1. Introduction
Tiller production in winter wheat (
Triticum aestivum L.) is the development of shoots from buds at the base of the main stem and is a critical factor to final yield [
1]. Early leaf and tiller development is crucial because the number of tillers per plant is a critical yield component [
2,
3,
4]. This is because tillers initiated in the fall make up the majority of spikes, compared to tillers initiated from 1 January until Zadok’s growth stage (GS) 30, and contribute up to 87% of grain yield, i.e., while tillers will continue to develop into the spring, the tillers produced in the fall will contribute the most toward yield [
5]. Scharf and Alley (1993) [
6] showed that when tiller density is low in spring, then N should be split and applied at GS 25 and GS 30 to stimulate additional yield. If tiller densities are not sufficient in late January to early February, then N should be applied to optimize and stimulate the needed tiller growth. Any tiller produced in March or later contributes less than 2% to the overall yield [
7]. The number of tillers per square meter (tiller density) is the proper method to determine whether a crop needs a split N application at GS 25. If the crop has a tiller density of 538 tillers per m
2 or greater, then a single application at GS 30 is sufficient [
8]. If the tiller density is less than 538 tillers per m
2, then N should be applied at GS 25. If the tiller density is 323 to 537 tillers per m
2, then 45 to 56 kg N ha
−1 should be applied. If tiller density is very thin with only 215 to 322 tillers per m
2, then 56 to 78 K N ha
−1 should be applied [
8].
Worldwide, 18% of N applied to cropping systems is applied to wheat [
9] and this is one of the largest expenses to producers, with the cost continuing to rise. From 2021 to 2022, the price of 24-0-0-3 (one of the most common liquid N fertilizers) increased by 267% (Gulasky, A. personal communication, 2022). However, wheat typically only uses approximately 33% of N application, with the rest being lost through leaching, volatilization, and denitrification, or immobilization which is not a loss [
10]. Leaching of N can pollute to ground and surface waters causing acidification, eutrophication of aquatic systems, and toxicity to animals [
11]. Therefore, a method to apply N based on need rather than a blanket application is crucial.
Even though applying N based on tiller density has been proven beneficial to wheat production, it is often not utilized due to tiller variability across the field and the amount of time involved in physically counting tillers [
12]. Vegetation indices incorporating red and near-infrared (NIR) canopy reflectance, such as NDVI, are now widely used for N monitoring models for several crops [
7,
13]. Previous work on 22 site-years in Virginia from 2000 to 2002, has shown that ground collected NDVI was well correlated with tiller density, with r
2 of 0.74, indicating that NDVI collected with hand-held optical sensors at GS 25 has the ability to predict tiller density without having to physically count the tillers [
14]. While they found that there is a saturation point at which NDVI will no longer increase as tiller density increases, this occurs beyond GS 25 when an N recommendation would be needed.
Aerial indices from UAV platforms or satellite can often outperform ground measurements as they provide an overall view of the entire area, not just a small area where the hand-held reflectance sensor is being pointed. For example, in peanut, ground indices predicted leaf loss from disease at an r
2 of 0.30, while aerial indices predicted leaf loss at an r
2 of 0.73 [
15]. Oakes and Balota (2016, unpublished data) also found that aerial NDVI was correlated with ground NDVI at an r
2 of 0.89. Similarly, Zhang et al. 2021 found that image data from an UAV could non-destructively diagnose wheat N status in China. Similarly, Jiang et al. (2020b) [
16] found that an active sensor mounted to an UAV platform could successfully monitor the growth and N nutrition status of winter wheat.
While abundant work has been carried out using ground NDVI and aerial sensors towards monitoring crop N status, little work has specifically examined the relationship between aerial indices and early season tiller density to determine N split application rates in winter wheat production. Therefore, the objectives of the study were to (1) determine the relationship between aerial indices and tiller density, (2) use aerial indices to develop a model for N application requirement at GS 25 in small plots, and (3) validate the model in large scale settings in growers’ fields.
4. Conclusions
To determine whether there was a relationship between tiller density and aerial indices (NDVI and NDRE), this study initially examined the relationship between these indices and tiller density. Aerial NDVI and aerial NDRE were significantly related with tiller density and these indices were excellent proxies for tiller density. Subsequent studies, both in small plots and in larger strips on growers’ fields, showed that these methods recommend similar amounts of N to be applied at GS 25 and that the grain yield was similar whether N was applied based on tiller density, NDVI, or NDRE. Based on the results from this study, we can recommend that aerial NDVI and NDRE can be used as proxies for tiller density using the equations reported here. Specifically, if NDVI is above 0.62, then N is not needed in that particular location and field. If below 0.62, then
Table 7 should be used for N application rates at GS 25. Similarly, for aerial NDRE, the threshold was 0.29. The ability to collect aerial NDVI or NDRE from a field gives growers a valuable tool in becoming able to assess the status of their winter wheat crop in January and February and help pinpoint the N needs at early growth stages.
In summary, tiller density is an important tool for growers to determine the rate and timing of mid-winter N application in winter wheat, but using vegetation indices NDVI and NDRE sensed remotely from a UAV is even more powerful because it can save growers time and N.