Effect of frequency of pasture measurement on estimation of pasture growth rates: field observations and predictions of a whole farm model

The frequency of measurements to generate pasture growth rate data varies. Commonly measurements are made once weekly or fortnightly and averaged monthly, using a variety of methods. To assess the effect of frequency of measurement on estimates of pasture growth rate, weekly visual assessment of farm pasture cover was compared with the fortnightly and monthly average and also with predictions by a Whole Farm Model. Weekly observed pasture growth rates had large fluctuations but these were removed when the weekly values were averaged monthly. The fluctuations are due to the variety of paddocks assessed, climatic factors, inconsistencies of operators and inherent errors in the technique used. Values calculated by a Whole Farm Model also showed daily variation in pasture growth rate but the fluctuations were not as severe as those in the observed pasture growth rate data because human error of assessment and error in the technique were removed. Observed monthly pasture growth rates were also compared with values calculated by the model. The model closely predicted observed pasture growth rates for most months. To obtain an accurate estimate for monthly growth rates it is better to average a number of assessments. In the field, this could be an average of weekly observations. Because the model calculates rates daily, it can be used to predict pasture growth rates on a more frequent basis (e.g. weekly) to aid feed budgeting. Keywords: dairy farm, herbage mass, model, pasture growth, simulation