Losing control: What happens when the control treatment is excluded in nitrogen fertilizer rate trials?

Authors: CLEVERINGA, ALEX - Iowa State University; Ransom, Curtis; MCDANIEL, MARSHALL - Iowa State University; MOORE, KENNETH - Iowa State University; NIEMI, JARAD - Iowa State University; MIGUEZ, FERNANDO - Iowa State University

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2025
Publication Date: 12/3/2025
Citation: Cleveringa, A., Ransom, C.J., Mcdaniel, M., Moore, K., Niemi, J.B., Miguez, F. 2025. Losing control: What happens when the control treatment is excluded in nitrogen fertilizer rate trials?. Agronomy Journal. 117. Article e70243. https://doi.org/10.1002/agj2.70243.
DOI: https://doi.org/10.1002/agj2.70243

Interpretive Summary: Farmers need accurate, reliable nitrogen fertilizer recommendations to maximize crop yields while minimizing environmental impacts. Scientists determine the optimal fertilizer rate using experiments that test different fertilizer rates and fit a model to the data. This study investigated how excluding the control treatment (no applied fertilizer) from these experiments affects the accuracy and precision of fertilizer recommendations. On-farm fertility trials sometimes abstain from using control treatments, but it was unclear how this decision impacts results. In this study, removing the control treatment led to the largest reduction of precision in estimating the optimal fertilizer rate, more so than removing any other fertilizer rate. This result supports the continued use of a control treatment in fertilizer trials to improve decision-making for sustainable agriculture.

Technical Abstract: Providing accurate and precise nitrogen (N) fertilizer recommendations remains a significant challenge. To arrive at a recommendation, researchers traditionally conduct hundreds or thousands of field experiments measuring the grain yield response to varying N fertilizer rates. A statistical model is then fit to the data to calculate the agronomic optimum nitrogen rate (AONR): – the fertilizer N rate that maximizes crop yield. We evaluated the impact of excluding individual fertilizer rates on the AONR estimate’s accuracy and precision using a mixed-effects quadratic-plateau (QP) model on a previously published dataset of 49 maize (Zea mays L.) fertility trials with 8 N fertilizer rates. When excluding the control treatment (0 kg N ha-1), the AONR deviated from the AONR calculated using all eight rates from 0 to 59 kg N ha^-1 for individual sites – a larger bias than when any other N rate was excluded. Excluding the control treatment also caused the greatest loss of precision in the AONR, with an average standard error increase of +43% without the control compared to +23% for other N rates. Furthermore, our simulations confirmed these findings and showed the largest losses of accuracy and precision when the control treatment was excluded. This also informed trends in bias and precision with different experimental designs. Our results demonstrate the importance of including the control treatment in N fertility trials designed to estimate AONR. Therefore, we recommend including a control treatment for a more robust N fertilizer recommendation program, especially in on-farm research.