Reduced Population and Nitrogen Rate Effects On Corn, Conventional and Brachytic Dwarf Forage Sorghum Yield and Plant Composition.

Low inputs are of interest in irrigation systems used for silage production in the Southern High Plains where water resources are declining. Forage sorghum (FS) provides an acceptable alternative to corn grown for silage as irrigation capabilities diminish. The effects of low plant population and nitrogen on yield, plant composition, and nutritive value of corn and FS are uncertain in these scenarios. A two-year study was initiated in 2009 to investigate the effects of three seeding (SR) and two nitrogen (NR) rates on yield, nutritive value, and plant composition of corn, conventional forage sorghum (C-FS), and brown midrib, brachytic dwarf forage sorghum (BMR-FS). Seeding rates were reduced from regional standards and were: low (38,890 and 124,445 seeds ha^-1); medium (47,775 and 151,110 seeds ha^-1); and high (55,575 and 185,185 seeds ha^-1) for corn and FS, respectively. Nitrogen rates for all crops were: low (168 kg ha^-1) and high (224 kg ha^-1). Irrigation amount was 33 mm wk^-1 to give a total of 282 mm yr^-1. Crops were harvested at optimum dry matter (DM) content for ensiling and were divided into stover and grain components. Over 2 years, C-FS and corn were similar in DM yield (19.4 Mg ha^-1), but yielded greater than BMR-FS (17.0 Mg ha^-1). Increasing SR increased DM yield in corn, indicating low SR was limiting in corn, but not FS. Nitrogen had no effect on yield or nutritive value. Corn contained the greatest in vitro true digestibility (IVTD; 830 g kg^-1 DM), but comparable neutral detergent fiber digestibility to BMR-FS (NDFD; 608 g kg^-1 NDF). Corn and dwarf BMR-FS had similar grain-to-stover ratios (GSR; 0.87 and 0.92, respectively). High SR resulted in the lowest GSR due to higher proportions of leaves and stalks combined with less grain. Results indicate that corn competes well with FS in limited water scenarios, but may be more sensitive to reducing populations. Brachytic dwarf sorghum has similar yield and nutritive value characteristics to other BMR types.