A Method for Estimating Yield Losses From Nutrient Rate Reductions.

Many farmers are finding themselves unable to pay for the entire quantity of nutrients recommended. A common question is, “How much yield loss can I expect if I cut my nutrient rates?” To answer this question, some estimate of the nature of crop response to nutrient additions is required. A commonly used mathematical model is a quadratic-plateau function. It is used both for its simplicity as well as its ability to adhere to the law of diminishing returns. Normally, least-squares techniques are used to fit this response function to data taken from studies involving incremental rate applications. However, at the farm level, such data sets normally do not exist. Instead, farmers may have data from a nutrient misapplication or from intentionally omitting a nutrient application. In cases where a lower than normal rate has been applied and a yield reduction observed, a quadratic-plateau response function can be reconstructed. Two data points are required: 1) the higher yield achieved at the normally applied rate and 2) the lower yield achieved at a reduced rate. The mathematical approach of estimating the coefficients of a quadratic response function requires that the normally applied rate be one of four possibilities: 1) an economically optimum rate; 2) a rate that maximizes yield; or a rate that is a known amount above that needed to either 3) optimize economic returns or 4) maximize yield. These four possibilities quantify a range of expected yield loss from rate reductions, helping the farmer to make more informed decisions about his or her nutrient management program.