Nematode population changes and forage yields of six corn and sorghum cultivars.

Two temperate corn (Zea mays) hybrids (Pioneer 3320 and Northrup King 508), two tropical corn cultivars (Pioneer X304C hybrid and Florida SYN-1 experimental open pollinated cultivar), the sorghum (Sorghum bicolor) x sudangrass (Sorghum sudanense) hybrid DeKalb SX-17, and the sorghum hybrid DeKalb FS25E were compared for effect on nematode densities and forage yield in three plantings (one single-crop and one double-crop system) in Florida. Final population densities of Meloidogyne incognita in the three plantings ranged from 0 to 13/100 cm(3) soil on the two Sorghum spp. and were lower (P </= 0.001) than those obtained on the corn cultivars (range 147 to 762/100 cm(3) soil). Early planted temperate corn and sorghum generally gave higher forage yields than did tropical corn. As second crops in double-cropping systems, tropical corn cultivars generally produced greater yields than temperate corn hybrids did. At 35% dry matter, double crop forage corn yield ranged from 51.1 to 64.8 ton/ha, and sorghum ranged from 79.8 to 102.2 ton/ ha. Tropical corn, forage sorghum, and sorghum-sudangrass were profitably grown at all planting dates. Late summer planting of temperate corn was unprofitable. DeKalb SX-17 sorghum x sudangrass first crop plus the ratoon double crop gave the highest net return of $1,133/ha. Among the corn cultivars, Florida SYN-1 gave the highest double crop net return of $652/ha.