Tall genotypes of elephantgrass (Pennisetum purpureum Schum.) and energycane (Saccharum spp.) are potential biomass energy plants in subtropical areas because annual dry matter (DM) yields often exceed 40 Mg ha−1. A field study was conducted to characterize crop performance of these C4 bunchgrasses during the 35‐ to 40‐wk warm season in north‐central Florida. Entries included two tall elephantgrasses (PI 300086 and N51), a tall energycane (L79‐1002), and intermediate pearlmillet [Pennisetum glaucum (L.) R. Br.] × elephantgrass hybrid (S41). In 1989 at Gainesville and in 1990 at another location 20 km away, the grasses were mowed on 28 March and fertilized 1 wk later with 200 kg N, 22 kg P, and 83 kg K ha−1. Onetime harvests of above‐ground biomass were made at 28‐d intervals from 16 May through 28 November. Final harvest DM yields (2‐yr means) were 46 Mg ha−1 yr−1 for PI 300086, 47 for N51, 49 for L79‐1002, and 37 for S41. Near‐linear DM accumulation continued for 140 to 196 d at mean crop growth rates ranging from 18 to 27 g m−2 d−1. For the tall bunchgrasses, DM production at any point during the season was accurately predicted from canopy height, since height was linearly related to the square root of DM yield (0.87 ≤ r2 ≤ 0.96). Gross energy yields (2‐yr means) in barrels of crude oil equivalent (158.8 L barrel −1) were 135 ha−1 yr−1 for PI 300086, 139 for N51, 141 for L79‐I002, and 106 for S41. The high‐yielding abilities of the grasses were related to the long linear DM accumulation period.

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Sustainable bioenergy cropping systems require not only high yields but also efficient use of inputs. Management practices optimizing production of sweet sorghum [Sorghum bicolor (L.) Moench] for bioenergy use are needed. The effects of N rate (45, 90, 135, and 180 kg N ha −1 ) and top removal (at boot stage, anthesis, and none) on biomass, brix, estimated sugar yield, and N and P recovery of sweet sorghum cv. M-81E were investigated in Florida at two sites differing in soil type. Across all data, dry biomass yields averaged 17.7 Mg ha −1 and were not affected by N fertilization rate at either site (P>0.10). Mean brix values ranged from 131 to 151 mg g −1 and were negatively related to N rate. Top removal, either at boot stage or anthesis, resulted in greater brix values and 13% greater sugar yields at both locations. Whole plant N recovery was positively and linearly related to N rate and ranged from 78 to 166 kg N ha −1 , approximately two thirds of which was in leaf and grain tissues. Based on yield and nutrient recovery responses, optimal nutrient requirements were 90 to 110 kg N ha −1 and 15 to 20 kg P ha −1 . Higher N fertilization led to greater N recovery, but little to modest gain in sugar yield. Thus, proper nutrient and harvest management will be needed to optimize sugar yields of sweet sorghum for production of biofuels and bio-based products. Further research is needed to refine management practices of sweet sorghum for bioenergy production, especially with regard to the use of leaf and grain tissues.

Elephantgrass (Pennisetum purpureum Schum.) is known throughout much of the wet tropics for its prolific growth and usage as a forage for ruminants. In a 3‐yr study conducted on a welldrained, infertile soil (loamy, siliceous, hyperthermic, Grossarenic Paleudult) and under subtropical conditions near Gainesville, FL, the response of this forage to three harvest frequency regimes was measured. Genotypes evaluated were four tall elephantgrasses (PI 300086, ‘Merkeron’, N‐43, and N‐51), a dwarf elephantgrass (Mott), and a semi‐dwarf Pennisetum glaucum (L.) R. Br. × P. purpureum Schum. hybrid (Selection 3). Forage dry matter (DM) yields for Merkeron, N‐43, and N‐51 did not differ. For these three genotypes, 3‐yr mean DM yields (1986‐1988) were 24.3, 21.1, and 17.0 Mg ha−1 yr−1 for one, two, and three harvests per year, respectively. Two‐year mean (1986‐1987) crude protein (CP) concentrations for all four tall genotypes were 40.3, 57.5, and 75.7 g kg−1 DM while in vitro digestible organic matter (IVDOM) concentrations were 399, 492, and 555 g kg−1 organic matter (OM) for one, two, and three harvests per year, respectively. For Mott, 3‐yr mean DM yields were 11.8, 11.8, and 11.7 Mg ha−1 yr−1 for one, two, and three harvests per year, respectively. Two‐year mean CP concentrations were 52.7, 69.4, and 85.6 g kg−1 DM whereas IVDOM concentrations were 396, 549, and 580 g kg‐1 OM, respectively. Merkeron, N‐43, N‐51, and Mott persisted well. In PI 300086 plots harvested multiple times, major stand losses occurred over the 1987–1988 winter. Selection 3 did not persist. Adapted elephantgrass genotypes can persist in colder regions of the subtropics and should provide substantial quantities of forage.

Florida dairies need year-round forage systems that prevent loss of N to ground water from waste effluent sprayfields. Our purpose was to quantify forage N removal and monitor nitrate N (NO3(-)-N) concentrations in soil water below the rooting zone for two forage systems during four 12-mo cycles (1996-2000). Soil in the sprayfield is an excessively drained Kershaw sand (thermic, uncoated Typic Quartzipsamment). Over four cycles, average loading rates of effluent N were 500, 690, and 910 kg ha(-1) per cycle. Nitrogen removed by the bermudagrass (Cynodon spp.)-rye (Secale cereale L.) system (BR) during the first three cycles was 465 kg ha(-1) per cycle for the low loading rate, 528 kg ha(-1) for the medium rate, and 585 kg ha(-1) for the high. For the corn (Zea mays L.)-forage sorghum [Sorghum bicolor (L.) Moench]-rye system (CSR), N removals were 320 kg ha(-1) per cycle for the low rate, 327 kg ha(-1) for the medium, and 378 kg ha(-1) for the high. The higher N removals for BR were attributed to higher N concentration in bermudagrass (18.1-24.2 g kg(-1)) than in corn and forage sorghum (10.3-14.7 g kg(-1)). Dry matter yield declined in the fourth cycle for bermudagrass but N removal continued to be higher for BR than CSR. The BR system was much more effective at preventing NO3(-)-N leaching. For CSR, NO3(-)-N levels in soil water (1.5 m below surface) increased steeply during the period between the harvest of one forage and canopy dosure of the next. Overall, the BR system was better than CSR at removing N from the soil and maintaining low NO3(-)-N concentrations below the rooting zone.