Sunn hemp (SH, Crotalaria juncea L.) is used in Florida strawberry production as an off-season summer legume cover crop to provide agroecosystem services such as weed and plant-parasitic nematode (PPN) suppression. Bicultures of SH with sorghum-sudangrass (SS) (Sorghum bicolor Moench × S. sudanense [Piper] Stapf) are of interest to reduce the cost of SH use. Our study objective was to determine whether bicultures retain the weed and PPN suppression benefits provided by SH grown in monoculture. A replacement series experiment was conducted in summer 2022 and repeated in 2023 in north-central Florida in a PPN-infested field. A factorial treatment arrangement of five SH:SS biculture proportions (100:0, 75:25, 50:50, 25:75, and 0:100) and three seeding rates (20, 40, and 60 lb/acre) were evaluated in a randomized complete block design with four replications. A no-cover crop, weedy control was also included in each block. Data were collected on cover crop biomass and carbon: nitrogen ratio (C:N), weed density, weed biomass, and PPN populations at eight weeks after planting. Bicultures consistently produced higher total cover crop biomass than the SH monoculture. Bicultures were as effective as the SH monoculture in reducing total weed density and resulted in either lower or equivalent total weed biomass. Averaged over year, only the 60 lb/acre seeding rate had a lower total weed density than the 20 lb/acre rate. In 2022, both the 40 and 60 lb/acre seeding rates resulted in lower weed biomass than the 20 lb/acre rate, while seeding rate had no significant effect on total weed biomass in 2023. Root-knot nematode populations in both years and sting nematode populations in 2022 were detected at levels too low for analysis. However, in 2023, the lowest and highest sting nematode populations occurred with the SH and SS monocultures, respectively; and sting nematode populations increased as the proportion of SS in the bicultures increased. Bicultures with ≤50% SS had significantly higher sting nematode populations than the weedy control. An increase in C:N was observed as SS proportion in bicultures increased. Thus, we conclude that SH/SS bicultures maintain the weed biomass suppression benefits of SH monocultures while increasing cover crop biomass. However, bicultures may lead to higher sting nematode populations, which will be of concern in organic strawberry production where soil fumigation is not permitted. Growers will also need to consider the effects of the higher C:N ratio of biculture residue on nitrogen fertilizer immobilization.
