Defoliation Tolerance of Bluebunch and Snake River Wheatgrasses
- Thomas A. Jones and
- Dale C. Nielson
Bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) A. Löve] has long been noted for poor tolerance to grazing during culm elongation in the spring. Bluebunch wheatgrass is an autoploid series of diploids (2x) and tetrapioids (4x). In the mid-1980s, group of populations were found to be allotetraploid and are now commonly referred to as Snake River wheatgrass. Relative grazing tolerance of 2x and 4x bluebunch wheatgrass and of Snake River wheatgrass is unknown. This study evaluated the effect of multiple defoliation on these grasses to simulate one consequence of grazing, removal of photosynthetic tissue. Fifteen entries representing the three taxa were established at North Logan, UT, on a Millville silt loam (coarse-silty, carbonatic, mesic Typic Rendolls; 2–4% slope) during 1988. Seedlings were vegetatively propagated into two clones apiece and transplanted adjacent to one another; one clone was subjected to multiple defoliation and the other served as an undefoliated control. Multiple defoliation at 10 cm was applied from late spring to midsummer from 1990 to 1992 at 4-wk intervals and dry matter yield was measured in late fall 1991, early spring 1992, late fall 1992, and late spring 1993. We used the defoliated-to-control ratio of plant response to estimate defoliation tolerance. The defoliated-to-control ratio for dry matter yield of Snake River wheatgrass (0.17 and 0.38 in late fall 1991 and early spring 1992, respectively) was greater than for 4x (0.06 and 0.18) or 2x (0.06 and 0.19) bluebunch wheatgrasses after the first year of multiple defoliation. The defoliated-to-control ratio for dry matter yield did not differ among taxa in late fall 1992, but the ratios for Snake River wheatgrass (0.20) and 4x bluebunch wheatgrass (0.21) were both greater than for 2x bluebunch wheatgrass (0.13) in late spring 1993. Less variation was seen among Snake River than bluebunch wheatgrass entries. Because Snake River wheatgrass had both higher yield and greater defoliation tolerance than bluebunch wheatgrass, Snake River wheatgrass is more likely to survive defoliation over the long term.
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