By Jason Clark
Introduction
Nitrogen (N) is an essential plant nutrient commonly applied to South Dakota (SD) oat crops and is critical for optimizing yield. It is important to apply an appropriate rate of N fertilizer as too low of a rate reduces economic return while too high of a rate can lead to potential negative environmental effects. Common N rate recommendation approaches at this time include the yield goal approach and the maximum return to N (MRTN) approach (Morris et al., 2018).
The yield goal approach was developed in the 1970s and was the main system for creating crop N recommendations until the maximum return to N approach was developed in 2005 (Morris et al., 2018; Sawyer et al., 2006). One of the greatest strengths of the yield goal approach is simplicity, but that simplicity is likely not able to account for some of the challenges in using the yield goal approach, including being able to estimate yield, internal corn plant N efficiency (i.e., lbs N/bu corn) and other fertilizer use efficiency factors at the beginning of the season (Morris et al., 2018). The MRTN approach determines N rate recommendations based on N response curves from N rate studies conducted in each state or region within a state and is continually updated with new research data. Some of the advantages of the MRTN approach are the ability to incorporate economics into the N recommendation and group sites by different criteria that can influence N response like previous crop, soil type, location, or tillage practice. Some potential disadvantages of the MRTN approach include not accounting for residual N that is especially important in drier climates, the inability to tailor a recommendation for an individual field or zone within a field, and it is often difficult to estimate the price of corn and fertilizer as the selling of corn and purchasing of fertilizer are often at different times of the year. Thus, both approaches have their advantages and disadvantages. South Dakota currently uses a yield goal-based system to determine N fertilizer recommendations. The purpose of this fact sheet is to evaluate the accuracy of both approaches in accurately estimating N fertilizer needs of an oat crop.
Materials and Methods
Twenty-eight oat N rate response trials were conducted at field locations across central and eastern SD from 2017-2022. Site locations varied in tillage practice, crop rotation, and soil type. Specifically, 9 were in conventional till and 19 in no-till fields. The previous crop was soybean at 25 locations, and corn at 3 locations. Nitrogen fertilizer was applied before planting at rates from 0 to 150 lbs N ac-1. Nitrogen fertilizer as urea (46-0-0) was broadcast on the soil surface. Fertilizer was incorporated if conventional tillage practices were used or remained on the soil surface when no tillage was used. Soil samples were collected before planting and fertilizer application from the 0-6 and 6-24 in. depth increments and analyzed for nitrate-N (Nathan et al., 2015). Oat grain yield was determined by harvesting the center five feet of each plot and adjusting grain weight to 13% moisture.
Economic optimal N rates (EONR) were determined by modeling the relationship between oat yield and N fertilizer rate by averaging the results from both the linear-plateau and quadratic-plateau models using a N fertilizer price to oat price ratio of 0.12 (Miguez & Poffenbarger, 2022). The oat price was $3.60/bu oat and $0.42/lb N. If no plateau was reached within the N rates used in the study, the economic optimal N rate was set to the maximum N rate used at that location. For the yield goal approach, the following recently updated equation was used:
Source : sdstate.edu