|Abstract or Summary
- Yellow starthistle (Centaurea solstitialis L.) is an introduced Asteraceae
that has become established on 10 million acres in the Pacific Northwest and
California. This weed functions as an annual or short-lived perennial and depends
on seeds for reproduction. Strategies of control that reduce plant fitness or lower
seed production or viability may help limit the rate of spread of yellow starthistle.
Previous work has shown that grazing and mowing can influence seed production.
This study tested the hypothesis that proper timing and frequency of defoliation can
reduce the number and viability of seeds produced. The study was conducted in
Umatilla County, Oregon using a randomized block design with 4 replications of
each of 4 defoliation treatments: (1) single defoliation at the bolting stage; (2)
single defoliation at the bud stage; (3) two defoliations, once at the bolting stage
and again at the bud stage; (4) non-defoliated control. Each of 4 blocks consisted
of a 12 x 12 m area, with 16 plots measuring 3 x 3 m. Plants were defoliated at
ground level using a gas-powered string-type mower. Response measurements
were collected at the end of the growing season (September) following potential
regrowth and included: (1) number of seedheads per plant; (2) number of seeds per
seedhead; (3) number of seeds per plant; (4) number of seeds m⁻², (5) seed viability
(% germination rates). Supporting measurements included: seedhead diameter;
plant height, number of branches per plant; pre-dawn xylem pressure; soil
moisture; and documentation of 5 biological control insect species. A single
defoliation at bolting resulted in fewer seeds per seedhead, and fewer seeds per
plant than non-defoliated controls. A single defoliation at the floral bud stage or
repeated defoliation (bolting and again at the bud stage) resulted in equally fewer
seeds per plant and fewer seeds m⁻² compared to non-defoliated controls. There
was no statistical difference in percent germination of seeds among treatments.
Defoliation had no effect on the infestation rates of seedheads by biological control
insects. A second study examined nutrient content of yellow starthistle during 6
phenological stages from sites in Union, Baker and Umatilla Counties, Oregon
during each of 2 years. Acid detergent fiber, lignin, cellulose and neutral detergent
fiber contents increased through phenological development. Crude protein ranged
from 16.7 to 5.0%. In Vitro dry matter digestibility ranged from 84.8% to 57.0%.
Mineral nutrients P, K, CA, Mg, Mn, Fe, Cu, Zn, and Na were analyzed and
determined to be adequate for maintenance needs of ewes.