Changes in the physiology and feed quality of cocksfoot (Dactylis glomerata L.) during regrowth

A glasshouse study was undertaken to determine the physiological and morphological changes in cocksfoot (Dactylis glomerata L.) during regrowth after defoliation. Individual plants were arranged in a mini-sward in a randomized complete block design. Treatments involved harvesting each time one new leaf had expanded (one-leaf stage), up to the six-leaf stage, with the plants separated into leaf, stubble (tiller bases) and roots. Stubble and root water-soluble carbohydrate (WSC), stubble and leaf dry matter (DM), tiller number per plant and leaf quality (crude protein (CP), estimated metabolizable energy (ME) and mineral content) were measured to develop optimal defoliation management of cocksfoot-based pastures. WSC concentration in stubble and roots was highest at the five- and six-leaf stages. Mean WSC concentration (g kg-1 DM) was greater in stubble than roots (32.7 ± 5.9 vs. 9.4 ± 1.5 respectively). There was a strong positive linear relationship between plant WSC concentration and leaf DM, root DM and tillers per plant after defoliation (Adj R2 = 0.72, 0.88 and 0.95 respectively). Root DM plant-1 and tiller DM tiller-1 decreased immediately following defoliation and remained low until the three-leaf stage, then increased from the four-leaf stage. Tillers per plant remained stable until the four-leaf stage, after which they increased (from 9.9 ± 0.5 to 15.7 ± 1.0 tillers plant-1). Estimated metabolizable energy concentration (MJ kg-1 DM) was significantly lower at the six-leaf stage (11.01 ± 0.06) than at any previous leaf regrowth stage, whereas CP concentration (g kg-1 DM) decreased with regrowth to the six-leaf stage. Both the levels of ME and CP concentrations were indicative of a high quality forage throughout regrowth (11.37 ± 0.04 and 279 ± 8.0 for ME and CP respectively). Results from this study give a basis for determining appropriate criteria for grazing cocksfoot-based pastures. The optimal defoliation interval for cocksfoot appears to be between the four- and five-leaf stages of regrowth. Delaying defoliation to the four-leaf stage allows time for replenishment of WSC reserves, resumption of root growth and an increase in tillering, and is before herbage is lost and quality falls due to onset of leaf senescence.