Mechanistic model of intake of tropical pasture, depending on the growth and morphology of forage at a vegetative stage

Abstract

The present model is targeted to simulate the diet of animals in tropical pastures, starting from underlying measurements carried out at the level of the tiller. Practically, we used measurements carried out at the following two levels: (1) on many tillers identified in the sward with rings and measured in various conditions (stages of regrowth, more or less fertilised and or irrigated), (2) on plots grazed individually by heifers tethered during 24 h, with measurements of both the forage and the animal faeces, for assessing their diet. The first step consisted in building a mechanistic model of the morphological grass growth calibrated on measurements performed on Dichanthium spp. at five vegetative stages of growth. In the process of growth, the principal stem was the major driving force, and the senescence of leaves was followed by their disappearance. The compartments represent lengths of stem or leaves. Taking into account the diversity of kinetics of the leaf appearance and growth, they were pooled in three types, according to their appearance process (two lower leaves, four leaves in the middle and the others at the upper level). In a second step, the impact of animal bites was included as an auxiliary variable in the model, taking into account the information at the level of the tillers and at the plot level. The impact of animal depends on the characteristics of the tiller, the length (or mass) and of the leaf characteristics, e.g. the leaves fraction in the sward. Afterwards, the information obtained at the tiller level and from the plot was compared. With a bottom-up approach it is possible to predict at a daily scale, starting from the tiller measurements, the dry matter intake and the organic matter digestibility, both measured at the plot level. Conversely, in a top-down approach, the information measured at the plot level were useful for adjusting the information acquired at the level of the tillers, e.g. the estimates of organic matter digestibility of the stems and the leaves at the plot level, allowing to assess the factors of variation of the organic matter digestibility of the consumed parts of a tiller, by the animal. These first results are encouraging and this model seems to promise a more complete mechanistic model of grazing cattle in tropical environment.