Seasonal variation in the quality of a lucerne-based pasture and its relationship with morphological and maturity estimates

To monitor seasonal changes in herbage quality, a lucerne-based pasture ( Medicago sativa, Bromus willdenowii and Dactylis glomerata) was sampled in Argentina every 2 weeks for 28 months. The pasture was strip-grazed and samples were taken from the regrowth of a previously grazed strip, ready for regrazing, for which herbage mass was estimated with a calibrated rising plate meter. Each sample was sorted into dead and green components, and the latter subsequently separated into grass and lucerne, and then into lamina plus leaflet and stem plus pseudostem fractions. Similarly, at each sampling date, quantitative maturity indexes-mean stage by count and mean stage by weight-were applied to grasses and lucerne. Samples were also analysed for in vitro dry matter digestibility (DMD), crude protein (CP), fibre and non-structural carbohydrates. The dataset was divided into morphological, maturity and nutritional variables. Analyses of variance by season for both groups of variables were carried out using year as a block. Multiple regression analyses were performed for each season between maturity indices and predictors of herbage quality. DMD, and consequently metabolisable energy (ME), was significantly lower in the autumn and CP was lower in the summer compared with overall averages, which were consistently high throughout the year (overall average of 11.5 MJ ME/ kg dry matter and 20.6% CP). The sward had a higher proportion of lucerne during summer and autumn, than winter and spring ( averages 59.3 and 48.8%, respectively). The highest leaf: stem ratio (2.82) was during winter and the highest green content (97.5%) was during spring. Grasses had a higher mean stage by count and mean stage by weight during spring-summer, whereas lucerne had a higher mean stage by count and mean stage by weight during summer-autumn. Morphological and maturity estimates predicted satisfactorily the changes in the energy and fibre within season, but CP content was not well predicted in summer or winter. These results provide the basis for tactical grazing practices with further calibration.