A pasture growth model for use in a whole-farm dairy production model

A generalised climate driven pasture growth model is described and evaluated by comparison to field observation. The model describes dry matter production and green-dead tissue flow dynamics for grazed temperate swards, especially perennial ryegrass (Lolium perenne). The model includes a unique feature to account for light interception by non photosynthetic tissue. Extrapolation across environments occurs by the interaction of climate variables with three parameters that tune the model to a site. These parameters reflect the influence of soil fertility and sward species composition on production patterns. They are: (1) the efficiency of radiant energy use for photosynthesis, (2) the timing of reproductive development and (3) the relative efficiency of radiant energy use in vegetative compared to reproductive swards. Initial parameter settings were derived from data from a sheep grazing experiment in New Zealand. In this paper the ability to describe pasture production under dairy grazing at a different site is confirmed. When the three available parameters were calibrated for the dairy site, all but two of the 42 seasonal estimates of pasture production were within the 95% confidence interval for mean measured production. The model is being used as a component of a whole-farm dairy production model.