Structural responses to selection for reduced fiber concentration in smooth bromegrass

Selection for reduced neutral detergent fiber (NDF) concentration has been practiced as a mechanism of increasing voluntary intake potential of forage crops. Because selection is usually conducted on the basis of whole-plant samples, there exists the potential for structural shifts in plant composit ion. The objective of this study was to quantify structural changes due to selection for reduced NDF concentration in smooth bromegrass (Bromus inermi s Leyss). Three cycles of phenotypic selection led to reduced whole-plant N DF concentration, primarily due to reductions in NDF of stems (-5.7 to -5.8 g kg(-1) cycle(-1)) and leaf sheaths (-3.1 to -4.9 g kg(-1) cycle(-1)). Se lection, at both the vegetative growth stage (primarily leaf blades) or hea ding growth stage (all shoot components present) led to similar changes in shoot-component NDF concentrations. Selection on the basis of vegetative sa mples did not lead to structural changes. Conversely, selection on the basi s of headed samples led to an average reduction of -7.5 % cycle(-1) in stem component concentration, which was compensated largely by increases in lea f blade and sheath component concentrations. Increases in leaf:stem ratio m ay partially explain reductions in forage yield associated with reduced NDF concentration. Future selection efforts should attempt to avoid this respo nse by using samples composed of a single shoot component. The NDF concentr ations in various shoot components of smooth bromegrass appear to be positi vely correlated with each other.