Assessment of spring defoliation to improve fall forage quality of bluebunch wheatgrass (Agropyron spicatum)

  • Published source details Pitt M.D. (1986) Assessment of spring defoliation to improve fall forage quality of bluebunch wheatgrass (Agropyron spicatum). Journal of Range Management, 39, 175-181.


The primary objective of this investigation was to determine the most judicious phenological stage at which bluebunch wheatgrass Agropyron spicatum should be grazed by cattle in order to improve autumn forage quality for wildlife. The study was undertaken in 1978 and 1979 at the University of British Columbia in Vancouver, western Canada.

In August of 1976, 200 bluebunch wheatgrass seeds were germinated in the greenhouse and transplanted into experimental plots. In 1978, plants were randomly assigned to each of five clipping treatments: Controls (45 plants unclipped), clipped at boot, emergence, flowering, and seed formation stages.

Moderate grazing was simulated by clipping to 15 cm height. Forage quality (digestibility evaluated with acid detergent fiber (ADF); nitrogen (N), calcium (Ca), and phosphorus (P) measured as quality assessments) of regrowth was compared with the non-clipped control plants during 1978 and 1979. Forage quality averages for nine 25-day intervals (9 April to 26 October) were estimated.

After 2 years of treatment, all clipped plants produced lower ADF levels and higher CP and P values than control plants at equivalent phenological stages. Clipping for 2 years at boot and emergence delayed flowering by 16 and 15 days, respectively. Flowering of plants clipped at flowering and seed formation was thereafter infrequent. Autumn forage quality assessments (26 October) indicated improvements compared to non-clipped plants.
Clipping at boot, emergence, flowering, and seed formation reduced percent foliar ADF, and increased relative proportions of CP, Ca and P compared to unclipped plants. Crude protein in plants clipped for 2 years at emergence, flowering, and seed formation averaged 11.9%, 12.5%, and 13.7% respectively. Phosphorus in regrowth foliage of plants clipped at flowering and seed formation was 0.22% and 0.26%, respectively.
This study indicates that spring defoliation increases bluebunch wheatgrass autumn forage quality. The values found exceed maintenance requirements of cattle and elk Cervus elaphus.
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