Tiller Persistence of Eight Intermediate Wheatgrass Entries Grazed at Three Morphological Stages

Intermediate wheatgrass [Thinopyrum intermedium (Host) Barkw. & D.R. Dewey subsp. intermedium] provides high quality forage for hay and pasture in the Great Plains of North America but lacks persistence under grazing. We investigated the effect of grazing at early vegetative, mid-culm elongation, and boot stages on tiller persistence of three cultivars and five experimental lines of intermediate and pubescent wheatgrass [T. intermedium subsp. barbulatum (Schur) Barkw. & D.R. Dewey]. The study, located near Mandan, ND was seeded in 1997, hayed in 1998 and 1999, and grazed from 2000 to 2002. Four tillers were marked on one plant per plot in May 2000. Number of newly emerged tillers (tiller recruitment) and tiller mortality were recorded on the four original tillers and subsequently recruited tillers in mid-May, July, and September. Tiller replacement ratios, which incorporated cumulative recruitment and mortality, were greatest on plots grazed during early vegetative and mid-culm elongation. Tiller recruitment, tiller mortality, and tiller replacement ratio had a year x treatment interaction. In 2000, grazing during the early vegetative stage resulted in significantly higher tiller replacement ratios than grazing during late boot or no grazing, and grazing during mid-culm elongation and late boot had greater tiller recruitment but also greater tiller mortality than the other two treatments. Tiller replacement ratio had a grazing treatment x entry interaction in 2001. ʹMandan 1871ʹ had the greatest tiller replacement ratio and tiller recruitment among entries. The results indicate time of grazing and cultivar selection influence the persistence of tillers of intermediate wheatgrass primarily through their effect on tiller recruitment.