & Morales-Payan, J.P. 2015 Apical dominance and planting density effects on weed suppression by sunn hemp (Crotalaria juncea L.) HortScience 50 263 267
Cho, A.H., Chase, C.A., Koenig, R.L., Treadwell, D.D., Gaskins, J., Morris, J.B. & Morales-Payan, J.P. 2016 Phenotypic characterization of 16 accessions of sunn hemp in Florida Agron. J. 108 2417 2424
Collins, A.S., Chase, C.A., Stall, W.M. & Hutchinson, C.M. 2008 Optimum densities of three leguminous cover crops for suppression of smooth pigweed (Amaranthus hybridus) Weed Sci. 56 753 761
Ellis, K.E. & Barbercheck, M.E. 2015 Management of overwintering cover crops influences floral resources and visitation by native bees Environ. Entomol. 44 999 1010
Elwakil, W.M. & Mossler, M. 2016 Florida crop/pest management profile: Cabbage. Univ. of Florida, IFAS Extension, CIR 1256, 18 p
Javaid, M.M., Bhan, M., Johnson, J.V., Rathinasabapathi, B. & Chase, C.A. 2015 Biological and chemical characterizations of allelopathic potential of diverse accessions of the cover crop sunn hemp J. Amer. Soc. Hort. Sci. 140 532 541
Mansoer, Z., Reeves, D.W. & Wood, C.W. 1997 Suitability of sunn hemp as an alternative late-summer legume cover crop Soil Sci. Soc. Amer. J. 61 246 253
Meagher, R.L., Nagoshi, R.N., Stuhl, C. & Mitchell, E.R. 2004 Larval development of fall armyworm (Lepidoptera: Noctuidae) on different cover crop plants Fla. Entomol. 87 454 460
Morris, J.B., Chase, C.A., Treadwell, D., Koenig, R., Cho, A., Morales-Payan, J.P., Murphy, T. & Antonious, G.F. 2015 Effect of sunn hemp (Crotalaria juncea L.) cutting date and planting density on weed suppression in Georgia, USA J. Environ. Sci. Health B 50 614 621
Mosjidis, J.A. 2007 Breeding of annual and perennial legumes and their utilization as forage and crops Field Veg. Crops Res. 44 7 11
Mosjidis, J.A. 2014 Sunn hemp cultivars capable of producing seed within the continental United States. United States Patent, US 8,680,369 B2
Osborne, J. 2010 Improving your data transformations: Applying the Box-Cox transformation Pract. Assess., Res. Eval. 15 12<http://pareonline.net/getvn.asp?v=15&n=12>
Pair, S.D. & Westbrook, J.K. 1995 Agro-ecological and climatological factors potentially influencing armyworm populations and their movement in the southeastern United States Southwest. Entomol. 18 101 118
Riedinger, V., Renner, M., Rundlöf, M., Steffan-Dewenter, I. & Holzschuh, A. 2014 Early mass-flowering crops mitigate pollinator dilution in late-flowering crops Landsc. Ecol. 29 425 435
Rotar, P.R. & Joy, R.J. 1983 ‘Tropic Sun’ sunn hemp Crotalaria juncea L. Hawaii Institute of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, Honolulu, HI. 7 pp. Research Extension Series 036. <http://www.ctahr.hawaii.edu/oc/freepubs/pdf/RES-036.pdf>
SAS Institute 2012 SAS for Windows, 9.4
Saunders, M.E., Luck, G.W. & Mayfield, M.M. 2013 Almond orchards with living ground cover host more wild insect pollinators J. Insect Conserv. 17 1011 1025
USDA-NASS 2014 Census of agriculture. 25 Aug. 2016. <https://agcensus.usda.gov./Publications/2012/Full_Report/Volume_1,_Chapter_2_County_Level/Florida/st12_2_029_029.pdf>
Wang, Q., Klassen, W., Li, Y., Codallo, M. & Abdul-Baki, A.A. 2005 Influence of cover crops and irrigation rates on tomato yields and quality in a subtropical region HortScience 40 2125 2131
Wang, M.L., Mosjidis, J.A., Morris, J.B., Dean, R.E., Jenkins, T.M. & Pederson, G.A. 2006 Genetic diversity of Crotalaria germplasm assessed through phylogenetic analysis of EST-SSR markers Genome 49 707 771