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In vitro and in vivo digestion trials with lambs were conducted to determine effects of ponderosa pine needles (PN; Pinus ponderosa Laws.) on digestibility of crested wheatgrass (CW; Agropyron desertorum [Link] Schultes) hay. Pine needles contained shikimic acid (15-28 mg/g) and several monomeric phenolics (p-hydroxy benzoid acid, caffeic acid, p-coumaric acid, ferulic acid) and flavonoids. Tannin concentration exceeded assay limits (>10%) and terpenes were not found, probably due to the drying procedure. In the in vitro trial, needles were mixed with CW in 10% increments from 0% to 100%. In the in vivo trial, PN were fed to lambs as follows: (1) 0%, (2) 12.5%, (3) 25%, and (4) 50%, with the remainder of the diet as CW. In vitro organic matter digestibility (IVOMD) was regressed on level of PN in the substrate. As the proportion of PN increased, IVOMD declined cubicly (P<0.01). The IVOMD values ranged from 54% for 100% CW to 24% for 100% PN. In vivo digestibility of organic matter, neutral detergent fiber and acid detergent fiber declined linearly (P<0.01) as PN were increased from 0% to 50% of the diet. Apparent crude protein digestibility and N retention by lambs declined cubicly (P = 0.02 and P<0.01, respectively) and urinary N increased cubicly (P<0.01) as dietary PN increased from 0% to 50%. We concluded that PN reduce in vitro and in vivo nutrient digestibility, reduced N retention by lambs, and effects were detectable even at low levels. This material was digitized as part of a cooperative project between the Society for Range Management and the University of Arizona Libraries. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020