Potassium Management for Brown Midrib Sorghum · Sudangrass as Replacement for Corn Silage in the North-eastern USA

In recent years, there has been a growing interest in brown midrib (BMR) sorghum (Sorghum bicolor (L.) Moench.) · sudangrass (Sorghum sudanense Piper) hybrids (SxS) as a replacement for silage corn (Zea mays L.) in the north-eastern USA. Recent studies suggest it is suitable for both rotational grazing and as a hay crop and could compete with corn harvested for silage in years when wet spring conditions prevent the timely planting of corn. However, little is known about its suitability as forage for nonlactating cows that require low potassium (K) forages to prevent health problems. Our objective was to evaluate the impact of K fertilizer management (0, 112 or 224 kg K2O ha cut) under optimum N management (112–168 kg N ha cut) on yield, quality and K concentrations of BMR SxS over a 2-year period. Field trials were established on a fine loamy, mixed, active, mesic Aeric Fragiaquepts with medium K-supplying capacity and characteristic of a large region in New York. Potassium application did not affect dry matter yields in either of the 2 years. Averaged over 2 years, neutral detergent fibre (NDF) significantly increased with K addition with similar but non-significant trends observed in each of the years individually. The digestibility of NDFwas unaffected byK application. Crude protein (CP) concentrations showed a significant decrease with K application in 2002 and similar trends were observed in 2003, although differences were not significant at P £ 0.05. The changes inNDF and CP did not significantly impact forage quality expressed as milk production per megagram of silage. Potassium application increased forage K concentration up to 13 mg K kg dry matter (in the first cut in 2003). Forage Ca and Mg concentrations decreased with K addition except for the first cut in 2002 where differences between 112 and 224 kg K2O ha )1 treatments were not significant. Without K addition in the 2-year period, K concentrations in the forage decreased from 23 g kg for the first cutting in 2002 to 15 g kg for the second cut in 2003. Low K forage was obtained for all second-cut forage unless 224 kg K2O ha )1 cut had been added. First-cut forage was suitable only when no additional K had been applied. These results suggest low K BMR SxS forage can be harvested from initially high K soils without loss in dry matter yield as long as no additional K is added.