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.
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