Grazing and Production in Seasonal Pastures: Analysis of a Simple Model

Abstract

also analytically. (3) The model allows for two effects of grazing on pasture dynamics, both through delays in the accumulation of biomass: (a) early in the season, lengthening the periods in which biomass is suboptimal for intake and growth rates; (b) later in the season, restraining the development of biomass levels which are supraoptimal for growth. The first effect is always prominent; the second one only occurs in fast-growing long-season pastures. (4) Some of the results of the model follow. (a) In contrast to previous results for continuously growing pastures, total production and consumption in seasonal pastures are continuous functions of animal density H; however, in long seasons the functions have a pronounced inflection. (b) Seasonal primary production in general decreases sigmoidally with increasing H, except in fast-growing long-season pastures where it may increase slightly at low densities. (c) Seasonal intake (and presumably production) per animal always decreases with H, in a generally sigmoidal fashion. (d) The production of a pasture in an ungrazed treatment may be poorly correlated with the production and seasonal intake it will allow under medium or heavy continuous grazing.. (e) The responses of pasture outputs to pasture and animal parameters (except te) are in general of diminishing slope. Primary production in the ungrazed pasture responds best to increases in the maximal growth rate (Ga) and the length of season (te). In grazed pastures (except at low H), both primary production and green season intake (animal production) are insensitive to Gx, but respond to increases in initial biomass V0, and in relative growth rate at low biomass g, over a wide range. (f) At very low animal density, the seasonal animal intake increases monotonically with pasture grazability (s) which is the effective grazing area rate at low biomass, and is a function of both physical accessibility and palatability. At any other density, there is an optimum s above which green season intake decreases; this optimum will be lower, the higher the density