Interactions between Forage Sorghum Cultivars and Defoliation Managements1

Abstract

AbstractYield and plant constituent responses of forage sorghum (Sorghum bicolor (L.) Moench) cultivars have usually been compared in uniform defoliation management tests. However, the influence of time of harvest and stubble height on differential response of cultivars needs more precise definition. The extent of cultivar × management interaction was studied for ‘Sweet Sioux III’3 (a leafy high‐producing cultivar similar to many others available), ‘FS‐531’ (a silage‐forage cultivar) and ‘Super‐Chowmaker 235’ (several weeks later in maturity than most other forage sorghum cultivars) for 2 years. The soil was a Sequatchie sandy loam (fine‐loamy, siliceous, thermic Humic Hapludults). Stubble height and defoliation managements designed to simulate grazing, greenchop, hay or silage harvests, in various combinations, were used with each cultivar. Chowmaker plants yielded more, had the same or higher N percent, and the same in vitro dry matter digestibility (IVDMD) when cut to a 15‐cm than when cut to an 8‐cm stubble. Sweet Sioux plants yielded more when cut back to 8 cm than to 15 cm, but their N percent and IVDMD were not affected by stubble height. Cutting when forage reached 50 cm previous to the boot (B) or early bloom (EB) cuts delayed subsequent development, and hence harvest, of FS‐531. The other two cultivars reached the B or EB stages at the same time, regardless of whether that harvest was preceded by a prior harvest or not, but the B or EB stages occurred at shorter heights if preceded by a harvest. Harvesting when forage reached 90 cm before the B or EB cut delayed harvest for all cultivars, probably because all regrowth had to originate from axillary meristems. The delay in B or EB harvests may be beneficial in situations where dry matter production later in the season is more desirable. The results suggested that, if the three different cultivars used here had been entered in a yield trial where all entries were managed uniformly, the information on the potential performance of these forage sorghum cultivars could have been misleading. Harvests at early stages of growth would have penalized Super‐Chowmaker 235 in comparison to Sweet Sioux III, and the reverse would have occurred if later stages of growth had been used to determine harvest time.