A theoretical analysis of grass growth under grazing

Abstract

A model of grass growth under grazing is constructed by combining a previously described crop growth model with an animal intake function. There are two main features of the crop growth model which make it appropriate for studying grazing. The first is that leaf area is treated as an independent state variable and is not, as in many crop models, inflexibly related to the crop weight by an empirical equation. The second feature is the treatment of senescence which has been shown to be an important component in grazing studies (due to the rapid turnover of material in the grass crop). Dynamic solutions for crop growth under grazing are examined. Equilibrium solutions of the model are then obtained and it is demonstrated that, under some conditions, two stable equilibria may exist for a given stocking density. The model is used to demonstrate that the usual theoretical assumption relating the growth of intermittently cut swards to that of grazed swards may be inappropriate and inaccurate. It is assumed that the crop is supplied with adequate water and nutrients: root growth, root:shoot partitioning, soil water and nutrients are assumed to be relatively unimportant for an established vegetative grass crop.