Predicting Leaf/Stem Ratio and Nutritive Value in Grazed and Nongrazed Big Bluestem

Abstract

Recent advancements in describing morphological development of perennial grasses have provided a useful index for identifying dates to harvest hay or graze pasture. The objective of this study was to determine the relationship between big bluestem (Andropogon gerardii Vitman) morphological developmental stage, days from 1 May, and accumulated growing degree days (GDD) and leaf/stem ratio, crude protein (CP), and neutral detergent fiber (NDF) in grazed and nongrazed swards. The grazing experiment was conducted in eastern Nebraska during 1997 and 1998. Six grazing treatments consisting of all combinations of two herbage allowances (22 or 66 kg of herbage dry matter per animal unit day) and three grazing dates (late May, early June, and mid‐June) were randomly assigned to seven paddocks (900 m2) in each of four blocks along with a nongrazed control treatment. Random samples consisting of 50 or more tillers were hand‐clipped weekly at ground level in each paddock beginning in late May and ending in early August. Morphological developmental stage of the samples were determined, and a mean morphological stage based on shoot count [mean stage count (MSC)] was calculated. Samples were hand‐separated to determine leaf/stem ratio and analyzed for CP and NDF. Regression analysis showed that good relationships were found between MSC (R2 = 0.61–0.81), days from 1 May (R2 = 0.45–0.76), and GDD (R2 = 0.44–0.74) and leaf/stem ratio in nongrazed big bluestem and high herbage allowance swards. All three were adequate predictors of nutritive value and leafiness, but MSC may be more useful to researchers who want a more descriptive measure of plant morphological development, particularly when comparing cultivars or species.