Thurber needlegrass: seasonal defoliation effects on forage quantity and quality.

Abstract

Although Thurber needlegrass (Stipa thurberiana Piper) is an important component of Palouse, sagebrush:steppe, and pine:forest rangelands, little is known of its qualitative and quantitative responses to defoliation. At 14-day intervals one of 7 cohorts of Thurber needlegrass plants was defoliated to a 2.5-cm stubble to describe initial growth rates, determine defoliation effects on subsequent regrowth accumulations, relate regrowth potential to available soil moisture, and determine the nutritional value of initial growth and regrowth for livestock. The study was conducted in 1985 and 1986 with a different group of plants used each year. Although crop-year precipitation for the 1985-86 treatment years was 77 and 111%, respectively, of the long term mean (25.2 cm), growth rates of tussocks were similar between years (P > 0.05). Seasonal yield of regrowth varied between years, however, and was well correlated (r2 = 0.76 to 0.80 P < 0.05) with soil moisture content when treatments were applied. Among 7 defoliation dates (24 April-17 July) only the first 5 yielded regrowth in 1985, and all produced regrowth in 1986. Among treatments regrowth averaged 22% of total herbage yield in 1985 and 50% of total yield in 1986. In both years total herbage accumulations were most suppressed (47-63% reduction) by defoliation during the early-boot stage of phenology. In 1985 when conditions were drier, any defoliation before mid-June depressed (P < 0.05) total herbage yield. Crude protein (CP) of needlegrass herbage was high (19-22%) when growth began in April but declined (P < 0.05) to marginal levels for cattle (6.7-7.7%) by mid-July. Regrowth harvested on 31 July ranged from 7 to 9% CP for the earliest (24 April) treatments and as high as 17% for the latest (17 July). Although Thurber needlegrass can produce highly nutritious regrowth for late-season use, managers face diminishing levels of regrowth as the initial cropping date is delayed later into the growing season. Managers contemplating 2-crop grazing regimes for Thurber needlegrass should base scheduling on plant phenology, soil moisture considerations and historic use rather than specific calendar dates. Further work is needed, however, to definitively determine Thurber needlegrass responses to long-term manipulative grazing regimes.