Using a mathematical framework to examine physiological changes in winter wheat after livestock grazing: 1. Model derivation and coefficient calibration

Existing crop-livestock models can largely be separated into two groups: those that simulate crop growth and grain yield with no grazing, and those that simulate pasture production during and after grazing but do not simulate grain yield. The purpose of this paper was to develop a crop-grazing model (hereafter, ‘WHTGRAZ’) capable of realistically simulating biophysical phenomena of grazed wheat crops in rainfed environments. This derivation revealed important physiological distinctions between typical crop simulation and changes in growth that occurred as a result of grazing, such as delayed phenological development. WHTGRAZ was based on the crop model SUCROS2, with modifications. The Penman approach to evapotranspiration simulation was adapted to the FAO Penman–Monteith method. Dry matter (DM) accumulation was modified from photosynthesis to radiation-use efficiency. Specific leaf area was modelled as a function of crop ontogeny, and rules for dead leaf decomposition and trampling of shoot DM were also added. Phenological delays induced by defoliation – and consequences thereof – were identified as a key determinant of growth after grazing, and were modelled as a function of temperature and the rate of shoot DM removal. Parameters were calibrated using measurements of phenology, soil water, shoot DM, and leaf area index from field experiments using two winter wheat cultivars and four grazing treatments. Two important physiological insights were gained from the derivation. First, accurate prediction (within one SEM) of shoot DM removal during grazing is a necessary prerequisite for accurate simulation of post-grazing shoot DM accumulation. Reasonable quantification of green leaf removal and trampling by livestock is needed because green leaf area governs light interception and growth in WHTGRAZ. Second, grazing-induced delays to phenological development cause greater proportions of shoot DM to be allocated to leaves during late vegetative development, decreasing the fraction of shoot DM that would otherwise be partitioned to stems. Delayed phenology of grazed crops also causes greater proportions of kernel DM to be sourced from post-anthesis photosynthesis, as opposed to DM retranslocated from stems.