Bromus-Poa response to defoliation intensity and frequency under three soil moisture levels

Smooth brome (Bromus inermis Leyss.) and Kentucky bluegrass (Poa pratensis L.) are important herbage for livestock and wildlife in Aspen-Boreal ecosystems in central Alberta, but there is paucity of information on the relationship between soil moisture and defoliation regimes on herbage production in these ecosystems. In a greenhouse experiment, we evaluated the effect of Bromus-Poa defoliation frequencies (2 or 4 wks) and intensities (2.5, 7.5, or 15 cm above the soil surface) under three soil moisture regimes [field capacity (wet), 50% field capacity (moist), 20% field capacity (dry)] on dry matter (DM) yield. Crude protein (CP) content, crude protein yield (CPY) and neutral detergent fiber (NDF) were also determined for herbage harvested. Total accumulated shoot DM decreased under defoliation compared to the undefoliated control, was higher if plants were clipped every 4 wks, rather than 2 wks, and increased with increasing soil moisture availability. Defoliation regimes decreased root DM Compared to the undefoliated control. Soil moisture regime did not significantly affect below-ground DM production, but root:shoot ratio increased significantly with decreasing moisture supply. The average CP content of grasses ranged from 12 to 23%, but was adequate to meet crude protein requirements of growing, pregnant or lactating grazing cattle (Bos spp.) The CPY decreased with increasing moisture stress, and was greatest when plants were clipped at a 7.5-cm height. Shoot NDF concentration increased with decreasing clipping frequency. These result indicate the need to investigate the relationship between soil moisture and management practices that affect the productivity of tame pastures in Aspen-Boreal ecosystems.