Defoliation regime effects on accumulated season-long herbage yield and quality in boreal grassland

Within boreal grasslands, little information exists on the effects of initial defoliation date, frequency, and intensity on accumulated herbage yield and quality. We investigated the effects of initial defoliation in May, June or July, at heights of 5, 10 or 15 cm, and repeated at 2-, 4- or 6-week intervals throughout the growing season. Harvested material was combined with year-end residual biomass, and examined for herbage removed, crude protein (CP), crude protein yield (CPY) and neutral detergent fibre (NDF). Compared to single defoliated check plots, total, grass and forb dry matter (DM) yields were lower under repeated defoliation by 25, 38 and 17 %, respectively. The majority of total herbage produced was harvested in the spring clipping. Total, grass and forb DM yields peaked under moderate (10 cm) clipping. Total DM and grass biomass were maximized with long (6 week) recovery periods. In contrast, forb biomass was greatest with May defoliation followed by a 4-week interval. While maximum grass CPY was found under 10-cm defoliation, forb CPY peaked with early and moderate to intensive defoliation. These results indicate that season-long herbage biomass, along with CPY, can be maximized in boreal grasslands through controlled defoliation.