Abstract

The herbage mass and nutritional value of harvested forage are fundamental determinants of the production potential of pastoral systems. The objective of this study was to evaluate the growth dynamics and accumulated herbage mass expressed in dry matter (DM) of perennial ryegrass (Lolium perenneL.) and pasture brome (Bromus valdivianusPhil.) pastures, using thermal time (TT) as a defoliation criterion. Thirty plots (15 ofL. perenneand 15 ofB. valdivianus) were distributed in three field blocks and subjected to five defoliation frequencies (DF) determined by TT, expressed as the accumulated growing degree-days (AGDD; DF1 = 90, DF2 = 180, DF3 = 270, DF4 = 360, and DF5 = 450 AGDD) for one year (2016), at the Austral Agricultural Experimental Station of the Universidad Austral de Chile. Every three days, the total leaf length (TLL) was measured, and the leaf elongation rate (LER, cm L-1), leaf growth rate (LGR, cm L-1), leaf appearance rate (LAR, d L-1), phyllochron (AGDD L-1), and accumulated herbage mass per hectare (kg DM ha(-1)) were calculated. Defoliations were scheduled according to AGDD, and a sample was taken from each cutting to determine (dry matter 'DM', crude protein 'CP', neutral detergent fiber 'NDF', acid detergent fiber 'ADF', water-soluble carbohydrates 'WSC' and metabolizable energy 'ME'). The pastures that were allocated to DF5 presented higher DM yields (12,600 kg DM ha(-1)year(-1)), TLL (54.6 cm), and LER (0.63 cm d(-1)) compared to pastures with high DF (90 and 180 ADGG).B.valdivianuspresented a lower phyllochron thanL.perenne(74.4 vs 87.9 AGDD L-1, respectively). Concentrations of CP and ME decreased from the shortest DF (90 AGDD) to the largest DF (450 AGDD), dropping from 221 to 138 g kg(-1)CP and from 2.6 to 2.4 Mcal kg(-1)DM of ME. All variables were affected by the season (Ssn) (p 0.001). The AGDD can be used as a defoliation criterion and a tool to balance yield with nutritive value according to the farmer's needs.