**2. Materials and Methods**

#### *2.1. Lint Yield and Fertilizer N Data*

Lint yield and fertilizer N application rate data for the farmer practice (FP) and OS-based VRN managemen<sup>t</sup> were from 21 study locations (Table 1). Farmers participating in the trials were eligible to receive payments to adopt VRN through USDA NRCS EQIP [8]. Stefanini et al. [24] previously reported di fferences in field-level fertilizer N use, lint yields, and profitability. The on-farm field trials were conducted between 2011 and 2014 at six locations in Tennessee, four locations in Mississippi, five locations in Louisiana, and six locations in Missouri. Most locations had only one year of data. However, several locations had two to three years of trials. Within each of the locations with multiyear trials, di fferent fields were used for each year. A total of 29 site-years of data were collected in the study.

The field trial experimental design for each site-year was a randomized complete block design with three fertilizer N treatments and three replications. A strip-plot running the entire field length was used as the plot for each treatment in each replicate. Each strip-plot was further divided into sub-plots. The sub-plots were used to implement the two VRN treatments evaluated in this study. Cotton was planted on the nine strip-plots at each site, each with 8 to 10 sub-plots, that measured approximately 30.5 by 11.6 m. A di fferent field on each participating farm was used for each site-year. While researchers attempted to choose similar field sizes in each year of the study, variation in field sizes resulted in di fferent numbers of sub-plots within the strip-plots among the site-years (Table 1). However, the same number of sub-plots for each strip-plot within each site-year was maintained during the study.

The trials evaluated FP N managemen<sup>t</sup> versus two OS-based VRN managemen<sup>t</sup> regimes. The FP treatment was N application based on the farmer's current practice. Cotton farmers and their crop consultants often formulate their fertilizer N rate for the field using University recommendations, their experience, and agronomic and soil considerations [6]. Optical sensing-based VRN treatment 1 (VRN 1) was VRN managemen<sup>t</sup> calculated using the normalized di fference vegetation index (NDVI) readings collected with the GreenSeeker ™ Crop Sensing System (Trimble, Sunnyvale, CA, USA) or Yara ™ N-Sensor (Yara North America, Tampa, FL, USA) canopy optical-sensing. The configurations of sensor arrays were di fferent in each state where the field trials took place. In Tennessee, a GreenSeeker ™ RT200 system with six sensors (1.93 m apart and 0.76 m above the cotton canopy) covering 12 rows of cotton (11.58 m wide) was used to collect about two scans s<sup>−</sup><sup>1</sup> at a field speed of about 7.64 km h−1. The second OS-based VRN treatment (VRN 2) was VRN managemen<sup>t</sup> based on NDVI readings but augmented with additional information.


**Table 1.** State and county/parish locations of the farm fields.

A The number in parentheses indicates the total number of sub-plots at each field site. The field trials were conducted using a randomized complete block design at each site. Two variable rate nitrogen treatments were compared against the existing farmer practice. Each treatment was replicated three times in three strip-plots at each site. Strip-plots were divided into 8–10 sub-plots to implement the two variable rate nitrogen treatments in the field trials. Different fields with dissimilar sizes were used on each farm in each year of the study and resulted in a variable number of sub-plots at each site. However, the same number of sub-plots for each strip-plot within each site-year was maintained during the study. Yields were measured in Missouri at the strip-plot rather than the sub-plot level.

Two split applications of fertilizer N were made for the three fertilizer N managemen<sup>t</sup> regimes. Starter fertilizer was applied at or before the planting of cotton and was determined by each farmer participating in the study. A uniform blanket rate of fertilizer N was applied to the entire field (covering all three treatment areas) with rates ranging from 33.6 to 78.4 kg N ha−1, depending on the farm field location. A second side dress application of fertilizer N for the FP was made at approximately the early bloom stage. For the two VRN treatments, crop N status was determined using canopy optical-sensing at about the early bloom stage for each site-year of the trial and fertilizer N was side dressed variably on the sub-plots, thereafter based on the NDVI readings for the VRN 1 treatment and NDVI readings and either digital yield maps (Mississippi and Tennessee), soil productivity zones (Louisiana), or soil zones (Missouri) for VRN 2. Each state used different algorithms for the VRN 1 and VRN 2 treatments because each state has different soils, climates, and managemen<sup>t</sup> practices for cotton. The unpublished algorithms were developed based on multiple-year and multiple-location data from previous research in each state.

The other production practices used to grow cotton on each field trial site were determined by the farmer cooperators in the study. Data collected for each sub-plot (strip-plot in Missouri) included harvested seed cotton yield, lint yields, applied fertilizer N rates, and latitude and longitudes for every field site, except in Missouri, where yield data were collected at the strip-plot level rather than by sub-plot (Table 2). In Louisiana, Mississippi, and Tennessee, cotton pickers with yield monitors were used to harvest cotton and determine sub-plot seed cotton and lint yields. Yield monitors were not available on cotton pickers at the Missouri sites so strip-plot yields rather than sub-plot yields were measured using

a weigh wagon. A measure of nitrogen use efficiency (NEFF), defined as lint yield divided by fertilizer N rate, was also calculated for each N managemen<sup>t</sup> regime (Table 2) [24].

**Table 2.** Field trial sub-plot mean, maximum, and minimum values and the number of sub-plot observations for lint yields, fertilizer nitrogen (N) rates, N e fficiency (lint yield/fertilizer N rate), and net returns for the three fertilizer N treatments that were collected from the 2011–2014 field trials.


a FP, farmer practice nitrogen managemen<sup>t</sup> on each field in the study. b VRN 1, variable rate nitrogen managemen<sup>t</sup> calculated using normalized difference vegetative index readings. c VRN 2, variable rate nitrogen managemen<sup>t</sup> based on normalized difference vegetative index readings and either digital yield maps (Mississippi and Tennessee), soil productivity zones (Louisiana), or soil zones (Missouri).
