Dry matter yield and nutritive value of corn, forage sorghum, and BMR forage sorghum at different plant populations and nitrogen rates

Total on-farm inputs must be reduced in order to ensure agricultural sustainability as input costs increase worldwide. Forage sorghum [FS; Sorghum bicolor (L.) Moench] may provide an acceptable, lower cost alternative to corn (Zea mays L.) grown for silage as available irrigation continues to decline in semi-arid regions of the world. Also, plant populations and nitrogen fertilizers need to be reduced as much as possible so that waste is minimized and resource use efficiency is maximized. Reducing plant populations and N fertilizer may affect yield and nutritive value of FS and corn when grown in limited irrigation situations. A 2-year study was conducted to investigate the effects of planting rates (PR) and nitrogen rates (NR) on corn, conventional forage sorghum (C-FS) and brown midrib forage sorghum (BMR-FS) on dry matter (DM) yield and forage quality. Irrigation applied was 33mmweek⁻¹, an amount that is limiting for corn production in warm, semi-arid regions. Planting rates were: low (55,575 and 185,250plantsha⁻¹); medium (66,690 and 222,300plantsha⁻¹); and high (74,100 and 247,000plantsha⁻¹) for corn and both FS, respectively. Nitrogen rates for C-FS and corn were: low (218kgha⁻¹) and high (291kgha⁻¹); and for BMR-FS were low (106kgha⁻¹) and high (140kgha⁻¹). All three crops were harvested at optimum DM content (35-40%) for ensiling. Over 2 years, there was no effect of PR or NR on DM yield, NDF, NDFD or NEL; however, crop did have an effect. Conventional FS and corn were similar in DM yield (24.4Mgha⁻¹), but greater than BMR-FS (21.1Mgha⁻¹). In general, crude protein (CP) was similar among all three crops; however, CP of C-FS was lower than that of corn and BMR-FS at the low NR. At the low NR, increasing PR to ‘medium' or ‘high' resulted in reduced CP. Corn contained the greatest net energy (NEL; 1.15Mcalkg⁻¹), but lowest neutral detergent fiber digestibility (NDFD; 62.0% of NDF). While BMR-FS had high NDFD (74.5% of NDF), its NEL was similar to that of C-FS (1.03Mcalkg⁻¹). Results indicate that production inputs of seed and N fertility can be reduced considerably without negative effects on performance of these silage crops grown with reduced irrigation.