Analysis of Herbage Mass and Herbage Accumulation Rate Using Gompertz Equations

Sigmoid equations are recognized as representative of the pattern of herbage accumulation during a growth period; however, the various equations and their variability among locations and during the growing season have not been well described. The objectives of this study were to find the most suitable, four-parameter sigmoid equations to fit measured herbage mass and to investigate how the patterns of herbage accumulation (i.e., equation parameters) varied with time of year and location. Herbage mass was measured approximately weekly during 11 to 12 growth periods with a rising plate meter (RPM) at three north-central United States locations (Columbus and Coshocton, OH, and Arlington, WI) during 2008, and those data were fit to Gompertz equations. There were four replicates for each growth period. We found predictable relationships between instantaneous herbage accumulation rate (HAR(i)) and herbage mass for each location and date. Time-independent HAR(i) vs. herbage mass curves have potential use for pasture management by defining the optimum herbage mass at which HAR(i) is maximum. The optimum herbage mass varied between 1600 and 4000 kg dry matter (DM) ha(-1) depending on location and date. Allowing herbage mass to exceed the optimum point (e.g., delayed harvest), or harvesting to below the optimum point, will reduce the HAR(i). The HAR(i)-herbage mass curves define a range of herbage mass within which pastures can be managed to achieve high HAR(i), and maintaining pastures within 90% of the maximum HAR(i) may be a practical target for producers.