Submitted to: Journal of Animal Science
Publication Type: Abstract Only
Publication Acceptance Date: 4/11/2006
Publication Date: 7/9/2006
Citation: Estell, R., Fredrickson, E., Remmenga, M. 2006. Antiherbivory compounds on the leaf surface of intact and resprouted tarbush [abstract]. 2006 Joint Annual Meeting American Society of Animal Science, American Dairy Science Association, July 9-13, 2006, Minneapolis, Minnesota. J. Anim. Sci. Vol. 84, Suppl. I / J. Dairy Sci. Vol. 89, Suppl. 1. p. 22.
Technical Abstract: Shrub encroachment into desert grasslands of the southwestern United States is a serious problem, resulting in loss of forage and rangeland productivity. Flourensia cernua (tarbush) is one such shrub contributing to the decline of Chihuahuan Desert grasslands. Our previous research has shown tarbush consumption by sheep and goats to be negatively related to leaf surface concentration of individual terpenes and epicuticular wax. Concentrations of antiherbivory compounds such as terpenes change with plant age and phenology. Our objective was to alter the vegetative state and potential palatability of tarbush by clipping intact plants and generating resprouts. We hypothesized resprouts would invest fewer resources into antiherbivory compounds and therefore have lower concentrations of volatile chemicals such as terpenes than intact plants. Ninety tarbush plants were randomly selected, and all biomass within 10 cm of the soil surface was removed from 45 plants during winter dormancy. Leaves were collected the following summer during active growth from the canopy of intact controls and resprouts. Leaf surface volatiles were extracted in ethanol and analyzed by gas chromatography-mass spectroscopy to determine their identity and concentration. Data were transformed to natural logarithms and subjected to univariate analysis of variance and stepwise discriminate analysis. Of the 87 compounds present on tarbush leaves, 35 were greater in canopy samples and 16 were greater in regrowth samples based on univariate analysis (P < 0.05). Mean concentration of total volatiles on canopy leaves tended to be less (P = 0.0622) than that of regrowth (3642 vs 4684 'g/g DM). Nine compounds in the discriminate analysis ('-muurolene, iso-borneol, unknown#6, p-cymen-8-ol, unknown#7, sabinene, '-caryophyllene, '-cadinene, and '-copaene) explained 95% of the variation between canopy and regrowth samples. In contrast to our hypothesis, lower cumulative concentration of volatile compounds in canopy than regrowth samples suggests repsrouts may be less vulnerable to herbivory than old growth tarbush. However, animal studies are needed and effects on other classes of secondary compounds must be examined.