- Research was conducted near the Starkey Experimental Forest and Range in
northeastern Oregon. Effects of defoliating bluebunch wheatgrass (Agropyron
spicatum (Pursh) Scribn. and Smith) to increase the quality of regrowth available on
elk (Cervus elaphus nelsoni) winter range were studied from 1988 through 1990.
Clipping treatments were implemented to condition the forage regrowth. Treatments
were no defoliation, spring defoliation (7.6 cm stubble height) in June, and fall
defoliation (7.6 cm stubble height) in September. Percent calcium, phosphorus, in
vitro dry matter digestibility (IVDMD), and available forage (kg/ha DM) of regrowth
present on control, spring defoliated, and fall defoliated plots were determined in
November and April of both years. Conditioned forage that was again defoliated in
the winter was also analyzed for nutrient quality and available forage.
Spring conditioning did not affect (p > 0.05) the forage in percent calcium,
phosphorus, or available forage, and only slightly increased the IVDMD, when
compared to the control in November. In November, the control and spring
conditioned forages were deficient in meeting elk requirements for phosphorus, and
contained wide calcium to phosphorus ratios. The forages were below 50% IVDMD,
and digestible energy levels were below animal requirements in year 1, indicating that
spring conditioning did not have an effect on the quality of winter range forage.
Defoliation in the vegetative phenology stage allowed the regrowth to complete the
growing season similarly as undefoliated plants.
Fall conditioning significantly increased the percent phosphorus and IVDMD,
while decreasing the available forage compared to the control and spring conditioned
forage in November. Fall conditioned forage exceeded elk requirements in both
calcium and phosphorus. The calcium to phosphorus ratio was near the optimum
absorption range. Digestibility was high, and digestible energy levels were above
animal requirements for both years. Fall conditioning however, may create a severe
deficit of forage if regrowth is not achieved.
In April, there were no differences among treatments in percent calcium,
phosphorus, or available forage. Forage from all treatments exceeded elk
requirements in calcium and phosphorus, and the calcium to phosphorus ratio would
allow optimum absorption of both minerals. Digestibility was high for forage from
all treatments. This indicated that the previous years defoliation did not effect
forage quality the following spring.
Conditioned forage that was again defoliated in the winter was not different
in percent calcium or phosphorus when compared to the control in April. Depending
on the year and conditioning treatment, there were statistically significant differences
in IVDMD and available forage between the control and the winter defoliated
samples in April. Conditioned forage that was not defoliated in the winter (April
(U)) and winter defoliated samples (April (W)) were comparable in forage quality
and available forage in April, though statistical differences were calculated for the
spring conditioned samples in year 1, and fall conditioned samples in year 2.