Growth responses of bromegrass (Bromus inermis Leyss) and orchardgrass (Dactylis glomerata L.) to intensities of defoliation

Bromegrass (Bromus inermis Leyss.) and orchard-grass (Dactylis glomerata L.), grown under greenhouse and growth room conditions, were subjected to various degrees of clipping. Established greenhouse-grown seedlings, planted three per crock, were clipped at different plant heights and to different stubble heights. In subsequent experiments on single plants reared in the growth room from cuttings, certain of the treatments were reversed mid-way through the experimental period.Plant and stubble height at harvesting both affected yield of dry matter but the latter had the greater effect. The combination of close and frequent clipping depressed yields considerably. The beneficial effect of an increase in plant height diminished as stubble height increased and vice versa. When treatments were reversed during the experimental period, yields of orchardgrass generally were higher in the second period from plants that had been subjected to the more lenient treatments in the first period. Yields of bromegrass tended to follow the same pattern as orchardgrass but did not respond as much to the previous cutting treatment.Root and crown weights were highly correlated with each other and with top growth weights. The number of stems increased with increasing stubble height but was little affected by plant height. When different clipping treatments were applied in each of two periods, root and crown weights and stem numbers were affected chiefly by the second-period treatments.The growth rate of plants cut to a 2-cm stubble height reached a maximum early in the experimental period and had begun to decline while the rate from the 6- and 10-cm stubble heights still was increasing. The growth rate was lower at the conclusion of the experiments than at the beginning when the plants were cut to the 2-cm stubble height, but remained above the initial rate when cut to the higher stubble heights.The distribution of yield was similar within a stubble height, especially when cut to 6 and 10 cm. Plant height did not alter the yield curve even though it affected total yield. There were differences in distribution between stubble heights. More uniform distribution of yield throughout the period was obtained by cutting to the higher stubble. The results indicate that bromegrass and orchardgrass should be clipped to a stubble height not less than 6 em at relatively infrequent intervals to obtain maximum yields distributed evenly throughout the growth period.